Title:	Vaal River System Analysis Update:

Hydrology of the Lower Vaal Catchment

Author:	MJ Tukker

Project Name:	Vaal River System Analysis Update

Project No:	7145

Status of report:	Final

DWAF Report No:	PC00/00/16596

First Issue:	July 1997

Final Issue:	January 1999

BKS (PTY) LTD / NINHAM SHAND (PTY) LTD / STEWART SCOTT INC. CONSORTIUM

Approved for Consortium by:

.................................................

RS McKenzie

Project Leader

DEPARTMENT OF WATER AFFAIRS AND FORESTRY

Directorate of Water Resources Planning

Approved for the Department of Water Affairs and Forestry by:

.................................................

JI Rademeyer

Deputy Chief Engineer

(Project Manager)

.................................................	..................................................

JA van Rooyen	MD Watson

Director	Chief Engineer

Vaal River System Analysis Update

Hydrology of the Lower Vaal Catchment

EXECUTIVE SUMMARY

INTRODUCTION

The Vaal River system is the most important water resource system in South Africa as it provides water to more than 40 % of the country's inhabitants and, with numerous industries and mines in the supply area, supports the production of more than 50 % of the country's gross domestic product. The first system analysis on the Vaal River System, covering the period October 1920 to September 1984, was carried out in 1985. As a result of various physical changes in the system the Vaal River System model was upgraded in 1993, although the hydrology from the first analysis was still used. In 1995 the Department of Water Affairs and Forestry (DWAF) decided to update the model and the Vaal River System Analysis Update (VRSAU) study was commissioned. The VRSAU study now includes the update of both the hydrology as well as the physical characteristics of the system. The hydrology has been extended by an additional 11 years (to cover the period October 1920 to September 1995, i.e. to the end of the 1994 water year) that include the last part of the drought in the eighties, as well as the extremely low inflow during the early nineties. This extension has also enabled the salinity models to be calibrated with greater confidence.

SCOPE OF THE REPORT

The Integrated Vaal River System consists of water supply schemes in the Vaal River Basin together with inter-basin transfer schemes linking the Vaal and adjoining basins. The system therefore includes several major subcatchments. Based on practical considerations, the total Vaal River System was sub-divided into eight subcatchments in order to facilitate hydrological analyses. These subcatchments are:

The Upper Vaal catchment (Vaal River catchment upstream of Vaal Dam)

The Barrage catchment (Vaal River catchment between Vaal Dam and Vaal Barrage)

The Middle Vaal catchment (Vaal River catchment downstream of Vaal Barrage and upstream of Bloemhof Dam)

The Lower Vaal catchment (Vaal River catchment downstream of Bloemhof Dam)

The Komati catchment (Upstream of Swaziland Border)

The Usutu catchment (Upstream of Swaziland Border)

The Tugela catchment

The Senqu catchment (Orange River catchment within the Lesotho Border).

This report presents the extension and upgrading of the hydrology of the Lower Vaal River and its tributaries between Bloemhof Dam and the Orange and Vaal River confluence.

The total catchment covers an area of over 88 000 km2 and comprises three distinct river systems, the Harts River, the Lower Vaal River and the Modder and Riet Rivers. The Harts River catchment lies towards the north and west of the study area upstream of the confluence with the Vaal. The Vaal River catchment lies in a band from west to east across the middle of the study area north of the third system, the Riet and Modder River catchment. The major centres within the area include Bloemfontein, Botshabelo, Thaba Nchu and Kimberley.

This document reports the hydrological investigations performed for the Lower Vaal system in order to provide input to the system model in the form of naturalised monthly run-off. This report contains a description of the catchment modelling procedures followed to obtain the naturalised flows, the results to be provided to the system model and recommendations on the future hydrological monitoring of the catchment.

The investigation and modelling strategy employed, followed a basic structure:

-	Collection and preparation of data required for catchment modelling :

-	requests for flow, meteorological, land-use, water supply and effluent data from the relevant authorities

-	coarse pre-screening of available flow records

-	study area overflight and video recording of main channel and riparian zone

-	sub-division of the study area into run-off modules according to the location of reliable flow gauging stations

-	evaluation and processing of available rainfall and evaporation records

-	processing of land-use data and the preparation of demand/supply sequences

-	collection of data on farm dams and diversion weirs

-	Modelling the catchment using SHELL (Berg and Beuster, 1992) which is similar to the WRSM90 modelling system (Midgley, Pitman and Middleton, 1994). SHELL incorporates the PITMAN model (Pitman,1973), a standard rainfall/run- off model, and various other routines to model reservoirs, forestry, abstractions and return flows. The PITMAN model was then calibrated at the selected flow gauging sites, taking into account historical land-use developments.

-	Monthly naturalised time series were produced for a 75 year period (hydrological years 1920-1994). The calibration parameters were used to produce the time series at the calibration sites and for the remaining uncalibrated portions of the catchment the WR90 (Midgley et al, 1994) regional parameters were used.

-	Recommendations for improving the hydrological monitoring network and knowledge about land-use development in the catchment were made.

RESULTS

Pitman model calibrations were undertaken for ten flow gauging stations. The naturalised incremental run-off at each station was then calculated; the summary of which is given in Table 1.

Table 1:	Naturalised incremental run-off at the calibration sites

C3H003	Taung	7975	530	1916	59.0	7.4
C3R002	Spitskop	9249	438	2039	77.5	8.4
C5H016	Aucampshoop	1847	350	2050	6.4	3.5
C5H018	Tweerivier	2236	422	1871	14.4	6.4
C5R001	Tierpoort	922	491	1640	23.8	25.8
C5R002	Kalkfontein	8781	412	1746	215.9	24.6
C5R003	Rustfontein	937	543	1600	30.7	32.7
C5R004	Krugersdrift	5391	508	1639	114.4	21.2
C9H009	De Hoop	3201	406	1963	12.9	4.0
C9R001	Vaalharts	2509	444	1946	11.2	4.5
TOTAL	43048	566.2	138.5

CONCLUSIONS AND RECOMMENDATIONS

Conclusions

This hydrological study has resulted in an extended set of naturalised run-off sequences for the Lower Vaal River catchment and producing the following conclusions:

-	The results of various studies of the study area appear to be comparable, with the main discrepancies being in the effective catchment areas.

-	The naturalised run-off estimates of the current study for the Harts River catchment compare very favourably with those of WR90 as the calibration parameters used varied, only slightly, from regional parameters and were similar to those used in WR90. The estimates for gauge C3R002 at Spitskop are, however, higher than those reported in the Long Term Salt Balance of the Vaalharts Irrigation Scheme as the observed record has been updated by DWAF. The unit run-off estimates vary due to the different effective catchment areas used.

-	For the Vaal River catchment between Bloemhof Dam and the Orange and Vaal River confluence, the calculated incremental naturalised run-off values are similar to those in WR90 as the same regional parameters were used.

-	In the Modder/Riet catchment, there are large differences in the estimates of incremental naturalised run-off reported in the current and previous studies. The naturalised run-off into Kalkfontein Dam at gauge C5R002 is significantly higher in the current study due to the updating of the flood events in the inflow record by DWAF.

-	In the current study, the incremental naturalised inflow to Krugersdrift Dam measured at C5R004 is significantly lower than those reported in previous studies. The impervious areas in the previous studies were either ignored or under-estimated by excluding Thaba Nchu and Botshabelo. The current study modelled more extensive urban impervious areas which resulted in increased surface run-off because an increase in the simulated historical flow results in a decreased "observed" natural flow.

-	In subcatchments where flows and irrigation are primarily supported by compensation releases from an upstream reservoir, caution must be exercised during calibration and naturalisation. The demands, irrigation or other, must be supplied by the actual compensation flows rather than the catchment run-off and river flows. Failure to model the abstraction of demands in this way results in extreme over-estimation of natural run- off when the irrigation demands are added back to the catchment run-off during the naturalisation process.

-	The standard method of estimating evaporative losses by calculating the net loss from an average river width and length needs to be reviewed.

-	The WR90 Regional Parameters appear to give a slightly conservative estimate of natural run-off in the catchments along the Vaal River and lower Modder and Riet catchments.

Recommendations for flow gauging

-	The Lower Vaal River catchment has a reasonable distribution of flow gauges, however, certain of these gauges produce unreliable records which need careful screening and patching. Some gauges are also not sited at optimal monitoring points.

-	The Harts River and Lower Vaal River catchments have numerous streamflow gauges in the main channel of both the Vaal and the Harts Rivers. There is significant irrigation in the region and therefore irrigation abstractions, transfers and return flows need to be monitored accurately. The following are streamflow gauging recommendations;

-	The inflow and spill records of Wentzel Dam are considered unreliable as there are no monitoring gauges upstream or downstream of the dam. It is recommended that this reservoir be gauged in order to accurately monitor the run-off and effects of the irrigation in the upper Harts River catchment. Further consideration should also be given to gauging the flows entering the Harts River from the Dry Harts River as these account for over 35% of the total flow from the Harts River catchment. As the return flows from the Vaalharts Irrigation Scheme have a significant effect on the inflows into Spitskop Dam, a more direct method of measuring these return flows should be sought.

-	A new flow gauging site is proposed for the Vaal River downstream of the Vaal and Harts River confluence and the Vaal Gamagara abstraction point. This would record the flow in the Vaal River, available for use by riparian irrigators along the Vaal River upstream of Douglas. The station at St Claire/Douglas (C9H007) on the lower Vaal River was closed in 1978. The weir has been upgraded (C9R003) and flows have been recorded since 1978 but the record is not available. This gauge is the most downstream station on the Vaal River and would provide useful information as to volume of water leaving the Vaal catchment.

-	The Modder and Riet catchments are essentially monitored by the inflows to the four major reservoirs, as most of the streamflow gauges are situated in tributaries of the two main rivers. After pre-screening the gauges and during the calibration process many of the streamflow gauges were rejected due to inaccurate records or a gauge's close proximity to a reservoir where the reservoir inflow record was preferred.

-	Records of the inflow into Mockes Dam would provide valuable information about the water availability and consumption in the upper reaches of the Modder River below Rustfontein Dam. It would also provide the opportunity to monitor, more closely, the developments in Thaba Nchu, Botshabelo and Bloemfontein and the effects of the Caledon-Modder Transfer. It would also provide more accurate information on the availability of water for abstractions at Mazelspoort Weir for consumption by the greater Bloemfontein. The same recommendation was made in the Orange River System Analysis (DWAF, 1991), but as yet there have been no upstream gauge constructed or calibration of the spillway at the dam.

-	There are a large number of uncertainties in terms of irrigated area and irrigation demand and supply in the Riet River catchment. The catchment therefore needs to be monitored more closely and therefore a number of potential sites for a new gauges in the Riet River are proposed.

-	A gauging station is required just upstream of the confluence of the Riet and Kaffer Rivers to monitor the run-off and effects of irrigation in the entire upper Riet River catchment. A further gauge could be located in the Riet River between the Riet and Kaffer River confluence and Kalkfontein Dam to monitor the effects of spills from Tierpoort Dam and the Kaffer River irrigation supplied by compensation releases from the dam. The inflow and spill record at Tierpoort Dam are considered doubtful and therefore a downstream gauge, other than Kalkfontein Dam (C5R002), would act as a check for the spills and releases from Tierpoort.

-	Another proposed site for useful gauging is in the lower reaches of the Kromellenboogspruit, which drains a large portion of the incremental catchment between Tierpoort Dam and Kalkfontein Dam and joins the Riet just upstream of Kalkfontein Dam.

-	A new weir was built at Tweerivier (C5H018), the Ritchie weir (C5H014) upgraded and a new weir built near the Vaal and Riet confluence, downstream of Aucampshoop (C5H016) in 1989. These records need to be maintained and managed to ensure they continue to provide valuable information.

Recommendations for catchment and landuse information

-	Irrigated agriculture is the main type of landuse in the entire catchment. There is no afforestation in the Lower Vaal catchment and livestock farming is very limited. There are several major Government Water Schemes in the region including the Vaalharts Scheme. None of these irrigation schemes have readily available information on actual water use, total irrigated area or crop types which is essential information needed for the representative modelling of the catchment developments in the catchment. The aerial extent and water use by smaller schemes and diffuse users was almost non-existent. In most cases no record of the historical development of these irrigation areas had been kept.

-	It is therefore recommended that the aerial photography and mapping of the region be updated in order to determine the current day extent of irrigated area. A database of all current and historical irrigation information and contact names should be compiled.

-	More accurate and standardised estimates of effective catchment areas need to be agreed upon.

Vaal River System Analysis Update

Hydrology of the Lower Vaal Catchment

TABLE OF CONTENTS

Page

EXECUTIVE SUMMARY	(i)

1.	INTRODUCTION	1 - 1

1.1	BACKGROUND	1 - 1

1.2	PURPOSE OF THE STUDY	1 - 2

1.3	SCOPE OF THE REPORT	1 - 2

1.4	RELEVANT PREVIOUS STUDIES AND REPORTS	1 - 3

1.5	CATCHMENT DESCRIPTION	1 - 3

1.5.1	The Harts River Catchment	1 - 4

1.5.2	The Vaal River	1 - 5

1.5.3	The Riet and Modder Rivers	1 - 5

1.6	MODELLING METHODOLOGY	1 - 7

2.	CATCHMENT DEVELOPMENTS	2 - 1

2.1	IMPOUNDMENTS	2 - 1

2.1.1	Large dams	2 - 1

2.1.2	Small dams	2 - 3

2.2	IRRIGATION	2 - 5

2.2.1	Introduction	2 - 5

2.2.2	Historical development	2 - 6

2.2.3	Present Irrigation areas	2 - 8

2.2.4	Water usage by irrigation	2 - 11

2.2.5	Historical growth in irrigated area	2 - 12

2.3	ABSTRACTIONS AND RETURN FLOWS	2 - 14

2.3.1	Abstractions, in-basin and inter-basin transfers	2 - 14

2.3.2	Irrigation return flows	2 - 15

2.4	URBANISED AREAS	2 - 15

2.4.1	Urban Return Flows	2 - 16

2.5	TRANSMISSION LOSSES	2 - 16

3.	HYDROLOGICAL DATA	3 - 1

3.1	PRECIPITATION	3 - 1

3.1.1	Selection, patching and verification	3 - 1

3.1.2	Catchment area rainfall inputs	3 - 2

3.2	STREAMFLOW	3 - 2

3.2.1	Gauges considered but not used	3 - 3

3.2.2	Gauges used for calibration	3 - 4

3.2.3	Patching of Streamflow Records	3 - 6

3.2.4	Subdivision of the study area	3 - 8

3.3	EVAPORATION	3 - 8

4.	HYDROLOGICAL MODEL CALIBRATION	4 - 1

4.1	INTRODUCTION	4 - 1

4.1.1	Component configuration	4 - 1

4.1.2	Calibration of the PITMAN Model	4 - 2

5.	CALIBRATION RESULTS AND DISCUSSION	5 - 1

5.1	Run-off MODULE GAUGED BY C3H003 (HARTS RIVER)	5 - 1

5.1.1	Summary of run-off module data	5 - 1

5.1.2	Description of the run-off module	5 - 1

5.1.3	Model configuration	5 - 2

5.2	RUN-OFF MODULE GAUGED BY C3R002 (HARTS RIVER)	5 - 3

5.2.1	Summary of run-off module data	5 - 3

5.2.2	Description of the run-off module	5 - 4

5.2.3	Model configuration	5 - 4

5.3	RUN-OFF MODULE GAUGED BY C5H016 (RIET RIVER)	5 - 6

5.3.1	Summary of run-off module data	5 - 6

5.3.2	Description of the run-off module	5 - 7

5.3.3	Model configuration	5 - 8

5.4	RUN-OFF MODULE GAUGED BY C5H018 (MODDER RIVER)	5 - 9

5.4.1	Summary of run-off module data	5 - 9

5.4.2	Description of the run-off module	5 - 10

5.4.3	Model configuration	5 - 10

5.5	RUN-OFF MODULE GAUGED BY C5R001 (KAFFER RIVER)	5 - 11

5.5.1	Summary of run-off module data	5 - 11

5.5.2	Description of the run-off module	5 - 12

5.5.3	Model configuration	5 - 12

5.6	RUN-OFF MODULE GAUGED BY C5R002 (RIET RIVER)	5 - 13

5.6.1	Summary of run-off module data	5 - 13

5.6.2	Description of the run-off module	5 - 13

5.6.3	Model configuration	5 - 14

5.7	RUN-OFF MODULE GAUGED BY C5R003 (MODDER RIVER)	5 - 15

5.7.1	Summary of run-off module data	5 - 15

5.7.2	Description of the run-off module	5 - 15

5.7.3	Model configuration	5 - 16

5.8	RUN-OFF MODULE GAUGED BY C5R004 (MODDER RIVER)	5 - 17

5.8.1	Summary of run-off module data	5 - 17

5.8.2	Description of the run-off module	5 - 17

5.8.3	Model configuration	5 - 18

5.9	RUN-OFF MODULE GAUGED BY C9H009 (VAAL RIVER)	5 - 20

5.9.1	Summary of run-off module data	5 - 20

5.9.2	Description of the run-off module	5 - 21

5.9.3	Model configuration	5 - 22

5.10	RUN-OFF MODULE GAUGED BY C9R001 (VAAL RIVER)	5 - 23

5.10.1	Summary of run-off module data	5 - 23

5.10.2	Description of the run-off module	5 - 23

5.10.3	Model configuration	5 - 24

5.11	RUN-OFF MODULE (LOWER VAAL RIVER)	5 - 25

5.11.1	Summary of run-off module data	5 - 25

5.11.2	Description of the run-off module	5 - 25

5.11.3	Model configuration	5 - 26

5.12	SUMMARY OF MODEL CALIBRATIONS	5 - 26

6.	NATURALISATION	6 - 1

6.1	NATURALISATION OF GAUGES C9R001, C9H009, C5H018,

C5H016 AND THE LOWER VAAL CATCHMENT	6 - 1

6.2	ELIMINATING NEGATIVE INCREMENTALS	6 - 3

6.3	COMPARISON OF RESULTS WITH PREVIOUS STUDIES	6 - 4

7.	CONCLUSIONS AND RECOMMENDATIONS	7 - 1

7.1	RECOMMENDATIONS FOR FLOW GAUGING	7 - 1

7.1.1	Harts and Lower Vaal River catchment	7 - 1

7.1.2	Modder and Riet River catchment	7 - 2

7.2	RECOMMENDATIONS FOR CATCHMENT AND LANDUSE

INFORMATION	7 - 3

7.3	CONCLUSIONS	7 - 3

8.	REFERENCES	8 - 1

LIST OF TABLES

Table No.

2.1	Details of the Major Dams in the Lower Vaal Catchment

2.2	Farm Dam Capacities for the Run-off Modules given per Quaternary

2.3	Original Scheduled Areas - Vaalharts Irrigation Scheme

2.4	Updated Scheduled Areas - Vaalharts Irrigation Scheme

2.5	Summary of Current Irrigated Areas in the Modder and Riet Catchments

2.6	Growth of Irrigated Areas in the Lower Vaal Catchment

2.7	Major In-Basin and Interbasin Transfers

3.1	Flow Gauging Stations Considered in the Lower Vaal Catchment

3.2	Patching of Tierpoort Reservoir Record (C5R001)

3.3	Details of Evaporation Stations in the Lower Vaal River Catchment

4.1	WR90 Regional Pitman Paramters for the Lower Vaal Catchment

4.2	Acceptable Limits for the Calibration of the Pitman Model

5.1	Summary of the Data for Run-off Module Gauged by C3H003

5.2	Summary of the Data for Run-off Module Gauged by C3R002

5.3	Summary of the Data for Run-off Module Gauged by C5H016

5.4	Summary of the Data for Run-off Module Gauged by C5H018

5.5	Summary of the Data for Run-off Module Gauged by C5R001

5.6	Summary of the Data for Run-off Module Gauged by C5R002

5.7	Summary of the Data for Run-off Module Gauged by C5R003

5.8	Summary of the Data for Run-off Module Gauged by C5R004

5.9	Summary of the Data for Run-off Module Gauged by C9H009

5.10	Summary of the Data for Run-off Module Gauged by C9R001

5.11	Summary of the Data for the Lower Vaal River Run-off Module

5.12	Final Pitman Parameters for the Lower Vaal Catchment

6.1	Results of the Naturalisation Process

6.2	Comparison of Results with Previous Studies

6.3	Comparison of Results with WR90

APPENDICES

Appendix A	Maps

Appendix B	Catchment Developments

Appendix C	Interbasin and In-basin Transfers

Appendix D	Rainfall

Appendix E	Raw Streamflow Data

Appendix F	Patched Streamflow Data

Appendix G	Model Calibration Results

Appendix H	Natural Streamflow

Appendix I	Miscellaneous

ABBREVIATIONS

CCWR	Computing Centre for Water Research

DWAF	Department of Water Affairs and Forestry

FSA	Full Supply Area

FSC	Full Supply Capacity

GWS	Government Water Scheme

MAE	Mean Annual Evaporation

MAP	Mean Annual Precipitation

MAR	Mean Annual Run-off

WR90	Water Resources of South Africa : 1990 (Midgley et al, 1994)

WRYM	Water Resources Yield Model

1.	INTRODUCTION

1.1	BACKGROUND

The Vaal River system is the most important water resource system in South Africa as it provides water to more than 40% of the country's inhabitants and, with numerous industries and mines in the supply area, supports the production of more than 50 % of the country's gross domestic product. It is therefore important that the management of water resources within the system receive special attention.

