Vaal River System Analysis Update

Hydrology of the Upper Vaal Catchment

EXECUTIVE SUMMARY

This report presents the hydrology of the catchment of the Vaal River upstream of Vaal Dam. The catchment has a total area of 38 638 km2 and stretches from the edge of the Drakensberg escarpment, inland to Vaal Dam. It covers the north-eastern part of the Orange Free State, the south-western part of Mpumalanga Province and the southern tip of Gauteng. There are four major rivers that drain this catchment. They are the Vaal River, the Wilge River, the Liebenbergsvlei River and the Klip River with a combined average natural runoff of almost 2000 million m3/a.

There are four major impoundments in the Vaal Dam catchment. They are Vaal Dam, Grootdraai Dam, Sterkfontein Dam and Saulspoort Dam with a combined capacity of more than 5 500 million m3. Using satellite imagery, small dams in the Vaal Dam catchment were estimated to constitute a total storage capacity of about 166 million m3, causing a significant decrease in the mean annual runoff from the catchment. Furthermore, losses from the system due to evaporation from small dam water surfaces, totalled about 60 million m3 (3% of the natural runoff) in 1994/5.

One of the most important water uses in the Vaal Dam catchment is irrigation. It was estimated that water providing a total irrigated area of 12 195 ha, was abstracted from rivers and small dams upstream of Vaal Dam in 1994/5.

Although most of the major industrial and urban water users are supplied directly by the Rand Water Board or major dams in the catchment, many small towns like Ermelo, Standerton and Volksrust abstract water from small dams, totalling about 33 million m3 in 1994/5. Return flow from users in the catchment was estimated to be about 35 million m3 in 1994/5 - consisting mainly of effluent from the Sasol industrial complex and related urban return flows.

There are currently three operational interbasin transfers schemes that supply water to the Vaal Dam catchment. They are the Tugela-Vaal Drakensberg scheme since October 1974, the Buffalo-Vaal scheme pumping water from Zaaihoek Dam since August 1992 and the Usutu- Vaal transfer scheme, supplementing Grootdraai Dam from Heyshope Dam since October 1985. An estimated total of about 639 million m3 was transferred to the Vaal Dam catchment in 1994/5.

Analysing the water resources of the Vaal River system, meant that reliable hydrological records were needed. For the purpose of generating representative historical catchment precipitation records, spanning the period October 1920 to September 1995, rainfall data for selected gauges, as requested from the Weather Bureau, were used.

Four streamflow gauges upstream of Vaal Dam were identified as being reliable and positioned geographically favourable. The streamflow records at these gauges were requested from DWAF, checked and then patched or extended to cover the total period of October 1920 to September 1995. Unreliable data were patched mainly using results from previous studies, following as guideline values from deterministic modelling, daily data and recorded values at nearby gauges. All patchings were checked against a mass balance of the streamflow at gauges upstream of Vaal Dam and the actual calculated inflows to Vaal Dam. Records were extended, where necessary, using deterministic modelling. The final flow records are considered to be as realistic and reliable as possible.

Using these four patched streamflow records, natural incremental streamflow for the period October 1920 to September 1995 was obtained for five subcatchments in the Vaal Dam catchment. This was achieved by taking account of any abstractions and return flows, including urban, industrial and irrigation users, as well as interbasin transfers and water losses, influencing the natural hydrology of the system. The subcatchments are Grootdraai (upstream of Grootdraai Dam), Delangesdrift (upstream of Delangesdrift Weir), Sterkfontein (upstream of Sterkfontein Dam), Frankfort incremental (upstream of Frankfort Weir and downstream of Sterkfontein Dam) and Vaal incremental (upstream of Vaal Dam and downstream of Grootdraai Dam, Frankfort Weir and Delangesdrift Weir). A summary of the results is given in the following table.

Table:	Summary of results from the hydrological analysis for the October 1920 to September 1995 period

Incremental subcatchment	Catch- ment area (km2)	Observed incr. MAR (106m3)	Average annual supply or return flow for the 1920 - 1994 water year period

(million m3)	Natural incr. MAR (106m3)
Irrigation supply	Irrigation return flow	Losses from small dams	Urban and industrial supply	Urban and industrial return flows	Interbasin transfers(1)
Grootdraai	7 995	453.7	5.6	-0.6	7.7	1.2	-1.6	-8.3	457.7
Delangesdrift	4 158	247.1	0.0	0.0	2.2	0.2	0.0	0.0	249.5
Frankfort incr.	15 498	696.2	28.3	-2.8	8.8	4.7	-1.8	0.0	733.3
Vaal incr.	10 792	493.2	14.9	-1.5	13.3	1.6	-2.8	0.0	518.7
Sterkfontein	195	97.3	0.0	0.0	0.0	0.0	0.0	-79.2(2)	18.1(3)
TOTAL FOR CATCHMENT	38 638	1987.5	48.8	-4.9	32.0	7.7	-6.2	-87.5	1977.3

Note:	(1)	The values for interbasin transfers have been taken to be the average over the period October 1920 to September 1995 to provide a meaningful mass balance. It should be noted, however, that, on average over the past 20 years, a far greater amount of water has been transferred annually into the Vaal Dam catchment.

(2)	This value is an estimate as the actual magnitude of the transfer from the Tugela system was not calculated. More information on this subject is given in Section 2.7.

(3)	As no observed streamflow record exists for the Sterkfontein Dam catchment, the natural mean annual runoff is based on simulated data using the regional catchment parameters (WRC, 1994).

In conclusion it was found that, although there are many streamflow gauges in the Vaal Dam catchment, only six of these could be used meaningfully as part of the hydrological analysis. They are Delangesdrift Weir (C1H002), Frankfort Weir (C8H001), Standerton Weir (C1H001), Engelbrechtsdrift Weir (C2H003), Vaal Dam (C1R001) and Grootdraai Dam (C1R002). The reason for this is that these gauges have long records with generally reliable data and are positioned geographically favourable. They are therefore considered to be key gauges in the Vaal Dam catchment.

Other gauges in the catchment are, however, of little use, as they have either short records or unreliable data. It is therefore recommended that the importance and reliability of gauges in the Vaal Dam catchment be assessed. Gauges that are found to be of little importance should consequently be closed and resources be used exclusively for the upgrading and maintaining of key gauges.

Vaal River System Analysis Update

Hydrology of the Upper Vaal Catchment

TABLE OF CONTENTS

Page

EXECUTIVE SUMMERY	.......................................................................................... (i)

TABLE OF CONTENTS	......................................................................................... (iv)

1.	INTRODUCTION	....................................................................................... 1 - 1

1.1	PURPOSE OF STUDY	...................................................................... 1 - 1

1.2	SCOPE OF REPORT	........................................................................ 1 - 2

1.3	DESCRIPTION OF CATCHMENT	...................................................... 1 - 3

2.	CATCHMENT DEVELOPMENTS	............................................................. 2 - 1

2.1	IMPOUNDMENTS	............................................................................. 2 - 1

2.1.1	Large dams	........................................................................... 2 - 1

2.1.2	Small dams	............................................................................ 2 - 2

2.2	IRRIGATION	.................................................................................... 2 - 9

2.2.1	Introduction	........................................................................... 2 - 9

2.2.2	Present irrigation areas	.......................................................... 2 - 9

2.2.3	Historical development	......................................................... 2 - 11

2.2.4	Water use by irrigation	......................................................... 2 - 13

2.3	ABSTRACTIONS AND RETURN FLOWS	......................................... 2 - 17

2.3.1	Abstractions	......................................................................... 2 - 17

2.3.2	Return flows	......................................................................... 2 - 21

2.4	URBANISED AREAS	....................................................................... 2 - 24

2.5	AFFORESTATION	........................................................................... 2 - 25

2.6	TRANSMISSION LOSSES	............................................................... 2 - 27

2.7	INTERBASIN TRANSFERS	............................................................. 2 - 30

2.8	WETLANDS	.................................................................................... 2 - 35

TABLE OF CONTENTS (Continued)

Page

3.	HYDROLOGICAL DATA	........................................................................... 3 - 1

3.1	RAINFALL	........................................................................................ 3 - 1

3.1.1	Selection, patching and verification	.......................................... 3 - 1

3.1.2	Catchment rainfall inputs	........................................................ 3 - 4

3.2	EVAPORATION	................................................................................ 3 - 7

3.3	STREAMFLOW	............................................................................... 3 - 10

3.3.1	Selection of gauging stations	................................................. 3 - 10

3.3.2	Verification of streamflow records	.......................................... 3 - 13

3.3.3	Patching of streamflow records	.............................................. 3 - 14

4.	MODEL CALIBRATION	............................................................................ 4 - 1

4.1	INTRODUCTION (APPROACH TO CALIBRATION)	............................. 4 - 1

4.2	CALIBRATION RESULTS	.................................................................. 4 - 2

5.	NATURAL STREAMFLOWS	.................................................................... 5 - 1

6.	CONCLUSIONS AND RECOMMENDATIONS	......................................... 6 - 1

7.	REFERENCES	.......................................................................................... 7 - 1

APPENDICES

Appendix A	Maps

Appendix B	Catchment developments

Appendix C	Interbasin transfers

Appendix D	Rainfall

Appendix E	Raw streamflow data

Appendix F	Patched streamflow data

Appendix G	Model calibration

Appendix H	Natural streamflow

Appendix I	Description of electronic data included with this report

1.	INTRODUCTION

1.1	PURPOSE OF STUDY

The Vaal River System is clearly the most important water supply system in South Africa as it supplies water to more than 40% of the country's inhabitants as well as numerous industrial consumers who are responsible for more than 50% of the country's annual Gross Geographical Product. It is therefore crucial that the water resources of the Vaal River System are managed to the maximum benefit of the country and it is very important that the water resource management in this region receives special attention.

In 1985 the first major water resource system analysis to be carried out in South Africa, the Vaal River System Analysis (VRSA) was undertaken by BKS (Pty) Ltd in association with ACRES International and Stewart Scott Inc. The study was completed approximately four years later during which time the project team developed various new computer models and analysis techniques. The VRSA 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 analyses techniques have since become the standard methodology used for all DWAF 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 up to September 1984, i.e. 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 always identified to be a potential problem and the importance of this could not be established without extending the hydrological database. The database can now be extended from the original 64 years to 76 years in length. The additional 12 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 also a major issue in the Vaal River System and salinity modelling was undertaken as part of the original VRSA. 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 verified. In some cases only one or two years of reliable data were available with which to calibrate the models. The latest 12 years of information and water quality data will enable the salinity models to be calibrated with greater accuracy and confidence.

In addition to the hydrological aspects of the original VRSA, the techniques developed during the study were then used in several other major water resource studies throughout South Africa including the Mgeni, Western Cape, Crocodile, and Orange river basins. The analysis techniques were continually improved during these studies, however, the improvements were not incorporated into the VRSA which had already been completed. The significance of farm dam storage, paved areas, irrigation return flows and air pollution are only a few of the numerous issues where a greater understanding now exists. As a result, the processes involved can be modelled with greater confidence.

The purpose of the Vaal River System Analysis Update (VRSAU) is basically to revise and update the hydrological and water quality databases used in the previous VRSA study and to re-assess the water quantity and quality capabilities of the whole Vaal River System using the most up-to-date information and techniques. The major shortcomings and improvements highlighted during the last 12 years can now be taken into account during the new study.

1.2	SCOPE OF REPORT

The hydrology of the Komati, Usutu and the Vaal River with all its tributaries needs to be extended and upgraded. This report covers the extension and upgrading of the hydrology of the catchment upstream of Vaal Dam.

Section 2 of the report contains information on the catchment developments which have a significant impact on streamflow. The actual streamflow as recorded may differ substantially from the natural flows owing to the impact of dams, irrigation, abstractions, effluent and industrial return flows and water transfers. Accordingly, Section 2 is of prime importance in the assessment of natural streamflow in the Vaal Dam catchment.

The data assembled for the hydrological analysis are given in Section 3 and include rainfall, evaporation and streamflow records. Section 3 also contains a description of the patching of rainfall and streamflow data, and generation of representative catchment rainfall.

Section 4 deals with rainfall-runoff modelling and calibration of the catchment using the WRSM90 model. A report is made of how the model was set up to allow the effects of all catchment developments to be included in the model. The calibration and results are discussed for each gauging point.

Section 5 presents the results with respect to natural streamflow characteristics at various key points, Comparisons are also made with results from previous studies.	The main conclusions emanating from the hydrological analysis are given in Section 6 along with recommendations regarding possible improvements to the gauging network.

1.3	DESCRIPTION OF CATCHMENT

The total Vaal Dam catchment area is 38 638 km2 and stretches from the edge of the Drakensberg escarpment, westward to Vaal Dam. There are four major rivers draining the Vaal Dam catchment, namely the Vaal River, the Wilge River, the Liebenbergsvlei River and the Klip River with a combined average natural runoff of almost 2 000 million m3 per annum.

There is very little urban development in the Vaal Dam catchment, with the only major towns being Standerton near Grootdraai Dam, Bethlehem in the upper reaches of the Liebenbergsvlei River and Harrismith and Phuthaditjhaba near the source of the Wilge River.

The vegetation of the interior catchment plateau is mainly tall grassland, and although such vegetation originally covered most of the catchment, some of the area is now under dryland cultivation. Wheat is grown primarily in the colder southern parts of the Wilge catchment while maize is grown in the warmer northern parts near Vaal Dam. In the Klip River catchment and the Vaal River catchment upstream of Grootdraai Dam, livestock farming is practised.

The catchment is covered with thousands of small farm dams, particularly in the Waterval River catchment (a tributary of the Vaal River), and in the Vaal River catchment upstream of Grootdraai Dam. Some small pans are found in the central parts of the Wilge River and the Liebenbergsvlei catchments, while two relatively large wetlands are situated at the confluence of the Vaal and Klip rivers and in the Klip River, north of Memel.

The geology of the catchment is relatively homogeneous and comprises mainly sedimentary rocks of the Karoo system. In the northern half of the catchment Ecca series, shale, sandstone, grits and coal occur, which are the oldest deposits in the catchment. Over most of the southern part of the catchment, younger deposits of the Beaufort and Stormberg series, consisting mainly of shale, mudstone and sandstone occur.

2.	CATCHMENT DEVELOPMENTS

For modelling purposes, the Vaal Dam catchment was subdivided into 5 smaller subcatchments: Delangesdrift (upstream of Delangesdrift Weir), Sterkfontein (upstream of Sterkfontein Dam), Grootdraai (upstream of Grootdraai Dam), Frankfort incremental, (upstream of Frankfort Weir and downstream of Sterkfontein Dam) and Vaal incremental (upstream of Vaal Dam and donwnstream of Delangesdrift Weir, Frankfort Weir and Grootdraai Dam). All information provided in Section 2 refers to developments within these incremental subcatchments.

2.1	IMPOUNDMENTS

2.1.1	Large dams

The major impoundments in the Vaal Dam catchment are Vaal Dam, Grootdraai Dam, Sterkfontein Dam and Saulspoort Dam with a combined capacity of more than 5 500 million m3. The location of these dams can be seen in Figure A-2.

Vaal Dam is situated at the confluence of the Vaal and Wilge Rivers, about 75 km south of Johannesburg and was constructed in the mid 1930s as the major storage unit of the Vaal River System. The net full supply capacity of Vaal Dam is currently 2 603 million m3 with a surface area of 320 km2 at full supply level. For the purpose of this analysis, a total catchment area of 38 638 km2 was assumed for Vaal Dam. This was taken from the Water Research Commission publications on the surface water resources of South Africa (WRC, 1994).

A large portion of the demand on Vaal Dam is from Rand Water, the largest supplier of potable water in South Africa. It supplies water to almost all municipal and industrial users in Gauteng province - the country's largest water user. Water is also supplied directly from Vaal Dam to the town of Villiers, other small urban users, the nearby power station at Grootvlei as well as farmers in the Vaal Dam basin for irrigation.