In 1985 the first major water resource system analysis study to be carried out in South Africa, the Vaal River System Analysis (VRSA) study, was undertaken by BKS (Pty) Ltd in association with ACRES International and Stewart Scott Inc, on behalf of the Department of Water Affairs and Forestry (DWAF). The study was completed approximately four years later during which time the project team developed various new computer models and analysis techniques. The VRSA study addressed the water resources of the Vaal River basin as well as all adjacent donor catchments, which augment supplies to the demand centres supported from the Vaal River. Both water quantity and water quality issues were addressed in the study and many important observations and conclusions were made, based on results from the new system models.

The computer models and analysis techniques developed in this process, have since become a standard methodology used for water resource studies in South Africa. The reliability of the various models and techniques is extremely important to ensure that new schemes are not built prematurely, and on the other hand, that the imposition of water restrictions are not made unnecessarily, as this could have serious influences on the economy of the country.

As the study started in 1985, it was only possible to process the hydrological records to cover the period October 1920 to September 1984, i.e. up to the end of the 1983 water year. Unfortunately the 1983 water year was still part of a very severe drought event which was only broken after the good rains of 1988. The ending of the hydrological records in the middle of a severe drought was recognised to be a potential problem but the significance of this could not be established before the drought was broken and the analysis repeated. As a result of various physical changes in the system the Vaal River System model was upgraded in 1993, although the hydrology from the first analysis was still used. The length of the records can now be extended from the original 64 years to 75 years (to cover the period October 1920 to September 1995, i.e. to the end of the 1994 water year). The additional 11 years include the last part of the eighties drought as well as 3 years of extremely low inflow during the early nineties, both of which may influence the system yield capabilities.

Salinity is a major issue in the Vaal River system and salinity modelling was undertaken as part of the original VRSA study. Unfortunately there was very little data with which to calibrate the salinity models, with the result that the conclusions and recommendations made in this regard could not be adequately verified. In some cases only one or two years of reliable data were available with which to calibrate the models. The availability of the additional 11 years of information with water quality data has allowed the salinity models to be calibrated with much greater confidence.

The analysis techniques developed during the VRSA study have been used for the assessment of several other major water resource systems throughout South Africa including the Mgeni, Western Cape, Crocodile and Orange River systems. The analysis techniques were improved and extended during these studies, but these improvements have not, prior to the current study, been incorporated into any re-assessment of the Vaal River system. The modelling routines have been significantly improved/extended for the analysis of farm dam storage, paved areas, irrigation return flows and air pollution. These are only some of the numerous routines for which improvements to the model have been made. As a result, the processes involved can now be modelled more confidently.

1.2	PURPOSE OF THE STUDY

The purpose of the Vaal River System Analysis Update (VRSAU) study is to revise and update the hydrological and water quality databases used in the earlier studies and to re-assess the water quantity and quality capabilities of the whole Vaal River System (for the period October 1920 to September 1995, i.e. up to the end of the 1994 water year) using the most up-to-date information and techniques. The major shortcomings of the previous studies can now be taken into account.

1.3	SCOPE OF THE REPORT

The Vaal River System is inter-linked with various other basins due to transfers from these basins to the Vaal Basin as well as transfers from the Vaal Basin to other basins. As part of the VRSAU study the hydrology of the Vaal River Basin was updated together with the hydrology for subcatchments within the Komati, Usutu, Tugela and Senqu river basins.

The total Vaal River System can therefore be sub-divided into various subcatchments. Based on practical considerations, the total Vaal River System was sub-divided into eight subcatchments in order to facilitate hydrological analyses. These subcatchments are:

The Upper Vaal catchment (Vaal River catchment upstream of Vaal Dam)

The Barrage catchment (Vaal River catchment between Vaal Dam and Vaal Barrage)

The Middle Vaal catchment (Vaal River catchment downstream of Vaal Barrage and upstream of Bloemhof Dam)

The Lower Vaal catchment (Vaal River catchment downstream of Bloemhof Dam)

The Komati catchment (Upstream of Swaziland Border)

The Usutu catchment (Upstream of Swaziland Border)

The Tugela catchment

The Senqu catchment (Orange River catchment within the Lesotho Border).

This report presents the extension and upgrading of the hydrology of the Lower Vaal River and its tributaries between Bloemhof Dam and the Orange and Vaal River confluence.

1.4	RELEVANT PREVIOUS STUDIES AND REPORTS

-	Orange River System Analysis - Riet and Modder River: Hydrology (DWAF, 1991)

-	Hydrology of the Harts River (DWAF, 1987)

-	Mzimvubu Transfer Options of the Vaal Augmentation Planning Study: Reconnaissance Phase (DWAF, 1995)

-	Long-term Salt Balance of the Vaalharts Irrigation Scheme (Stewart Scott, 1994)

1.5	CATCHMENT DESCRIPTION

The total study area covers an area of over 88 000 km2 and is comprised of three distinct river systems, the Harts River, the Lower Vaal River and the Modder and Riet Rivers shown in Appendix A1. The Harts River catchment lies towards the north and east of the study area upstream of the confluence with the Vaal. The Vaal River catchment lies in a band from north east to south west across the middle of the study area north of the Riet and Modder River catchment. All three catchments are in the summer rainfall region of South Africa.

1.5.1	The Harts River Catchment

The Harts River drains a catchment area of approximately 31 000 km2 and has one major tributary, the Dry Harts which joins the Harts just downstream of Taung. The topography of the catchment is generally flat (the average slope of the Harts River is one in a thousand) and this results in a very low drainage density and an ill-defined watershed. To the west of the Dry Harts and the lower Harts, the geology consists predominantly of dolomite and limestone whereas extrusive igneous rocks underlie the rest of the catchment. The catchment consists of moderate to deep soils, clayey in the western region, sandy loams along the middle band running north to south and clayey loams in the north east region. Most of the western sector is Kalahari thornveld and scrub bushveld covers merging into grassland in the north east.

The MAP for the whole region is 460 mm but the MAP ranges between 500 and 600 mm in the upper reaches of the Harts decreasing gradually to just below 400 mm at the confluence with the Vaal. The MAE is relatively high and varies between 1 800 mm in the north east to 2 600 mm in the far south west.

A combination of the flat topography, low rainfall and high potential evaporation result in very low MAR from this region, between just 5 and 10 mm. In addition approximately 40 % of the total catchment is characterised by endoreic areas which do not contribute to streamflow.

There are a number of significant water usage features in this region. Barberspan is a natural off-channel storage system in the upper reaches of the Harts. Further downstream Wentzel Dam was constructed and relies totally on the natural flow of the Harts River. A very recent construction is the Taung Dam further downstream on the Harts River which at present does not supply irrigation or urban demands.

The most important water usage feature adjacent to the Harts River is the Vaalharts Irrigation Scheme. Water for irrigation is abstracted and transferred, via a complex system of canals, from the Vaalharts Weir on the Vaal River to the irrigators between the lower Harts River and the Vaal River. Spitskop Dam was constructed on the lower Harts River to supply the irrigators in the region and stores the significant return flows from the Vaalharts Scheme.

1.5.2	The Vaal River

The stretch of the Vaal River considered in this study is the reach between Bloemhof Dam and the Orange and Vaal River confluence. The total catchment area is almost 22 500 km2.

The vegetation is predominantly Kalahari thornveld with areas of grassland in the east and small zones of Karoo in the south and west. The geology is comprised of three distinct groups. In the far west the geology is predominantly dolomite with extrusive igneous rocks north of the Vaal and argillaceous strata south of the river. There is a broad centre band running north to south of moderate to deep sandy loam soils. The rest of the catchment has moderate to deep clayey loam soils. The catchment is flat becoming undulating in the far south.

The MAP of the region is 398 mm with a large part of the eastern catchment experiencing a MAP of 400 - 500 mm diminishing to under 400 mm in the lower reaches at the confluence with the Orange River. The MAE is 2 070 mm and ranges from 1 800 mm in the east to 2 600 mm in the west.

Of the total catchment area only 35 % contributes directly to the river network. The remainder drains into the many pans and enclosed drainage basins and is evaporated. As a result of these endoreic areas, the low rainfall and high potential evaporation, the MAR from the catchment is only between 0 and 5 mm.

Bloemhof Dam is at the head of this catchment but is not within the scope of this study, however, water is released from Bloemhof Dam to support downstream riparian irrigators and is diverted at the Vaalharts Weir to the Vaalharts Irrigation Scheme. Further compensation releases are made from Vaalharts Weir down the Vaal River to support the requirements of the various abstractors, urban industrial and irrigation.

There are two major transfers within this stretch of the Vaal, namely the Kimberley transfer from Riverton and the Vaal-Gamagara transfer just below the Vaal and Harts River confluence.

1.5.3	The Riet and Modder Rivers

The combined areas of the Riet and Modder catchments is approximately 35 000 km2. The Modder River, Kaffer River and Kromellenboogspruit are the three main tributaries of the Riet. The Kaffer River is a right bank tributary of the Riet and the confluence is downstream of Tierpoort Dam. The confluence with the Kromellenboogspruit is just upstream of the Kalkfontein Dam. The confluence of the Riet and the Modder Rivers is just upstream of Ritchie. The Modder River has a number of tributaries and these are, working downstream, the Kromspruit upstream of Rustfontein Dam, the Kgabanyane, Koranna, Os, Renosterspruit and Kaalspruit. The contributions of these catchments to the Vaal will only be significant during major flood events.

The vegetation comprises grassland in the east and Kalahari thornveld, invaded by Karoo, in the west. The catchment is mostly flat becoming gently undulating in the south east and far north west. The soils range from moderate to deep clayey loams and sandy loams in the east to shallow sandy soils in the west. The geology is predominantly tillite at the Vaal confluence changing to argillaceous strata in the central sector and intercalated arenaceous and argillaceous strata in the west.

The catchment MAP is 421 mm ranging from just under 600 mm in the east to well below 400 mm in the west. The catchment MAE is 1 832 mm and ranges between 1 500 mm in the east to 2 600 mm in the west. Of the three regions the Riet and Modder catchments have the highest MAR, from 20 - 50 mm in the east to 0 - 2.5 mm in the west. This is due to the steeper topography and more clearly defined river networks in the upper catchments of both rivers.

There has been extensive development in this region on both the Riet and the Modder River. The Tierpoort Dam was constructed on the Kaffer River for irrigation. Further downstream is the Kalkfontein Dam used to supply water to the Riet River Government Water Scheme (RRGWS), Riet River Settlement and various other irrigation schemes on the lower Riet.

Three major dams are located on the Modder River which is the main tributary of the Riet. Rustfontein Dam, near the head of the catchment is the primary source of supply to the Bloemfontein area. Mockes Dam is a balancing dam further downstream to regulate the flow through Mazelspoort Weir where water is abstracted for urban consumption in Bloemfontein. Krugersdrift Dam in the lower catchment supplies water to the Modder River Government Water Scheme in controlled releases.

1.6	MODELLING METHODOLOGY

The investigation and modelling strategy employed, followed a basic structure:

-	Collection and preparation of data required for catchment modelling :

-	requests for flow, meteorological, land-use, water supply and effluent data from the relevant authorities

-	coarse pre-screening of available flow records

-	study area overflight and video recording of main channel and riparian zone

-	sub-division of the study area into run-off modules which are defined as subcatchments of the total study area. These run-off modules are determined by the location of reliable flow gauging stations

-	evaluation and processing of available rainfall and evaporation records

-	processing of land-use data and the preparation of demand/supply sequences

-	collection of data on farm dams and diversion weirs.

-	To model the catchment an in-house modelling system known as SHELL (Berg and Beuster,1992) was used. SHELL is similar to the WRSM90 modelling system (Midgley, Pitman and Middleton, 1994) and incorporates the PITMAN model (Pitman, 1973), a standard rainfall/run-off model, and various other routines to model reservoirs, forestry, abstractions and return flows. The PITMAN model was then calibrated at the selected flow gauging sites, taking into account historical land-use developments.

-	Monthly naturalised time series were produced for a 75 year period (hydrological years 1920-1994). The calibration parameters were used to produce the time series at the calibration sites and for the remaining uncalibrated portions of the catchment the WR90 (Midgley et al, 1994) regional parameters were used.

-	Recommendations for improving the hydrological monitoring network and knowledge about land-use development in the catchment were made.

2.	CATCHMENT DEVELOPMENTS

2.1	IMPOUNDMENTS

Due to the extreme water shortages in substantial areas of the lower Vaal catchment, many dams have been constructed in an attempt to augment and stabilise water supply for irrigation.

2.1.1	Large dams

Most of the major dams have already been mentioned briefly in the previous section, however each one will be dealt with in detail here.

The Wentzel Dam on the Harts River was completed in 1935 and was built to supply water to the demands of Schweizer Reneke town and limited areas of scheduled irrigated land.

Taung Dam was built on the Harts River in 1993 to augment irrigation supplies to the Taung irrigation area and possibly support new irrigation areas in the Pudimoe area. However, at the time of this study the dam was at 60 % of FSC and was not being used for irrigation supplies. The construction of a pumping station, weir and canal system to transfer the water from the dam for irrigation purposes would be uneconomical. In future drought periods, water could be released from Taung Dam to augment supplies to Spitskop Dam (Visser, 1996).

Spitskop Dam was constructed in 1975 in order to supply irrigators along the lower Harts upstream of the Vaal confluence. The dam wall was partly washed away in the floods of March 1988 but reconstructed by September 1989. The dam is situated downstream of the Vaalharts Irrigation Scheme and therefore substantial volumes of return flows seep into the dam.

Vaalharts Weir was constructed on the Vaal River as part of the Vaalharts Government Water Scheme in the early 1930's. Water from the Vaal River which is supplemented by releases from Bloemhof Dam is diverted from the weir to supply water to the extensive areas of scheduled irrigation land adjacent to the Harts River.

In the Modder/Riet catchments there are numerous large dams most of which were constructed to supply or support irrigation requirements. Tierpoort Dam on the Kaffer River was built in the early 1920's for irrigation purposes. The capacity of the dam was significantly reduced due to siltation which has limited the irrigation area the dam is able to support to 7.4 km2, just over 15 % of the original 1922 scheduled area. The dam wall was washed away in the 1988 floods but rebuilt to the original capacity in early 1990. Downstream of Tierpoort Dam on the Riet River is Kalkfontein Dam which was commissioned in 1938 to supply water to the Riet River Government Water Scheme.

Rustfontein Dam was constructed in 1954 as a major supply source for the Bloemfontein area. Water released from Rustfontein into the Modder River and spills from Groothoek Dam which (Groothoek Dam supplies the demands of Thaba Nchu) reach Mockes Dam which acts as a balancing dam to stabilise flows to the Mazelspoort purification works at Mazelspoort Weir from where the water is distributed to the Bloemfontein area. Krugersdrift Dam is located further down the Modder River and was commissioned in 1971 to supply water to the Modder River Government Water Scheme.

The capacity and area tables for the dams were requested from DWAF and from these, a unique area capacity relationship (see Equation 2.1) was established for each of the dams.

Table 2.1:	Details of the major dams in the Lower Vaal Catchment (DWAF)

Wentzel	C3R001	9219	5.988	2.91	0.926	0.771
Taung	C3R006	-	60.000	-	-	-
Spitskop	C3R002	26914	56.664	25.09	2.833	0.540
Tierpoort	C5R001	922	33.709	89.98	1.395	0.503
Kalkfontein	C5R002	10268	318.936	45.32	1.367	0.590
Rustfontein	C5R003	940	72.207	11.57	0.936	0.536
Krugersdrift	C5R004	6315	73.194	18.53	0.671	0.753
Groothoek	C5R005	116	13.243	2.49	0.811	0.310
Vaalharts Weir	C9R001	115055	48.663	21.19	1.238	0.783

Note:	Coefficients a and b are defined in Equation 2.1

2.1.2	Small dams

It is becoming increasingly apparent that farm dams have a significant effect on the run- off of a catchment. Therefore, all farm dams with a surface area of greater than two hectares were identified and lumped together to form one large "dummy" dam in each run-off module (DWAF, 1995). In order to model these dummy dams the following information is required :

-	Catchment area of each farm dam digitised off 1:50 000 topographical maps;

-	Full Supply Area (FSA), digitised off 1:50 000 topographical maps;

-	Full Supply Capacity (FSC);

-	A relationship between area and capacity.

To derive an approximation of an appropriate area-capacity relationship for dummy dams the area-capacity relationship of registered dams in the study area was developed. All the larger dams are excluded from the analysis as most of these were modelled individually and would have significantly biased the regression analysis. A list of the registered dams, their FSCs and FSAs was obtained from the DWAF and are listed in Appendix B12.

A generalised area-capacity relationship was developed by the following method:

-	the registered dams were ranked in ascending order according to their capacities;

-	total capacity of all the registered dams was calculated;

-	the 10 % capacity (million m3) intervals were determined and the accumulative areas (km2) associated with each interval was calculated;

-	a regression analysis was then used to determine the a and b parameters in the relationship below :

Accumulated Surface Area = a * (Capacity)b	2.1

The parameter values derived using Equation 2.1 for the study area were as follows:

a = 0.755

b = 0.883

The parameters were applied to each farm dam to obtain the total capacity for a dummy farm dam in each run-off module. These capacities are recorded in Table 2.2.

Table 2.2:	Farm dam capacities for the run-off modules given per quaternary

Run-off module	Quaternary	Area

(km2)	Total farm dam capacity

(million m3
C3H003	C31A	1.881	2.909
C31B	0.468	0.426
C31C	3.575	3.569
C31D	16.040	30.399
C31E	0.665	0.634
C31F	1.542	1.376
C3R002	C32A	0.275	0.243
C32B	0.472	0.422
C32C	0.858	1.005
C32D	1.431	1.564
C33A	2.098	2.458
C33B	0.426	0.427
C5H016	C51K	2.394	3.668
C51L	0.131	0.078
C5H018	C52H	0.560	0.527
C52J	3.904	4.754
C52K	1.661	1.628
C52L	0.402	0.537
C5R001	C51D	5.200	5.392
C5R002	C51A	4.879	5.363
C51B	11.932	13.248
C51C	2.815	2.914
C51E	4.441	6.080
C51F	1.247	1.197
C51G	11.237	14.829
C51H	10.170	11.817
C51J	0.803	0.761
C5R003	C52A	7.350	10.333
C5R004	C52B	3.396	4.190
C52C	1.451	1.691
C52D	1.905	1.818
C52E	3.357	5.321
C52F	3.838	4.120
C52G	2.926	2.744
C9H009	C91C	0.000	0.000
C91D	0.717	0.709
C9R001	C91A	0.000	0.000
C91B	0.157	0.154
LOWER VAAL	C33C	1.216	1.226
C51M	0.157	0.170
C91E	0.346	0.323
C92A	0.163	0.177
C92B	0.370	0.335
C92C	0.022	0.017

In order to determine the catchment area upstream and downstream of the farm dams, the catchment boundaries of farm dams with a surface area of two hectares or greater were determined on 1:50 000 topographical maps. The catchment boundaries were then digitised and GIS was used to calculate the catchment areas upstream of farm dams. The area downstream of farm dams was considered to be the difference between the total run-off module area and the area upstream of farm dams within the run-off module.

2.2	IRRIGATION

2.2.1	Introduction

Irrigation generally falls into the following four groups:

-	Government Water Schemes (GWS)

-	Government Water Control Areas (GWCA)

-	Irrigation Boards

-	Private or diffuse irrigation.

A GWS consists of water works such as dams pipelines, canals, purification works and other associated structures built and operated by the State. A GWCA is declared in order to protect public water sources from exploitation and to ensure that users obtain their rightful share of water. In practice little control is exercised on irrigation within a GWCA and only GWCA irrigation that also lies within a GWS is metered.

Irrigation boards are statutory bodies which protect the rights of a group of irrigation farmers. These boards can generally provide a list of scheduled areas on which is indicated the quantity of water to which each land owner is entitled.

Private irrigation consists of all irrigation that does not fall under a GWS or Irrigation Board. Water is abstracted (legally or illegally) from a river, dam or borehole. This form of irrigation usage is not metered and the areas under irrigation and types of crops are usually not available and have to be estimated from information provided by local sources.

The lower Vaal catchment area is characterised by a number of significant irrigation schemes. Probably the most important is the Vaalharts Irrigation Scheme between the Harts and the Vaal Rivers. In the Riet and Modder catchments there are two Government Water and three Irrigation Board schemes. The Riet River Government Water Scheme (RRGWS) is the major irrigation scheme in this region and includes irrigation along the Riet River downstream of Kalkfontein Dam, the Riet River Settlement and the Ritchie and Scholtzburg irrigation areas (DWAF, 1991). The Modder River Government Water Scheme (MRGWS) includes the irrigation along the Modder River from upstream of the Krugersdrift Dam to the confluence of the Riet and Modder Rivers. The three Irrigation Board schemes are the Kaffer River Irrigation Board scheme downstream of Tierpoort Dam, the Lower Riet Irrigation Board scheme along the Riet River from the confluence of the Riet and Modder Rivers to just upstream of the confluence of the Vaal and Riet Rivers, and the Roodeheuvel Irrigation district on the Modder River upstream of Krugersdrift Dam.

In addition to the controlled irrigation there is a significant amount of diffuse irrigation which is supported by farms dams and run-of-river abstractions. The reliability of supply for this type of irrigation is considerably lower than that of the controlled irrigation as it is supported by little or no storage.

2.2.2	Historical development

Upper Harts and Vaalharts Irrigation

Work on the Vaalharts Government Water Scheme began in 1933 and the first plots in the North Canal area were allocated in 1938 north of Jan Kempdorp. The West Canal development began in 1945 after the completion of the North Canal and the first plots were allocated in this area in 1957. The Barkly West Canal area was developed in the late 1950's. Between 1976 and 1979 a comprehensive drainage system was installed in the North and West Canal areas. The Bloemhof Dam was completed in 1971 as an additional storage dam which releases water on the basis of daily demand upstream of the Vaalharts Weir. Bloemhof Dam is in turn augmented by releases from the Vaal Dam which was completed in 1938.

The scheme receives its irrigation water from the Vaal River via the Vaalharts Weir and the Harts River then serves as a drainage canal for the irrigation return flow from the scheme. Irrigation water is diverted from the Vaalharts Weir between Christiana and Warrenton into the Main Canal system. The Main Canal bifurcates into the West and North Canals and Klipdam-Barkly Canal. The North Canal also supplies water to the Taung Irrigation area. The historical irrigated areas associated with each canal are summarised in Table 2.3 (Stewart Scott Inc, 1994).

Irrigation water is provided throughout the year (250 days) and farmers apply for a weekly quota which they are entitled to if their application is successful. In the North and West Canal areas about 97 % of the total area is under flood irrigation with the remaining area under overhead spray irrigation (2.5 %) and drip irrigation (0.5 %). Most of the Taung area has been upgraded to centre pivot irrigation with only 0.1 km2 of flood irrigation remaining out of the total 35.7 km2.

Table 2.3:	Original scheduled areas - Vaalharts Irrigation Scheme

Irrigated Area	Scheduled Area

(km2)
North Canal Area	240.0
West Canal Area	50.0
Taung Area	35.7
Klipdam-Barkly Area	29.0

In the upper Harts catchment there is a small area of scheduled irrigation near Schweizer Reneke which is supplied with water from the nearby Wentzel Dam. The remainder of the irrigation is diffuse supplied either by farm dams or run-of-river irrigation.

Vaal River

Scheduled irrigation areas in the Middle Vaal area between Bloemhof and Vaalharts Weir were scheduled as early as 1934. The permits allow for 0.79 x million m3 per 24 hour period which is equivalent to 46.67 km2 at an annual quota of 620 mm/a. In addition to the scheduled quotas there are further quotas of 3.06 and 3.92 x million m3 per annum. Bloemhof Dam was built to support this riparian irrigation and provide a reliable supply to the Vaalharts Weir.

An area of 64.37 km2 between Vaalharts and Schmidtsdrift was scheduled in 1956 with an annual quota of 914 mm/a. These scheduled allowances as well as the demands of the Vaal-Gamagara Government Regional Water Supply are supported by compensation releases from Vaalharts Weir (DWAF, 1996b).

Modder/Riet Irrigation

The Riet River Government Water Scheme consists of the Kalkfontein Irrigation, Riet River Settlement and the Ritchie and Scholtzburg Irrigation areas. The water requirements were supplied in full by releases into the canal from Kalkfontein Dam. Since the construction of Orange-Riet/Sarel Hayward Canal in 1987 additional water is supplied from the Van der Kloof Dam which supplies all the water requirements of the Riet River Settlement and the Ritchie and Scholtzburg Irrigation Areas. The excess water from the Orange-Riet Canal has also provided potential for further agricultural development. The remaining irrigation areas along the Riet River downstream of Kalkfontein Dam are still supplied from Kalkfontein Dam.

Several weirs along the Riet River supply the irrigation requirements of the 27 km2 of land listed under the control of the Lower Riet River Irrigation Board. These weirs were originally supplied entirely by releases from Krugersdrift Dam but since 1987 have been supplemented by excess flow from the Orange-Riet Canal.

The Kaffer River Irrigation Board Scheme was introduced in 1922 with 47.8 km2 listed. Due to the inability of Tierpoort Dam to supply the irrigation requirements this area was reduced to 11 km2 in 1935 and still further to 7.4 km2 in 1986.

There are 35 km2 of scheduled irrigation in the Modder River Government Water Scheme. Krugersdrift Dam was built in 1971, primarily to provide a stable water supply to the downstream irrigators. Releases are made from the dam to 52 weirs along the Modder from which irrigation water is pumped.

The Roodeheuvel Irrigation district consists of 1.8 km2 of irrigation, scheduled in 1984 on both banks of the Modder River. Water supply to this area is from weirs at Eureka and Roodekop upstream of Krugersdrift Dam.

Diffuse irrigation is well developed in the Modder and Riet catchments and most of it occurs in the upper regions of the catchments. The areas of diffuse irrigation are not fixed and estimates of their actual area are often not available or unreliable (DWAF, 1991).

2.2.3	Present Irrigation areas

Vaalharts

Paper records of the detailed, updated information on Vaalharts irrigation are kept by DWAF in Jan Kempdorp. However, DWAF was only able to collate the most recent years of monthly abstractions before the completion of the hydrological data base in this study. Actual irrigated areas and the annual quotas provided by DWAF are summarised in Table 2.4.

Vaal River

The scheduled areas in the Middle and Lower Vaal regions have not changed significantly over the years and in theory the water demands should also have remained constant. However, the freely available water supply in the Middle Vaal region gives the farmers no incentive to maintain their meters and as a result the DWAF have no means of monitoring the exact volume of abstraction, which may regularly exceed the quota. This water is not necessarily all used for irrigation but often for domestic purposes (DWAF, 1996b).

Table 2.4:	Updated scheduled areas - Vaalharts Irrigation Scheme

Irrigated Area	Scheduled Area

(km2)	Annual Quota

(million m3/km2/a)	Water Volume

(million m3/a)
Vaalharts Canal
1. Vaalharts (North and West)	276.69	0.914	252.89
2. Volkswelsyn	3.00	0.914	2.74
3. Dept. of Agriculture	3.81	0.914	3.48
Klipdam-Barkly Canal
1. Klipdam-Barkly	22.08	1.186	26.17
2. Delportshoop	0.55	1.186	0.65
3. Barkly West	1.03	1.186	1.22
4. Small farms	3.00	1.186	3.55
Taung (Vaalharts)
1. Taung	64.24	0.770	49.46

Modder and Riet River

Information on irrigated areas and crop types in the Modder and Riet catchments was obtained from local sources (DWAF, 1996a) and previous studies. In the upper Modder River above Rustfontein Dam there is a total of 4.50 km2 of irrigated land of which 2.30 km2 is scheduled. The remaining irrigated area is under diffuse irrigation. The irrigation water is supplied from run-of-river and farm dam abstractions. For the Rustfontein Dam catchment and the incremental catchment between Rustfontein and Krugersdrift Dam, not more than 40 % of the farm dams are used for irrigation.

The total irrigated area between Rustfontein and Krugersdrift Dam is 23.90 km2, 30 % of which is upstream of Mockes Dam and 70 % downstream, which includes the Roodeheuvel Irrigation district. Upstream of Mockes Dam irrigation demands are again supplied by run-of-river abstractions and farm dams. However, downstream of Mockes Dam all irrigation demands along the Modder River are supplied directly from the river only. Approximately, 7.50 km2 of the irrigated land downstream of Mockes Dam is supported by flows in the Renoster River and urban return flows from the Bloemspruit treatment works. Flows from the treatment works are released into the Bloemspruit which feeds into the Renosterspruit.

The scheduled irrigation between Krugersdrift Dam and Tweerivier falls under the Modder River Government Water Scheme. Fairly extensive areas have been upgraded to centre pivot irrigation resulting in more efficient water usage. The total irrigated area is 33.64 km2 and has an annual quota of 28.26 x million m3 per annum, although the demand is probably closer to 33.00 x million m3.

In the upper reaches of the Riet and Kaffir Rivers water for irrigation is very scarce with the rivers drying up almost every year. There is at most 32 km2 of irrigated land, mostly lucerne for cattle fodder supported by abstractions from the farm dams which are fed either by catchment run-off or boreholes. No new information was available on the irrigated area controlled by the Kaffir River Irrigation Board and therefore it was assumed that the area has remained constant at 7.4 km2, as it was before the Tierpoort Dam wall was washed away. The scheduled area was greatly reduced from the original listed area of 47.8 km2 due to the inability of Tierpoort Dam to supply the irrigation requirements.

Conditions in the middle reaches of the Riet River upstream of Kalkfontein Dam and along the Kromellenboogspruit are much the same as the upper catchment. The rivers dry up most years and the estimated 10 km2 of irrigated land is supported by farm dams and minor run-of-river abstractions. Boreholes are a significant source of water for domestic use, stock watering and limited irrigation in this region (Brink, 1996).

There has been extensive agricultural development along the Riet River between Kalkfontein Dam and the Vaal and Riet River confluence. There are 30.46 km2 listed under the control of the Kalkfontein Irrigation Board and the water requirements are supplied via the canal from Kalkfontein Dam. There is further 4.8 km2 of scheduled land just below Kalkfontein Dam that is irrigated directly from the Riet River when the river is flowing.

Further downstream is the Riet River Settlement (78.12 km2) and the Ritchie (0.99 km2) and Scholtzburg (6.37 km2) Irrigation Areas. The Orange-Riet Canal, which transfers water from the Van der Kloof Dam supplies the entire needs of these irrigated areas. The excess flows from the Orange-Riet Canal and flows in the Riet River, augmented by flows from the Modder River, supply the irrigation requirements of the 39.37 km2 listed under the Lower Riet River Irrigation Board.

The current day irrigated areas and annual water quotas for scheduled areas are summarised in Table 2.5 and recent Government Water Control Areas and Water Board boundaries are shown in Appendix A1.

Table 2.5:	Summary of current irrigated areas in the Modder and Riet catchments

Irrigated Area	Irrigated Area (km2)	Water Quota

(million m3/km2/a)	Water Volume

(million m3)
Upstream of Rustfontein	Scheduled - 2.30

Diffuse - 2.20	0.605

-	1.39

-
Between Rustfontein and Krugersdrift	Diffuse - 23.90	-	-
Between Krugersdrift and Tweerivier

- Modder River Government Water Scheme

Scheduled - 33.64

0.840

28.26
Upstream of Tierpoort	Diffuse - 2.00	-	-
Between Tierpoort and Kalkfontein	Scheduled - 7.40

Diffuse - 40.00	-

-	-

-
Between Kalkfontein and Aucampshoop

- Riet River Settlement

- Ritchie Irrigation

- Scholtzburg Irrigation

- Kalkfontein Irrigation

- Lower Riet River Irrigation

Scheduled - 78.12

Scheduled - 0.99

Scheduled - 6.37

Scheduled - 30.46

Scheduled - 19.69

Diffuse - 4.80

1.100

0.914

0.914

1.100

0.914

-

85.93

0.88

5.62

33.51

18.00

-

2.2.4	Water usage by irrigation

In order to quantify the water demands related to agriculture, areas of irrigation and crop distributions (where possible) were obtained from the local sources, which are listed in Appendix I1. The irrigation was differentiated into three types:

-	diffuse irrigation from farm dams,

-	diffuse irrigation supported by run-of river abstractions,

-	controlled irrigation under various water schemes and irrigation boards.

The routine called IRRDEM developed by BKS Inc. was used to estimate the monthly demands of all irrigated crops where the actual demand was not known. IRRDEM produces a time series of irrigation demands for the specified irrigated area using the following information:

-	irrigation area at the end of the calibration period,

-	growth rate per annum in irrigation area,

-	run-off module MAP,

-	monthly rainfall as percentage of the MAP,

-	area under each different crop (% of total irrigated area),

-	monthly crop factors for each crop,

-	monthly A-Pan evaporation,

-	effective rainfall factor in each month,

-	irrigation efficiency.

When running the irrigation demand routine an effective rainfall factor of 60 % was used for all months and the irrigation efficiency varied according to the type of irrigation. An irrigation efficiency of 65 % was used for all diffuse irrigation and 75 % was used for controlled irrigation. In some run-off modules the total irrigation demand was then factored to obtain the portion of irrigation demand from farm dams and that from rivers downstream of farms dams. The factor varied for each run-off module depending either on local knowledge of the use of farm dams or on the total capacity or expected failure rate of the all the farm dams in the module. The irrigation demand time series and crop factor files are included in Appendix B.

2.2.5	Historical growth in irrigated area

Irrigation patterns, crop types and irrigated areas change over time and need to be modelled accordingly. To represent abrupt changes in irrigation methods, the IRRDEM routine is run using a different set of input parameters and files for each homogeneous time period. These files are then concatenated to form a complete time series. In the case where only the irrigated area changes, the original and final areas are used to calculate a growth rate over the modelled period. The growth rate and final irrigated area are supplied as parameters to IRRDEM which then produces a time series representing the changing irrigation demand over time. Table 2.6 summarises the irrigation areas used during catchment modelling and the time series of irrigation demands can be found in Appendix B.

Table 2.6:	Growth of irrigated areas in the Lower Vaal Catchment

Irrigated Area	Type	Year	Area (km2)	Source	Comment
Upstream of Wentzel Dam	Diffuse	1935-1994	4.00	Farm dams	Estimated
Schweizer Reneke IB	Scheduled	1934-1994	-	Wentzel Dam	Demand equal to irrigation releases from Wentzel Dam
Taung Irrigation upstream of Gauge C3H003	Scheduled	1935

1977

1994	0.00

8.01

11.00	Vaalharts Irrigation Scheme
Dry Harts River Catchment	Diffuse	1975-1994	5.00	25% Farm Dams

75% Dry Harts River	Estimated
Harts River upstream of Spitskop Dam	Diffuse	1975-1994	-	Harts River	Based on allocation quotas from the Harts River
Downstream of Kalkfontein Dam

Scheduled

Scheduled

Diffuse	1960-1994

1987

1994

1960-1994	-

82.22

85.03

10.00	Kalkfontein Dam

Orange-Riet Canal

Riet River	Demand equal to irrigation releases from Kalkfontein Dam -supplies Riet River Settlement, Ritchie,Scholtzburg and Kalkfontein IB (1960-1987)

and Kalkfontein IB (1987-1994)

Supply Riet River Settlement,Ritchie, Scholtzburg IB

Estimated
Upstream of Aucampshoop	Scheduled	1952

1987

1994	15.74

26.98

39.38	Lower Riet River
Krugersdrift Dam to Tweerivier	Diffuse	1971-1994	50.00	30 % Farm Dams

70% Compensation releases and spills from Krugersdrift Dam	Estimated
Upstream of Tierpoort Dam	Diffuse	1923-1976	2.00	70% Farm Dams

30% Kaffer River	Estimated
Below Tierpoort Dam	Scheduled	1937-1976	-	Tierpoort Dam	Demand equal to irrigation releases from Tierpoort Dam for Kaffer River IB
Tierpoort Dam to Kalkfontein Dam	Diffuse	1937-1976	40.00	70% Farm Dams

30% Riet River	Estimated
Upstream of Rustfontein Dam	Scheduled/Diffuse	1954

1986

1994	0.00

Sched - 1.10

Diff - 1.70

Sched - 2.26

Diff - 2.24	25% Farm Dams

75% Modder River
Rustfontein Dam to Mockes Dam	Diffuse	1948

1987

1994	0.00

1.69

7.35	25% Farm Dams

75% Modder River
Mockes Dam to Krugersdrift Dam	Diffuse	1948

1987

1994	0.00

3.81

9.00	Modder River
Renoster River Catchment	Diffuse	1971-1994	7.55	25% Farm Dams

75% Renoster River	Estimated

Vaalharts Weir to De Hoop	Diffuse	1968

1985

1994	10.00

30.00

30.00	Vaalharts Weir Compensation Releases	Estimated
Bloemhof Dam to Vaalharts Weir	Diffuse	1985

1994	0.00

46.67	Compensation Releases from Bloemhof Dam	Estimated from permit information

For the current study, each area of irrigation was analysed individually and modelled accordingly. For diffuse irrigation areas which did not fall under a specific irrigation board and in most cases had to estimated, the irrigated area remained constant as there was insufficient information on growth rates and cropping patterns.

2.3	ABSTRACTIONS AND RETURN FLOWS

2.3.1	Abstractions, in-basin and inter-basin transfers

The Lower Vaal River catchment is sparsely populated and therefore most of the major abstractions and transfers are for irrigation purposes. The Vaalharts Irrigation Scheme is the most significant abstraction from the Vaal River between Bloemhof Dam and the confluence with the Harts River. The return flows from the irrigation scheme augment flows in the Harts River upstream of Spitskop Dam. Water supplies for Kimberley are abstracted from the Vaal at Riverton and pumped to Kimberley.

The Vaal-Gamagara Government Regional Water Supply Scheme pumps water from the Vaal River just downstream of the confluence of the Vaal and the Harts Rivers. Water from this scheme is delivered to Postmasburg, Sishen, Hotazel, Blackrock and the surrounding farms.

In the Modder River catchment water is abstracted from Mazelspoort Weir to supply, in part, Bloemfontein's urban water supply (de Beer, 1996). The remainder of the urban water demand is supplied by the Caledon-Modder Government Water Scheme, an inter- basin transfer from Welbedacht Dam on the Caledon River. The water supplied from Welbedacht is distributed among a number of consumers including Botshabelo township, Bloemdustria industrial area and greater Bloemfontein. Knellpoort Dam has also been recently constructed with the aim of augmenting the releases from Welbedacht Dam which has a diminishing capacity due to high siltation levels. Water will either be transferred from Knellpoort to Welbedacht or conveyed to Rustfontein Dam in the upper Modder River catchment to be released to Mazelspoort Weir (Bloem Water, 1996). The urban demands of Thaba Nchu and its surrounds are supplied in full by Groothoek and Seroalo Dams but will in time also be augmented by water from the inter-basin transfer.

Water is transferred into the lower Riet River catchment from the Van der Kloof Dam via the Orange-Riet/Sarel Hayward Canal. These supplies meet the irrigation requirements of the Riet River Settlement and the Ritchie and Scholtzburg Irrigation Areas. The excess flows are released into the Riet River near Jacobsdal upstream of the Modder-Riet confluence. Since the completion of the canal, the excess flows were used to augment releases from Krugersdrift Dam to meet the requirements of the scheduled areas under the control of the Lower Riet River Irrigation Board. The in-basin and inter-basin transfers are summarised in Table 2.7 and Appendix C contains the available time series for the transfers.

Table 2.7:	Major in-basin and inter-basin transfers

Name of Scheme	Source	Destination
Caledon-Modder GWS	Welbedacht Dam, Caledon River and Knellpoort Dam, Rietspruit River	Supplement supplies to Thaba Nchu and Bloemfontein
Riet River GWS	Vanderkloof Dam, Orange River via the Sarel Hayward/ Orange-Riet Canal	Riet River Settlement near Jacobsdal to supplement releases from Kalkfontein Dam, Riet River and Krugersdrift Dam, Modder River
Vaal-Gamagara Government Water Regional Water Supply Scheme	Vaal River, downstream of the confluence of the Vaal and Harts Rivers	Postmasburg,Sishen,

Hotazel, Blackrock and surrounding farms
Vaalharts GWS	Vaalharts Weir on the Vaal River	Vaalharts scheduled irrigation areas
Riverton-Kimberley transfer	Vaal River at Riverton	Kimberley
Orange-Douglas GWS	Orange River	Douglas Irrigation Scheme

2.3.2	Irrigation return flows

The estimation of return flows is particularly important when modelling water quality. For this study, with exception of the Vaalharts region, all return flows were calculated as 10% of the actual irrigation supply. The intensive irrigation along the Harts river yields return flows greater than 10 % of the supply from the Vaalharts Weir. A more accurate method of calculating the actual return flows used by Stewart Scott Inc. (1994), was employed and is described in detail in Section 5.2.3 as part of the model calibration for gauge C3R002.

2.4	URBANISED AREAS

There are only four major population centres in the Lower Vaal Catchment namely, Kimberley, Bloemfontein, Botshabelo and Thaba Nchu. Kimberley is situated well within the broad band of endoreic area between the Modder River and the Vaal River and therefore the impermeability of the urban surfaces which causes increased run-off have no effect on the total catchment run-off. Kimberley's urban water demand is met by abstractions from the Vaal River at Riverton which are pumped from the treatment works at Riverton to Kimberley.

The other three urban centres all fall in the Krugersdrift Dam catchment. An original estimate of 10 % of the total urbanised area was considered to be impervious and contribute directly to overland run-off. This figure was later adjusted to generate realistic low flow sequences and to represent the extent of development and infrastructure within each centre. The original, exclusive source of water for Bloemfontein was Mazelspoort Weir on the Modder River, but this resource has since become insufficient. The interbasin transfer from Welbedacht Dam on the Caledon River now supplies a large portion of the water requirements of Bloemfontein, Botshabelo, Dewetsdorp and various other small consumers en route. Thaba Nchu is supplied with water from the treatment works at Groothoek and Seroalo Dam.

Most of the other small towns in the Modder and Riet catchments use boreholes for their water supply with the exception of Koffiefontein town and mine and Jacobsdal which receive water from the Kalkfontein Irrigation canal.

2.4.1	Urban Return Flows

There are various sewage treatment plants (STW) within the Bloemfontein area which release water either straight back into the rivers or provide limited irrigation water supplies. Bloemspruit STW releases into the Bloemspruit River (a tributary of the Renoster River) and a significant portion of this water is used for diffuse irrigation in the lower Renoster River catchment. Welvaart STW releases return flows from Bloemfontein into the Kaalspruit River downstream of Krugersdrift Dam. Return flows from the Thaba Nchu and Botshabelo STW reach the Modder River upstream of Mazelspoort Weir.

There are currently two STWs in Kimberley and a third is under construction. The total capacity of these plants will be 0.164 x 106 m3/d. All the outflow from the treatment works is released either into Kamfersdam Pan or Du Toits Pan and is not used for irrigation or returned to the river system.