Grootdraai Dam is located just upstream of Standerton in the Vaal River. It was constructed in 1978 and has a wall height of 42 m. The net full supply capacity of Grootdraai Dam is 356 million m3 with a surface area of 38.8 km2 at full supply level. The total catchment area upstream of Grootdraai Dam is 7 995 km2 (WRC, 1994). Water from Heyshope Dam in the Usutu catchment is transferred to the Grootdraai Dam subcatchment via the Klein Vaal River. It supplies water to various power stations and industrial users, including Tutuka power station and the Sasol II and III petro- chemical plants.

Sterkfontein Dam is located close to the origin of the Nuwejaarspruit River near Harrismith. It has a wall height of 93 m and was constructed in 1974. Although the nett full supply capacity of Sterkfontein Dam is 2 616 million m3 which is more than Vaal Dam, the surface area is only 67.3 km2 and it is therefore a very efficient reservoir with regards to evaporation. As Sterkfontein is used to store transferred water from the Tugela River system, it has a relatively small catchment area of 195 km2 (WRC, 1994). Driekloof Dam, which is situated at the backwaters of the Sterkfontein reservoir, forms part of Eskom's Drakensburg Pumped Storage Scheme.

Saulspoort Dam was constructed in 1968 in the Liebenbergsvlei River near Bethlehem with a wall height of 10 m. It is a fairly small dam with a net full supply capacity of only 17 million m3 and a surface area of 4 km2 at full supply level. Saulspoort Dam has a catchment area of 746 km2 (WRC, 1994) and supplies mainly municipal water to the town of Bethlehem.

Other important water users supplied from the Vaal River System include various power stations in Mpumalanga, Vaalreefs and Western Transvaal Region Water Companies in the North West province, Goudveld Water in the Orange Free State and the Vaalharts irrigation scheme in the Northern Cape.

2.1.2	Small dams

Small dams have a major impact on the runoff from a catchment and therefore have to be taken into account when modelling the hydrological process. In the Vaal Dam catchment, the large number of farm dams represent a significant storage capacity that results in a decrease in the mean annual runoff (MAR) and an increased assurance of supply to irrigation and small rural demands. Evaporation losses from farm dam surface areas are estimated to be in the order of 60 million m3/a at 1994/5 development level, which represents approximately 3% of the total MAR.

The processing of small dam data involves estimating the total area and capacity of all the small dams in a given subcatchment. These dams are then lumped together to form one or more representative dams, referred to as "dummy dams". The effect of a dummy dam on runoff in the catchment is modelled to correspond as closely as possible with the combined effect of the numerous individual dams.

The DWAF Dam Safety Register provides information on most dams in South Africa with a capacity greater than 50 000 m3 and/or a wall height greater than 5 m. Details of these dams such as location, date of construction, hight of dam wall, full supply capacity and surface area are available from the Department. Determining the total area and capacity of registered dams in any subcatchment is therefore a simple procedure of adding together their individual areas and capacities as it is given in the Dam Safety Register. The location of these dams can be seen in Figure A-2.

It should be noted, however, that in many instances the information from the Dam Safety register is incomplete. The capacities of some dams are missing, as are the surface areas in many cases. Before the Dam Safety data can be used in any calculations, the missing values must be infilled.

Dams from the Dam Safety Register with a complete record (both full supply area and capacity given), were used to determine a typical area-capacity relationship for registered dams in the catchment. This area-capacity relationship can then be used to infill missing data, ie if the area of a dam is known, the capacity can be estimated and if the capacity is known, the area can be estimated. In cases where both values are missing, an area is estimated directly from the 1:50 000 topographical map. According to the State Printer, the area shown on a 1:50 000 map closely corresponds with the area of water in a dam at full supply level. The reason for this is that these maps are created using aerial photographs on which it is usually easy to identify the highest water level, even if a dam is not full.

As a first estimate of the full supply area of unregistered dams, 1:50 000 maps were also used. All dams within a specific catchment were numbered and their areas measured. To avoid double counting, overlays on tracing paper were plotted, showing the position of all registered dams which were considered separately.

To determine the capacities of small dams from the measured full supply areas, a typical area-storage relationship was derived. Using infilled Dam Safety information as discussed earlier, an estimate was made of the typical storage to area relationship for the Dam Safety dams in the catchment. Using this relationship, the capacity of each unregistered dam was estimated from its measured full supply area.

The most serious drawback of using 1:50 000 maps for estimating unregistered dam areas, however, is that many of the maps are based on surveys undertaken as far back as the early seventies with an average date of somewhere in the mid-1980s. It is therefore necessary to adjust the measured areas and calculated capacities in some way to represent 1994/5 development levels. One possible approach is to use the growth characteristics of registered dams and extrapolate the growth to unregistered dams.

The growth characteristics for the registered dams in the catchment were established using their construction dates given in the Dam Safety register. Plots were made of the total cumulative capacity and area of registered dams in the catchment against time. By employing simple curve fitting techniques, area and capacity growth curves of registered dams were derived. These growth curves were applied to the unregistered dams and, in this way, an estimate of the total area and capacity in 1994/5 was obtained. Figure B-1 shows the growth curves of the total area and capacity of small registered dams in the Vaal Dam catchment.

An attractive alternative solution for estimating the area of unregistered dams at current development level, is offered by satellite remote sensing imagery. Satellite remote sensing involves the capturing of electromagnetic radiation emitted or reflected by objects on the surface of the earth. Special techniques are applied to process and interpret this imagery and the information can be used to produce conventional maps and resource surveys on agriculture, forestry and geology for example.

Satellite images can be used in the same way to acquire information on the surface areas of water bodies on the earth's surface. S-spot images from the Landsat TM satellite with a pixel resolution of 30 x 30 m, covering the whole of the Vaal dam catchment were obtained from the CSIR. The images were obtained during March 1996, after an extremely wet period, and therefore show all the dams in the catchment at full supply level. The result are shown in Figure A-2.

Using the satellite images, the Satellite Application Centre of Mikomtek at the CSIR was able to provide BKS with digital information on the exact size and geographical location of every water body within the Vaal Dam catchment. The water bodies were then plotted to 1:50 000 scale on sheets of tracing paper which could be overlaid onto the corresponding 1:50 000 topographical maps. In this manner it was possible to identify all the registered and unregistered dams as well as pans, wetlands, etc. By removing all water bodies except the unregistered dams, it was possible to obtain a digital coverage of only unregistered dams inside the catchment from which their total surface area could be determined.

Table 2.1 provides the total capacity, total surface area and average depth of all unregistered dams in the Vaal Dam catchment. Three sets of values are given. The first set is based on areas estimated from 1:50 000 maps that are scaled up to 1994/5 development levels as discussed earlier, the second is the same but the areas are not scaled up in any way. The third set is based on areas from the satellite imagery.

Table 2.1:	Total capacity and surface area of unregistered dams, based on different sources of information.

Source of information	Unregistered dams in the Vaal Dam catchment
Total capacity

(106m3)	Total area

(km2)
1:50 000 maps (scaled up to 1994/5 development level)	115.549	113.479
1:50 000 maps (not scaled up)	71.146	69.748
Satellite imagery	68.013	66.558

As this simple comparison shows, the areas derived from the satellite imagery support the areas obtained directly from the 1:50 000 maps without any adjustment. This suggests that the unregistered dams have not increased in number over the last ten years. The areas and capacities finally adopted in the study were based on the results from the satellite imagery, and are given in Table 2.2.

Table 2.2:	Adopted combined areas and capacities of small dams at full supply level for incremental subcatchments at 1994/5 development levels

Incremental subcatchment	Unregistered dams	Registered dams	All small dams
Total capacity

(106m3)	Total area in

(km2)	Total capacity

(106m3)	Total area in

(km2)	Total capacity

(106m3)	Total area in

(km2)	Ave depth

(m)
Delangesdrift	7.5	7.3	3.0	1.2	10.5	8.5	1.2
Frankfort incremental	20.0	19.6	50.1	17.1	70.0	36.7	1.9
Grootdraai	16.7	16.4	27.1	14.2	43.8	30.6	1.4
Sterkfontein	0.0	0.0	0.0	0.0	0.0	0.0	-
Vaal incremental	23.8	23.2	18.4	7.3	42.2	30.5	1.4
TOTAL	68.0	66.5	98.5	39.8	166.5	106.3	1.6

As mentioned previously, the small dams in a subcatchment are lumped together to form one or more representative dummy dam. The reason for this is that, when including small dams in a hydrological analysis, it is not practical to model each dam separately, as this would make the simulation of the system practically impossible.	The number and size of dummy dams per subcatchment is determined from the complexity of the subsystem as well as the geographical position of the individual farm dams within the catchment. In general, however, the number of dummy dams is kept to a minimum, without compromising the physical representativeness of the model.

The historical full supply capacities and areas of dummy dams are given in Tables 2.3 and 2.4, and their localities are shown in Figure A-2. As mentioned previously, the growth of the unregistered dams is based on the growth characteristics of registered dams for the period 1920 to the mid 1980s. After this time, the number of unregistered dams was estimated to remain constant.

Table 2.3:	Historical full supply capacity of dummy dams

Incremental subcatchment	Name of dummy dam	Full supply capacity in year (106m3)
1920	1960	1970	1982	1995
Delangesdrift	Delangesdrift	0.95	4.15	6.01	9.36	10.50
Frankfort incremental	Liebenbergsvlei	1.89	8.30	12.01	18.72	26.56
Frankfort incremental	Saulspoort	0.02	0.10	18.94	19.02	19.06
Frankfort incremental	Wilge River	2.06	9.02	13.06	20.36	24.42
Grootdraai	Upper Vaal	0.59	2.58	3.73	5.82	7.91
Grootdraai	Grootdraai	2.79	12.25	17.73	27.63	35.87
Vaal incremental	Vaal-Wilg	2.01	8.81	12.76	19.88	22.28
Vaal incremental	Waterval River	1.54	6.77	9.80	15.28	19.89
TOTAL	11.84	51.98	94.04	136.07	166.49
Table 2.4:	Historical area at full supply level of dummy dams

Incremental subcatchment	Name of dummy dam	Area at full supply level in year (km2)
1920	1960	1970	1982	1995
Delangesdrift	Delangesdrift	0.81	3.58	5.19	8.11	8.58
Frankfort incremental	Liebenbergsvlei	1.10	4.84	7.01	10.95	14.07
Frankfort incremental	Saulspoort	0.02	0.08	4.37	4.43	4.44
Frankfort incremental	Wilge River	1.64	7.25	10.51	16.40	18.20
Grootdraai	Upper Vaal	0.45	1.97	2.85	4.45	5.73
Grootdraai	Grootdraai	2.07	9.15	13.26	20.70	24.85
Vaal incremental	Vaal-Wilg	1.81	8.01	11.60	18.11	19.68
Vaal incremental	Waterval River	0.96	4.25	6.16	9.61	10.83
TOTAL	8.86	39.11	60.94	92.75	106.37

It is interesting to note that the volumes of the dummy dams as given in Tables 2.3 and 2.4 are relatively large compared to the amount of irrigation that they typically support. This is due to the fact that many of the small dams represented by such a dummy dam, do not necessarily support irrigation themselves. According to the Land Use Planning Directorate of the Department of Agriculture in Ermelo, this is especially true of small dams within the Vaal Dam catchment. The reason for this is that in the Highveld region, winter temperatures are typically too cold to allow irrigation. Instead, small dams are mostly used for live stock farming and in some cases recreation. This was confirmed by the office of the Department of Agriculture in Secunda, the DWAF regional office at Vaal Dam and several farmers in the area.

Ideally, the system should be modelled in such a way that irrigation is supported from dummy dams that in reality support irrigation. In this way, the availability and probability of supply of water for irrigation will not be overestimated. Obtaining such information, however, was found to be very time consuming and could not be justified by the relatively small increase in accuracy that could be obtained.

In addition to the information given in Tables 2.3 and 2.4, area-capacity characteristics of every dummy dam are also needed for the WRSM90 runoff model. This information is used for the purpose of calculating the surface area of a dummy dam when it is not full. Area-capacity characteristics are described by the following equation:

A=kCP, where:	A is surface area in km2,

C is volume or capacity in million m3

k is a constant with dimensions m-1and

P is a dimensionless constants.

The runoff model requires the value of P, which was easily determined by means of simple curve fitting techniques. The adopted value of P for each dummy dam is given Table 2.5.

Determining the percentage of runoff routed through each dummy dam, was undertaken manually, using maps similar to that in Figure A-2. The results are also given in Table 2.5.

Table 2.5:	Area-capacity characteristics and percentage of runoff routed through dummy dams

Incremental subcatchment	Area of incremental subcatchment (km2)	Name of dummy dam	Average depth

(m)	Value of P

(in the equation A=kCP)	% of subcatchment runoff to dummy dam
Delangesdrift	4158	Delangesdrift	1.22	0.78	42.8
Frankfort incremental	15489	Liebenbergsvlei	1.89	0.69	6.1
Frankfort incremental	15489	Saulspoort	4.29(1)	0.70	3.9
Frankfort incremental	15489	Wilge River	1.34	0.70	26.2
Grootdraai	7995	Upper Vaal	1.38	0.65	13.2
Grootdraai	7995	Grootdraai	1.44	0.68	37.3
Vaal incremental	10792	Vaal-Wilg	1.13	0.95	27.4
Vaal incremental	10792	Waterval River	1.84	0.60	14.5

Notes:	(1)	This value is very high due to the fact that the dummy dam in question mainly comprises Saulspoort Dam, with an average depth of 4.42m.

2.2	IRRIGATION

2.2.1	Introduction

The Vaal Dam catchment was proclaimed a Government Water Control Area (GWCA) in 1970 (proclamation 181, 1970) in terms of section 59(1) of the Water Act. Due to the ever-increasing demands on the resources of the Vaal River system, it was considered of crucial importance to protect this public source of water against over-exploitation.

Although, in practice, little control is exercised on irrigation inside a GWCA, legislation has been implemented, in terms of proclamation 181, to limit the development of irrigation inside the catchment. This legislation prohibits irrigation on registered riparian farms of more than 21.4 ha at a maximum quota of 6100 cubic metres per hectare per annum and abstraction rate of 42.5 l/s. If, at the time of implementation, more than 21.4 ha was under irrigation, it was allowed to continue.

2.2.2	Present irrigation areas

The current area of irrigation in the Vaal Dam catchment is difficult to determine accurately without conducting a detailed and comprehensive survey of the whole area. Since such an undertaking would be unjustifiably expensive, it was decided to make use of the best available information.

A detailed survey of mainstream irrigation was recently undertaken by Schoeman & Partners, covering the Vaal River upstream of Vaal Dam, as well as two of its main tributaries - the Liebenbergsvlei River and the Wilge River. For every farm irrigating directly from these rivers, Schoeman & Partners supplied information on areas and systems of irrigation by crop type as well as plant dates where applicable. As the Schoeman & Partners survey was considered to be the most reliable source of available information, their areas of main stream irrigation were accepted for use in the current study. Areas covered by the survey are shown in Figure A-3.

Estimated areas of irrigation from smaller tributaries, not included by the Schoeman & Partners survey, were obtained directly from the DWAF regional office at Vaal Dam. Included with this information was the area of irrigation from the Vaal River upstream of its confluence with the Kleinvaal.

The final adopted areas of irrigation from surface water sources upstream of Vaal Dam in 1994/5 are given in Table 2.6. Areas are given for subdivisions of incremental subcatchments, and distinction is made between main stream irrigation and irrigation from smaller tributaries. The latter is referred to as "dummy dam" irrigation since the water is supplied from small dams. Section 2.1.2 of this report provides more information on dummy dams and Figure A-2 shows the localities of the dams.

The area of irrigation given in Table 2.6 for the Vaal incremental subcatchment includes an estimated area of irrigation in the Vaal Dam basin. This area is based on measured abstractions from the Vaal dam at the allowable quota of 6100 cubic metres per hectare per annum. Abstractions directly from Vaal Dam are metered by DWAF since the area falls within a Government water scheme.

It is interesting to note that according to the DWAF regional office there is no irrigation in the Delangesdrift subcatchment. This information is contradicted by the Water Research Commission in their report on the surface water resources of South Africa (WRC, 1994). They estimate the irrigated area in Delangesdrift to be 310 ha. It was decided to accept the DWAF information, however, as it was considered to be more reliable. Their information was also supported by personal communication with the Land Use Planning Directorate of the Department of Agriculture in Ermelo and farmers in the district.

Table 2.6:	Areas under irrigation in incremental subcatchments, at 1994/5 development levels (1)

Incremental subcatchment	Area of subcatchment (km2)	Description of irrigation	Total area of irrigation (ha)
Delangesdrift	4158	None	0.0
Frankfort incremental	15498	Liebenbergsvlei dummy dam irrigation	604.9
Frankfort incremental	15498	Liebenbergsvlei River main stream irrigation	1757.1
Frankfort incremental	15498	Saulspoort dummy dam irrigation	168.3
Frankfort incremental	15498	Wilge River dummy dam irrigation	2311.3
Frankfort incremental	15498	Wilge River main stream irrigation	2170.7
Grootdraai	7995	Upper Vaal main stream irrigation	796.6
Grootdraai	7995	Grootdraai main stream irrigation	440.6
Sterkfontein	195	None	0.0
Vaal incremental	10792	Vaal-Wilg dummy dam irrigation	13.9
Vaal incremental	10792	Vaal-Wilg main stream irrigation	1549.5
Vaal incremental	10792	Waterval River dummy dam irrigation	874.3
Vaal incremental	10792	Waterval River main stream irrigation	1230.4
Vaal incremental	10792	Irrigation in the Vaal Dam basin	277.2
TOTAL	38638	-	12194.8

Note:	(1)	From Schoeman & Partners surveys and personal communication with the DWAF regional office at Vaal Dam.

2.2.3	Historical development

Personal communication with the DWAF regional office at Vaal Dam indicated that there was considerable growth in irrigation inside the Vaal Dam catchment during the late 1980s. The past few years, however, the irrigation within the catchment has remained almost constant.

Irrigated areas at current development levels, were estimated using information obtained from DWAF and the Schoeman & Partners surveys as discussed in the previous section. For irrigated areas before this time, the most reliable source of information was considered to be the reports from the Central Statistical Services on their censuses of agriculture (CSS, 1968/9, CSS, 1976, CSS, 1988a and CSS, 1988b). Data were used from the censuses for 1968, 1976 and 1988.

Information provided by the CSS has one major drawback, in that all of the data concerning land utilisation is given by municipal district. It was therefore necessary to convert the data to drainage regions or catchments. This was accomplished by assuming the areas of irrigation within a municipal area to be evenly distributed, unless information to the contrary was available.

As historical data were needed over the full period of calibration, an estimate had also to be made of irrigation before 1968. The only information available was that from a report of the Commission of Inquiry into Water Matters (1970), detailing estimated water consumption for irrigation in the Republic of South Africa since 1910. This was used to estimate historical areas of irrigation in the catchment.

The historical data from DWAF, the Schoeman & Partners survey, the Agricultural censuses and the report of the Commission of Inquiry into Water Matters are summarised in Table 2.7.

By plotting the total areas of irrigation in Table 2.7 against time, and employing simple curve fitting techniques, an estimate could be made of the historical growth curve of the total area under irrigation in the Vaal Dam catchment. Applying this growth curve to the estimated present areas of irrigation, resulted in the areas given in Table 2.8

Table 2.7:	Estimated areas of irrigation for incremental subcatchments

Incremental subcatchment	Total area of irrigation in year by source of information (ha)
Commission of Enquiry into Water Matters(1) (1920)	Agricultural

census(2)

(1968)	Agricultural

census(3)

(1976)	Agricultural

census(4)

(1988)	Total from S&V survey(5) and DWAF(6)

(1994/5)
Delangesdrift	0	0	0	0	0
Frankfort incremental	1295	6858	3389	6815	7012
Grootdraai	228	1210	598	1202	1237
Sterkfontein	0	0	0	0	0
Vaal incremental	728	3859	1907	3834	3945
TOTAL	2252	11927	5894	11852	12195

Note:	(1)	Report of the Commission of Inquiry into Water Matters (1970), Figure 2.

(2)	From report CSS (1968/9), Table 2.3.

(3)	From report CSS (1976), tables 2.3 and 2.4.

(4)	From report CSS (1988a), Table 3.2 and (1988b), Table 3.2.

(5)	From personal communication with Schoeman & Partners.

(6)	From personal communication with the DWAF regional office, Vaal Dam.

Table 2.8:	Adopted historical development of irrigation

Incremental subcatchment	Subcatchment area

(km2)	Total area of irrigation (ha)
1920	1968	1988	1995
Delangesdrift	4158	0.0	0.0	0.0	0.0
Frankfort incremental	15498	1294.7	4255.2	6815.0	7012.3
Grootdraai	7995	228.4	750.8	1202.4	1237.2
Sterkfontein	195	0.0	0.0	0.0	0.0
Vaal incremental	10792	728.4	2394.1	3834.2	3945.3
TOTAL	38638	2251.6	7400.0	11851.6	12194.8

Comparing the values given in Table 2.8 to those from other sources, indicates that the adopted areas are realistic. The previous BKS report on the hydrology of the Vaal Dam catchment (BKS (Pty) Ltd, 1986) used a total figure of 8340 ha and the report on the assessment of irrigation demands, using satellite imagery (BKS (Pty) Ltd, 1985), suggested a figure of 12221 ha. Both are at 1983/4 development level. Although the second figure may seem a bit high, one must remember that satellite imagery should be used with caution when estimating irrigated areas, as it is often difficult to differentiate between dry land and irrigated agriculture. The Water Research Commission report (WRC, 1994) indicates a total area of 12 560 ha at 1989/90 development levels.

2.2.4	Water use by irrigation

The WRSM90 runoff model can be used to simulate the monthly irrigation requirement within a catchment, using as input the total area under irrigation within the catchment, the historical rainfall record for the catchment, the monthly effective rainfall factors as well as the evapotranspiration from the catchment for a given month. It calculates the monthly evapotranspiration for an irrigated area by multiplying class A-pan evaporation with a value called a "crop factor". Crop factors are empirically estimated and are, amongst others, a function of crop type, growing season and climatic circumstances.

Monthly crop factors for individual crops were obtained from the Department of Agriculture and Water Supply publication on the estimation of irrigation requirements of crops in South Africa (1985). Crop combinations were obtained from the Schoeman & Partners information. They supplied areas of irrigation by crop type as well as plant dates. Although the Schoeman & Partners data applies to main stream irrigation only, it was considered to be representative for all of the irrigation in a subcatchment. The adopted figures are given in Table 2.9 and Figure B-2 provides a graphical representation of crop distributions and growing seasons for the various subcatchments upstream of Vaal Dam.

It should be noted that the total percentages of crop types given in Table 2.9 exceed 100 % of irrigated area for the Frankfort and Vaal incremental subcatchments. This is due to double cropping. Also, note that Delangesdrift and Sterkfontein subcatchments are not included in Table 2.9 as there is no irrigation in these areas.

Table 2.9:	Adopted percentages, plant dates and crop types for subcatchments(1)

Crop type	Plant date

(where applicable)	% of crop type to total irrigated area
Frankfort incremental	Grootdraai	Vaal incremental
Beans	1 September	1	0	0
Deciduous fruit	1 September(2)	0	0	0
Lucern	1 July	10	6	1
Maize	1 October / 15 December(3)	15	26	43
Pastures	-	27	48	18
Potatoes	1 January	17	3	21
Soya beans	1 December	0	12	1
Vegetables	1 May(4)	13	0	5
Wheat	15 June	21	5	41
TOTAL	103	100	130

Note:	(1)	From personal communication with Schoeman & Partners.

(2)	Deciduous fruit taken to be mid-season.

(3)	Some maize crops in the Vaal incremental subcatchment are planted on 15 December.

(4)	The plant date for vegetables was based on personal communication with Schoeman & Partners.

When estimating irrigation requirements, the WRSM90 runoff model requires one set of monthly crop factors for the whole of a catchment. It was therefore necessary to convert the combination of monthly crop factors for different crops in a catchment to one set of representative monthly catchment crop factors. This was done simply by taking the weighted average crop factor for a subcatchment by percentage of crop type. The results are given in Table 2.10.

Table 2.10:	Representative monthly crop factors for subcatchments(1)

Incremental subcatchment	Representative monthly crop factors
Oct	Nov	Dec	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep
Frankfort incr.	0.62	0.56	0.51	0.55	0.55	0.55	0.52	0.37	0.26	0.31	0.43	0.53
Grootdraai	0.55	0.68	0.76	0.77	0.64	0.57	0.46	0.34	0.28	0.29	0.31	0.37
Vaal incr.	0.74	0.65	0.65	0.64	0.47	0.40	0.38	0.26	0.17	0.26	0.45	0.56

Note:	(1)	Delangesdrift and Sterkfontein subcatchments are not shown as there is no irrigation.

It is important to note that the crop factors shown in Table 2.10 are used to calculate nett irrigation requirements, ie the amounts of water that would have to be applied assuming a perfectly efficient irrigation system. Such a system has an efficiency factor of 1.0. On the other hand, the gross irrigation requirement represents the amount of water that must be delivered by an irrigation system with an efficiency factor of less than 1.0, making allowances for losses due to runoff, evaporation, deep drainage and others. The gross irrigation requirement is calculated by dividing the nett irrigation requirement by the actual application efficiency of the system of irrigation.

The WRSM90 runoff model, does not compensate for application efficiencies and effectively simulates only nett irrigation requirements. To solve this problem, the crop factors given in Table 2.10 were divided by the average application efficiencies for each subcatchment. Using this altered crop factor, the model could directly simulate realistic gross irrigation requirements.

The average application efficiency for each subcatchment was calculated using the information from the Schoeman & Partners survey, since they also indicated the type of irrigation system used. Data on the efficiency of specific systems were obtained from a previous BKS report on the hydrology of the Crocodile River (BKS (Pty) Ltd, 1992). The results are given in Table 2.11.

Table 2.11:	Irrigation efficiency information for subcatchments(1)

System of irrigation	Irrigation efficiency of system as a percentage	% of irrigation system to total area of irrigation for subcatchment(2)
Frankfort incremental	Grootdraai	Vaal

incremental
Micro	90%	3	0	0
Centre pivot	85%	41	8	85
Sprinkle	75%	56	92	15
Flood	60%	0	0	0
TOTAL	100	100	100
Average irrigation efficiency for subcatchment as a percentage:	80%	76%	84%

Note:	(1)	Delangesdrift and Sterkfontein subcatchments are not shown as there is no irrigation.

(2)	From personal communication with Schoeman & Partners.

In practice, irrigation requirements are not always met, since less water may be available in the system than is actually required for irrigation. The WRSM90 runoff model takes this effect into consideration and abstracts from the system only the amount of water that can actually be supplied. The output files of simulated monthly irrigation supplies are given in Appendix B-3.

For this study, return flows into the system from irrigated areas were assumed to be 10% and a monthly effective rainfall factor of 0.6 was used.

2.3	ABSTRACTIONS AND RETURN FLOWS

The process of naturalisation, as described in Section 5 of this report, involves the generation of natural flow at various points in the system called system nodes. This is achieved by adding abstractions by users and subtracting return flows from the observed flows.

2.3.1	Abstractions

There are various urban and industrial water users inside and outside the Vaal Dam catchment who are dependant on the catchment runoff for their water. In most cases, the individual abstractions of users are small and not significant when compared to the overall water balance. When totalled, however, the combined abstraction can cause a significant decrease in the MAR of the catchment.

The major abstractions from Vaal and Grootdraai Dams are measured by DWAF and already taken into account during the calculation of the inflow to the reservoirs. The magnitude of such abstractions is therefore not needed to regenerate the natural flow. They are however given in Table 2.12.

Table 2.12:	Direct urban and industrial abstractions from Vaal and Grootdraai Dams

Point of abstraction	Description	DWAF dam balance component number	Period of abstraction (calender years)
Vaal Dam	Pipeline to Deneysville	C1H013-M01	1979-1994
Vaal Dam	Pipeline to Grootvlei power station	C1H023-M01	1969-1994
Vaal Dam	Large pipeline to Rand Water	C1H024-M01	1989-1994
Vaal Dam	Small pipeline to Rand Water	C1H025-M01	1966-1994
Grootdraai Dam	Sasol II abstraction	C1H020-A01 and C1H022-M01	1979-1994
Grootdraai Dam	Pipeline to Tutuka power station	C1H021-M01	1984-1991

Estimated figures of historical abstractions by most of the users in the catchment were obtained from the Vaal Augmentation Planning Study report on future demands and return flows (BKS (Pty) Ltd, 1995). Other figures were based on information supplied by the various users, estimates based on population, figures from the previous BKS report on the hydrology of the Vaal Dam catchment (BKS (Pty) Ltd, 1986) or figures from the BKS report on demands and return flows in the Vaal River system (BKS 1988). The adopted historical abstractions are given in Table 2.13.

Abstraction files representing the total historical monthly abstractions by urban and industrial users from different parts of the system, are given in Appendix B-4. Note that linear growth was assumed between first and last abstraction.

Table 2.13:	Urban and industrial abstractions in the Vaal Dam catchment(1)

Incremental subcatchment and demand centre	Last or current abstraction	Comment
Hydro- logical year	Abstrac- tion (106m3/a)	Hydro- logical year	Abstrac- tion (106m3/a)
Delangesdrift subcatchment
Memel	1956	0.026	1994	0.100	Abstraction from the Klip River.
Volksrust	1990	0.126	1994	0.400	Abstraction from the Balfour and Schuil- hoek Dams on a Sandspuit tributary. Additional water is supplied to Volksrust from the Tugela catchment.
Total for subcatchment	1994	0.500
Frankfort incremental subcatchment
Bethlehem	1924	0.162	1994	5.480	Abstraction from the Saulspoort Dam on the Liebenbergsvlei River.
Frankfort	1920	0.024	1994	1.365	Abstraction from the Wilge River.
Harrismith	1920	0.066	1994	2.390	Abstraction from dams on tributaries of the Wilge River.
Kestell	-	-	-	-	Supplied from municipal and private boreholes.
Phutaditjhaba (Qwa-Qwa)	1920	0.047	1994	9.366	Abstraction from the Elands River. Estimated from population figures. Additional water has been abstracted from Sterkfontein Dam since April 1988. It was 0.674 million cubic metres in 1994 hydrological year.
Reitz	1954	0.173	1994	0.805	Abstraction from weirs in the Liebenbergsvlei River.
Warden	1955	0.080	1994	0.816	Abstraction from a dam on the Cornelis River.
Total for subcatchment	1994	20.222
Grootdraai subcatchment
Amersfoort	1936	0.008	1994	0.172	Abstraction from the Amersfoort Dam on the Skulpspruit.
Bethal	-	-	-	-	Supplied by the Rand Water Board.
Breyten	1920	0.005	1994	0.330	Abstraction from Torbenite Dam on the upper Vaal River.
Camden power station	-	-	-	-	Supplied from the Usutu catchment.
Davel	-	-	-	-	Supplied from the Usutu catchment.
Ermelo	1947	0.491	1994	3.650	Abstraction from the Douglas and Brummer Dams on the Klein Kafferspruit.
Majuba power station	-	-	-	-	Supplied from the Zaaihoek Dam which lies outside the catchment.
Morgenzon	-	-	-	-	Supplied from municipal and private boreholes.
Perdekop	-	-	-	-	Supplied from municipal and private boreholes.
Tutuka power station	-	-	-	-	Supplied by Grootdraai Dam component (pipeline C1H021).
Total for subcatchment	1994	4.152
Vaal incremental subcatchment
Balfour	-	-	-	-	Abstraction from the Suikerbosrand River which lies outside the catchment.
Cornelia	-	-	-	-	Supplied from municipal and private boreholes.
Deneysville	-	-	-	-	Supplied by Vaal Dam component (pipeline C1H013).
Embalenhle township	-	-	-	-	Supplied by the Rand Water Board.
Evander	-	-	-	-	Supplied by the Rand Water Board.
Greylingstad	-	-	-	-	Supplied from municipal and private boreholes.
Grootvlei power station	-	-	-	-	Supplied by Vaal Dam component (pipeline C1H023).
Heilbron	-	-	-	-	Supplied by Vaal Dam component (pipeline C1H013).
Kinross	-	-	-	-	Supplied by the Rand Water Board.
Leandra	-	-	-	-	Supplied by the Rand Water Board.
Sasol II and III industrial	-	-	-	-	Supplied by Grootdraai Dam component (pipeline C1H020).
Sasol urban	-	-	-	-	Supplied by the Rand Water Board.
Secunda	-	-	-	-	Supplied by the Rand Water Board.
Small users(3)	1920	0.005	1994	0.310	Abstraction from the Vaal Dam.
Standerton	1920	0.089	1994	6.400	Abstraction from the Vaal River (com- pensation released from Grootdraai Dam).
Trichardt	-	-	-	-	Supplied by the Rand Water Board.
Tweeling	-	-	-	-	Supplied from municipal and private boreholes.
Twisdraai, Brandspruit, Bossiespruit and Middelbult Mines	-	-	-	-	Supplied by the Rand Water Board and recycled mine water.
Villiers	1920	0.006	1994	0.510	Abstraction from the Vaal Dam.
Vrede	1952	0.093	1994	0.740	Abstraction from the Spruitsonderdrif River, a tributary of the Klip River.
Total for subcatchment	1994	7.960
TOTAL FOR CATCHMENT	1994	32.834

Note:	(1)	Entries in the table with no abstraction values, either abstract water from outside the catchment or are already taken into account by the calculation of the flow data.