2.5	TRANSMISSION LOSSES

In the Vaal catchment, channel bed losses are significant, while the contribution of flows from incremental catchments along the main Vaal River channel are relatively small. According to the agreed method of the hydrology team, an aerial video of the region and the 1:50 000 topographical maps were used to calculate the evaporative losses from the water surface and the riparian vegetation. The length and average width of the river and vegetation were estimated and the net loss (mm) was calculated from the evaporation and rainfall records. By multiplying the surface area and net loss depth, a time series of monthly volumetric losses was created.

Applying these evaporative losses in the calibration and the subsequent naturalisation, large discrepancies were found that might be related to the over-estimation of the losses by the foregoing method. Therefore, a new method for estimation of evaporative losses had to be derived.

Evaporative losses are particularly important where the channel is consistently wide providing a large surface of water for evaporation. This is often the case downstream of large dams that release compensation flows for irrigation into the downstream river such as Bloemhof, Vaalharts and Krugersdrift. The evaporative losses could therefore be proportional to the releases made from the dams except in flood months. A value of 0.5 % of the compensation releases was "calibrated" to be lost to evaporation and the net loss during months where there were spills recorded, ie. flood months, was assumed to be zero. This method removed the discrepancies caused by the previous approach.

3.	HYDROLOGICAL DATA

3.1	PRECIPITATION

3.1.1	Selection, patching and verification

Selection

The rainfall stations used in the WR90 study were considered to be sufficiently long, reliable and representatively distributed over each run-off module. Therefore, all these stations were accepted as being spatially and temporally representative and were supplemented with additional data (where available) up to September 1995. The monthly rainfall data was obtained from the Computing Centre for Water Research (CCWR) at the University of Natal, Pietermaritzburg and from the Weather Bureau.

Patching and Verification

The WR90 patched values in the rainfall files were accepted and the supplementary data was appended and patched using the same patching procedure used by the WR90 team. The program searches for stations within a certain radius of the target station and with at least 10 years of overlapping record with the target station. The station groupings that the program yielded were therefore the same as for WR90. The program then performs a linear regression of target station values on candidate station values to be used for patching. The candidate station is the station with highest correlation coefficient with the target station.

Approximately 98 % of the records were longer than thirty years and 60 % of the stations were still open. Therefore, due to the adequate number of representative, long records, no records required extending by regression. The period of rainfall and flow sequences required by the system analysis is 1920 to 1994.

The stations were checked for reliability, stationarity and consistency using single mass plot analysis. An in-house program was used to extract details about the completeness of all the selected rainfall records which are shown in Appendix D12.1.

MAP values for each quaternary were obtained from WR90. A total MAP for each run-off module was calculated by using an area weighted average of the MAPs of the quaternaries in each run-off module.

3.1.2	Catchment area rainfall inputs

Rainfall stations were grouped according to quaternary boundary but representative stations outside the quaternary were used if the simulation period was not adequately covered by the stations within the quaternary.

Rainfall stations groupings were used that produced the best representation of catchment run-off at each calibration point. Data for these groups of rainfall stations were processed through the CATCHRN routine to produce the required percentage rainfall input for the PITMAN routine within the SHELL model.

3.2	STREAMFLOW

The information for all stations whose streamflow records were considered to be of adequate length and reliability were requested from the DWAF. The details of these flow gauges are given in Table 3.1 below. The location of the gauges is illustrated in Appendix A.4 and the raw streamflow data and graphical representation of the availability of data at the gauges can be found in Appendix E.

Table 3.1:	Flow gauging stations considered in the Lower Vaal Catchment

Gauge

Number	Gauge Name	River	Latitude

(D M)	Longitude

(D M)	Approximate Cumulative

Catchment Area

(km2)	Start Date of Automatic Recording	Period

of

Record
C3H003	Taung	Harts	27 34	24 44	10990	01/1968	1927-1994
C3H007	Espagsdrift	Harts	27 55	24 36	24097	-	1948-1994
C3R001	Schweizer Reneke	Harts	27 10	25 20	2919	12/1984	1935-1993
C3R002	Spitskop	Harts	28 07	24 30	26914	06/1975	1975-1994
C5H007	Shannon Valley	Renoster	29 08	26 19	348	01/1965	1948-1994
C5H008	Riviera	Riet	29 48	26 12	593	06/1967	1931-1985
C5H012	Kromdraai Rietwater	Riet	29 39	25 58	2372	12/1972	1953-1995
C5H015	Stoomhoek	Modder	28 48	26 06	6009	03/1974	1948-1983
C5H016	Aucampshoop	Riet	28 57	24 14	33351	07/1962	1952-1994
C5H018	Tweerivier	Modder	29 02	24 38	17315	10/1974	1959-1994
C5R001	Tierpoort	Kaffer	29 25	26 08	922	08/1979	1937-1976
C5R002	Kalkfontein	Riet	29 29	25 13	10268	06/1978	1937-1994
C5R003	Rustfontein	Modder	29 16	26 37	940	07/1982	1954-1994
C5R004	Krugersdrift	Modder	28 53	25 57	6315	11/1970	1971-1994
C9H009	De Hoop 65	Vaal	28 30	24 36	121052	08/1968	1968-1994
C9R001	Vaalharts	Vaal	28 06	24 55	115055	08/1968	1971-1994

3.2.1	Gauges considered but not used

Espagsdrift - C3H007

The record was not considered for model calibration as higher discharges are significantly under-estimated at the weir. The gauge is, however, sufficiently accurate at low flows and was therefore used to calculate return flows from the Vaalharts irrigation scheme.

Schweizer Reneke - C3R001

The spills from the dam are not gauged downstream and therefore the calculation of inflows will be inaccurate during spilling periods.

Shannon Valley - C5H007

The record, although long, required extensive patching. The flows measured at gauge C5H007 only represented a small catchment area (348 km2) and could not provide extra information to justify a separate calibration. Irrigation was assumed to be negligible in the Orange River System Analysis (DWAF, 1991) and even though no data on actual irrigation areas and demands was available, the number of farm dams in the area suggests that there is fairly significant irrigation activity. The catchment is also on the outskirts of Bloemfontein and the run-off is therefore affected by increased overland flow due to the impervious urban areas and urban returns. The catchment area was included in the incremental catchment between Rustfontein and Krugersdrift Dam gauged by C5R004.

Riviera - C5H008

This catchment is relatively small (593 km2) and contains no significant demands, irrigation or other, which need to be considered individually. It was therefore included in the incremental Kalkfontein Dam incremental catchment where there is a complete flow record.

Rietwater - C5H012

This station is downstream of C5H008 and although it has a significantly larger catchment (2 372 km2) it has similar rainfall, run-off and irrigation characteristics. The flow record requires considerable patching and as it only forms a relatively small part of the Kalkfontein Dam catchment it was not dealt with individually but included in the Kalkfontein Dam incremental catchment analysis.

Stoomhoek - C5H015

This gauge is situated between Mockes Dam and Krugersdrift Dam. Although the gauge is considered to be reliable for the period 1949-1971 there are some doubts concerning the accuracy of the low flows. The observed flows at C5H015 and the downstream gauge C5R004 at Krugersdrift Dam were compared for the overlapping period (1971- 1983) with the aim of appending the earlier part of the C5H015 record to the shorter C5R004 record in order to extend the calibration period. However, the C5H015 record required too much patching in the overlapping years for the comparison to be meaningful. It was also noted that most of the abstraction and return flow volumes have only been accurately recorded since the construction of Krugersdrift Dam, thereby limiting the calibration period.

The incremental catchment between the two gauges is approximately 150 km2 and contains no significant abstractions or return flows to motivate the analysis of both gauges.

Douglas - C9H007

The record at the most downstream station, C9H007 at Douglas, only covers the period 1947-1972 of which the portion prior to 1959 requires considerable patching. The station was closed in 1978, but the weir has since been upgraded (C9R003) and flows have been recorded since 1978. However, when the record was requested it was found to be unavailable.

3.2.2	Gauges used for calibration

Taung - C3H003

The flow record at Taung appears to be reliable although the long broad crest is not ideal for measuring low flows. There seems to be some discrepancies in the earlier record (1926 -1937) where the datum level or actual weir structure may have changed. There was no irrigation upstream of this gauge prior to 1978. Taung Dam was constructed approximately 10 km upstream of the gauge and will affect the record after October 1992.

Spitskop Dam - C3R002

The earlier part of the inflow record for this station had to be calculated from flows measured at the downstream weir (C3H013), because reservoir releases from Spitskop Dam were not measured prior to 1989. The C3H013 gauge under-estimated outflows from Spitskop due to ungauged seepage under the weir, irrigation abstractions and an incorrect discharge table at the weir. The records have since been revised by the DWAF according to the new rating table calculated after the 1987 floods.

Aucampshoop - C5H016

This record is not considered completely reliable and requires a fair degree of patching. However, the location of the gauge made it necessary to include the gauge in the study. It is a storage weir in the lower reaches of the Riet River between the confluences of the Riet and Modder Rivers and the Riet and the Vaal Rivers. It is in a prime location to monitor the effects of the intensive irrigation in the upstream reaches of the Riet and Modder Rivers.

Tweerivier - C5H018

This station is situated just upstream of the Modder and Riet River confluence, on the Modder River and is in a good position to monitor the effects of the Modder River Government Water Scheme on the flows released at Krugersdrift Dam.

Tierpoort Dam - C5R001

The record for Tierpoort Dam appears to be reliable and requires limited patching for the period 1923-1976. There were no recordings made at the station between 1977 and 1984 and after this period the DWAF labelled the record as extremely unreliable. The dam wall was washed away in 1987/8 and conscientious recording only recommenced in 1991.

The recorded spills from the dam may be unreliable as the downstream weir has flow limitations and in turn, the calculated inflows may also be inaccurate during periods of spill.

Kalkfontein Dam - C5R002

For Kalkfontein Dam, the data appears to be reliable for the available record 1937-1994. The previous and updated records were compared and some extreme adjustments to the flood events in the record before the automatic recording in 1978 were noted. The DWAF was consulted about the discrepancy, but they were confident that the new record was correct and that the old record had been subject to unit conversion problems. A comparison plot of the original and updated records is shown in Appendix G6.4.

Rustfontein Dam - C5R003

The record was assessed to be reasonably reliable for the period 1954-1994. The flows in the months when the dam was spilling have been updated by DWAF for the years prior to the installation of an automatic gauge. In most cases the updated flows exceed the original flows although only marginally and appear to be satisfactory.

Krugersdrift Dam - C5R004

The flow record at Krugersdrift Dam was considered to be of satisfactory quality from 1971-1994. There were some discrepancies in the spill and spill plus compensation records where occasionally the spills recorded in the spillway exceeded the spills plus the compensation releases recorded just downstream of the dam. The values in the spill plus compensation file which were exceeded by the spills only value were replaced by this value.

The record at Stoomhoek (C5H015) was also assessed and found to be reasonably reliable, although sometimes doubtful during low flows. It was considered more reliable to append the recorded flows at C5H015 to the Krugersdrift record to extend the length of the calibration period for the downstream gauge (C5H018).

De Hoop 65 - C9H009

This gauge is located just downstream of Riverton on the Vaal River. It is in a position to record the remainder of the compensation releases from the Vaalharts Weir, after irrigation demands have been subtracted and the urban demand of Kimberley and the water requirements of various industries and private users have been supplied. The record is considered reliable, although the discharge table is limited.

Vaalharts Weir - C9R001

The inflow record at this station is reliable for the period 1971-1994. The inflows reflect very closely the compensation releases from Bloemhof Dam upstream.

3.2.3	Patching of Streamflow Records

All missing or exceeded values in the observed records are flagged by DWAF as unreliable. These values were patched in order to make the comparison between observed and simulated flows meaningful. The observed record was patched with the simulated value obtained from using the regional parameters until a final calibration set was obtained. The PITMAN parameters were adjusted in order to "fine tune" the correlation between the observed and simulated records and the simulated values from the final simulation run were used to replace the initial patched values.

All reservoir inflow records were analysed and all months where spills were recorded prior to automatic logging, were flagged in the inflow records. These values were considered unreliable and were patched if necessary during calibration. The spill records were also patched using simulated values where values were missing. Details of records that required specific patching are discussed below.

C5R001 - Tierpoort

Spill values were flagged as missing from February to May 1967 and months, June and July recorded very high spills. The inflow record to Kalkfontein Dam downstream showed no correlation to the spill record and therefore it was decided to use simulated values to patch the inflow and spill record of Tierpoort Dam.

The model for the run-off module was calibrated for the period 1923-1965 and then extended to include the 1966 water year to produce inflow values for the unreliable period of observed flows. The patched inflow record was then used as input to the RESSIM reservoir simulation routine. The observed irrigation demand and evaporative losses were subtracted during the RESSIM routine and the simulated spills calculated. The observed spill record was then patched using the simulated values. A summary of the monthly recorded and simulated inflows and spills for Tierpoort and inflows for Kalkfontein for February to July 1967 are given in Table 3.2.

Table 3.2:	Patching of Tierpoort Reservoir Record (C5R001)

Flow Record

(million m3)	Feb	Mar	Apr	May	Jun	Jul
Raw Inflow to Tierpoort Dam	0.00	0.00	0.00	0.00	11.30+	6.49+
Simulated Inflow to Tierpoort Dam	2.88	0.71	18.31	10.20	1.14	0.00
Raw Spill from Tierpoort Dam	0.00+	0.00+	0.00+	0.00+	46.92	35.70
Simulated Spill from Tierpoort Dam	0.00	0.00	10.58	10.23	0.76	0.00
Raw Inflow into Kalkfontein Dam (C5R002)	29.00	18.70	110.00	53.60	7.22	0.96

3.2.4	Subdivision of the study area

The Lower Vaal catchment was divided into 10 separate run-off modules, determined by the positions of reliable flow gauges. Within each run-off module containing farm dams a division was made to separate the catchment area upstream and downstream of dummy farm dams. This enabled the determination of the approximate volume of water entering the farm dams, which was available for irrigation and subject to evaporative losses. The positions of the calibrated gauges and the groupings of quaternary catchments in each run-off module are shown in Appendix A4.

3.3	EVAPORATION

In Pitman catchment modelling, mean monthly S-pan evaporation data are required. There are numerous evaporation stations in this region recording either Class A-pan or Symons Pan (some record both) evaporation data. It was debated whether or not to update the WR90 evaporation records with a further 15 years of data or to simply accept the distribution of MAE calculated in the Addendum to "Surface Water Resources of South Africa (1981)" (Midgley, Pitman and Middleton, 1993), and used in WR90. It was concluded that reworking the evaporation data was unnecessary for the following reasons:

-	the shortest time series of monthly evaporation used to determine the monthly evaporation as a percentage of MAE for each evaporation zone was 30 yrs and extending this period by a few years would not significantly improve the monthly estimate,

-	the stable nature of the seasonal evaporation cycle.

However, for completeness the name, length of record, type of evaporation data collected and percentage missing data are recorded in Table 3.3.

MAE values for each quaternary were obtained from WR90. A total MAE for each run-off module was calculated by using an area weighted average of the MAEs of the quaternaries in each run-off module.

Table 3.3:	Details of evaporation stations in the Lower Vaal River catchment

Gauge No.	Station Name	Latitude	Longitude	Record

Used	No. Years	% Missing	MAE

S-Pan

(mm)	MAE

A-Pan

(mm)	Evap. Zone
C3E001	Vergenoeg	26 35	25 33	1929-1970	42	4.0	1815	8A
C3E002	Armoedsvlakte	26 57	24 38	1957-1983	27	5.6	2513	8A
C3E003	Zandvallei	26 34	25 35	1960-1991	32	5.2	2069	8A
C3E004	Doornlaagte	26 37	26 06	1961-1975	15	5.0	2292	8A
C3E005	Bospoort	26 37	26 11	1960-1960	1	0.0	2533	8A
C3E006	Hartswater	27 43	24 47	1967-1969	3	5.6	2481	8A
C3E007	Guldenskat	27 57	24 50	1967-1983	17	6.9	2217	8A
C3E008	Koopmansfontein	28 12	24 04	1967-1983	17	7.4	2553	8A
C3E011	Ottosdal	26 49	26 01	1976-1983	8	24.0	2555	8A
C3E012	Spitskop	28 07	24 30	1977-1995	19	3.9	2339	8A
C5E001	Tierpoort	29 25	26 08	1933-1991	59	8.8	1757	19A
C5E002	Kalkfontein	29 29	25 13	1941-1995	55	2.6	1968	19A
C5E003	Mazelspoort	29 01	26 24	1951-1987	37	2.5	1629	19C
C5E004	Rustfontein	29 16	26 35	1953-1995	43	5.4	1663	19C
C5E005	Bloemfontein Airport	29 07	26 11	1957-1960	4	0.0	2409	19C
1956-1961	6	20.8	1917
C5E006	Fauresmith	29 46	24 19	1957-1978	22	14.8	2547	19A
C5E007	B.J. Vorster Airport	28 48	24 46	1957-1979	23	0.4	2804	19B
1956-1992	37	3.2	2208
C5E008	J.B.M. Hertzog Airport	29 06	26 18	1962-1979	18	0.0	2364	19C
C5E009	Uitvlugt-Wes	28 53	25 57	1968-1995	28	5.7	2171	19C
C9E001	Kimberley	28 31	24 42	1930-1956	27	5.2	1687	9B
C9E002	Newton-Kimberley	28 45	24 45	1932-1956	25	2.0	2060	19B
C9E003	Vaalharts Weir	28 06	24 56	1947-1994	48	1.9	2067	9B
C9E004	Bloemhof Dam	27 40	25 37	1969-1994	26	2.6	2148	9B
C9E005	Atherton	29 02	23 50	1969-1994	26	6.4	2289	7A
C9E006	Douglas	29 03	23 46	1976-1982	7	2.4	2509	7A

4.	HYDROLOGICAL MODEL CALIBRATION

4.1	INTRODUCTION

4.1.1	Component configuration

Following the collection and preparation of all the information required to model the catchment the network diagrams were developed and each run-off module was configured in SHELL. Each component is unique but contains elements of the same basic structure. This structure is composed of the following elements:

-	PITMAN rainfall-run-off model

-	RESSIM reservoir routine

The PITMAN model was used to simulate run-off from areas draining into farms dams and areas draining directly into rivers downstream of farm dams. The input data required for these operations is as follows:

-	catchment area upstream and downstream of farm dams (NB. The catchment areas used in the calibrations have been taken from the WR90 study and may differ slightly from those reported by the DWAF.)

-	quaternary and run-off module MAP

-	monthly time series of rainfall as a % of MAP

-	mean monthly run-off module evaporation (S-Pan)

-	start and end year of simulation period

-	PITMAN parameter set

-	area of afforestation (not applicable to the Lower Vaal catchment)

-	urban area and its growth over time (if applicable)

The RESSIM routine was used to simulate the operation of the "dummy" farm dams and certain larger dams where the reservoir gauge was not used as a calibration point. Input data required for this operation was:

-	monthly rainfall (in tenths of mm)

-	inflow sequence to the reservoir (simulated using the PITMAN model)

-	mean monthly S-Pan evaporation factored for open water surfaces

-	monthly time series of reservoir demands (irrigation and urban)

-	a and b coefficients describing the area/capacity curve (a single set of coefficients to describe all the "dummy" farm dams and a unique set for each major reservoir)

-	volume of water in storage at the start of the simulation (% FSC)

-	start and end year of simulation period

-	FSC and dead storage for each "time-slice" (these values reflect the growth or reduction in the number of farm dams or capacity of major reservoirs with time)

Spills from the "dummy" farm dam simulations and any irrigation return flows were added to the simulated run-off from the catchment downstream of the farm dams. From this flow sequence, run-of-river irrigation and urban demands were then subtracted. If the resulting flow sequence contained negative values, these were set to zero and considered to be unmet demands. Return flows (equal to 10% of the actual irrigation supply) from the run-of-river irrigation were added back to the system. The end result of the above procedure was a simulated run-off sequence for each hydrological calibration component.

4.1.2	Calibration of the PITMAN Model

Following the completion of the sub-system configuration in SHELL, the next process was the calibration of each of the run-off modules.

Rainfall input files for the PITMAN model and model parameters were used to generate simulated run-off for each run-off module. The Regional Parameters given in WR90 were used as starting values and are presented in Table 4.1.

Table 4.1:	WR90 Regional PITMAN parameters for the Lower Vaal Catchment

Gauge Number	Zmin	Zmax	ST	FT	GW	R	POW	PI	TL	GL	SL
C3H003	50	900	140	0	0	0	0	1.5	0.33	0	0
C3R002	20	900	140	0	0	0	0	1.5	0.50	0	0
C5H016	50	900	200	0	0	0	0	1.5	0.30	0	0
C5H018	50	900	200	0	0	0	0	1.5	0.30	0	0
C5R001	35	480	100	0	0	0	0	1.5	0.30	0	0
C5R002	20	460	100	0	0	0	0	1.5	0.30	0	0
C5R003	35	480	100	0	0	0	0	1.5	0.30	0	0
C5R004	20	480	100	0	0	0	0	1.5	0.30	0	0
C9H009	50	900	200	0	0	0	0	1.5	0.25	0	0
C9R001	50	900	200	0	0	0	0	1.5	0.25	0	0
The generated run-off files were compared with the relevant observed record. The statistics used to compare the simulated and observed records were:

-	MARs

-	standard deviations (SD)

-	log of MARs

-	log of SDs

-	time series comparing monthly simulated and observed flows, seasonal distributions, yield-storage and drought sequence curves.

In most observed flow records there were missing or exceeded values. These values need to be patched in order to make the comparison between observed and simulated flows meaningful. The observed record was patched with the simulated value obtained from using the regional parameters until a final calibration set was obtained. The PITMAN parameters were adjusted in order to "fine tune" the correlation between the observed and simulated records and the simulated values from the final simulation run were used to replace the initial patched values.

Acceptable limits for calibration were set and these are summarised in Table 4.2.

Table 4.2:	Acceptable limits for the calibration of the PITMAN model

Property	Acceptable Limit
MAR	<5%
SD	<20%
Time Series
Seasonal Distribution	as similar as possible
Storage vs Yield
Drought Sequences

A concerted effort was made to achieve these limits for all calibration points. However, the "goodness-of-fit" of a calibration was, to some extent, related to the accuracy of the flow gauging station and the availability of accurate, reliable and representative rainfall stations and landuse data.

5.	CALIBRATION RESULTS AND DISCUSSION

5.1	Run-off MODULE GAUGED BY C3H003 (HARTS RIVER)

5.1.1	Summary of run-off module data

Flow Station Number	:	C3H003

River Name	:	Harts River

Place	:	Taung

Table 5.1:	Summary of data for run-off module gauged by C3H003

Description	Value
WR90 quaternaries comprising the run-off module	C31A,C31B,C31C,C31D,C31E,C31F
Run-off module area	10 949 km2
Contributing run-off module area	7 975 km2
Downstream gauge	C3R002
Urbanised area (% of total area)	0 %
Diffuse irrigation area

Scheduled irrigation area - Taung area upstream of C3H003 (Vaalharts)	4 km2 (estimated)

11 km2
Farm dam capacity

Barberspan (incl. Leeupan)	8.913 million m3

30.399 million m3
Run-off module MAP	530 mm
Run-off module MAE (S-Pan)	1 916 mm
Calibration period	1935-1994
Observed MAR for calibration period (raw)	42.52 million m3
Observed MAR for calibration period (patched)	56.45 million m3
Simulated MAR for calibration period	57.04 million m3
Incremental Naturalised MAR (1920-1994)	58.96 million m3
Incremental Naturalised Run-off Coefficient (1920-1994)	0.014
Cumulative Naturalised MAR (1920-1994)	58.96 million m3

5.1.2	Description of the run-off module

This run-off module is drained by the upper reaches of the Harts River which arises near Lichtenburg. It contains two large off-channel storages, Leeupan and Barberspan. Barberspan is surrounded by the Barberspan Flora and Fauna Nature Reserve. The area is relatively unpopulated other than the small towns of Lichtenburg and Schweizer Reneke. The water supply for Lichtenburg is from boreholes and Schweizer Reneke obtains water from the Wentzel Dam. The dam also supplies irrigation water to the limited areas of irrigation near Schweizer Reneke.

Between Wentzel Dam and the newly built Taung Dam there is no significant irrigation or development. Taung Dam was completed in 1993 but as yet does not supply water for irrigation or urban use. Below Taung Dam there is the Taung scheduled irrigation area which is part of the Vaalharts Irrigation Scheme. Only a portion (11 km2) of the irrigation area is upstream of the C3H003 gauge and therefore only a percentage of the irrigation return flows contribute to the flow measured at the gauge. The urban water supply to Taung, the largest town in the catchment, is also supplied by abstractions at the Vaalharts Weir, via the canal network.

5.1.3	Model configuration

The system diagram and calibration results for this run-off module are included in Appendix G1.

Separate PITMAN runs were completed for:

-	area upstream of farm dams for each quaternary;

-	area downstream of farm dams for each quaternary.

The flow from the catchment area upstream of Barberspan and Leeupan was routed through a single dummy dam and there was no demand applied. The flow from the remaining areas upstream of farm dams above Wentzel Dam was routed through a separate dummy dam. An irrigation demand on the dam was applied in order to cause dam supply failure on average every three years (which was adjudged to be an appropriate frequency of failure for this region). The spills from the dummy dams and return flows from the upstream irrigation were added to the PITMAN simulated flow from all areas downstream of farm dams. The resulting flow was taken as the simulated flow entering Wentzel Dam. The dam was modelled using RESSIM and an irrigation and urban demand applied.

The flow from all areas upstream of farm dams below Wentzel Dam was routed through a single dummy dam using RESSIM. As there is only limited diffuse irrigation and most of the irrigation requirements are supplied from the Vaalharts Weir, no demand was applied to the dummy dam. The spills from the dummy dam and Wentzel Dam were then added to the PITMAN simulated flow from the area downstream of farm dams. The irrigation return flow from the Schweizer Reneke irrigation was estimated as 10% of the supply from Wentzel Dam and was added to the simulated flow. The resulting flow was routed through Taung Dam, modelled using RESSIM and the dummy dam area-capacity relationship (no area and capacity tables were available). No demand was applied to the dam. The spills from the dam were added to the return flow from the Taung irrigation upstream of the gauge, which was estimated as 10 % of the monthly irrigation volumes supplied from the Vaalharts Weir, to obtain the simulated flows at C3H003. The WR90 parameters were adjusted slightly during model calibration. The moisture holding capacity (ST) of the modelled catchment was reduced, thereby producing more run-off. The final calibration parameters are summarised in Table 5.2.

5.2	RUN-OFF MODULE GAUGED BY C3R002 (HARTS RIVER)

5.2.1	Summary of run-off module data

Flow Station Number	:	C3R002

River Name	:	Harts River

Place	:	Spitskop Dam

Table 5.2:	Summary of data for run-off module gauged by C3R002

Description	Value
WR90 quaternaries comprising the run-off module	C31F, C32A, C32B, C32C, C32D, C33A, C33B
Run-off module area:

Incremental

Cumulative

15 947 km2

26 896 km2
Contributing run-off module area	9 249 km2
Downstream gauge	-
Urbanised area (% of total area)	0 %
Irrigation area	5 km2 (estimated)
Farm dam capacity	6.119 million m3
Run-off module MAP	438 mm
Run-off module MAE (S-Pan)	2 039 mm
Calibration period	1975-1994
Observed MAR for calibration period (raw)	138.11 million m3
Observed MAR for calibration period (patched)	185.41 million m3
Simulated MAR for calibration period	185.95 million m3
Incremental Naturalised MAR (1920-1994)	77.49 million m3
Incremental Naturalised Run-off Coefficient (1920-1994)	0.019
Cumulative Naturalised MAR (1920-1994)	136.45 million m3
5.2.2	Description of the run-off module

Vryburg is the largest town in this run-off module and is situated in the upper reaches of the Dry Harts River which is a right bank tributary of the Harts River. The confluence is just downstream of Taung. There are fairly extensive parts of the extreme northern and western regions of the run-off module that do not contribute to the catchment run-off due to the extremely flat topography.

The lower reaches of this run-off module are highly developed in terms of agriculture as a significant portion of the Vaalharts irrigation scheme is adjacent to the Harts River in this region. There are also limited abstractions for diffuse irrigation from the same reach.

Gauge C3R002 measures the inflow to Spitskop Dam which supplies irrigation water to the downstream users in the form of compensation releases.

5.2.3	Model configuration

The system diagram and calibration results for this run-off module are included in Appendix G2.

Separate PITMAN runs were completed for:

-	area upstream of farm dams for each quaternary;

-	area downstream of farm dams for each quaternary, i.e contributing to Spitskop Dam.

The flow from all areas upstream of farm dams was routed through a single dummy dam modelled using RESSIM. The catchment area upstream of the farm dams was an estimated value as the catchment boundaries could not be accurately identified due to the flatness of the topography. An estimate was obtained by calculating the relationship between the number of dams and the total catchment area upstream of farm dams in the adjacent C3H003 catchment where the catchment boundaries were more easily defined. An irrigation demand on the farm dams was estimated as no irrigation information was available for the region. The demand was chosen to cause the dummy dam to fail on average, once every two to three years. The spill from the farm dams was then added to the PITMAN simulated flow from all areas downstream of farm dams.

The irrigation return flows from the North Canal irrigation area and Taung irrigation area are fairly significant and therefore a more accurate estimate rather than a simple percentage of supplied water was required. The same method was adopted as that used by Stewart Scott Inc. (1994). The observed records for the C3H003 and C3H007 gauges were utilised for this purpose, as the return flow enters the Harts River between these gauges.

The following equation was used to calculate the return flow:

IRRIG.RET	=	C3H007.OBS - C3H003.OBS - C3H007.FLW	(5.2.1)

where:

IRRIG.RET	=	return flow for the North Canal area

C3H007.OBS	=	observed flow at station C3H007

C3H003.OBS	=	observed flow at station C3H003

C3H007.FLW	=	incremental inflow (determined using the PITMAN model ) for the C3H007 run-off module.

The file IRRIG.RET was amended to exclude monthly flows where the occurrence of any of the following conditions casts doubts on the accuracy of the calculation using Equation 5.2.1:

-	flow at C3H007 > DT limit of 3.2 m3/s (implies that the flow gauging is inaccurate);

-	flow at C3H003 > 3 x 106 m3 for the month. (When flow at the upstream station is high, small errors in the estimation of river flows can result in large percentage errors in the return flow calculated using Equation 5.2.1).

-	incremental catchment run-off between stations C3H003 and C3H007 is high relative to the calculated return flow (which implies that small errors in the estimated catchment run-off can result in large percentage errors in the calculated return flow).

The low DT for weir C3H007 meant in effect that irrigation return flows could only be calculated during relatively dry months and not for most months between November and May. The final amended flow file is shown in Appendix B2.

A similar procedure was used to calculate the total return flow (including West Canal area) from the area between C3H003 and C3R001 using the flow record at C3R002 in place of C3H007.OBS and the run-off for the entire Spitskop Dam catchment in place of C3H007.INF. The return flows calculated using Equation 5.2.1 were marginally lower than those in IRRIG.RET indicating losses in the West Canal area. The West Canal area is also about 15 km from the Harts River and the numerous pans evaporate any run-off that could potentially enter the river. The irrigation return flow factor from this region was therefore taken to be zero.

The irrigation return flows were then added to the catchment run-off after which the diffuse irrigation demands from the lower Harts River were subtracted and the return flows added back to the system. The resulting file is the simulated inflow into Spitskop Dam at C3R002.

During model calibration, only the ZMIN parameter was adjusted. The effect of reducing ZMIN was to increase the volume of surface run-off and thereby increase the MAR.

5.3	RUN-OFF MODULE GAUGED BY C5H016 (RIET RIVER)

5.3.1	Summary of run-off module data

Flow Station Number	:	C5H016

River Name	:	Riet River

Place	:	Aucampshoop

Table 5.3:	Summary of data for run-off module gauged by C5H016

Description	Value
WR90 quaternaries comprising run-off module	C51K, C51L
Run-off module area:

Incremental

Cumulative

5 657 km2

33 296 km2
Contributing run-off module area	1 847 km2
Downstream gauge	-
Urbanised area (% of total area)	0 %
Actual scheduled irrigation area

Estimated diffuse irrigation area	135.6 km2

10.0 km2
Farm dam capacity	-
Run-off module MAP	350 mm
Run-off module MAE (S-Pan)	2 050 mm
Calibration period	1960-1994
Observed MAR for calibration period (raw)	63.75 million m3
Observed MAR for calibration period (patched)	262.52 million m3
Simulated MAR for calibration period	265.72 million m3
Incremental Naturalised MAR (1920-1994)	6.37 million m3
Incremental Naturalised Run-off Coefficient (1920-1994)	0.010
Cumulative Naturalised MAR (1920-1994)	405.49 million m3

5.3.2	Description of the run-off module

A significant portion of this run-off module is endoreic and therefore does not contribute to the run-off. There is a broad corridor on either side of the Riet River which is under intensive irrigation. The water requirements of the Kalkfontein scheduled irrigation, just downstream of Kalkfontein Dam, are supplied directly from the dam via an irrigation canal. If there is water available in the Riet River downstream of Kalkfontein Dam due to spills or irrigation return flows, this water is used for irrigation.

Further downstream is the Riet River Settlement and Ritchie and Scholtzburg Irrigation Boards whose requirements were supplied by Kalkfontein Dam prior to 1987 and are now supplied by the Orange-Riet Canal. There is extensive controlled irrigation along the Orange-Riet Canal within the catchment. As this falls within the endoreic zone it is assumed that no return flow from this region reaches the Riet River. Excess flows from the Orange-Riet Canal are released into the Riet River near Jacobsdal. The water requirements of Koffiefontein town and mine and Jacobsdal are supplied by the Kalkfontein Irrigation canal.

The Modder River joins the Riet River near Ritchie and these flows, the remaining flows in the Riet River and all the excess from the canal are used to supply the requirements of the Lower Riet River Irrigation Board, which extends from Ritchie to the confluence of the Riet and the Vaal Rivers. The C5H016 gauge is situated almost halfway down this reach and it is therefore assumed that only half of the total demand from the Lower Riet River irrigation is abstracted in this reach. Therefore only 10% of the actual supply of the halved demand is assumed to occur above C5H016 gauge as return flows.

5.3.3	Model configuration

The system diagram and calibration results for this run-off module are in Appendix G3.

Separate PITMAN runs were completed for the contributing area of each quaternary.

There are no farm dams modelled in this catchment. The simulated run-off from the total quaternary catchments were summed and added to the spills from Kalkfontein Dam. For the period 1960-1986 the return flows for the Kalkfontein irrigation area, Riet River Settlement, Ritchie Irrigation Board and Scholtzburg Irrigation Board were collectively assumed to be 10 % of the irrigation releases from Kalkfontein Dam. For the period after 1987, the water demand for the Riet River Settlement, Ritchie and Scholtzburg Irrigation areas were calculated individually. It was assumed that these demands were met in full by the Orange-Riet Canal and the return flows were calculated as 10 % of the demand. The return flow from the Kalkfontein Irrigation area was again calculated as 10 % of the irrigation releases from Kalkfontein Dam.

The observed flows measured at station C5H018 were then added and a run-of-river abstraction was made for the extra irrigated area below Kalkfontein Dam. This area was scheduled at 4.80 km2. This was increased during calibration to 10 km2 because systematic over-estimation of low flows indicated that irrigation abstractions were being under-simulated.

The excess flows from the Orange-Riet Canal were added to the calculated return flows, simulated run-off and spills from Kalkfontein Dam. The irrigation demand for the Lower Riet River Irrigation Board was estimated and halved as the C5H016 gauged is situated in the middle of the Lower Riet River Irrigation area. The Orange-Riet Canal excess flows were added to the Lower Riet River irrigation demand to eliminate a large portion of the low flows in a further attempt to remedy the under-simulated irrigation abstractions. The demand was subtracted from the simulated flows in the Riet River and the return flow added back to the system. The resulting flows were the simulated flows at gauge C5H016.

WR90 Regional Parameters were used during model calibration. This was due to the incomplete monthly water demand information with the result that many assumptions were made concerning irrigation water requirements and return flows. The observed record also required extensive patching which explains the significant difference between the raw and patched MARs for the calibration period. The time series of the raw and patched files are included in Appendix E and F respectively.

5.4	RUN-OFF MODULE GAUGED BY C5H018 (MODDER RIVER)

5.4.1	Summary of run-off module data

Flow Station Number	:	C5H018

River Name	:	Modder River

Place	:	Tweerivier

Table 5.4:	Summary of data for run-off module gauged by C5H018

Description	Value
WR90 quaternaries comprising the run-off module	C52H, C52J, C52K, C52L
Run-off module area:

Incremental

Cumulative

11 029 km2

17 360 km2
Contributing run-off module area	2 236 km2
Downstream gauge	C5H016
Urbanised area (% of total area)	0%
Actual scheduled irrigation area

Estimated additional irrigation area	33.64 km2

11.36 km2
Farm dam capacity	7.447 million m3
Run-off module MAP	422 mm
Run-off module MAE (S-Pan)	1 871 mm
Calibration period	1971-1994
Observed MAR for calibration period (raw)	143.64 million m3
Observed MAR for calibration period (patched)	125.73 million m3
Simulated MAR for calibration period	114.28 million m3
Incremental Naturalised MAR (1920-1994)	14.38 million m3
Incremental Naturalised Run-off Coefficient (1920-1994)	0.015
Cumulative Naturalised MAR (1920-1994)	159.48 million m3
5.4.2	Description of the run-off module

A large percentage of this run-off module does not contribute to the total run-off due to the extremely flat topography. There is assumed to be a narrow corridor along the Modder River which contributes to the flows in the river. Most of this area is scheduled irrigation land known as the Modder River Government Water Scheme. Releases from the Welvaart STW are made into the Kaalspruit augments the flows in the Modder River in the upper reaches of the run-off module just downstream of Krugersdrift Dam. There are numerous farm dams which store a portion of the catchment run-off as well as acting as a storage facility/balancing dam for the water pumped from the many weirs in the Modder River for irrigation.

A section of the Kimberley urban area falls within this catchment, however, all releases from the sewage treatment works are to the Du Toits Pan and Kamfersdam Pan and therefore do not enter the Modder River system. Kimberley lies well within an endoreic area and therefore the run-off from the impervious urban areas will have no effect on the catchment run-off.

5.4.3	Model configuration

The system diagram and calibration results for this run-off module are included in Appendix G4.

Separate PITMAN runs were completed for:

-	area upstream of farm dams for each quaternary;

-	area downstream of farm dams for each quaternary.

The total irrigation demand for this run-off module was applied, in unequal portions, to the dummy dam and compensation releases. An irrigation demand equal to 70 % of the total demand was subtracted from the combined spills and compensation releases from Krugersdrift Dam, as were the evaporative losses, to produce a net inflow file. The evaporative losses from the river were assumed to be a function of the compensation releases and was therefore "calibrated" as 0.5 % of the compensation releases. The evaporative losses were set to zero in flood months.

The simulated flow from the catchment upstream of farm dams was routed through a dummy farm dam. The remaining irrigation demand was applied to the dummy dam (30 % of the total irrigation demand) and the spills from the dam and return flows from the irrigation supported by the dam were added to the simulated flows from the catchment area downstream of farm dams.

The return flows from Welvaart STW, the return flows from the irrigation supported by compensation releases and the net inflow were added to the total catchment run-off to produce the simulated flows at Tweerivier (C5H018).

5.5	RUN-OFF MODULE GAUGED BY C5R001 (KAFFER RIVER)

5.5.1	Summary of run-off module data

Flow Station Number	:	C5R001

River Name	:	Kaffer River

Place	:	Tierpoort Dam

Table 5.5:	Summary of data for run-off module gauged by C5R001

Description	Value
WR90 quaternaries comprising the run-off module	C51D
Run-off module area	922 km2
Contributing run-off module area	922 km2
Downstream gauge	C5R002
Urbanised area (% of total area)	0 %
Irrigation area	7.0 km2
Farm dam capacity	5.39 million m3
Run-off module MAP	491 mm
Run-off module MAE (S-Pan)	1 640 mm
Calibration period	1923-1976
Observed MAR for calibration period (raw)	20.51 million m3
Observed MAR for calibration period (patched)	22.28 million m3
Simulated MAR for calibration period	22.70 million m3
Incremental Naturalised MAR (1920-1994)	23.76 million m3
Incremental Naturalised Run-off Coefficient (1920-1994)	0.053
Cumulative Naturalised MAR (1920-1994)	23.76 million m3

5.5.2	Description of the run-off module

The Tierpoort Dam catchment has some diffuse irrigation which is supplied mostly by abstractions from the numerous farm dams as the river is frequently dry. There is no recent information on the extent of irrigation in the area.

5.5.3	Model configuration

The system diagram and calibration results for this run-off module are included in Appendix G5.

Separate PITMAN runs were completed for:

-	area upstream of farm dams;

-	area downstream of farm dams, i.e. contributing to Tierpoort Dam.

The catchment area contributing to farm dams was reduced to 10 % of the measured area to ensure that the flows entering farm dams was sufficiently small to cause dam supply failure almost every year (in line with failure frequency recommended by the local agricultural extension officer (Brink, 1996)). The irrigation demand from farm dams, estimated as 70 % of the total demand, was subtracted during the reservoir modelling routine (RESSIM). The spills from this dummy dam were then added to the PITMAN simulated flow from the area downstream of farm dams. The remainder of the diffuse irrigation demand was subtracted from the total flow and return flows of 10 % of the actual supply were added back to the system. The resulting flows are the simulated inflows to Tierpoort Dam.

The original estimate of 13 km2 of irrigated area reported in the Orange River System Analysis (DWAF, 1991) was found to use up too large a portion of the low flows. The area was reduced to 2 km2 in order to simulate the low flows acceptably. The WR90 Regional Parameters were also adjusted to simulate more run-off during model calibration.

5.6	RUN-OFF MODULE GAUGED BY C5R002 (RIET RIVER)

5.6.1	Summary of run-off module data

Flow Station Number	:	C5R002

River Name	:	Riet River

Place	:	Kalkfontein Dam

Table 5.6:	Summary of data for run-off module gauged by C5R002

Description	Value
WR90 quaternaries comprising the run-off module	C51A,C51B,C51C,C51E,

C51F,C51G,C51H,C51J
Run-off module area:

Incremental

Cumulative

9339 km2

10 261 km2
Contributing run-off module area	8 085 km2
Downstream gauge	C5H016
Urbanised area (% of total area)	0 %
Irrigation area	40 km2
Farm dam capacity	56.21 million m3
Run-off module MAP	412 mm
Run-off module MAE (S-Pan)	1 746 mm
Calibration period	1937-1976
Observed MAR for calibration period (raw)	210.09 million m3
Observed MAR for calibration period (patched)	225.35 million m3
Simulated MAR for calibration period	215.55 million m3
Incremental Naturalised MAR (1920-1994)	215.88 million m3
Incremental Naturalised Run-off Coefficient (1920-1994)	0.060
Cumulative Naturalised MAR (1920-1994)	239.64 million m3

5.6.2	Description of the run-off module

This catchment contains many small farm dams which store a small portion of the catchment run-off and borehole water. They are used for very limited irrigation and livestock watering. The rivers in this run-off module regularly run dry and are therefore not a reliable source of water.