(2)	Abstractions are taken into account only from 1920 onwards.

(3)	Small users are the Jim Fouché resort, Oranjeville, the Vaal Marina and the Vaal Dam Recreation Complex. They are grouped together as their combined demands are very small.

2.3.1	Return flows

Most of the urban users in the Vaal Dam catchment generate no return flow to the river system as their effluent is discharged to evaporation pans or used for irrigation. In cases where return flow does occur, adopted figures were mostly based on typical abstraction to return flow ratios.

Return flows from most of the industrial users are of little importance as they lie mainly within the Tugela and Olifants River catchments. The most important industrial return flows inside the Vaal Dam catchment are from Sasol II and III and seepage from Camden, Grootvlei and Tutuka power stations. These figures have been obtained directly from Sasol and Eskom.

All the adopted historical return flows by urban and industrial users, are given in Table 2.14. Data files representing the total historical monthly return flows by urban and industrial users from different parts of the system, are given in Appendix B-4. Note that linear growth was assumed between first and last return flow.

Table 2.14:	Urban and industrial return flows in the Vaal Dam catchment(1)

Incremental subcatchment and return flow centre	Last or current abstraction	Comment
Hydro- logical year	Return flow (106m3/a)	Hydro- logical year	Return flow (106m3/a)
Delangesdrift subcatchment
Memel	-	-	-	-	Effluent discharged to oxidation dams.
Volksrust	-	-	-	-	Effluent discharged to the Tugela catchment.
Total for subcatchment	1994	0.000
Frankfort incremental subcatchment
Bethlehem	1924	0.081	1994	2.740	Effluent discharged to Loch Lomond on the Liebenbergsvlei River.
Frankfort	-	-	-	-	Effluent used for irrigation.
Harrismith	1920	0.030	1994	1.099	Effluent to the Nuwejaarspruit.
Kestell	-	-	-	-	Effluent used for irrigation.
Phutaditjhaba (Qwa-Qwa)	1920	0.018	1994	3.916	Effluent discharged to the Elands River.
Reitz	-	-	-	-	Effluent discharged to oxidation dams and used for irrigation.
Warden	-	-	-	-	Effluent discharged to oxidation dams.
Total for subcatchment	1994	7.755
Grootdraai subcatchment
Amersfoort	-	-	-	-	Effluent discharged to oxidation dams.
Bethal	1954	0.165	1994	2.210	Effluent discharged to the Blesbokspruit.
Breyten	-	-	-	-	Effluent discharged to oxidation dams.
Camden power station	1967	0.730	1988	0.730	Seepage to the upper Vaal River.
Davel	-	-	-	-	Effluent discharged to oxidation dams.
Ermelo	1947	0.388	1994	2.884	Effluent discharged to a tributary of the Vaal River, upstream of Grootdraai Dam.
Majuba power station	-	-	-	-	No historical effluent as Majuba Power Station was only commissioned in 1996.
Morgenzon	-	-	-	-	Effluent used for irrigation.
Perdekop	-	-	-	-	Effluent discharged to oxidation dams.
Tutuka power station	1986	0.900	1994	0.900	Seepage to the Leeuspruit, upstream of Grootdraai Dam.
Total for subcatchment	1994	5.994
Vaal incremental subcatchment
Balfour	-	-	-	-	Effluent discharged to the Suikerbosrand River which lies outside the catchment.
Cornelia	-	-	-	-	Effluent discharged to gravel pits.
Deneysville	-	-	-	-	Effluent discharged to evaporation dams.
Embalenhle township	1976	0.222	1994	2.235	Effluent to the Trichardspruit, a tributary of the Waterval River.
Evander	1955	0.225	1994	2.252	Effluent discharged to a tributary of the Waterval River.
Greylingstad	-	-	-	-	Effluent used for irrigation..
Grootvlei power station	1970	0.560	1989	0.560	Seepage to the Molspruit, upstream of Vaal Dam.
Heilbron	-	-	-	-	Effluent discharged to a tributary of the Renoster River, outside the catchment.
Kinross	-	-	-	-	Effluent discharged to the Olifants Catchment.
Leandra	-	-	-	-	Effluent discharged to the Olifants Catchment.
Sasol II and III industrial	1993	6.570	1994	5.475	Effluent discharged to a tributary of the Waterval River since 1993.
Sasol urban	1976	0.758	1994	7.639	Effluent discharged to a tributary of the Waterval River.
Secunda	-	-	-	-	Effluent from Secunda is treated at the Sasol sewerage works. It is included with the Sasol figure for urban effluent.
Small users(3)	-	-	-	-	Effluent discharged to evaporation dams and used for irrigation.
Standerton	1920	0.050	1994	3.584	Effluent is discharged to the Vaal River, upstream of Vaal Dam.
Trichardt	-	-	-	-	Effluent discharged to the Olifants Catchment.
Tweeling	-	-	-	-	Effluent used for irrigation.
Twisdraai, Brandspruit, Bossiespruit and Middelbult Mines	-	-	-	-	Effluent is recycled or discharged to evaporation dams.
Villiers	-	-	-	-	Effluent used for irrigation.
Vrede	-	-	-	-	Effluent discharged to oxidation dams.
Total for subcatchment	1994	21.185
TOTAL FOR CATCHMENT	1994	34.934

Note:	(1)	Entries in the table with no return flow values, either discharge water outside the catchment or are already taken into account by the calculation of the flow data.

(2)	Return flows are taken into account only from 1920 onwards.

(3)	Small users are the Jim Fouché resort, Oranjeville, the Vaal Marina and the Vaal Dam Recreation Complex. They are grouped together as their combined demands are very small.

(4)	Irrigation from urban effluent was not included with the irrigation demands as discussed in Section 2.2.4 - Water use by irrigation of this report.

2.4	URBANISED AREAS

In some catchments, such as the Vaal Barrage catchment downstream of Vaal Dam, a significant contribution to the runoff takes place from paved areas in urban development centres. The timing of the runoff generated within such a catchment, is largely influenced by the percentage of paved areas. Generally, an increase in paved areas will result in an increase in runoff, similar in magnitude to the amount of precipitation.

Urban development in the Vaal Dam catchment is relatively small, and the increased runoff due to paved areas was considered to have a negligible effect on the overall hydrology. It was therefore ignored in the modelling exercise.

2.5	AFFORESTATION

Owing to the small areas of afforestation in the Vaal Dam catchment, the previous BKS report on the hydrology (BKS (Pty) Ltd, 1986), accepted the areas of afforestation given by the Water Research Commission in their report on the surface water resources of South Africa (WRC, 1981). A zero area of afforestation was accepted in 1920.

These results, including areas of afforestation from the new Water Research Commission report (WRC, 1994), are given in Table 2.15. The table provides an indication of historical areas and growth of afforestation upstream of Vaal Dam.

Table 2.15:	Areas of afforestation for incremental subcatchments

Incremental subcatchment	Gauge number	(km2)
(1920)(1)	(1975)(2)	(1983)(1)	(1990)(3)
1	Delangesdrift	C1H002	0.0	0.0	0.0	0.0
2	Frankfort incremental	C8H001	0.0	18.0	20.7	0.0
3	Grootdraai	C1H001 / C1R002	0.0	0.0	2.0	0.0
4	Sterkfontein	C8R003	0.0	0.0	0.0	0.0
5	Vaal incremental	C2H003 / C1R001	0.0	2.0	0.3	0.0
TOTAL	0.0	20.0	23.0	0.0

Note:	(1)	From BKS report (BKS (Pty) Ltd, 1986), Table 2, p 2-3.

(2)	From HRU report (WRC, 1981).

(3)	From WRC report (WRC, 1994), Appendix 8 , p 8.1.

On the recommendation of Dr van der Zel of the Department of Water Affairs and Forestry, the areas of afforestation from the DWAF report "Commercial timber resources and primary roundwood processing in South Africa" (DWAF, 1993/4) were accepted for this study. This report presents the final results of the DWAF survey on South Africa's commercial timber resources. It provides areas of afforestation per magisterial district in South Africa for 1993.

The areas of species of afforestation for subcatchment in the Vaal Dam catchment, based on figures from the "Commercial timber resources and primary roundwood processing in South Africa" are given in Table 2.16.

Table 2.16:	1993 areas of afforestation in the Vaal Dam catchment(1)

Incremental subcatchment	Gauge number
Pine	Eucalypt	Wattle
1	Delangesdrift	C1H002	0.0	0.0	0.0
2	Frankfort incremental	C8H001	0.0	0.9	11.6
3	Grootdraai	C1H001 / C1R002	0.0	0.0	1.8
4	Sterkfontein	C8R003	0.0	0.0	0.0
5	Vaal incremental	C2H003 / C1R001	0.0	0.0	0.0
TOTAL	0.0	0.9	13.4

Note:	(1)	From the report "Commercial timber resources and primary roundwood processing in South Africa", (DWAF, 1993/4) Table a8, p91.

As shown in Table 2.15 and Table 2.16, there is almost no afforestation in the Vaal Dam catchment. The effect on the catchment runoff was considered to be negligible and was therefore ignored in the calibration and naturalisation process.

2.6	TRANSMISSION LOSSES

Regulated river reaches flowing through arid regions, often experience substantial transmission losses, primarily through the process of evaporation from the water surface. Although transmission losses in reaches of the Vaal River between Vaal Barrage and Douglass Weir play an important role, they are less significant in the river reaches upstream of Vaal Dam.

Based on a publication by DWAF on the determining of streamflow losses on the Wilge River trajectory between Sterkfontein Dam and Vaal Dam (DWAF, 1985), it was found that a considerable part of the water released from Sterkfontein Dam is lost from the system before it reaches the downstream weir at Frankfort (gauge C8H001).

It was found that, as the water is released, a significant part is initially lost from the river through infiltration. Infiltration losses in this river reach are particularly severe as releases typically occur during periods of drought when the river bed is dry. After a certain amount of time, however, the system stabilises as the river bed becomes wet. Infiltration losses therefore occur primarily at the beginning of the release, and are therefore referred to as initial losses. The actual magnitude of an initial loss was estimated to be about 3.54 million cubic metres.

After the system has stabilised, the infiltration losses decrease, and evaporation losses become more important. These evaporation losses remain constant for the duration of the releases and are referred to as continuous losses. Their magnitude is dependent mainly on the rate of release. The following information was available:

a)	At a flow rate of 45.76 cumecs, the continuous loss is about 6.9%.

b)	At a flow rate of 60.90 cumecs, the continuous loss is about 5.4%.

Using this information and the daily flow data from DWAF, an estimate could be made of the continuous losses from the system for all releases. As most lie within the range 30 to 70 cumecs, linear interpolation and extrapolation was considered to be sufficiently accurate to calculate the percentage loss at other rates of flow.

Another factor that has to be taken into account when modelling the effect of a Sterkfontein Dam release, is that it obviously does not appear downstream at Vaal Dam instantaneously, but takes a considerable amount of time to reach Vaal Dam. Using the same DWAF report, it was found that the travel time from Sterkfontein Dam to Frankfort Weir for a releases of 45.76 cumecs is approximately five days. The average flow rate for releases was found to be of this order of magnitude and it was decided to accept a lag time of five days throughout.

The streamflow data records of the Sterkfontein releases are given in Appendix E-2. Using these data, information on initial and continuous losses and also the accepted time lag of five days, new monthly totals could be calculated. The monthly totals were based on daily flow calculations, assuming constant release rates for the duration of each month. The result is given in Appendix F-3, and is representative of the effect of the Sterkfontein releases, downstream at Frankfort Weir. The results are also summarised in Table 2.17.

Table 2.17:	Annual total releases at Sterkfontein Dam, before and after considering the effects of transmission losses and time lag of five days(1)

Hydrological year	Annual total release from

Sterkfontein Dam

(106m3)	Release after considering the effects of transmission losses and time lag of five days

(106m3)
1974	0.00	0.00
1975	0.00	0.00
1976	0.00	0.00
1977	0.00	0.00
1978	0.00	0.00
1979	0.02	0.02
1980	0.00	0.00
1981	0.00	0.00
1982	250.94	214.67
1983	40.74	57.13
1984	408.38	359.11
1985	562.11	519.99
1986	301.13	294.81
1987	0.23	0.24
1988	0.00	0.00
1989	0.00	0.00
1990	0.00	0.00
1991	112.19	82.66
1992	1306.47	1229.81
1993	25.46	42.26
1994	1140.29	1036.82

Note:	(1)	Based on a publication by DWAF on the determining of streamflow losses on the Wilge River trajectory between Sterkfontein Dam and Vaal Dam (DWAF, 1985).

2.7	INTERBASIN TRANSFERS

Due to the fact that most of South Africa's urban and industrial development is dependant on the Vaal Dam catchment as its main supplier of water, the demand on this system has for a long time exceeded its yield capability. Water therefore has to be transferred across watersheds and over great distances to supplement the local resources. As part of the naturalisation process, the water transfer must be subtracted from the observed flows.

2.7.1	The Tugela-Vaal / Drakensberg scheme

There are currently three operational interbasin transfers schemes that supply water to the Vaal Dam catchment. The first of these is the Tugela-Vaal Drakensberg scheme. Since October 1974, water has been pumped from a network of storage dams and diversion weirs in the upper reaches of the Tugela basin over the Drakensberg to Driekloof Dam - a small dam adjoining Sterkfontein Dam. Some of this water spills into Sterkfontein Dam during the weekend when demand for power is low, and in this way is transferred to the Vaal Dam catchment. The purpose of the scheme is therefore twofold, namely to transfer water to the Vaal dam basin and to generate power during periods of peak demand. This scheme is known as the Drakensberg Pumped Storage Scheme.

The actual monthly volume of water transferred into the Vaal Dam catchment is difficult to calculate, due to the complexity of the transfer and the number of components involved in the mass balance. No information is available from DWAF on the water balance of Driekloof Dam, thus making it impossible to correctly analyse the water balance of the Sterkfontein subcatchment.

Although the scheme is designed to allow a maximum annual transfer of approximately 700 million m3, it is rarely possible or desirable to transfer the full amount. An estimate of the transfer was made by subtracting the monthly natural runoff from the Sterkfontein catchment, from the total calculated monthly inflows to Sterkfontein Dam. The results are given in Appendix C-1. Note that small negative values resulting from the calculation have been zeroed out.