The catchment incorporates the entire upper catchment of the Riet River and some of its major tributaries namely, the Fouriespruit and Kromellenboogspruit. The Kaffer River also joins the Riet River downstream of Tierpoort Dam.

5.6.3	Model configuration

The system diagram and calibration results for this run-off module are included in Appendix G6.

Separate PITMAN runs were completed for:

-	area upstream of farm dams;

-	area downstream of farm dams.

The catchment area contributing to farm dams was reduced to 10 % of the measured area and the same area added to the catchment downstream of farm dams. This was to ensure that the flows entering farm dams was sufficiently small to cause dam supply failure almost every year (in line with failure frequency recommended by the local agricultural extension officer). The irrigation demand for the upper and lower quaternaries was calculated individually according to the irrigation information provided by local sources. The total irrigation demand for this run-off module was applied, in unequal portions, to the dummy dam and the main river. The irrigation demand from farm dams, calculated as 70 % of the total demand, was abstracted from a single dummy dam for the whole run-off module. The spills from this dummy dam and Tierpoort Dam were then added to the PITMAN simulated flow from the area downstream of farm dams. The return flows from the irrigation area listed under the Kaffer River Irrigation Board were calculated as 10 % of the supply from Tierpoort Dam and added to the simulated flows. The remainder of the diffuse irrigation demand (30 %) was subtracted from the total flow and return flows of 10 % of the actual supply were added back to the system. The resulting flows are the simulated flows into Kalkfontein Dam.

The WR90 parameters were only adjusted slightly in an attempt to reduce the simulated MAR. However, the model calibration for this gauge aimed to achieve a slightly lower MAR than the observed file due the extreme alterations by DWAF of the flood events in the record before automatic recording started in 1978. None of the monthly rainfalls recorded during the flood events were large enough to produce simulated inflows of the same magnitude as those recorded in the inflow record. Therefore, in adjusting the model parameters to produce a similar MAR to the observed record, the modeller may attempt to compensate for the exaggerated peak events and consequently produce more run-off. As a result the low flows end up being over-estimated. The DWAF was consulted about the apparent discrepancy, who reported back that the earlier records had been subject to unit conversion errors and that the new records were accurate. The observed flow record into Kalkfontein Dam before and after the adjustment are compared in Appendix G6.2. The comparison shows an increase in the MAR of the new record of over 20 %.

5.7	RUN-OFF MODULE GAUGED BY C5R003 (MODDER RIVER)

5.7.1	Summary of run-off module data

Flow Station Number	:	C5R003

River Name	:	Modder River

Place	:	Rustfontein dam

Table 5.7:	Summary of data for run-off module gauged by C5R003

Description	Value
WR90 quaternaries comprising the run-off module	C52A
Run-off module area	937 km2
Contributing run-off module area	937 km2
Downstream gauge	C5R004
Urbanised area (% of total area)	0 %
Present irrigation area	4.5 km2
Farm dam capacity	10.333 million m3
Run-off module MAP	543 mm
Run-off module MAE (S-Pan)	1 600 mm
Calibration period	1954-1994
Observed MAR for calibration period (raw)	32.07 million m3
Observed MAR for calibration period (patched)	32.66 million m3
Simulated MAR for calibration period	33.20 million m3
Incremental Naturalised MAR (1920-1994)	30.67 million m3
Incremental Naturalised Run-off Coefficient (1920-1994)	0.060
Cumulative Naturalised MAR (1920-1994)	30.67 million m3

5.7.2	Description of the run-off module

The catchment upstream of Rustfontein consists of only one quaternary and is situated in the headwaters of the Modder River. The other major river in the catchment is the Kromspruit which also feeds directly into Rustfontein Dam. There are areas of scheduled and diffuse irrigation and the demand is supplied either by run-of-river abstractions or from the numerous farm dams in the area. All irrigation return flows and catchment run- off downstream of the farm dams flow into Rustfontein Dam.

5.7.3	Model configuration

The system diagram and calibration results for this run-off module are included in Appendix G7.

Separate PITMAN runs were completed for:

-	area upstream of farm dams;

-	area downstream of farm dams i.e. contributing to the Rustfontein Dam.

The growth of scheduled and diffuse irrigation demand was estimated from known irrigation areas and crop distribution information using the IRRDEM routine. The diffuse and scheduled demands were added together to create a total demand file. The catchment areas upstream of farm dams were reduced to 10 % of their original area to ensure the dummy dam failed to supply the demand, on average, once every three years according to information supplied by a local agricultural organisation. The flow from this area upstream of farm dams was routed through two dummy dams. Local information indicated that no more than 40 % of the farm dams are used for irrigation. Therefore, as a conservative assumption, only 30 % of the run-off was routed through the irrigation dummy dam and the remaining 70 % through a token dummy dam which functions simply as an evaporation pan and contributes spills in flood events. A 25 % portion of the irrigation demand was ajudged to be supplied from the irrigation dummy dam.

The area downstream of farm dams was increased by the same area subtracted from the area upstream of farm dams. The simulated flows from this area were added to the spills from the dummy dams and the return flow from the farm dam irrigation to give the total catchment run-off. The remaining 75 % irrigation demand was then subtracted and the return flows added back to the system to produce the simulated flow into Rustfontein Dam at gauge C5R003.

The WR90 Regional Parameters produced a reasonably good simulation of the observed flows and therefore only ZMAX (which determines, in conjunction with ZMIN, the average infiltration into the soil) was reduced to simulate slightly more run-off.

5.8	RUN-OFF MODULE GAUGED BY C5R004 (MODDER RIVER)

5.8.1	Summary of run-off module data

Flow Station Number	:	C5R004

River Name	:	Modder River

Place	:	Krugerdrift Dam

Table 5.8:	Summary of data for run-off module gauged by C5R004

Description	Value
WR90 quaternaries comprising the run-off module	C52B, C52C, C52D, C52E, C52F, C52G
Run-off module area:

Incremental

Cumulative

5 394 km2

6 331 km2
Contributing run-off module area	5 391 km2
Downstream gauge	C5H018
Urbanised area (% of total area)	12.4 %
Present irrigation area	23.9 km2
Farm dam capacity (above Mockes Dam)

Farm dam capacity (below Mocles Dam)

Farm dam capacity (quaternary C52F)	7.699 million m3

8.065 million m3

4.120 million m3
Run-off module MAP	508 mm
Run-off module MAE (S-Pan)	1 639 mm
Calibration period	1971-1994
Observed MAR for calibration period (raw)	154.72 million m3
Observed MAR for calibration period (patched)	157.49 million m3
Simulated MAR for calibration period	154.00 million m3
Incremental Naturalised MAR (1920-1994)	114.43 million m3
Incremental Naturalised Run-off Coefficient (1920-1994)	0.042
Cumulative Naturalised MAR (1920-1994)	145.10 million m3

5.8.2	Description of the run-off module

This run-off module is highly developed in terms of agriculture and urbanisation. Botshabelo township and Thaba Nchu are both situated in the upper reaches of the incremental catchment. Botshabelo receives its water supply from the Caledon-Modder transfer scheme. Thaba Nchu's water supplies come from Groothoek and Seraolo Dams. The urban return flows from both Botshabelo and Thaba Nchu eventually augment flows in the Modder River upstream of Mockes Dam. As there are no reliable records of the volume of water consumed or returned, the return flows had to be estimated from the water supplied. There is intensive diffuse irrigation throughout the area and irrigation water supplies are from run-of-river abstractions and farm dams.

Mockes Dam divides the catchment into two, almost equal areas and acts as a balancing dam to ensure a constant supply of water to the purification works at Mazelspoort Weir. From the weir, water is pumped to the Bloemfontein area for urban and industrial use. Bloemfontein's main water supply, however, is the Caledon-Modder transfer from Welbedacht Dam. This water only reaches the Modder River much further downstream as effluents from Bloemspruit and Welvaart Sewage Treatment Works.

Almost all the treated water from the Bloemspruit STW, released into the Bloemspruit which feeds into the Renosterspruit is used to supply the demands of the intensive diffuse irrigation in the lower Renosterspruit catchment.

5.8.3	Model configuration

The system diagram and calibration results for this run-off module are included in Appendix G8.

Separate PITMAN runs were completed for:

-	area upstream of farm dams for each quaternary;

-	area downstream of farm dams for each quaternary;

All the information concerning the farm dams and proportioning of irrigation demand between farm dams and the main river in this run-off module was supplied by a local agricultural organisation (Terblanche, 1996). Not more than 40 % of the farm dams in this region are used for irrigation. Therefore, in order to model the effect of the dams the run-off was divided and routed through two dummy dams, an irrigation and an urban dummy dam. As a conservative assumption the total capacity of the farm dams in the area was divided; the irrigation dummy dam storing 30 % and the urban dummy dam storing 70 % of the total capacity. The total inflow into the dummy dams was divided in the same proportions. Catchment areas upstream and downstream of the farm dams were adjusted to produce sufficient run-off to cause the farm dams to fail with the frequency recommended.

The area upstream of farm dams was reduced to 30 % of the original area and the main catchment was increased by the same area. This was in order to decrease the run-off entering the farm dams and ensure supply failure, on average, every three years.

The flows from the area upstream of farm dams in quaternary C52B, C52C and C52D upstream of Mockes Dam were summed. Approximately 19 % of quaternary C52B is urbanised of which 2 % was considered impervious. The flow was divided, in the proportions mentioned above, and routed through the irrigation dummy dam and the urban dam. A 25 % portion of the total irrigation demand upstream of Mockes Dam was abstracted from the irrigation dummy dam through the reservoir modelling routine (RESSIM). No demand was placed on the urban dummy dam which simply acts as an evaporation pan. The spills from both the irrigation and urban dummy dams and return flows from the irrigation supplied by the farm dams were then added to the PITMAN simulated flow from the area downstream of farm dams above Mockes Dam. The spills and compensation releases from Rustfontein Dam (irrigation releases via pipes were considered negligible) and spills from Groothoek Dam, as well as urban return flows from Botshabelo and Thaba Nchu (estimated as a percentage of water supplied), were added to the main catchment run-off. The sum of these files produced the total contributing catchment run-off for the region above Mockes Dam. A further 75 % of the irrigation demand was subtracted from the flows and the irrigation return flows were added back to the system. The resulting monthly flows were routed through Mockes Dam just upstream of Mazelspoort Weir.

At the Mazelspoort Weir, the urban demand for Bloemfontein and the surrounding areas was subtracted. The run-off from the catchments upstream of farm dams in quaternary C52E and C52G downstream of Mockes Dam was routed through a single dummy dam, without demands, as all irrigation demands are supplied directly from the Modder River and not farm dams. The spills from the dummy dam were added to the main catchment run-off. The irrigation demand was subtracted and the return flows added back to the system to produce the simulated flows in the Modder River upstream of the confluence with the Renoster River.

The catchment area upstream of farm dams in quaternary C52F was reduced to 50 % of the original measured area and the same area added to the downstream catchment area. Bloemfontein and the surrounding areas occupy approximately 72 % of this quaternary. Again only 2 % of the urbanised area was considered to be impervious. All the run-off from upstream of farm dams was routed through a single irrigation dummy dam. A 25 % portion of the total irrigation demand was subtracted from the irrigation dummy dam. The spills from the dummy dam and return flows from the irrigation supplied by the farm dams were then added to the PITMAN simulated flow from the area downstream of farm dams. The urban return flows from Bloemfontein were then added to these flows after which a large portion of the flows in the Renoster River were subtracted to satisfy the remaining 75 % of the diffuse irrigation demand. The remaining flows as well as the calculated irrigation return flows were added to the simulated flows in the Modder River to produce the simulated inflow into Krugersdrift Dam.

As this run-off module is characterised by extensive irrigation, and urban development, the WR90 Regional Parameters were altered quite significantly in order to simulate the observed flows more accurately. The model was consistently over-estimating the catchment run-off and peaks of smaller events. This was a clear indication that the irrigation and domestic water use information used in model calibration was significantly under-estimating the actual water use in the catchment. However, with no justification for increasing the urban or irrigation demands, the ZMIN, ZMAX and ST parameters had to be increased in order to simulate more catchment run-off.

5.9	RUN-OFF MODULE GAUGED BY C9H009 (VAAL RIVER)

5.9.1	Summary of run-off module data

Flow Station Number	:	C9H009

River Name	:	Vaal River

Place	:	De Hoop 65

Table 5.9:	Summary of data for run-off module gauged by C9H009

Description	Value
WR90 quaternaries comprising the run-off module	C91C, C91D, C91E
Run-off module area:

Incremental

Cumulative (below Bloemhof Dam)

5 997 km2

13 218 km2
Contributing run-off module area	3 201 km2
Downstream gauge	-
Urbanised area (% of total area)	0 %
Irrigation area	30 km2 (estimated)
Farm dam capacity	0.709 million m3
Run-off module MAP	418 mm
Run-off module MAE (S-Pan)	1 942 mm
Calibration period	1971-1994
Observed MAR for calibration period (raw)	818.65 million m3
Observed MAR for calibration period (patched)	978.92 million m3
Simulated MAR for calibration period	1 061.22 million m3
Incremental Naturalised MAR (1920-1994)	12.93 million m3
Incremental Naturalised Run-off Coefficient (1920-1994)	0.010
Cumulative Naturalised MAR (1920-1994)	24.09 million m3

5.9.2	Description of the run-off module

This run-off module lies between Vaalharts Weir and just downstream of Riverton. To the west of the run-off module there are vast endoreic areas which significantly reduce the area of the run-off module that contributes to the run-off. For quaternary C91C the whole catchment is considered by WR90 to be endoreic. This study found that to produce a meaningful calibration and later naturalisation the contributing catchment area had to be increased to half of the gross catchment area.

There is irrigation along the main channel supported by compensation releases from Vaalharts Weir. Various industries and diggers are also supplied with water from the Vaal River in this reach. In addition just over 10 million m3/a is allocated as "free water" for various users. Abstractions of approximately 28 million m3 per month are made from the Vaal at Riverton to supply the urban and industrial demands of Kimberley, south of the Vaal River. Urban return flows from the various treatment works in Kimberley are released into either the Kamfersdam Pan or the Du Toits Pan. Therefore, none of the return flows are used for irrigation or are returned to the river. A section of the Kimberley urban area falls within this catchment, however, Kimberley lies well within an endoreic area and therefore the impervious urban areas is considered to have no effect on the catchment run-off.

5.9.3	Model configuration

The system diagram and calibration results for this run-off module are included in Appendix G9.

Separate PITMAN runs were completed for:

-	area upstream of farm dams for each quaternary;

-	area downstream of farm dams for each quaternary.

The total monthly volume abstracted for irrigation was estimated from permit information, provided by DWAF in Jan Kempdorp, and then subtracted from the compensation releases from Vaalharts Weir. The water demands of various industries, urban users and diggers as well as the supply to Kimberley was subtracted from the compensation releases. The evaporative losses were assumed to be a function of the compensation releases and were calculated as 0.5 % of the compensation releases. The losses were set to zero in flood months. The evaporative losses from the river were then also subtracted from the remaining compensation flow resulting in a net inflow into the catchment.

The flow from all are as upstream of farm dams was routed through a single dummy dam modelled using RESSIM. The spill from the dummy dam was then added to the PITMAN simulated flow from all areas downstream of farm dams to give the total contributing catchment run-off. The irrigation return flows were then added back to the catchment run-off and net inflows. The resulting flow file was the simulated flow at De Hoop (C9H009).

5.10	RUN-OFF MODULE GAUGED BY C9R001 (VAAL RIVER)

5.10.1	Summary of run-off module data

Flow Station Number	:	C9R001

River Name	:	Vaal River

Place	:	Vaalharts Weir

Table 5.10:	Summary of data for run-off module gauged by C9R001

Description	Value
WR90 quaternaries comprising the run-off module	C91A, C91B
Run-off module area

Incremental

Cumulative (Below Bloemhof Dam)

7 221 km2

7 221 km2
Contributing run-off module area	2 509 km2
Downstream gauge	C9H009
Urbanised area (% of total area)	0 %
Irrigation area	46.67 km2
Farm dam capacity	0.154 million m3
Run-off module MAP	444 mm
Run-off module MAE (S-Pan)	1 946 mm
Calibration period	1971-1994
Observed MAR for calibration period (raw)	1 544.53 million m3
Observed MAR for calibration period (patched)	1 571.47 million m3
Simulated MAR for calibration period	1 712.21 million m3
Incremental Naturalised MAR (1920-1994)	11.16 million m3
Incremental Naturalised Run-off Coefficient (1920-1994)	0.010
Cumulative Naturalised MAR (1920-1994)	11.16 million m3

5.10.2	Description of the run-off module

This run-off module stretches between Bloemhof Dam and Vaalharts Weir. There is only a narrow corridor along the Vaal River which is judged to contribute to the catchment run-off. Within this corridor there is irrigation controlled by permits, which have not been revised since their implementation in the sixties and therefore only provide an estimate of actual irrigation demand. Compensation releases are made from Bloemhof Dam to support the needs of the irrigators along the Vaal, the requirements of the Vaalharts Irrigation Scheme and the Lower Vaal consumers, urban and agricultural. The flows to the Lower Vaal (incl. the Vaal-Gamagara transfer) are controlled at the Vaalharts Weir.

5.10.3	Model configuration

The system diagram and calibration results for this run-off module are included in Appendix G10.

Separate PITMAN runs were completed for:

-	area upstream of farm dams for quaternary C91B;

-	area downstream of farm dams for each quaternary.

The total monthly volume abstracted for irrigation was estimated from permit information and then subtracted from the spills and compensation releases from Bloemhof Dam. The evaporative losses were assumed to be a function of the compensation releases and were therefore calculated as 0.5 % of the compensation releases. The losses were set to zero in flood months. The evaporative losses from the river were then subtracted from the spills and compensation flow, resulting in a net inflow into the catchment.

The flow from all areas upstream of farm dams was routed through a single dummy dam, without demands, modelled using RESSIM. The spill from the dummy dam was then added to the PITMAN simulated flow from all areas downstream of farm dams to give the total contributing catchment run-off. The irrigation return flows were then added back to the catchment run-off and net inflows. The resulting flow file was the simulated inflow into the Vaalharts Weir.

Due to the shortage of irrigation and abstraction information there was insufficient evidence to motivate adjusting the WR90 Regional Parameters to improve the model simulation. This explains the discrepancy between the raw and observed MARs for the calibration period in Table 5.10.

5.11	RUN-OFF MODULE (LOWER VAAL RIVER)

5.11.1	Summary of run-off module data

River Name	:	Vaal River

Place	:	Vaal and Orange River Confluence

Table 5.11:	Summary of data for the Lower Vaal River run-off module

Description	Value
WR90 quaternaries comprising the run-off module	C33C, C51M, C91E, C92A, C92B
Run-off module area

Incremental

Cumulative (Below Bloemhof Dam)

14 838 km2

28 056 km2
Contributing run-off module area	6 096 km2
Downstream gauge	-
Urbanised area (% of total area)	0 %
Irrigation area	Unknown
Farm dam capacity	2.231 million m3
Run-off module MAP	361 mm
Run-off module MAE (S-Pan)	2 210 mm
Incremental Naturalised MAR (1920-1994)	31.52 million m3
Incremental Naturalised Run-off Coefficient (1920-1994)	0.014
Cumulative Naturalised MAR (1920-1994)	597.55 million m3

5.11.2	Description of the run-off module

The run-off module incorporates quaternaries from three different secondary drainage regions.

There is diffuse irrigation along the lower Harts River between Spitskop Dam and the Vaal and Harts River confluence. Compensation releases are made from Spitskop Dam to support the run-of-river abstractions for irrigation.

The scheduled irrigation area near Barkly West north of the Vaal River is supplied by water from the Klipdam-Barkly canal which is part of the Vaalharts Irrigation Scheme. There is also fairly intensive riparian irrigation along the remaining stretch of the Vaal River.

Just downstream of the Vaal and Harts River confluence, abstractions are made for the Vaal-Gamagara transfer which supplies water to the urban communities of Sishen, Hotazel and Postmasburg, Blackrock and the surrounding farms. There is more irrigated land adjacent to the Vaal below the confluence with the Harts River and it intensifies closer to the Vaal and Riet River confluence.

The remaining irrigation along the Riet River falls under the control of the Lower Riet Irrigation Board. Run-of-river abstractions from the Riet River supply the irrigation requirements of the scheduled irrigation.

5.11.3	Model configuration

No calibration of this gauge was attempted as there was insufficient irrigation information available along the Vaal River. The record at the most downstream station, C9H007 at Douglas, covers the period 1947-1972, of which the portion prior to 1959 requires considerable patching. In addition, the information required for the model calibration such as the Vaal-Gamagara transfer, spills from Spitskop Dam and observed flows at station C9H009 were only available for approximately the last 25 years and therefore only overlapped with the observed record at C9H007 for between three and four years. This was considered insufficient to obtain a meaningful calibration. It was therefore decided to use simulated naturalised flows for the entire run-off module to the Orange-Vaal confluence with the same set of WR90 Regional Parameters used for model calibration of the upstream gauges on the Vaal River, ie. C9R001 and C9H009.

5.12	SUMMARY OF MODEL CALIBRATIONS

The final PITMAN parameters used for the calibration of each station are summarized in Table 5.12 below. The comparison plots of observed and simulated streamflow characteristics can be found in Appendix G.