A summary of the annual totals is given in Table 2.18.

Table 2.18:	Estimated historical annual total transfers from the Tugela Basin to Sterkfontein Dam

Hydrological year	Annual total transfer from the Tugela Basin

(106m3)
1974	34.36
1975	52.31
1976	77.76
1977	102.14
1978	96.87
1979	70.14
1980	64.22
1981	202.56
1982	155.09
1983	303.09
1984	299.16
1985	363.65
1986	587.11
1987	563.71
1988	527.69
1989	385.46
1990	320.20
1991	293.46
1992	499.57
1993	468.44
1994	473.38

To simplify the analysis, it was decided not to try to calculate the actual amount of water transferred over the watershed into Driekloof Dam. Instead, the natural runoff from the Sterkfontein catchment was generated by simulation. Since the catchment area is very small, this approach is considered to be sufficiently accurate.

2.7.2	The Zaaihoek transfer

The transfer from Zaaihoek Dam in the Buffalo system to the Vaal Dam catchment has been in operation since August 1992. The scheme supplies water to the Majuba power station, supplements the water supply to Volksrust and transfers surplus water to the Vaal Dam catchment.

Water is pumped from Zaaihoek Dam, to the watershed where, when required, some of it is released upstream of Mahawane Dam for Volksrust. From here, the remaining water flows via a gravity main to Majuba and the surplus is released into the Perdewaterspruit, a tributary of the Schulpspruit. This water passes through the Amersfoort Dam before it flows into the Rietspruit and then into the Vaal River upstream of Grootdraai Dam.

Measured monthly releases to the Perdewaterspruit were obtained directly from the DWAF Highveld regional office and is given in Appendix C-2. It should be noted that losses in the river reach between the Perdewaterspruit point of release and Grootdraai Dam, were not taken into account in the analysis, as no information on these losses was available. The transfers are however relatively small, and it was found that, even though losses were ignored, this did not lead to any discernible problems during calibration.

Table 2.19 summarises the total annual transfers.

Table 2.19:	Historical annual transfers from Zaaihoek Dam to the Perdewaterspruit River

Hydrological year	Annual transfer from Zaaihoek Dam

(106m3)
1991	8.290
1992	73.369
1993	34.383
1994	52.370

2.7.3	The Heyshope transfer

The Usutu-Vaal transfer scheme supplements water abstracted from the Vaal River at Grootdraai Dam and has been in operation since October 1985. A pumping station at Heyshope Dam pumps water into a canal (DWAF component W5H027-A01) that gravitates to a pumping station at Geelhoutboom. This pumping station is used to transfer the water into a second canal, the Balmoral canal, which then gravitates into the Little Vaal River where it is finally released before flowing into Grootdraai Dam.

Not all of the water in the Balmoral Canal is transferred to the Vaal Dam catchment. Some water is occasionally supplied via the Balmoral emergency pipeline to an industrial user, the Taaiboschspruit Collieries, in the Usutu catchment. Apart from this, it is envisaged that the Balmoral emergency pipeline will also be used in times of drought to augment Morgenstond Dam from Heyshope, should the level in Morgenstond drop to less than 10% of its capacity. Historically, this mode of operation has never been used.

Historical monthly data for the W5H027-A01 canal were obtained directly from the DWAF dam balance for Heyshope while the abstractions via the Balmoral emergency pipeline were obtained from the DWAF regional office at Jericho Dam. Subtracting the two yields the actual historical monthly transfers to the Little Vaal River, and is given in Appendix C-3. It should be noted that losses in the river reach between the Little Vaal River point of release and Grootdraai Dam, were not taken into account in the analysis, as no information on these losses was available. The transfers are however relatively small, and it was found that, even though losses were ignored, this did not lead to any discernible problems during calibration.

Table 2.20 summarises the total annual transfers.

Table 2.20:	Historical annual total transfer from Heyshope Dam to the Kleinvaal River

Hydrological year	Annual total transfer from Heyshope Dam

(106m3)
1985	64.947
1986	51.047
1987	13.258
1988	0.085
1989	3.326
1990	0.028
1991	50.428
1992	65.836
1993	93.295
1994	113.181

2.8	WETLANDS

There are two significant areas covered by wetland upstream of Vaal Dam. The first is situated at the confluence of the Vaal and the Klip rivers, in the Vaal incremental subcatchment, and the second lies north of Memel on the upper reaches of the Klip River within the Delangesdrift subcatchment.

As explained in Section 2.1.2, S-spot images from the Landsat TM satellite, covering the whole of the Vaal Dam catchment and showing surface water bodies, were obtained from the CSIR. These images, taken during March 1996, show very clearly the locality of both the Vaal incremental and Delangesdrift wetlands, and is reproduced in Figure A-2. The corresponding wetland surface areas are given in Table 2.21.

Table 2.21:	Surface areas of wetlands in the Vaal Dam catchment
Incremental subcatchment	Locality of wetland	Surface area(1)

(km2)
Delangesdrift	Klip River	5.0
Vaal incremental	Confluence of the Vaal and Klip rivers	14.4

Note:	(1)	Based on Landsat TM satellite image, March 1996.

It is important to note that, although the surface areas quoted in Table 2.21 are relatively large, these are representative of very extreme conditions (after a period of unusually high rainfall), and should not be seen as typical. Simulating the response of the wetlands under general conditions realistically, was also found te be impossible, as no information on the area-capacity characteristics of the wetlands was available. It was therefore decided not to include wetland components in the modelling of the system.

Instead, the impact of the wetlands, in terms of lag in runoff and evaporation, was incorporated implicitly in the analysis as part of the calibration process. This approach was considered to be realistic, as calibration of both subcatchments was undertaken downstream of the wetland. Also, the impact of wetlands could be considered a natural influence, and can be expected to remain constant over the period of calibration.

3.	HYDROLOGICAL DATA

3.1	RAINFALL

3.1.1	Selection, patching and verification

The Vaal Dam catchment is subdivided into 5 smaller catchments as shown in Figure A-5. They are:

Delangesdrift (upstream of Delangesdrift Weir),

Sterkfontein (upstream of Sterkfontein Dam),

Grootdraai (upstream of Grootdraai Dam),

Frankfort incremental, (upstream of Frankfort Weir and downstream of Sterkfontein Dam) and

Vaal incremental (upstream of Vaal Dam and downstream of Delangesdrift Weir, Frankfort Weir and Grootdraai Dam).

For each subcatchment a set of evenly distributed representative rain gauges was required as input for the WRSM90 runoff model. The combined rainfall data must span the whole record period from 1920 to 1994, although the individual records need only cover part of the full period.

Rainfall gauges were selected from those used by the Water Research Commission in their report on the surface water resources of South Africa (WRC, 1994) and those in the original BKS report (BKS (Pty) Ltd, 1986). The selection was based on length of record, reliability of data, geographical position and the consistency of data. The concistency of the data for a rain gauge is evaluated by using a mass plot - a graph of cumulative rainfall plotted against time.

The data for gauges in the Vaal Dam catchment were generally of high quality and the following criteria for selection were used:

A record length of 40 years or more.

No more than 8% of the total observed record should consist of unreliable data.

A gauge should lie geographically inside of the subcatchment or within 10 km2 of the subcatchment.

The mass plot for the total record, should form a straight line, indicating consistent data. Small deviations caused by wet or dry years are of course acceptable. Examples of mass plots are shown in Appendix D-2.

To ensure that the set of gauges selected for a subcatchment span the total study period, bar charts were created. These bar charts show graphically the span of record for each gauge and can be seen in Figure D-1. It was sometimes necessary to select gauges that did not adhere to the selection criteria and in such cases, any periods of data from the gauge with a high percentage of unreliable data were excluded. In cases where a gauge was initially rejected because of the mass plot showing a change in gradient, the record was sometimes split into two portions with different MAPs and one or both blocks used separately.

Starting with a total of 127 gauges, a final selection for each subcatchment was made. The final gauges are listed in Table 3.1 and the locations of the gauges shown in Figure A-5. The table shows the period of record used, an evaluation of the mass plot, as well as the mean, standard deviation and coefficient of variation for the data in that period.

Table 3.1:	Adopted rainfall gauges for incremental subcatchments in the Vaal Dam catchment

Gauge No.	Gauge Name	Lat.

South	Long.

East	Mass

plot	Period of used record

(Hydrological years)	Years	Mean

(mm)	Standard

deviation

(mm)	CV
Delangesdrift subcatchment (C1H002)
370 486	Loskop	27 36	29 47	good	1941-1994	54	756.1	159.2	0.21
405 295a	Vrede	27 25	29 10	good	1920-1984	65	692.5	133.1	0.19
406 221	Paardekopplaas	27 11	29 38	good	1920-1970	51	725.0	147.4	0.20
406 607	Rietpoort	27 07	29 51	good	1920-1994	75	735.8	149.8	0.20
Frankfort incremental subcatchment (C8H001)
298 512	Witsieshoek	28 32	28 48	good	1920-1994	75	785.0	203.5	0.26
331 271	Vaalbank	28 01	28 10	good	1920-1994	75	638.2	164.4	0.26
331 740	Spitskrans	28 20	28 25	good	1920-1981	62	717.4	150.7	0.21
331 828	Nil Desperandum	28 18	28 28	good	1940-1994	55	685.2	161.7	0.24
333 051	Rosedale	28 21	29 02	good	1920-1960	41	648.7	187.0	0.29
369 238(1)

Buckland Downs	27 58	29 08	good

good	1920-1943

1952-1994	24

43	721.3

670.8	143.5

136.3	0.20

0.20
403 054	Schoongezicht	27 24	28 02	good	1925-1994	70	675.4	153.9	0.23
405 295a	Vrede	27 25	29 10	good	1920-1984	65	692.5	133.1	0.19
Grootdraai subcatchment (C1H001 / C1R002)
406 221	Paardekopplaas	27 11	29 38	good	1970-1994	25	595.6	115.1	0.19
406 607	Rietpoort	27 07	29 51	good	1920-1994	75	735.8	149.8	0.20
407 639	Groot Rietvlei	27 09	30 09	good	1929-1994	66	767.7	180.4	0.24
441 777	Berrings	26 57	29 26	good	1920-1978	59	700.8	129.7	0.19
442 068	Hendrikspan	26 38	29 33	good	1920-1976	57	687.4	125.3	0.18
442 660	Amersfoort	27 00	29 52	good	1920-1994	75	679.8	141.0	0.21
443 196	De Emigratie	26 46	30 07	good	1920-1994	75	702.2	113.7	0.16
443 451	Blaauwkop	26 31	30 16	good	1928-1994	67	793.1	155.7	0.20
479 348	Tevreden	26 18	29 42	good	1920-1959	40	681.0	110.5	0.16
Sterkfontein subcatchment (C8R003)
298 871	Boschkloof	28 31	28 60	good	1925-1988	64	756.8	175.0	0.23
333 051	Rosedale	28 21	29 02	good	1920-1960	41	648.7	187.0	0.29
333 120	New Leeds	28 30	29 04	good	1920-1928	9	686.0	173.7	0.25
333 226	Harrismith	28 16	29 08	good	1956-1994	49	622.6	169.3	0.27
Vaal incremental subcatchment (C2H003 / C1R001)
403 054	Schoongezicht	27 24	28 02	good	1925-1994	70	675.4	153.9	0.23
405 295a	Vrede	27 25	29 10	good	1920-1984	65	692.5	133.1	0.19
406 221	Paardekopplaas	27 11	29 38	good	1920-1970	51	725.0	147.4	0.20
439 389	Oranjeville	26 59	28 13	good	1922-1994	73	581.2	119.8	0.21
440 435	Greylngstad	26 45	28 45	good	1920-1994	75	653.0	138.4	0.21
441 777	Berrings	26 57	29 26	good	1920-1978	59	700.8	129.7	0.19
477 772	Leslie	26 22	28 56	good	1920-1994	75	692.8	128.0	0.18

Note:	(1)	This record was split. The period of 1944-1951 contains a high percentage of unreliable or missing data and was excluded from the analysis.

In the rainfall data sets, months with no data, as well as months with unreliable data often occur, and have to be infilled. This process, called patching, was performed with the use of two computer software programs called CLASSR and PATCHR, developed by BKS (Pty) Ltd for the Department of Water Affairs and Forestry as part of the Lesotho Highlands hydrological study.

For the purpose of this study, it was decided to accept any patching already carried out by the Water Research Commission as publicised in their report on the surface water resources of South Africa (WRC, 1994). As a result, new patching was only required for the 1990 - 1994 period. Naturally, gauges that were not used by the WRC had to be patched over their full record.

The CLASSR program has as its main function the classing of rainfall gauges into groups with hydrologically similar data. From the output of CLASSR, the user is able to make a selection of well-correlated gauges to be used in the patching process. Problems were often encountered during the process of classing with gauges that did not seem to fit into any group. These gauges were, where possible, excluded from the data set and substituted by others.

The PATCHR program patches all the flagged monthly values of the gauges in a group simultaneously. It uses the method of multiple linear regression in combination with the EM (Expectation Maximization) algorithm. This is described in a report by BKS for the Department of Water Affairs and Forestry (BKS (Pty) Ltd, 1994).

Both CLASSR and PATCHR also assist in the process of identifying outlier values - values that are not flagged as unreliable by the Weather Bureau, but obviously do not fit. These values have to be flagged and patched. Although CLASSR gives a good indication of possible outliers, it was found that PATCHR is easier to use and more reliable. The reason for this is that the user can decide which values to flag as outliers depending on the relative weight PATCHR assigns to their "differentness". It is therefore somewhat less subjective than decisions based on the CLASSR output. In cases where it was difficult to decide whether a value is an outlier or not, the original recorded value was accepted.

After all outliers are flagged, PATCHR is run for a second time. Now all the unreliable data, the missing values, as well as the outliers are patched in a single run. Among the rain gauges used, for the period patched by BKS, no value could positively be identified as an outlier

3.1.2	Catchment rainfall inputs

After the data of the initially selected rainfall gauges in a subcatchment are patched, the set is used to create one representative rainfall record for the subcatchment, called a secondary rainfall file. This file consists of monthly values expressed as a percentage of the mean annual precipitation (MAP) and are generated by the HDY P08 program. The secondary rainfall file for each subcatchment is given in Appendix D-3.

It should be noted that the percentage of rain for December 1973 in the Delangesdrift record was lowered from the original 44.37% to a value of 34.51%. The reason for this is that the original value was considered to be unrealistic due to abnormally high rainfall at gauge 405 295 for that month. This gauge registered rainfall of 424.6 mm compared to the much lower average of 245.7 mm based on seven other gauges in the vicinity. The value in the secondary rainfall file is derived from this lower average.

The Water Research Commission in their report on the surface water resources of South Africa (WRC, 1994) subdivide the main drainage regions into quaternary subcatchments and also give the MAP of each. Based on this information, the incremental MAP for each subcatchment was calculated by taking the weighted average MAP by area of quaternary catchments lying inside each subcatchment. The results can be seen in Table 3.2. The table shows the name of the subcatchment, its area and the calculated MAP.

Table 3.2 :	Mean annual precipitation for incremental subcatchments

Incremental subcatchment	Gauge number	Area(1)

(km2)	MAP(2)

(mm)
1	Delangesdrift subcatchment	C1H002	4 158	712
2	Frankfort incremental subcatchment	C8H001	15 498	679
3	Grootdraai subcatchment	C1H001 / C1R002	7 995	690
4	Sterkfontein subcatchment	C8R003	195	735
5	Vaal incremental subcatchment	C2H003 / C1R001	10 792	657

Note:	(1)	From WRC report (WRC, 1994). The subcatchment area was calculated by taking the total area of quaternary catchments lying inside each subcatchment.

(2)	From WRC report (WRC, 1994). The subcatchment MAP was calculated by taking the weighted average MAP of quaternary catchments lying inside each subcatchment.