Table 5.12:	Final PITMAN parameters for the Lower Vaal Catchment

Gauge Number	Zmin	Zmax	ST	FT	GW	R	POW	PI	TL	GL	SL
C3H003	50	900	150

(140)	0	0	0	0	1.5	0.33	0	0
C3R002	60

(20)	900	140	0	0	0	0	1.5	0.50	0	0
C5H016	50	900	200	0	0	0	0	1.5	0.30	0	0
C5H018	50	900	200	0	0	0	0	1.5	0.30	0	0
C5R001	30

(35)	500

(480)	150

(100)	0	0	0	0	1.5	0.30	0	0
C5R002	20	430

(460)	100	0	0	0	0	1.5	0.30	0	0
C5R003	35	485

(480)	100	0	0	0	0	1.5	0.30	0	0
C5R004	40

(20)	500

(480)	140

(100)	0	0	0	0	1.5	0.30	0	0
C9H009	50	900	200	0	0	0	0	1.5	0.25	0	0
C9R001	50	900	200	0	0	0	0	1.5	0.25	0	0

Note:	Where calibration parameters differ from WR90 Regional Paramters, the WR90 values are given in brackets.

6.	NATURALISATION

After the completion of the calibration process, the finalised parameters were used to re-
simulate each run-off module. All land use components were "switched off" in order to simulate the virgin conditions or natural hydrology of each run-off module.

The historical flow sequence obtained in the calibration process was subtracted from the virgin sequence to obtain a flow sequence representing the overall effect of all land use developments in each run-off module. The flow sequence was added to the patched observed historical record to reconstruct the "obse vbbbrved" naturalised flow sequence.

The procedure follows the approach used in the WR90 study, as follows:

On = Oh + (Sv - Sh)	(6.1)

Where:

Sh = Simulated historical flow

Sv = Simulated virgin/natural flow

Oh = Observed historical flow

On = "Observed" natural flow

The result is a time series of natural monthly flows at each gauge.

To obtain the extended naturalised records for the whole period under consideration (1920-1994), the virgin flows, simulated using the calibration parameters and the percentage rainfall record for the extended period, were appended to the naturalised observed flows. In all cases a single mass plot was used to check the stationarity of the naturalised sequence.

6.1	NATURALISATION OF GAUGES C9R001, C9H009, C5H018, C5H016 AND THE LOWER VAAL CATCHMENT

In order to naturalise the flows at gauges C9R001, C9H009 on the Vaal River and C5H018 on the Modder River a different procedure was adopted. All abstractions and losses were added back individually to the observed historical flow and all return flows and water gains were subtracted individually for the calibration period.

For station C9R001, the net inflow file calculated during the calibration process in Section 5.10.3 was subtracted from the patched observed historical flow. The losses due to farm dams was calculated by subtracting the spills from the inflows and then added back to the observed record. The return flows from the compensation release irrigation were subtracted. This record was appended to the simulated natural flows using regional parameters for the earlier part of the record before 1971 to form an "observed" naturalised sequence for the period 1920-1994. The single mass plot of the incremental naturalised sequence is included in Appendix G10.

A similar procedure was followed for station C9H009. Firstly, the net inflows were subtracted from the patched observed historical flow. The losses due to farms dams were calculated by subtracting the spills from the inflows and then added back to the observed record. The return flows from the irrigation supported by compensation releases were subtracted. Again the record was appended to the simulated natural flows for the earlier part of the record before 1968 to form an "observed" natural flow sequence for the period 1920-1994.

The unit run-off produced from this incremental catchment was slightly higher than the upstream incremental catchment. This is clearly an anomaly as the MAP of the downstream catchment is lower and the MAE is higher. An under-estimation of the contributing catchment area was assumed to be the reason for the higher unit run-off, particularly in quaternary C91C. In the WR90 report there are conflicting estimates of endoreic area in this catchment and therefore a contributing catchment area was estimated that would produce a lower unit run-off for the downstream catchment gauged by C9H009. The single mass plot of the incremental naturalised sequence is shown in Appendix G9.

It was also clear from the mass plots for both of the above gauges that the irrigation demand was being over-estimated, particularly in the very dry mid-eighties when water restrictions were in place. The demands were reduced iteratively during recalibration.

The flow record for station C5H018 was naturalised in the same manner as for the above two records. The net inflows were subtracted from the patched observed historical flow. The losses due to farms dams were calculated by subtracting the spills from the inflows and then added back to the observed record. The return flows from the farm dams irrigation and irrigation supported by compensation releases were subtracted as were the return flows from Welvaart Treatment Works. The record was appended to the simulated natural flows for the earlier part of the record before 1971 to form an "observed" natural flow sequence for the period 1920-1994. The single mass plot of the incremental naturalised sequence is included in Appendix G4.

For station C5H016, the simulated record was taken as the final natural incremental, flow as the observed flow record was unreliable and much of the land-use data was estimated or assumed. The simulation was undertaken using the same parameters as for the gauge at Tweerivier (C5H018), as the incremental catchments upstream of these gauges were considered to have similar characteristics to each other and the lower Vaal catchments. The single mass plot of the incremental naturalised sequence is shown in Appendix G3.

For the remainder of the lower Vaal catchment the regional parameters for C9R001 were used to produce a simulated natural flow record for the period 1920-1994. The results of the naturalisation are presented in Table 6.1.

Table 6.1:	Results of the naturalisation process

Gauge Number	Station Name	Effective Incremental

Area

(km2)	MAP

(mm)	MAE

(mm)	Naturalised

MAR

million m3/a	Unit Run-off

mm/a
C3H003	Taung	7975	530	1916	59.0	7.4
C3R002	Spitskop	9249	438	2039	77.5	8.4
C5H016	Aucampshoop	1847	350	2050	6.4	3.5
C5H018	Tweerivier	2236	422	1871	14.4	6.4
C5R001	Tierpoort	922	491	1640	23.8	25.8
C5R002	Kalkfontein	8781	412	1746	215.9	24.6
C5R003	Rustfontein	937	543	1600	30.7	32.7
C5R004	Krugersdrift	5391	508	1639	114.4	21.2
C9H009	De Hoop	3201	406	1963	12.9	4.0
C9R001	Vaalharts	2509	444	1946	11.2	4.5
-	Lower Vaal	6096	361	2210	31.5	5.2
TOTAL	49144	597.7	143.7

6.2	ELIMINATING NEGATIVE INCREMENTALS

Negative incremental flows between two gauges are inevitable for numerous reasons. Firstly, the most common cause of negative incremental values is gauging errors. These errors are caused by gauge capacity exceedence, incorrect rating tables, structural damage or leaking at the gauging or storage weir or poor maintenance. Unmetered abstractions for irrigation or domestic use and evaporative and river bed losses may also account for the discrepancies when subtracting the upstream from the downstream naturalised flow record.

A simple method was used to eliminate negative incremental flows in the naturalised observed flows. A short routine was written to subtract the upstream record from the downstream record, and where the monthly difference was negative, the upstream flow was reduced by the monthly difference and this was repeated sequentially in an upstream direction. This method is conservative, as opposed to adjusting the monthly flows or "creating water" at the downstream site in an attempt to eliminate negative incrementals.

6.3	COMPARISON OF RESULTS WITH PREVIOUS STUDIES

Various other studies have produced naturalised run-off estimates for similar run-off modules as used in this study. The results from this study are compared with the WR90 results as well as the Stewart Scott Inc. report for the WRC on the Long Term Salt Balance of the Vaalharts Irrigation Scheme (Stewart Scott Inc, 1994) for gauges C3H003 and C3R002. The naturalised run-off in the Modder and Riet catchments are compared with those reported in the Orange River System Analysis, Riet and Modder River: Hydrology (DWAF, 1991). Table 6.2 and Table 6.3 summarise the naturalised MAR, effective catchment area and unit run-off at each gauge obtained from the current study, and those obtained from the previous studies and WR90 for the relevant time period.

The naturalised run-off estimates of the current study for the Harts River catchment compare very favourably with those of WR90 as the calibration parameters used varied, only slightly, from regional parameters and were similar to those used in WR90. The estimates for gauge C3R002 at Spitskop are, however, higher than those reported in the Long Term Salt Balance of the Vaalharts Irrigation Scheme as the observed record has been updated by DWAF. The unit run-off estimates vary due to the different effective catchment areas used.

For the Vaal River catchment between Bloemhof Dam and the Orange and Vaal River confluence, the calculated incremental naturalised run-off values are similar to those in WR90 as the same regional parameters were used.

In the Modder/Riet catchment, there are large differences in the estimates of incremental naturalised run-off reported in the current and previous studies. The naturalised run-off into Kalkfontein Dam at gauge C5R002 is significantly higher in the current study due to the updating of the flood events in the inflow record by DWAF as discussed in Section 5.6.3. In the current study, the incremental naturalised inflow to Krugersdrift Dam measured at C5R004 is significantly lower than those reported in previous studies. The impervious areas in the previous studies were either ignored or under-estimated by excluding Thaba Nchu and Botshabelo. The current study modelled more extensive urban impervious areas which resulted in increased surface run-off. According to Equation 6.1, an increase in the simulated historical flow results in a decreased "observed" natural flow.

Table 6.2:	Comparison of results with previous studies

Gauge Number	Gauge Name	Effective Incremental Naturalised Run-off
VRSAU Study	Study C and D
Effective

Incr. Area

(km2)	million m3/a	mm/a	Effective

Incr. Area

(km2)	million m3/a	mm/a
C3H003 (1923-1990)	Taung	7975	59.1	7.4	10990*	54.5	5.0
C3R002 (1923-1990)	Spitskop	9249	81.3	8.8	15924*	62.6	3.9
C5H016 (1920-1987)	Aucampshoop	1847	6.7	3.6	3090	13.7	4.4
C5H018 (1920-1987)	Tweerivier	2236	14.0	6.3	4770	23.7	5.0
C5R001 (1920-1987)	Tierpoort	922	23.9	25.9	922	24.7	26.8
C5R002 (1920-1987)	Kalkfontein	8781	208.8	23.8	9096	196.2	21.6
C5R003 (1920-1987)	Rustfontein	937	31.1	33.2	940	28.6	30.4
C5R004 (1920-1987)	Krugersdrift	5391	116.7	21.6	4459	126.9	28.4
C9R001 (1920-1995)	Vaalharts	2509	11.2	4.5
LOWER VAAL (1920-1995)	9297	43.2	4.6

*	refers to the Long Term Salt Balance of the Vaalharts Irrigation Scheme (Stewart Scott Inc., 1994) - the remaining natural run-off values in the column are from the report on the hydrology of the Modder and Riet Rivers (DWAF, 1991).

Table 6.3:	Comparison of results with WR90

Gauge Number	Gauge Name	Effective Incremental Naturalised Run-off
VRSAU Study	WR90
Effective Incr. Area

(km2)	million m3/a	mm/a	Effective

Incr. Area

(km2)	million m3/a	mm/a
C3H003 (1920-1989)	Taung	7975	60.9	7.6	8284	64.9	7.8
C3R002 (1920-1989)	Spitskop	9249	79.7	8.6	9275	72.8	7.9
C5H016 (1920-1989)	Aucampshoop	1847	6.6	3.6	1853	4.5	2.4
C5H018 (1920-1989)	Tweerivier	2236	14.3	6.4	2241	8.9	4.0
C5R001 (1920-1989)	Tierpoort	922	24.2	26.2	922	27.0	29.3
C5R002 (1920-1989)	Kalkfontein	8781	208.2	23.7	8790	179.9	20.5
C5R003 (1920-1989)	Rustfontein	937	31.1	33.2	937	27.9	29.8
C5R004 (1920-1989)	Krugersdrift	5391	115.6	21.4	5394	148.8	27.6
C9R001 (1920-1989)	Vaalharts	2509	11.2	4.5	2508	10.5	4.2
LOWER VAAL (1920-1989)	9297	44.7	4.7	10460	48.7	4.7

7.	CONCLUSIONS AND RECOMMENDATIONS

7.1	RECOMMENDATIONS FOR FLOW GAUGING

The Lower Vaal River catchment has a reasonable distribution of flow gauges, however, certain of these gauges produce unreliable records which need careful screening and patching. Some gauges are also not sited at optimal monitoring points.

7.1.1	Harts and Lower Vaal River catchment

This catchment has numerous streamflow gauges in the main channel of both the Vaal and the Harts Rivers. There is significant irrigation in the region and therefore irrigation abstractions, transfers and return flows need to be monitored accurately. The following are streamflow gauging recommendations:

-	The inflow and spill records of Wentzel Dam are considered unreliable as there are no monitoring gauges upstream or downstream of the dam. It is recommended that this reservoir be gauged in order to accurately monitor the run-off and effects of the irrigation in the upper Harts River catchment.

-	Some consideration should be given to gauging the flows entering the Harts River from the Dry Harts River as these account for over 35 % of the total flow from the Harts River catchment.

-	A more direct method of measuring the return flows from the Vaalharts Irrigation Scheme should be sought.

-	A new flow gauging site is proposed for the Vaal River downstream of the Vaal and Harts River confluence and the Vaal Gamagara abstraction point. This would record the flow in the Vaal River, available for use by riparian irrigators along the Vaal River upstream of Douglas.

-	The station at St Claire/Douglas (C9H007) on the lower Vaal River was closed in 1978. The weir has been upgraded (C9R003) and flows have been recorded since 1978 but the record is not available. This gauge is the most downstream station on the Vaal River and would provide useful information as to volume of water leaving the Vaal catchment.

7.1.2	Modder and Riet River catchment

This catchment is essentially monitored by the inflows to the four major reservoirs, as most of the streamflow gauges are situated in tributaries of the two main rivers. After pre- screening the gauges and during the calibration process many of the streamflow gauges were rejected due to inaccurate records or a gauge's close proximity to a reservoir where the reservoir inflow record was preferred. The following recommendations are made:

-	Records of the inflow into Mockes Dam would provide valuable information about the water availability and consumption in the upper reaches of the Modder River below Rustfontein Dam. It would also provide the opportunity to monitor, more closely, the developments in Thaba Nchu, Botshabelo and Bloemfontein and the effects of the Caledon-Modder Transfer. It would also provide more accurate information on the availability of water for abstractions at Mazelspoort Weir for consumption by the greater Bloemfontein. The same recommendation was made in the Orange River System Analysis (1991), but as yet there have been no upstream gauge constructed or calibration of the spillway at the dam.

-	There are a large number of uncertainties in terms of irrigated area and irrigation demand and supply in the Riet River catchment. The catchment therefore needs to be monitored more closely and therefore a number of potential sites for a new gauges in the Riet River are proposed.

-	A gauging station is required just upstream of the confluence of the Riet and Kaffer Rivers to monitor the run-off and effects of irrigation in the entire upper Riet River catchment. A further gauge could be located in the Riet River between the Riet and Kaffer River confluence and Kalkfontein Dam to monitor the effects of spills from Tierpoort Dam and the Kaffer River irrigation supplied by compensation releases from the dam. The inflow and spill record at Tierpoort Dam are considered doubtful and therefore a downstream gauge, other than Kalkfontein Dam (C5R002), would act as a check for the spills and releases from Tierpoort.

-	Another proposed site for useful gauging is in the lower reaches of the Kromellenboogspruit, which drains a large portion of the incremental catchment between Tierpoort Dam and Kalkfontein Dam and joins the Riet just upstream of Kalkfontein Dam.

-	A new weir was built at Tweerivier (C5H018), the Ritchie weir (C5H014) upgraded and a new weir built near the Vaal and Riet confluence, downstream of Aucampshoop (C5H016) in 1989. These records need to be maintained and managed to ensure they continue to provide valuable information.

7.2	RECOMMENDATIONS FOR CATCHMENT AND LANDUSE INFORMATION

Irrigated agriculture is the main type of landuse in the entire catchment. There is no afforestation in the Lower Vaal catchment and livestock farming is very limited. There are several major Government Water Schemes in the region including the Vaalharts Scheme. None of these irrigation schemes have readily available information on actual water use, total irrigated area or crop types which is essential information needed for the representative modelling of the catchment developments in the catchment. The aerial extent and water use by smaller schemes and diffuse users was almost non-existent. In most cases no record of the historical development of these irrigation areas had been kept.

It is therefore recommended that the aerial photography and mapping of the region be updated in order to determine the current day extent of irrigated area. A database of all current and historical irrigation information and contact names should be compiled, although a list of contacts used in this study are included in Appendix I.

More accurate and standardised estimates of effective catchment areas need to be agreed upon.

7.3	CONCLUSIONS

This hydrological study has resulted in an extended set of flow sequences for the Lower Vaal River catchment.

-	In subcatchments where flows and irrigation are primarily supported by compensation releases from an upstream reservoir, caution must exercised during calibration and naturalisation. The demands, irrigation or other, must be supplied by the actual compensation flows rather than the catchment run-off and river flows. Failure to model the abstraction of demands in this way results in extreme over-estimation of natural run-off when the irrigation demands are added back to the catchment run-off during the naturalisation process.

-	The standard method of estimating evaporative losses by calculating the net loss from an average river width and length needs to be reviewed.

-	The WR90 Regional Parameters appear to give a slightly conservative estimate of natural run-off in the catchments along the Vaal River and lower Modder and Riet catchments.

8.	REFERENCES

Berg RR, Beuster H, (1992)	SHELL - A computational aid facilitating water resource modelling in developed catchments. Paper presented at the 1st Young Water Engineers Conference (YWEC), Eskom Training College, Midrand.

Bloem Water, (1996)	Personal communication, records of water released from Welbedacht Dam and sold to consumers.

Brink S, (1996)	Personal communication. Agricultural extension officer. Edenburg, Redesburg and Trompsburg.

De Beer B, (1996)	Personal communication, Bloemfontein Municipality.

DWAF, (1987)	Directorate of Planning. Hydrology of the Harts River. Prepared by Stewart, Sviridov and Oliver. DWAF Report No.P C330/00/0187.

DWAF, (1991)	Hydrology. Orange River System Analysis. Riet and Modder River : Hydrology. Prepared by BKS Inc. and Stewart Sviridov and Oliver. DWAF Report No. PC000/00/0291.

DWAF, (1995)	Hydrology. Prepared by J Larsen of Consult 4 Consortium as part of the Mzimvubu Transfer Options of the Vaal Augmentation Planning Study: Reconnaissance Phase. DWAF Report No. P C000/0014894 (S6).

DWAF, (1996a)	Personal communication, records of scheduled areas, irrigation quotas and crop distribution below Kalkfontein Dam, Jacobsdal.

DWAF, (1996b)	Personal communication, records of scheduled areas and irrigation quotas for Vaalharts and the Middle and Lower Vaal, Jan Kempdorp.

Midgley D C, Pitman W V	Addendum to "Surface water resources of South Africa

& Middleton BJ (1993)	(1981)". Hydrological Research Unit.

Midgley D C, Pitman W V	Surface water resources of South Africa (1990),

& Middleton BJ, (1994)	Volume II, Vaal. Prepared for the Water Research Commission. WRC Report No. 298/1/94.

Pitman W V, (1973)	A mathematical model for generating monthly streamflows from hydrometeorological data in South Africa. Report No. 2/73, Hydrological Research Unit, University of the Witwatersrand, Johannesburg.

Stewart Scott Inc, (1994)	Long Term Salt Balance of the Vaalharts Irrigation Scheme. Prepared by Herold C.E. and Bailey A.K. for the Water Research Commission. WRC Report No. 420/1/94.

Strydom M & Malala J,	Department of Water Affairs and Forestry - Pretoria:

(1996)	Reservoir records and flow data (respectively).

Terblanche L, (1996)	Personal communication. Agricultural extension officer. Department of Agriculture and Environmental Affairs. Bloemfontein.

Visser D, (1996)	Personal communication. Department of Water Affairs and Forestry. Bloemfontein.

APPENDIX A

MAPS

APPENDIX

NUMBER	CONTENTS	PAGE

NUMBER
A1	Catchment Developments	A - 1
A2	Rainfall Isoheyts and Rainfall Stations	A - 2
A3	Evaporation Isoheyts and Evaporation Stations	A - 3
A4	Model Calibration	A - 4

APPENDIX B

CATCHMENT DEVELOPMENTS

APPENDIX

NUMBER	CONTENTS	PAGE

NUMBER
B1	Catchment Developments in Run-off Module C3H003
B1.1	Diffuse Irrigation Demand from Farm Dams upstream of Wentzel Dam	B - 1.1
B1.2	Diffuse Irrigation Supply from Farm Dams upstream of Wentzel Dam	B - 1.2
B1.3	Controlled Irrigation Demand/Supply from Wentzel Dam	B - 1.3
B1.4	Controlled Irrigation Demand/Supply for Taung Irrigation upstream of C3H003	B - 1.4
B1.5	Irrigation Return Flows from Taung Irrigation Area upstream of C3H003	B - 1.5
B1.6	Urban Supply to Schweizer Reneke from Wentzel Dam	B - 1.6
B2	Catchment Developments in Run-off Module C3R002
B2.1	Diffuse Irrigation Demand from Farm Dams in the Dry Harts Catchment	B - 2.1
B2.2	Diffuse Irrigation Supply from Farm Dams in the Dry Harts Catchment	B - 2.1
B2.3	Diffuse Irrigation Demand from the Dry Harts River	B - 2.2
B2.4	Diffuse Irrigation Supply from the Dry Harts River	B - 2.2
B2.5	Diffuse Irrigation Demand from the Lower Harts River upstream of Spitskop Dam	B - 2.3
B2.6	Diffuse Irrigation Supply from the Lower Harts River upstream of Spitskop Dam	B - 2.3
B2.7	Irrigation Return Flows from the North and West Canal and Taung Irrigation Areas	B - 2.4
B3	Catchment Developments in Run-off Module C5H016
B3.1	Controlled Irrigation Demand/Supply from Kalkfontein Dam	B - 3.1
B3.2	Controlled Irrigation Demand/Supply from the Orange-Riet Canal	B - 3.1
B3.3	Diffuse Irrigation Demand from the Riet River between Kalkfontein and Aucampshoop	B - 3.2
B3.4	Diffuse Irrigation Supply from the Riet River between Kalkfontein and Aucampshoop	B - 3.3
B3.5	Controlled Irrigation Demand from the Lower Riet River upstream of Aucampshoop - original	B - 3.4
B3.6	Controlled Irrigation Demand from the Lower Riet River upstream of Aucampshoop - increased	B - 3.5
B3.7	Controlled Irrigation Supply from the Lower Riet River upstream of Aucampshoop	B - 3.6

APPENDIX B

CATCHMENT DEVELOPMENTS CONT.

APPENDIX

NUMBER	CONTENTS	PAGE

NUMBER
B4	Catchment Developments in Run-off Module C5H018
B4.1	Controlled Irrigation Demand from Farm Dams between Krugersdrift and Tweerivier	B - 4.1
B4.2	Controlled Irrigation Supply from Farm Dams between Krugersdrift and Tweerivier	B - 4.1
B4.3	Controlled Irrigation Demand from Spills and Compensation Releases from Krugersdrift Dam	B - 4.2
B4.4	Controlled Irrigation Supply from Spills and Compensation Releases from Krugersdrift Dam	B - 4.2
B4.5	Return Flows from Welvaart Sewage Treatment Works	B - 4.3
B4.6	Evaporative Losses between Krugersdrift Dam and Tweerivier	B - 4.4
B5	Catchment Developments in Run-off Module C5R001
B5.1	Diffuse Irrigation Demand from Farm Dams upstream of Tierpoort Dam	B - 5.1
B5.2	Diffuse Irrigation Supply from Farm Dams upstream of Tierpoort Dam	B - 5.2
B5.3	Diffuse Irrigation Demand from the Kaffer River upstream of Tierpoort Dam	B - 5.3
B5.4	Diffuse Irrigation Supply from the Kaffer River upstream of Tierpoort Dam	B - 5.4
B6	Catchment Developments in Run-off Module C5R002
B6.1	Diffuse Irrigation Demand from Farm Dams between Tierpoort and Kalkfontein	B - 6.1
B6.2	Diffuse Irrigation Supply from Farm Dams between Tierpoort and Kalkfontein	B - 6.2
B6.3	Diffuse Irrigation Demand from the Riet River upstream of Kalkfontein	B - 6.3
B6.4	Diffuse Irrigation Supply from the Riet River upstream of Kalkfontein	B - 6.4
B6.5	Controlled Irrigation Demand/Supply to Kaffer River Irrigation from Tierpoort Dam	B - 6.5
B7	Catchment Developments in Run-off Module C5R003
B7.1	Irrigation Demand from Farm Dams upstream of Rustfontein Dam	B - 7.1
B7.2	Irrigation Supply from Farm Dams upstream of Rustfontein Dam	B - 7.2
B7.3	Irrigation Demand from the Modder River upstream of Rustfontein Dam	B - 7.3
B7.4	Irrigation Supply from the Modder River upstream of Rustfontein Dam	B - 7.4

APPENDIX B

CATCHMENT DEVELOPMENTS CONT.

APPENDIX

NUMBER	CONTENTS	PAGE

NUMBER
B8	Catchment Developments in Run-off Module C5R004
B8.1	Diffuse Irrigation Demand from Farm Dams upstream of Mockes Dam	B - 8.1
B8.2	Diffuse Irrigation Supply from Farm Dams upstream of Mockes Dam	B - 8.1
B8.3	Diffuse Irrigation Demand from the Modder River upstream of Mockes Dam	B - 8.2
B8.4	Diffuse Irrigation Supply from the Modder River upstream of Mockes Dam	B - 8.2
B8.5	Diffuse Irrigation Demand from the Modder River downstream of Mockes Dam	B - 8.3
B8.6	Diffuse Irrigation Supply from the Modder River downstream of Mockes Dam	B - 8.3
B8.7	Diffuse Irrigation Demand from Farm Dams in the Renoster River Catchment	B - 8.4
B8.8	Diffuse Irrigation Supply from Farm Dams in the Renoster River Catchment	B - 8.4
B8.9	Diffuse Irrigation Demand from the Renoster River	B - 8.5
B8.10	Diffuse Irrigation Supply from the Renoster River	B - 8.5
B8.11	Return Flows from Botshabelo	B - 8.6
B8.12	Return Flows from Thaba Nchu	B - 8.6
B8.13	Water Supply to Bloemfontein from Mazelspoort Weir	B - 8.7
B8.14	Return Flows from Blowmspruit Sewage Works	B - 8.7
B9	Catchment Developments in Run-off Module C9H009
B9.1	Irrigation Demand from Compensation Releases from Vaalharts Weir	B - 9.1
B9.2	Irrigation Supply from Compensation Releases from Vaalharts Weir	B - 9.1
B9.3	Various Demands (Urban, Industrial and Diggers)	B - 9.2
B9.4	Supply to Various Demands (Urban, Industrial and Diggers)	B - 9.2
B9.4	Evaporative Losses between Vaalharts Weir and C9H009	B - 9.3
B10	Catchment Developments in Run-off Module C9R001
B10.1	Irrigation Demand from Spills and Compensation Releases from Bloemhof Dam	B - 10.1
B10.2	Irrigation Supply from Spills and Compensation Releases from Bloemhof Dam	B - 10.1
B10.3	Evaporative Losses between Bloemhof Dam and Vaalharts Weir	B - 10.2
B11	Crop Factor Files
B11.1	Area Upstream of Wentzel Dam	B - 11.1
B11.2	Taung Irrigation Area Upsteam of Station C3H003	B - 11.1
B11.3	Area Upstream of Spitskop Dam	B - 11.1
B11.4	Area Upstream of Rustfontein Dam	B - 11.2
B11.5	Area between Rustfontein Dam and Mockes Dam	B - 11.2
B11.6	Area between Mockes Dam and Krugersdrift Dam	B - 11.2

APPENDIX B

CATCHMENT DEVELOPMENTS CONT.

APPENDIX

NUMBER	CONTENTS	PAGE

NUMBER
B11.7	Renoster River Catchment	B - 11.3
B11.8	Area between Krugersdrift Dam and Tweerivier	B - 11.3
B11.9	Area Upstream of Tierpoort Dam	B - 11.3
B11.10	Upper Kalkfontein Dam Catchment	B - 11.4
B11.11	Lower Kalkfontein Dam Catchment	B - 11.4
B11.12	Diffuse Irrigation between Kalkfontein Dam and Aucampshoop	B - 11.4
B11.13	Scholtzburg Irrigation Area	B - 11.5
B11.14	Ritchie Irrigation Area	B - 11.5
B11.15	Riet River Settlement	B - 11.5
B11.16	Lower Riet River Irrigation Area	B - 11.6
B11.17	Area between Vaalharts Weir and Station C9H009	B - 11.6
B11.18	Area between Bloemhof Dam and Vaalharts Weir	B - 11.6
B12	General Information
B12.1	Registered Dams in the Lower Vaal River Catchment	B - 12.1

APPENDIX C

INTERBASIN AND IN-BASIN TRANSFERS

APPENDIX

NUMBER	CONTENTS	PAGE

NUMBER
C1	Irrigation Transfer from Vaalharts Weir	C - 1.1
C2	Water Supply to Kimberley from the Vaal River at Riverton	C - 1.2
C3	Water Delivered to the upper Modder River from Welbedacht Dam	C - 1.3
C4	Water Supplied from the Vaal River for the Vaal-Gamagara Transfer	C - 1.3
C5	Irrigation Transfer into the Orange-Riet Canal from Van der Kloof Dam	C - 1.4
C6	Excess Flows from the Orange-Riet Canal released near Jacobsdal	C - 1.4

APPENDIX D

RAINFALL

APPENDIX

NUMBER	CONTENTS	PAGE

NUMBER
D1	Percentage Rainfall Files for Calibration of C3H003
D1.1	Average Rainfall on Catchment C31A	D - 1.1
D1.2	Average Rainfall on Catchment C31B	D - 1.2
D1.3	Average Rainfall on Catchment C31C	D - 1.3
D1.4	Average Rainfall on Catchment C31D	D - 1.4
D1.5	Average Rainfall on Catchment C31E	D - 1.5
D1.6	Average Rainfall on Catchment C31F	D - 1.6
D1.7	Average Rainfall on Catchment C3H003	D - 1.7
D1.8	Average Rainfall on Wentzel Dam	D - 1.8
D2	Percentage Rainfall Files for Calibration of C3R002
D2.1	Average Rainfall on Catchment C32A	D - 2.1
D2.2	Average Rainfall on Catchment C32B	D - 2.2
D2.3	Average Rainfall on Catchment C32C	D - 2.3
D2.4	Average Rainfall on Catchment C32D	D - 2.4
D2.5	Average Rainfall on Catchment C33A	D - 2.5
D2.6	Average Rainfall on Catchment C33B	D - 2.6
D2.7	Average Rainfall on Catchment C3R002	D - 2.7
D3	Percentage Rainfall Files for Calibration of C5H016
D3.1	Average Rainfall on Catchment C51K	D - 3.1
D3.2	Average Rainfall on Catchment C51L	D - 3.2
D4	Percentage Rainfall Files for Calibration of C5H018
D4.1	Average Rainfall on Catchment C52H	D - 4.1
D4.2	Average Rainfall on Catchment C52J	D - 4.2
D4.3	Average Rainfall on Catchment C52K	D - 4.3
D4.4	Average Rainfall on Catchment C52L	D - 4.4
D4.5	Average Rainfall on Catchment C5H018	D - 4.5

APPENDIX D

RAINFALL CONT.

APPENDIX

NUMBER	CONTENTS	PAGE

NUMBER
D5	Percentage Rainfall Files for Calibration of C5R001
D5.1	Average Rainfall on Catchment C51D	D - 5.1
D6	Percentage Rainfall Files for Calibration of C5R002
D6.1	Average Rainfall on Catchment C51A	D - 6.1
D6.2	Average Rainfall on Catchment C51B	D - 6.2
D6.3	Average Rainfall on Catchment C51C	D - 6.3
D6.4	Average Rainfall on Catchment C51E	D - 6.4
D6.5	Average Rainfall on Catchment C51F	D - 6.5
D6.6	Average Rainfall on Catchment C51G	D - 6.6
D6.7	Average Rainfall on Catchment C51H	D - 6.7
D6.8	Average Rainfall on Catchment C51J	D - 6.8
D6.9	Average Rainfall on Catchment C5R002	D - 6.9
D6.10	Average Rainfall on the Upper Kalkfontein Catchment	D - 6.10
D6.11	Average Rainfall on the Lower Kalkfontein Catchment	D - 6.11
D7	Percentage Rainfall Files for Calibration of C5R003
D7.1	Average Rainfall on Catchment C52A	D - 7.1
D8	Percentage Rainfall Files for Calibration of C5R004
D8.1	Average Rainfall on Catchment C52B	D - 8.1
D8.2	Average Rainfall on Catchment C52C	D - 8.2
D8.3	Average Rainfall on Catchment C52D	D - 8.3
D8.4	Average Rainfall on Catchment C52E	D - 8.4
D8.5	Average Rainfall on Catchment C52F	D - 8.5
D8.6	Average Rainfall on Catchment C52G	D - 8.6
D8.7	Average Rainfall on the Catchment Upstream of Mockes Dam	D - 8.7
D8.8	Average Rainfall on the Catchment Downstream of Mockes Dam	D - 8.8
D9	Percentage Rainfall Files for Calibration of C9H009
D9.1	Average Rainfall on Catchment C91C	D - 9.1
D9.2	Average Rainfall on Catchment C91D	D - 9.2
D9.3	Average Rainfall on Catchment C91E	D - 9.3
D9.4	Average Rainfall on Catchment C9H009	D - 9.4
D10	Percentage Rainfall Files for Calibration of C9R001
D10.1	Average Rainfall on Catchment C91A
D10.2	Average Rainfall on Catchment C91B

APPENDIX D

RAINFALL CONT.

APPENDIX

NUMBER	CONTENTS	PAGE

NUMBER
D11	Percentage Rainfall Files for the Simulation of the Lower Vaal Run-off
D11.1	Average Rainfall on Catchment C91E	D - 11.1
D11.2	Average Rainfall on Catchment C92A	D - 11.2
D11.3	Average Rainfall on Catchment C92B	D - 11.3
D11.4	Average Rainfall on Catchment C92C	D - 11.4
D11.5	Average Rainfall on Catchment C33C	D - 11.5
D11.6	Average Rainfall on Catchment C51M	D - 11.6
D12	Availability of Rainfall Data
D12.1	Data Availability : Lower Vaal River Gauges	D - 12.1

APPENDIX E

RAW FLOW DATA

APPENDIX

NUMBER	CONTENTS	PAGE

NUMBER
E1
E1.1	Raw Run-off at Taung (C3H003)	E - 1.1
E2
E2.1	Raw Inflow at Spitskop (C3R002)	E - 2.1
E2.2	Raw Spills and Compensation Releases from Spitskop	E - 2.1
E3
E3.1	Raw Run-off at Aucampshoop (C5H016)	E - 3.1
E4
E4.1	Raw Run-off at Tweerivier (C5H018)	E - 4.1
E5
E5.1	Raw Inflow at Tierpoort (C5R001)	E - 5.1
E5.2	Raw Spills from Tierpoort Dam	E - 5.2
E6
E6.1	Raw Inflow at Kalkfontein (C5R002 - adjusted)	E - 6.1
E6.2	Raw Inflow at Kalkfontein (C5R002 - unadjusted)	E - 6.2
E6.3	Raw Spills from Kalkfontein Dam	E - 6.3
E7
E7.1	Raw Inflow at Rustfontein (C5R003)	E - 7.1
E7.2	Raw Spills from Rustfontein Dam	E - 7.2
E7.3	Raw Compensation Releases from Rustfontein Dam	E - 7.3
E8
E8.1	Raw Inflow at Krugersdrift (C5R004)	E - 8.1
E8.2	Raw Compensation Releases from Krugersdrift Dam	E - 8.1
E8.3	Raw Spills from Krugersdrift Dam	E - 8.2
E8.4	Raw Spills from Groothoek Dam	E - 8.2
E9
E9.1	Raw Run-off at De Hoop (C9H009)	E - 9.1
E10
E10.1	Raw Inflow at Vaalharts (C9R001)	E - 10.1
E10.2	Raw Compensation Releases from Vaalharts Weir	E - 10.1

APPENDIX E

RAW FLOW DATA CONT.

APPENDIX

NUMBER	CONTENTS	PAGE

NUMBER
E11
E11.1	Raw Spills and Compensation Releases from Bloemhof Dam	E - 11.1
E12
E12.1	Availability of Streamflow Data	E - 12.1
E12.2	Degree of Patching in Streamflow Records	E - 12.1

APPENDIX F

PATCHED FLOW DATA

APPENDIX

NUMBER	CONTENTS	PAGE

NUMBER
F1
F1.1	Patched Run-off at Taung (C3H003)	F - 1.1
F2
F2.1	Patched Inflow at Spitskop (C3R002)	F - 2.1
F3
F3.1	Patched Run-off at Aucampshoop (C5H016)	F - 3.1
F4
F4.1	Patched Run-off at Tweerivier (C5H018)	F - 4.1
F5
F5.1	Patched Inflow at Tierpoort (C5R001)	F - 5.1
F5.2	Patched Spills from Tierpoort Dam	F - 5.2
F6
F6.1	Patched Inflow at Kalkfontein (C5R002)	F - 6.1
F7
F7.1	Patched Inflow at Rustfontein (C5R003)	F - 7.1
F8
F8.1	Patched Inflow at Krugersdrift (C5R004)	F - 8.1
F8.2	Patched Spills and Compensation Releases from Krugersdrift Dam	F - 8.1
F9
F9.1	Patched Run-off at De Hoop (C9H009)	F - 9.1
F10
F10.1	Patched Inflow at Vaalharts (C9R001)	F - 10.1

APPENDIX G

MODEL CALIBRATION RESULTS

APPENDIX

NUMBER	CONTENTS	PAGE

NUMBER
G1
G1.1	Model Configuration of Subcatchment C3H003	G - 1.1
G1.2	Graphical Comparison of Observed and Simulated Flows at C3H003	G - 1.2
G1.3	Low Flow-Duration-Frequency Curves for Observed and Simulated Flows at C3H003	G - 1.3
G2
G2.1	Model Configuration of Subcatchment C3R002	G - 2.1
G2.2	Graphical Comparison of Observed and Simulated Flows at C3R002	G - 2.2
G2.3	Low Flow-Duration-Frequency Curves for Observed and Simulated Flows at C3R002	G - 2.3
G3
G3.1	Model Configuration of Subcatchment C5H016	G - 3.1
G3.2	Graphical Comparison of Observed and Simulated Flows at C5H016	G - 3.2
G3.3	Low Flow-Duration-Frequency Curves for Observed and Simulated Flows at C5H016	G - 3.3
G3.4	Single Mass Plot of the Incremental Naturalised Flows at C5H016	G - 3.4
G4
G4.1	Model Configuration of Subcatchment C5H018	G - 4.1
G4.2	Graphical Comparison of Observed and Simulated Flows at C5H018	G - 4.2
G4.3	Low Flow-Duration-Frequency Curves for Observed and Simulated Flows at C5H018	G - 4.3
G4.4	Single Mass Plot of the Incremental Naturalised Flows at C5H018	G - 4.4
G5
G5.1	Model Configuration of Subcatchment C5R001	G - 5.1
G5.2	Graphical Comparison of Observed and Simulated Flows at C5R001	G - 5.2
G5.3	Low Flow-Duration-Frequency Curves for Observed and Simulated Flows at C5R001	G - 5.3

APPENDIX G

MODEL CALIBRATION RESULTS CONT.

APPENDIX

NUMBER	CONTENTS	PAGE

NUMBER
G6
G6.1	Model Configuration of Subcatchment C5R002	G - 6.1
G6.2	Graphical Comparison of Observed and Simulated Flows at C5R002	G - 6.2
G6.3	Low Flow-Duration-Frequency Curves for Observed and Simulated Flows at C5R002	G - 6.3
G6.4	Graphical Comparison of Original and Adjusted Observed Flows at C5R002	G - 6.4
G7
G7.1	Model Configuration of Subcatchment C5R003	G - 7.1
G7.2	Graphical Comparison of Observed and Simulated Flows at C5R003	G - 7.2
G7.3	Low Flow-Duration-Frequency Curves for Observed and Simulated Flows at C5R003	G - 7.3
G8
G8.1	Model Configuration of Subcatchment C5R004	G - 8.1
G8.2	Graphical Comparison of Observed and Simulated Flows at C5R004	G - 8.2
G8.3	Low Flow-Duration-Frequency Curves for Observed and Simulated Flows at C5R004	G - 8.3
G9
G9.1	Model Configuration of Subcatchment C9H009	G - 9.1
G9.2	Graphical Comparison of Observed and Simulated Flows at C9H009	G - 9.2
G9.3	Low Flow-Duration-Frequency Curves for Observed and Simulated Flows at C9H009	G - 9.3
G9.4	Single Mass Plot of Incremental Naturalised Flow at C9H009	G - 9.4
G10
G10.1	Model Configuration of Subcatchment C9R001	G - 10.1
G10.2	Graphical Comparison of Observed and Simulated Flows at C9R001	G - 10.2
G10.3	Low Flow-Duration-Frequency Curves for Observed and Simulated Flows at C9R001	G - 10.3
G10.4	Single Mass Plot of Incremental Naturalised Flow at C9R001	G - 10.4

APPENDIX G

MODEL CALIBRATION RESULTS CONT.

KEY TO THE SYMBOLS USED IN THE SUBCATCHMENT MODEL CONFIGURATIONS

APPENDIX H

NATURAL STREAMFLOW

APPENDIX

NUMBER	CONTENTS	PAGE

NUMBER
H1	Incremental Naturalised Flow Sequence at Taung (C3H003)	H - 1.1
H2	Incremental Naturalised Flow Sequence at Spitskop (C3R002)	H - 1.2
H3	Incremental Naturalised Flow Sequence at Aucampshoop (C5H016)	H - 1.3
H4	Incremental Naturalised Flow Sequence at Tweerivier (C5H018)	H - 1.4
H5	Incremental Naturalised Flow Sequence at Tierpoort (C5R001)	H - 1.5
H6	Incremental Naturalised Flow Sequence at Kalkfontein (C5R002)	H - 1.6
H7	Incremental Naturalised Flow Sequence at Rustfontein (C5R003)	H - 1.7
H8	Incremental Naturalised Flow Sequence at Krugersdrift (C5R004)	H - 1.8
H9	Incremental Naturalised Flow Sequence at De Hoop (C9H009)	H - 1.9
H10	Incremental Naturalised Flow Sequence at Vaalharts (C9R001)	H - 1.10
H11	Incremental Simulated Naturalised Flow Sequence at Vaal-Orange Confluence	H - 1.11

APPENDIX I

MISCELLANEOUS

APPENDIX

NUMBER	CONTENTS	PAGE

NUMBER
I1	Sources of Data and Information
I1.1	List of Contact Names and Numbers in the Lower Vaal River Catchment	I - 1.1