To test the continuity and consistency of the data in the secondary rainfall files, a monthly subcatchment rainfall record was generated using the secondary record and the calculated MAP. Mass plots were made and the results can be seen in Appendix D-2. Clearly, all the mass plots form almost perfectly straight lines, indicating consistent and reliable data sets. A further test was performed by plotting the cumulative deviation of rainfall from the mean against time and these plots are also included in Appendix D-2.

The secondary rainfall records, together with the subcatchment MAPs are used as input for the WRSM90 program which is used to generate natural runoff from a subcatchment. The secondary rainfall records are also used together with the appropriate dam MAPs to generate point rainfall files for the various major dams or proposed dams in the catchment. This is undertaken using a simple computer program that creates a file containing monthly rainfall in millimetres for the total record period considered in this study. This rainfall file is referred to as the .RAN file. These files are not included in the hydrology report but will be included in the system analysis reports.

3.2	EVAPORATION

The average annual Symons pan evaporation (MAE) in the Vaal Dam catchment ranges from over 1 600 mm in the Vaal Dam basin to less than 1 300 mm in the vicinity of Sterkfontein Dam. Potential evaporation changes significantly from one month to the next, but is relatively constant from year to year. In order to simulate streamflow for a subcatchment, these annual fluctuations are considered small enough to ignore and historical monthly evaporation records are therefore not required.

In their report on the surface water resources of South Africa (WRC, 1994), the Water Research Commission subdivide the main drainage regions into quaternary subcatchments and also provide the MAE of each. Based on this information, the incremental MAE for each subcatchment was calculated by taking the weighted average MAE by area of quaternary catchments lying inside each subcatchment. The report also gives monthly evaporation as percentage of the MAE for various evaporation zones. Using this information, the results in Table 3.3 were obtained. The table shows the name of the subcatchment, the calculated MAE as well as the average monthly Symons pan evaporation.

Table 3.3:	Average monthly and annual Symons pan evaporation data

Incremental subcatchment	Average Symons pan evaporation in month (mm)
Oct	Nov	Dec	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Total(1)
Delangesdrift	146	144	155	153	132	126	97	82	68	74	102	129	1407
Frankfort incr.	153	153	165	160	132	123	93	76	61	66	96	129	1407
Grootdraai	149	147	159	157	135	129	99	84	70	76	105	132	1444
Sterkfontein	142	143	153	149	123	115	87	70	57	62	89	120	1310
Vaal incremental	166	167	179	174	144	135	102	83	68	73	106	141	1540

Note:	(1)	From WRC report (WRC, 1994). The subcatchment MAE was calculated by taking the weighted average MAE of quaternary catchments lying inside each subcatchment.

It should be noted, however, that the figures in Table 3.3, have to be converted within the WRSM90 runoff model to represent potential catchment evapotranspiration. Conversion factors for this purpose were also obtained from the Water Research Commission report (WRC, 1994) and are given in Table 3.4.

Table 3.4:	Factors converting Symons pan data to catchment evapotranspiration(1)

Oct	Nov	Dec	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep
0.80	1.00	1.00	1.00	1.00	1.00	1.00	1.00	1.00	0.80	0.80	0.80

Note:	(1)	From WRC report (WRC, 1994).

To enable the WRSM90 runoff model to simulate evaporation from dummy dam surfaces, Symons pan to lake evaporation factors are needed. These factors were obtained directly from DWAF and are given in Table 3.5. They are based on a study by RA Chapman and have been in use by DWAF since 1 October 1991.

Table 3.5:	Pan factors converting Symons pan evaporation to lake evaporation(1)

Oct	Nov	Dec	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep
0.81	0.82	0.83	0.84	0.88	0.88	0.88	0.87	0.85	0.83	0.81	0.81

Note:	(1)	Obtained by personal communication from DWAF.

It is important to note that historically, DWAF has used three sets of factors to convert pan evaporation to lake evaporation. Because of the fact that these factors were used to calculate reservoir evaporation, it has the implication that calculated inflows to reservoirs given in DWAF dam balances are also affected. Recalculating the original inflows based on old conversion factors, using the new factors given in Table 3.5, showed little difference. It was therefore decided to adopt the inflows given in the DWAF dam balances, irrespective of the conversion factors used.

Finally, monthly crop irrigation requirements are calculated by the WRSM90 runoff model using class A-pan evaporation data. The conversion of average monthly Symons pan evaporation to A-pan evaporation for a catchment is achieved using a simple formula given in the Water Research Commission report (WRC, 1994) as:

(A-pan)=26.3622+1.0786×(S-pan), where:

A-pan is monthly class A-pan evaporation in mm and

S-pan is monthly Symons pan evaporation in mm.

The results are given in Table 3.6. As a check, the figures were compared with the results of the evaporation studies undertaken at the University of Natal, under the direction of Prof Roland Schultze, and it was found to compare favourably. The mean annual A-pan evaporation per subcatchment in Table 3.6 differs with an average of only about 3% with Schultze's A-pan equivalent MAE values. On this basis, it was decided to retain the SWR90 values.

Table 3.6:	Average monthly and annual class A-pan evaporation values

Incremental subcatchment	Average class A-pan evaporation in month (mm)(1)
Oct	Nov	Dec	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Total
Delangesdrift	183	181	193	191	169	162	131	115	100	107	137	165	1834
Frankfort incr.	191	191	204	199	169	159	127	108	93	98	130	166	1834
Grootdraai	188	185	198	196	172	166	134	117	101	109	139	169	1874
Sterkfontein	180	180	192	187	159	150	120	102	88	93	123	156	1729
Vaal incremental	206	206	220	214	182	172	137	116	99	105	140	179	1977

Note:	(1)	From WRC report (WRC, 1994).

3.3	STREAMFLOW

3.3.1	Selection of gauging stations

The most important streamflow gauges in the Vaal Dam catchment are listed in Table 3.7 and the position of these gauges are shown in Figure A-7. Unfortunately, although there are many gauges inside the catchment, most are of little use for calibration purposes. The reasons are a combination of short periods of record, high percentages of unreliable data and, also, unfavourable geographical position.

Table 3.7:	Basic information on streamflow gauges

Gauge number	Gauge name	Total catchment area (km2)(1)	Period of available data (hydro years)	% of un- reliable months	Comment(2)
C1H001	Standerton	8 193	1904-1988	16.39	Good. From 1978 reflects Grootdraai releases.
C1H002	Delangesdrift	4 152	1906-1994	24.02	Many unreliable values due to submergence during high flows. Use in combination with C1H015 for high flows.
C1H004	Roodebank	901	1959-1994	1.19	Data good, but rejected due to small catchment area.
C1H006	Rietvley	1 094	1964-1994	16.29	Data good, but rejected due to small catchment area.
C1H012	Gladdedrift	1 550	1985-1994	1.68	Rejected because of short record.
C1H015	Delangesdrift (flood section)	4 152	1906-1994	91.57	Rejected because almost no data is available. Used to check high flows of gauge C1H002.
C2H003	Engelbrechts-drift	38 564	1923-1992	11.59	Good. From 1936 reflects Vaal Dam releases
C8H001	Frankfort	15 673	1913-1994	28.56	Many unreliable values due to submergence during high flows. Use in combination with C8H022 for high flows.
C8H003	Warden	806	1953-1994	14.50	Data is reasonable, but rejected due to small catchment area.
C8H004	De Welkom	3 527	1965-1994	13.20	Data good, but rejected due unfavourable geographical position.
C8H011	Killarney	1 486	1971-1994	9.87	Data good, but rejected due to short record and unfavourable geographical position.
C8H014	Bavaria	7 497	1972-1994	13.22	Data good, but rejected due to short record and unfavourable geographical position.
C8H020	Roodekraal	3 578	1974-1994	8.58	Data good, but rejected due to short record and unfavourable geographical position.
C8H022	Kimberley	15 466	1961-1994	9.63	Data reasonable, but rejected because low flows inaccurate. Used to check high flows of gauge C8H001.
C8H026	Frederiksdal	4 650	1984-1994	22.96	Data good, but rejected due to short record.
C8H027	Ballingtomp	10 489	1984-1994	2.66	Data good, but rejected due to short record.
C1R001	Vaal Dam	38 505	1936-1994	0.00	Good.
C1R002	Grootdraai Dam	7 924	1978-1994	0.98	Good.
C8R003	Sterkfontein Dam	58	1974-1994	0.00	Good.

Note:	(1)	From the List of hydrological gauging stations, volume 1, (DWAF 1990a).

(2)	Using Report on quality of river flow data (DWAF, 1996).

Based on the information in Table 3.7, a final set of gauges was selected. These gauges are given in Table 3.8, their geographical positions are shown in Figure A-7 and bar charts showing the period of available record in Figure E-1.

Table 3.8:	Selected streamflow gauges

Gauge number	Gauge name	Total catchment area (km2)(1)	Period of used record (hydrological years)	% of unreliable months
C1H001	Standerton weir	7 995	1920-1977	15.8
C1H002	Delangesdrift weir	4 158	1920-1994	22.7
C2H003	Engelbrechtsdrift weir	38 638	1923-1935	26.9
C8H001	Frankfort weir	15 693	1920-1994	32.3
C1R001	Vaal Dam	38 638	1936-1994	0.0
C1R002	Grootdraai Dam	7 995	1978-1994	1.0
C8R003	Sterkfontein Dam	195	1974-1994	0.0

Note:	(1)	Estimated from the WRC report (WRC, 1994).

The following should be noted:

a)	The flow at C1H001 (Standerton weir) has been affected by the construction of a municipal dam upstream of the weir since 1958. According to the DWAF report on the quality of river flow data in drainage regions C and W, (DWAF, 1996), however, the measured flows during this period can still be considered as reasonable. Since 1978 onwards, flow at this weir represents only releases and spills from Grootdraai Dam. For the Grootdraai catchment runoff, the calculated inflows to Grootdraai Dam (gauge C1R002) will be used for this later period. A complete record can thus be obtained by combining the two sets of data.

A downstream component for Grootdraai Dam, component C1H019, was completed in 1979. Gauge C1H001 (Standerton) was kept in use to check the flows measured by C1H019. In 1983, however, due to a severe water shortage, storage weirs were constructed across the Vaal River to pump water from Vaal Dam up to Grootdraai Dam. The upstream pools of one of these weirs caused the submergence of C1H001 (Standerton) and it was finally closed on 21 February 1989

b)	As with C1H001 (Standerton), the flow at gauge C2H003 (Engelbrechtsdrift Weir) reflects the releases and spills from Vaal Dam since 1936. A complete record was obtained by combining C2H003 (Engelbrechtsdrift) with the calculated inflows to Vaal Dam, gauge C1R001.

c)	Although the gauges C1H002 (Delangesdrift Weir) and C8H001 (Frankfort Weir) both have a high percentage of unreliable data, these gauges were selected as they were considered to be key gauges upstream of Vaal Dam. Both were used in the original BKS report on the hydrology of the Vaal Dam catchment (BKS (Pty) Ltd, 1986) and the WRC report (WRC, 1994).

Unreliable data occurs at both, mainly during high flows. Use was made, however, of two upstream gauges, C1H015 and C8H022, to check the high flows of C1H002 (Delangesdrift) and C8H001 (Frankfort) respectively. In this way, the number of unreliable values were effectively reduced. It should also be noted that the percentages of unreliable data given in Table 3.8, represent the number of months containing missing or unreliable days. Calculating the percentage of unreliable or missing data based on days, yields much lower figures, ie 11.6% for C1H002 (Delangesdrift) and 3.7% for C8H001 (Frankfort). Their catchments, amongst them, include almost the whole of the Klip, the Wilge and the Liebenbergsvlei Rivers with their tributaries. Additionally, available data for both these gauges span the period of 1920 to 1994, with the result that the extension of data by simulation would not be necessary since historical data can be used for the whole study period.

According to the DWAF report on the quality of river flow data (DWAF, 1996), the data for gauges C1H002 (Delangesdrift) and C8H001 (Frankfort) are considered to be reasonable.

Listings of the raw streamflow records for the four gauges mentioned above, are given in Appendix E-2. It should be noted that the flags in the listings have the following meaning:

a)	The "*" symbol indicates that the value for a month is unreliable because one or more days in the month are missing.

b)	The "+" symbol indicates that the value for a month is unreliable (or too small) because the gauge capacity for one or more days in the month has been exceeded.

c)	The "#" symbol indicates that the value for a month is unknown.

3.3.2	Verification of streamflow records

Streamflow data for the four selected gauges were received from DWAF in daily and monthly format. The data were screened to detect and eliminate any obvious problems or errors. The first step was to check the monthly data against similar data from other sources.

Comparing the new DWAF data to patched records for the same gauges from the WRC report (WRC, 1994), indicated that the records were virtually identical. Some minor differences were found, but it was decided to ignore these differences as they were never higher than approximately 5%. The months that were flagged in the DWAF data as unreliable or missing, were also flagged as being patched in the SWR90 data.

Patched data from the previous BKS hydrology report (BKS (Pty) Ltd, 1986) however, were found to differ in some instances with the new DWAF data. Although the differences in most cases were below 5 %, another problem was detected. Many months that were previously unflagged, and therefore accepted to be reliably measured, were flagged as unreliable in the new data. For gauges upstream of Vaal Dam, ie C1H001 (Standerton), C1H002 (Delangesdrift) and C8H001 (Frankfort), the number of flagged values more than doubled.

Personal communication with DWAF indicated that this could be the result of the fact that flow data is constantly being upgraded or revised, during which it is sometimes necessary to flag months that were previously thought to be reliable.

As a final verification check, a detailed analysis was made of the mass balance of the three upstream gauges, and the inflows to Vaal Dam. Obviously, to satisfy such a mass balance, the total monthly runoff from the upstream gauges can not be larger than the inflows to Vaal Dam for that month, otherwise negative incremental flows are implied upstream of Vaal Dam.

This mass balance on the inflows to Vaal Dam was executed as part of the patching process and will therefore be discussed in Section 3.3.3.

3.3.3	Patching of streamflow records

The patching of streamflow records involves the infilling of missing or unreliable data, with values that are both realistic and viable. This has to be done before the flow data can be naturalised.

The process of patching is complex and time consuming. The patching of a single gauge can be done iteratively by deterministic modelling, using a runoff model such as the WRSM90. The improvement of a simulation is achieved by adjusting certain process parameters that represent the physical system. This procedure is known as "calibration" and is described in Section 4.

The gauges in the Vaal Dam catchment can however not be infilled in this way. Simulated runoff can only serve as an indication of the magnitude of the patched value. The reason for this is that the runoff at upstream gauges are constrained by the inflow to Vaal Dam. As mentioned earlier, the sum of the runoff at these upstream gauges can never be higher than the inflow to Vaal Dam, as this would imply a negative flow for the incremental catchment upstream of Vaal Dam. It is therefore necessary to patch these gauges simultaneously.

The gauges C1H001 (Standerton), C1H002 (Delangesdrift) and C8H001 (Frankfort) were therefore patched simultaneously using a spreadsheet. The spreadsheet was designed to execute a simple mass balance for every single month of available historical data. The mass balance consisted of subtracting the sum of the runoff at the upstream gauges from the inflow to Vaal Dam, alerting the user of any negative result. A negative on the mass balance indicates a negative flow for the Vaal incremental catchment, indicating that the patching of one or more of the upstream gauges is too high.

Previous patchings by BKS (BKS (Pty) Ltd, 1986) or the WRC (WRC, 1994) were accepted for all flagged months where possible, using the mass balance as a guideline, together with the simulated values, daily data and recorded values at nearby gauges. In cases where none of these values suggested a realistic patching, the month was infilled using the mass balance.

Tables showing details on patched flow data for gauges C1H001 (Standerton), C2H001 (Delangesdrift) and C8H001 (Frankfort) are given in Appendix F-1. The tables include the original values from DWAF, the previous BKS patchings, the SWR90 patchings, the WRSM90 simulated values and the final adopted values. The table also provides comments explaining the origin of all patchings.

It is important to note that the tables in Appendix F-1 also indicate some months that were intentionally lowered. As explained earlier, this had to be done in some cases to prevent negative incremental flows.

a)	Details for gauge C1H001 (Standerton)

The data for gauge C1H001 (Standerton) generally seemed to be of good quality. There are no periods of missing data, and the record required only minor patching. It should be noted that patching was only undertaken up to September 1978, since the flows passing through C1H001 (Standerton) represent only the releases and spills from Grootdraai Dam after this date.

Combining the patched record from C1H001 (Standerton) for the period 1920 to 1978 with that of C1R001 (inflows to Grootdraai Dam) for the period 1979 to 1994, a complete record was obtained. The combined record yields a recorded catchment MAR of 453.70 million cubic metres for the 1920 to 1994 period.

b)	Details for gauge C1H002 (Delangesdrift)

The data at gauge C1H002 (Delangesdrift) seems to be reliable, although it required extensive patching. The large number of unreliable months were due to the submergence of the weir at medium and high flows. DWAF recommend in their report on the quality of river flow data in regions C and W, that a flood section C1H015, just upstream of C1H002 (Delangesdrift), should be used to assist in the patching of C1H002 (Delangesdrift), since the high flows measured at C1H015 are considered to be more accurate.

The record for gauge C1H002 (Delangesdrift) contains no prolonged periods of missing data that had to be infilled. It also spans the total period of 1920 to 1994, making any extension of data unnecessary. The recorded catchment MAR for the patched record of gauge C1H002 (Delangesdrift) is 247.10 million cubic metres for the 1920 to 1994 period.

c)	Details for gauge C2H003 (Engelbrechtsdrift)

The patching of gauge C2H003 (Engelbrechsdrift) was not undertaken by BKS, since the gauge is included in the Middle Vaal catchment and had therefore already been patched by Stewart Scott Incorporated who were responsible for the Middle Vaal hydrology.

No data are available for C2H003 (Engelbrechsdrift) prior to October 1923, leaving three years that have to be extended using simulated flows. With exception of the first three years, the data seem to be reliable and there were very few values that required patching. It is important to note that, as with gauge C1H001 (Standerton), patchings were only undertaken on data prior to October 1936, since the flows passing through C2H003 (Engelbrechsdrift) after this date, represent the releases and spills from Vaal Dam.

Combining the patched and extended record from C2H003 (Engelbrechsdrift) for the period 1920 to 1935 with that of C1R001 (inflows to Vaal Dam) for the period 1936 to 1994, yields a recorded catchment MAR of 1 917.91 million cubic metres for the 1920 to 1994 period.

d)	Details for gauge C8H001 (Frankfort)

As with gauge C1H002 (Delangesdrift), the data at gauge C8H001 (Frankfort) appears to be reliable, although it required extensive patching. The large number of unreliable data is caused by the submergence of the weir at medium and high flows. DWAF recommend in their report on the quality of river flow data in regions C and W (DWAF, 1996), that the upstream gauge, C8H022, be used to assist in the patching of C8H001 (Frankfort) as high flows are measured at this gauging station with greater accuracy.

The record contains no periods of missing data that had to be infilled. It also spans the total period of 1920 to 1994, making any extension of data unnecessary. The recorded catchment MAR for the patched record of gauge C8H001 (Frankfort) is 760.38 million cubic metres for the period of 1920 to 1994.

e)	Details for gauge C1R001 (Vaal Dam)

The streamflow record for gauge C1R001 (Vaal Dam) was obtained from the calculated inflows on the dam balance for Vaal Dam. It spans the total period from 1936, when Vaal Dam was constructed, to 1994 water year. No patching was required.

A total streamflow record was obtained by combining the patched record from gauge C2H003 (Engelbrechsdrift) for the period 1923 to 1935 with that of C1R001 (Vaal Dam) for the period 1936 to 1994. This record yields a recorded catchment MAR of 1 858.25 million cubic metres for the period of 1923 to 1994.

As the existing record did not contain any data for the period 1920 to 1922, the record had to extended to also cover these missing years. This was undertaken using simulated data from the WRSM90 runoff model. By adding simulated flows to the recorded data, a complete record was obtained, which indicates a catchment MAR of 1 917.91 million cubic metres for the 1920 to 1994 period.

f)	Details for gauge C1R002 (Grootdraai Dam)

The streamflow record for gauge C1R002 (Grootdraai Dam) was obtained from the calculated inflows on the dam balance for Grootdraai Dam. It spans the total period from 1978, when Grootdraai Dam was constructed, to the present. Only two months are missing as shown in Table 3.9. The table gives the original DWAF value, the gauge reading at downstream C1H001 (Standerton) and the final adopted values. Comments are also given on the origin of the patched values.

Table 3.9:	Patched flow data for gauge C1R002 (Grootdraai Dam)

Calender	Original DWAF value (106m3)	C1H001 gauge reading (106m3)	Adopted value

(106m3)	Comment
Year	Month
1978	October	0.00*	0.00*	13.97	Adopt SWR90 patching.
1978	November	0.00*	15.33	15.71	Based on C1H001 reading and dam balance.

As mentioned previously, storage weirs were constructed across the Vaal River in 1983, to pump water from Vaal Dam up to Grootdraai Dam, due to a severe water shortage. These artificial additions are included in the calculated inflows to Grootdraai Dam. In order to obtain a true inflow record, these transfers had to be subtracted from the inflows in the dam balance. The actual corrections are shown in Table 3.10.

Table 3.10:	Corrections made to Grootdraai Dam inflows, taking into account artificial additions from Vaal Dam

Calender	Inflows to Grootdraai Dam	(106m3)
Year	Month	Inflows as given in dam balance	Artificial additions from Vaal Dam	Inflows excluding addition from Vaal Dam
1983	September	15.02	13.60	1.42
1983	October	21.49	19.30	2.19
1983	November	170.21	15.70	154.51

Apart from the inflows to Grootdraai dam, releases and spills are also needed in order to naturalise the flow into Vaal Dam. These releases are given in the dam balance as component C1H019-A01, and span the period from 1978, when Grootdraai Dam was constructed, to date. The record contains a few unreliable values that were patched using information given in the Grootdraai Dam dam balance. Details of the patchings are given in Table 3.11.

Table 3.11:	Patched flow data for gauge C1H019 (Releases and spills from Grootdraai Dam)

Calender	Original DWAF value

(106m3)	Adopted value as given in Grootdraai Dam dam balance

(106m3)
Year	Month
1978	December	0.00*	0.38
1979	January	0.00*	2.07
1979	February	0.00*	1.38
1979	March	0.00*	1.05
1979	April	0.00*	0.53
1979	May	0.00*	0.34
1979	June	0.00*	0.23
1979	July	0.00*	0.32
1979	August	0.00*	0.56
1979	September	0.00*	0.17
1979	October	0.00*	1.05
1983	July	0.00*	0.20
1983	August	0.00*	0.00
1983	September	0.00*	0.00
1983	October	0.00*	0.00
1983	November	0.00*	0.00
1983	December	0.00*	0.00
1984	January	0.00*	0.00
1984	February	0.00*	105.00
1984	March	0.00*	0.04

As with the Grootdraai Dam inflows given in Table 3.9, the values of spills and releases for the months October and November of 1978, are not given in the dam balance. For this reason, these values had to be patched in a similar manner to those in Table 3.9. Table 3.12 gives the original DWAF value, the gauge reading at downstream C1H001 (Standerton) and the final adopted values. Comments are also given on the origin of the patched values.

Table 3.12:	Patched flow data for gauge C1R002 (Grootdraai Dam)

Calender	Original DWAF value (106m3)	C1H001 gauge reading (106m3)	Adopted value

(106m3)	Comment
Year	Month
1978	October	0.00*	0.00*	13.97	Adopt SWR90 patching.
1978	November	0.00*	15.33	15.33	Based on C1H001 reading.

g)	Details for gauge C8R003 (Sterkfontein Dam)

It was decided not to use the gauging station C8R003 (inflows to Sterkfontein Dam) in the hydrological analysis, since the surface area of Sterkfontein is relatively large compared to its catchment area (it comprises almost 35% at full supply level). As a result, the inflows to the dam are not representative of the runoff from the catchment and cannot be used for calibration. In addition, there was no streamflow gauge at the Dam site prior to the construction of the dam and therefore no data are available prior to 1974.

In order to naturalise the downstream gauge of C8H001 (Frankfort), however, the releases from Sterkfontein Dam are needed which are given in the dam balance as component C8H032-A01. The record covers the period from 1974, when Sterkfontein Dam was constructed, to date, and is complete.

It is important to note that the releases from Sterkfontein Dam as provided by C8H032, cannot be subtracted directly from C8H001 (Frankfort) during the naturalising process. This is due to the losses which occur from the system through the processes of evaporation and infiltration before the water reaches Frankfort weir. Infiltration losses from the releases are particularly severe, since the releases are made during periods of drought when the river bed is dry. Additionally, the water released from Sterkfontein takes about five days to reach Frankfort, some 273.3 km downstream.

In order to estimate the influence of Sterkfontein releases on flows recorded at Frankfort weir, the river losses and lag time had to be taken into account. This process is described in more detail in Section 2.6.

As Sterkfontein Dam was only built in 1974, the existing record had to be extended to cover the whole period down to 1920. Since no gauge existed prior to the construction of Sterkfontein Dam, the record had to be extended using simulated natural streamflows. Since it was not possible to calibrate flows in this catchment, it was decided to use suggested regional parameters given by the Water Research Commission in their report on the surface water resources of South Africa (WRC, 1994). Estimating the flows in this manner, was considered to be sufficiently accurate, in view of the very small catchment area involved. The natural runoff generated from the area is very small compared to the streamflows recorded at Frankfort.

Listings of the final set of patched data are given in Appendix F. The flags have the following meanings:

a)	The "*" symbol indicates that the original value for a month was unreliable because one or more days in the month were missing.

b)	The "+" symbol indicates that the original value for a month was unreliable (or too small) because the gauge capacity for one or more days in the month had been exceeded.

c)	The "#" symbol indicates that the original value for a month was unknown.

d)	The "&" symbol indicates that the values were intentionally lowered to prevent negative incremental flows in downstream catchments.

e)	The "$" symbol was used in the record of gauge C1R002 (Grootdraai Dam) to indicate that the value for a month has been altered to a true inflow by excluding artificial additions from Vaal Dam.

f)	The "@" symbol indicates data that has been extended using simulated data.

Closer inspection will reveal that some of the unflagged values in the unpatched listings, are in fact flagged with an "&"-sign in the patched listings. These months, even though not flagged as unreliable, were lowered to prevent negative flows in the Vaal incremental subcatchment. A list of these values is given in Appendix F-2 for each gauge, together with their original and adjusted values.

4.	MODEL CALIBRATION

4.1	INTRODUCTION (APPROACH TO CALIBRATION)

Simulated monthly streamflow data are usually used to extend or infill available data so as to obtain a complete record covering a required analysis period. In order to simulate data for streamflow gauges in the Vaal Dam catchment, use was made of the WRSM90 runoff model. This model is an upgraded version of the Pitman (or HDYP09) model that was used in the previous study on the hydrology of the Vaal Dam catchment.

In order to simulate runoff that is comparable with the observed streamflow at given gauges in the catchment, various sets of data files have to be prepared as input for the WRSM90 runoff model. These include files representative of historical catchment rainfall and catchment evaporation, files containing historical data of the various abstractions and return flows within the catchment, files containing data of transfers into the system, files containing a description of the impoundments within the catchment and data on catchment developments such as irrigation and afforestation. All these files are created using the data discussed in the previous sections on catchment developments and hydrological data. Finally, the model requires a physical description of the system, consisting of a network file and a set of so-called Pitman catchment parameters. This set of parameters governs the way in which runoff is generated within a specific catchment.

Due to the fact that the catchment parameters for a specific catchment are initially unknown, they have to be estimated by means of a process of calibration. This process involves changing various process parameters within certain limits, until the characteristics of simulated flows compare favourably with those of the actual observed flows. As a first estimate, values were based on the recommended regional catchment parameters given by the Water Research Commission in their report on the surface water resources of South Africa (WRC, 1994).

Of the total of eleven parameters needed for simulation, mainly three parameters were altered to improve the fit of the synthetic flows. They are the following:

a)	ST =	Soil moisture capacity in mm. If this value is increased, surface runoff is reduced with a consequent reduction in mean annual runoff.

b)	FT =	Subsurface flow at full soil moisture capacity in mm/month. If FT is increased, dry weather flows increase.

c)	TL =	Lag of flow (excluding groundwater) in months. This parameter is mainly used to control the distribution of high and low flows. It effectively has no influence on the total runoff calculated over a long period of time.

It is important to note that calibrations for gauges in the Vaal Dam catchment were undertaken on the actual observed flows, and not on the naturalised flows. The effects of historical catchment developments and imports were therefore added to or subtracted from the simulated natural runoffs before they were compared with the recorded runoffs.

A total of four gauges were calibrated and results are discussed individually in the following sections.

4.2	CALIBRATION RESULTS

4.2.1	Gauge C1H001 / C1R002 (Standerton weir / Grootdraai Dam)

The combined record of gauges C1H001 (Standerton) and C1R002 (Grootdraai Dam) was calibrated on the total catchment upstream of Grootdraai Dam with a catchment area of 7 995 km2. The calibration is based on the 1920 - 1994 period. The results are given in Table 4.1 and Table 4.2 and they show that this catchment could be calibrated to a high degree of accuracy. The statistics of the observed data compare very well with those of the simulated data. For this reason it was decided to accept the parameters as shown in Table 4.1.

Observed and simulated data are compared graphically in Appendix G.

Table 4.1:	Pitman parameters for the Grootdraai Dam subcatchment

(area = 7 995 km2)

Pitman catchment parameters	Units of parameter	Value for parameter
Previous BKS study (BKS, 1986)	From SWR90 regional

(WRC, 1994)	Final adopted value based on the period

1920-1994
POW	-	3.00	3.00	3.00
SL	mm	0.00	0.00	0.00
ST	mm	131.00	70.00	95.00
FT	mm/month	6.00	3.60	5.70
GW	mm/month	0.00	0.00	0.00
ZMIN	mm/month	999	999	999
ZMAX	mm/month	999	999	999
PI	mm	1.50	1.50	1.50
TL	months	0.50	0.24	0.60
GL	months	0.00	0.00	0.00
R	-	0.50	0.50	0.50

Table 4.2:	Results of calibration on gauge C1H001 / C1R002 (Standerton weir / Grootdraai Dam)

Description	Period of record	Total area upstream

(km2)	MAR

(106m3)	Standard deviation

(SD)	Seasonal index (SI)
Observed data	1920-1994	7 995	453.70	355.10	40.02
Simulated data	1920-1994	7 995	453.65	352.23	37.43
Percentage difference	-0.01%	-0.81%	-6.47%

4.2.2	Gauge C1H002 (Delangesdrift weir)

Streamflow gauge C1H002 at Delangesdrift was calibrated on the total catchment upstream of Delangesdrift weir with a catchment area of 4 158 km2. The calibration is based on the 1920 - 1994 period. The results are given in Table 4.3 and Table 4.4 and they show that this catchment was calibrated to a high degree of accuracy. The statistics of the observed data compare very well with those of the simulated data. For this reason it was decided to accept the parameters given in Table 4.3. Observed and simulated data are compared graphically in Appendix G.

Table 4.3:	Pitman parameters for the Delangesdrift subcatchment

(area = 4 158 km2)

Pitman catchment parameters	Units of parameter	Value for parameter
Previous BKS study (BKS, 1986)	From SWR90 regional

(WRC, 1994)	Final adopted value based on the period

1920-1994
POW	-	3.00	3.00	3.00
SL	mm	0.00	0.00	0.00
ST	mm	150.00	105.00	111.98
FT	mm/month	7.00	6.00	4.85
GW	mm/month	0.00	0.00	0.00
ZMIN	mm/month	999	999	999
ZMAX	mm/month	999	999	999
PI	mm	1.50	1.50	1.50
TL	months	0.50	0.30	0.70
GL	months	0.00	0.00	0.00
R	-	0.50	0.50	0.50

Table 4.4:	Results of calibration on gauge C1H002 (Delangesdrift weir)

Description	Period of record	Total area upstream

(km2)	MAR

(106m3)	Standard deviation

(SD)	Seasonal index (SI)
Observed data	1920-1994	4 158	247.10	206.12	36.02
Simulated data	1920-1994	4 158	247.10	198.72	35.80
Percentage difference	0.00%	-3.59%	-0.61%

4.2.3	Gauge C8H001 (Frankfort weir)

Gauge C8H001 at Frankfort was calibrated on the incremental subcatchment, upstream of Frankfort weir and downstream of Sterkfontein Dam. It has a total area of 15 498 km2. It is based on the period 1920 to 1994. The results are given in Table 4.5 and Table 4.6 and observed and simulated data are compared graphically in Appendix G.

Although this catchment was calibrated satisfactoraly, some difficulty was experienced with the seasonal index. Since the remaining characteristics of the fow record were well modelled, however, it was decided to accept the results.

It should be noted that the parameters given in Table 4.5 from the previous BKS report on the hydrology of the Vaal Dam catchment (BKS (Pty) Ltd, 1986) are not directly comparable to the results from the current study, since they refer to the total catchment upstream of Frankfort with a catchment area of 15 693 km2, which includes the small Sterkfontein Dam catchment.

Table 4.5:	Pitman parameters for the Frankfort incremental subcatchment

(area = 15 498 km2)(1)

Pitman catchment parameters	Units of parameter	Value for parameter
Previous BKS study (BKS, 1986)	From SWR90 regional

(WRC, 1994)	Final adopted value based on the period

1920-1994
POW	-	3.00	3.00	3.00
SL	mm	0.00	0.00	0.00
ST	mm	160.00	99.00	98.04
FT	mm/month	6.00	6.50	8.55
GW	mm/month	0.00	0.00	0.00
ZMIN	mm/month	40.00	999	999
ZMAX	mm/month	999	999	999
PI	mm	1.50	1.50	1.50
TL	months	0.25	0.20	0.10
GL	months	0.00	0.00	0.00
R	-	0.50	0.50	0.50

Note:	(1)	The parameters from the previous BKS report (BKS (Pty) Ltd, 1986) in column 3, are based on the total catchment area upstream of Frankfort weir (15 693 km2), and not the incremental subcatchment that excludes Sterkfontein Dam subcatchment.

Table 4.6:	Results of calibration on gauge C8H001 (Frankfort weir)

Description	Period of record	Total area upstream

(km2)	MAR

(106m3)	Standard deviation

(SD)	Seasonal index (SI)
Observed data	1920-1994	15 693	760.38	612.32	32.07
Simulated data	1920-1994	15 693	760.28	611.78	25.39
Percentage difference	-0.01%	-0.09%	-20.83%

4.2.4	Gauge C2H003 / C1R001 (Engelbrechtsdrift weir / Vaal Dam)

The combined record of gauges C2H003 (Engelbrechsdrift) and C1R001 (Vaal Dam) was used for the calibration of the Vaal incremental catchment - upstream of Vaal Dam but downstream of gauges C1H001 (Standerton), C1H002 (Delangesdrift) and C8H001 (Frankfort). It has an incremental catchment area of 10 792 km2. The calibration is based on the available period of 1923 to 1994. The results are given in Tables 4.7 and 4.8 and they show this calibration to be excellent. Observed and simulated data are compared graphically in Appendix G.

It should be noted that because the model was calibrated against the observed flows at Vaal Dam, observed patched streamflow records for the three upstream gauges had to be used as known inflows to the Vaal incremental simulation network. A similar approach was used in the previous BKS study, and the parameters given in column 3 of Table 4.7 from the previous BKS report on the hydrology of the Vaal Dam catchment (BKS (Pty) Ltd, 1986), also represent a calibration on the incremental catchment upstream of Vaal Dam.

Table 4.7:	Pitman parameters for the Vaal incremental subcatchment

(area = 10 792 km2)

Pitman catchment parameters	Units of parameter	Value for parameter
Previous BKS study (BKS, 1986)	From SWR90 regional

(WRC, 1994)	Final adopted value based on the period

1923-1994
POW	-	3.00	3.00	3.00
SL	mm	0.00	0.00	0.00
ST	mm	85.00	61.00	58.05
FT	mm/month	11.00	3.30	2.50
GW	mm/month	0.00	0.00	0.00
ZMIN	mm/month	999	999	999
ZMAX	mm/month	999	999	999
PI	mm	1.50	1.50	1.50
TL	months	0.20	0.21	0.21
GL	months	0.00	0.00	0.00
R	-	0.50	0.50	0.50

Table 4.8:	Results of calibration on gauge C2H003 / C1R001 (Engelbrechtsdrift weir / Vaal Dam)

Description	Period of record	Total area upstream

(km2)	MAR

(106m3)	Standard deviation

(SD)	Seasonal index (SI)
Observed data	1923-1994	38 638	1858.25	1423.01	35.30
Simulated data	1923-1994	38 638	1858.20	1380.43	35.45
Percentage difference	0.00%	-2.99%	0.42%

4.2.5	Gauge C8R003 (Sterkfontein Dam)

As stated earlier, it was decided not to use the flow record of the inflows to Sterkfontein Dam (C8R003) as part of the hydrological analysis. Alternatively, the suggested regional parameters given by the Water Research Commission in their report on the surface water resources of South Africa (WRC, 1994) were used. This was considered to be sufficiently accurate since the catchment area of Sterkfontein Dam is very small and so too is the generated runoff.

The results are given in Table 4.9. It should be noted that the adopted values are of course identical to those based on the SWR90 regional parameters and no calibration results or comparison between observed and simulated data are applicable.

Table 4.9:	Pitman parameters for the Sterkfontein subcatchment

(area = 195 km2)

Pitman catchment parameters	Units of parameter	Value for parameter
Previous BKS study (BKS, 1986)	From SWR90 regional

(WRC, 1994)	Final adopted value based SWR90 parameter
POW	-	-	3.00	3.00
SL	mm	-	0.00	0.00
ST	mm	-	85.00	85.00
FT	mm/month	-	6.00	6.00
GW	mm/month	-	0.00	0.00
ZMIN	mm/month	-	999	999
ZMAX	mm/month	-	999	999
PI	mm	-	1.50	1.50
TL	months	-	0.20	0.20
GL	months	-	0.00	0.00
R	-	-	0.50	0.50

5.	NATURAL STREAMFLOWS

Natural streamflows are obtained by eliminating the influence of developments within a catchment, from the observed streamflow of that catchment. There are various methods of accomplishing this. By using the WRSM90 runoff model it is possible to incorporate the catchment demands, and calibrate directly against the observed flows. This has the advantage that the model automatically calculates the actual historical monthly supplies from the system. Although these supplies are based upon the WRSM90 simulated runoff, they are of sufficient accuracy, since the model was previously calibrated against observed flows.

The catchment developments considered are:

irrigation demands and return flows,

the effect of small dams,

abstractions by and return flows from urban and industrial users,

runoff or releases from upstream catchments and

interbasin transfers.

As the magnitude of urban and industrial abstractions, runoff from upstream catchments and interbasin transfers are known, and not dependant upon calibration results, these flows are added onto observed flows directly. The same approach is used for urban and industrial return flows which are subtracted. Water supplied for irrigation is obtained directly from the WRSM90 output data and these demands are also added. The return flows from irrigated areas, taken to be 10%, is finally subtracted from the observed flow to obtain the natural flow record.

Water lost from dummy dams through evaporation, is calculated by taking the difference between the water coming into the dam from the system and the water spilling from the dam into the system. These are again obtained directly from the model results.

The natural streamflow for the 1920 - 1994 period was obtained for the five subcatchments in the Vaal Dam catchment, namely Grootdraai, Delangesdrift, Frankfort incremental, Vaal incremental and Sterkfontein. A summary of the results is given in Table 5.1.

Table 5.1:	Summary of results from the hydrological analysis for the

1920 - 1994 period

Incremental subcatchment	Catch- ment area (km2)	Observed incr. runoff or inflow (106m3)	Average annual supply or return flow for the 1920 - 1994 period

(106m3)	Natural incr. MAR (106m3)
Irrigation supply	Irrigation return flow	Losses from small dams	Urban and industrial demands	Urban and industrial return flows	Interbasin transfers(1)
Grootdraai	7 995	453.7	5.6	-0.6	7.7	1.2	-1.6	-8.3	457.7
Delangesdrift	4 158	247.1	0.0	0.0	2.2	0.2	0.0	0.0	249.5
Frankfort incr.	15 498	696.2	28.3	-2.8	8.8	4.7	-1.8	0.0	733.3
Vaal incr.	10 792	493.2	14.9	-1.5	13.3	1.6	-2.8	0.0	518.7
Sterkfontein	195	97.3	0.0	0.0	0.0	0.0	0.0	-79.2(2)	18.1(3)
TOTAL FOR CATCHMENT	38 638	1987.5	48.8	-4.9	32.0	7.7	-6.2	-87.5	1977.3

Note:	(1)	The values for interbasin transfers have been taken to be the average over the period 1920-1994 to provide a meaningful mass balance. It should be noted, however, that, on average over the past 20 years, a far greater amount of water has been transferred annually into the Vaal Dam catchment.

(2)	This value is an estimate as the actual magnitude of the transfer from the Tugela system was never calculated. More information on this subject is given in Section 2.7.

(3)	As no observed streamflow record exists for the Sterkfontein Dam catchment, the natural MAR is based on simulated data using the regional parameters suggested in the WRC report (WRC, 1994).

5.1	Naturalisation of the Grootdraai Dam subcatchment

The Grootdraai Dam subcatchment has a total catchment area of 7 995 km2. Although it contains fairly little development, the subsystem is rather complex due to transfers from other catchments. These transfers are from Heyshope Dam in the Usutu catchment since October 1985 and Zaaihoek Dam in the Tugela catchment since October 1992. The total transfer to the Grootdraai catchment therefore amounts to approximately 50 million cubic metres per annum.

The Grootdraai subsystem contains two dummy dams with a combined capacity of about 43.8 million cubic metres and mainstream irrigation with a total demand of about 12.4 million cubic metres at 1994 development levels. Additionally, two urban abstractions were modelled, including demands from the towns of Amersfoort, Breyten and Ermelo with a total water demand of 4.2 million cubic metres in 1994. Return flows include Bethal, Ermelo and seepage from the Camden and Tutuka power stations which total approximately 6.0 million cubic metres in 1994. A schematic layout of the system is shown in Figure G-1.

Results of the naturalisation are given in Table 5.2.

Table 5.2:	Naturalisation results for the Grootdraai Dam subcatchment(1)

Description	Period	MAR

(106m3)	Standard deviation

(SD)	Seasonal index

(SI)	Coefficient of variation

(CV)
Observed data	1920-1994	453.70	355.10	40.02	0.78
Naturalised data	1920-1994	457.68	358.01	39.89	0.78
1920-1989	476.39	362.12	39.94	0.76
1920-1983	489.93	367.67	40.29	0.75

Note:	(1)	Based on a catchment area of 7 995 km2 and an MAP of 690 mm.

5.2	Naturalisation of the Delangesdrift subcatchment

The Delangesdrift subcatchment has a total catchment area of 4 158 km2. There is very little development in the Delangesdrift subsystem as indicated in the schematic layout given in Figure G-1. It contains one dummy dam with a capacity of approximately 10.5 million cubic metres at full supply level and one abstraction. This modelled abstraction consists of urban demands from the towns of Volksrust and Memel with a total water use of 0.5 million cubic metres in 1994. Results of the naturalisation are given in Table 5.3.

Table 5.3:	Naturalisation results for the Delangesdrift subcatchment1

Description	Period	MAR

(106m3)	Standard deviation

(SD)	Seasonal index

(SI)	Coefficient of variation

(CV)
Observed data	1920-1994	247.10	206.12	36.02	0.83
Naturalised data	1920-1994	249.49	205.53	35.53	0.82
1920-1989	259.61	207.37	35.49	0.80
1920-1983	257.27	203.75	35.22	0.79

Note:	(1)	Based on a catchment area of 4 158 km2 and an MAP of 712 mm.

5.3	Naturalisation of the Frankfort incremental subcatchment

The Frankfort incremental subcatchment lies upstream of Frankfort weir and downstream of Sterkfontein Dam and has a total catchment area of 15 498 km2.

The most important step in naturalising the runoff from Frankfort incremental subcatchment is to subtract water released from Sterkfontein Dam, from the observed flows. These flows were obtained from the Sterkfontein dam balance, where they are given as compensation releases measured by component C8H032-A01. The record spans the period from 1974, when Sterkfontein Dam was constructed, to date.

The Frankfort incremental subcatchment is highly developed. The subsystem includes three dummy dams with a combined capacity of about 70 million cubic metres. It contains both main stream and dummy dam irrigation with a total demand of about 62.3 million cubic metres in 1994. Additionally, there are four modelled abstractions which include urban demands from the towns of Bethlehem, Frankfort, Harrismith, Qwa-Qwa, Reitz and Warden, with a total of 20.2 million cubic metres in 1994. Return flows modelled include Bethlehem, Harrismith and Qwa-Qwa, with a total of 7.8 million cubic metres in 1994. A schematic layout of the system is shown in Figure G-1.

Results of the naturalisation are given in Table 5.4.

Table 5.4:	Naturalisation results for the Frankfort incremental subcatchment(1)

Description	Period	MAR

(106m3)	Standard deviation

(SD)	Seasonal index

(SI)	Coefficient of variation

(CV)
Observed data	1920-1994	696.20	619.44	35.89	0.89
Naturalised data	1920-1994	733.31	613.49	34.33	0.84
1920-1989	758.99	621.01	34.22	0.82
1920-1983	754.29	622.58	33.91	0.83

Note:	(1)	Based on a catchment area of 15 498 km2 and an MAP of 679 mm.

5.4	Naturalisation of the Vaal Dam incremental subcatchment

The Vaal Dam incremental subcatchment lies upstream of Vaal Dam and downstream of Delangesdrift weir (C1H002), Frankfort weir (C8H001) and Grootdraai Dam (C1R002). It has an incremental catchment area of 10 792 km2.

The subsystem includes two dummy dams with a combined capacity of about 42 million cubic metres. It contains both mainstream and dummy dam irrigation with a total demand of about 32.4 million cubic metres at 1994 development levels. Additionally, there is are modelled abstractions from Standerton and Vrede with a total water use of 7.1 million cubic metres in 1994. Urban return flows include those from Evander, Sasol and Standerton, and industrial effluent from Sasol II and III, as well as seepage from the Grootvlei power station. The total return flow is approximately 17.6 million cubic metres at 1994 development levels. A schematic layout of the system is shown in Figure G-1.

Results of the naturalisation are given in Table 5.5.

Table 5.5:	Naturalisation results for the Vaal incremental subcatchment(1)

Description	Period	MAR

(106m3)	Standard deviation

(SD)	Seasonal index

(SI)	Coefficient of variation

(CV)
Observed data	1920-1994	493.22	381.57	35.46	0.77
Naturalised data	1920-1994	518.65	378.26	34.78	0.73
1920-1989	542.17	380.19	34.81	0.70
1920-1983	550.64	381.58	34.81	0.69

Note:	(1)	Based on a catchment area of 10 792 km2 and an MAP of 657 mm.

5.5	Natural streamflows for the Sterkfontein subcatchment

As explained in Section 3.3, it was decided not to use the gauging station C8R003 (inflows to Sterkfontein Dam) as part of the hydrological analysis as it does not represent runoff from the catchment. The reason for this is that Sterkfontein Dam and its inflows are unusually large compared to its catchment area. Instead, suggested regional parameters given by the Water Research Commission in their report on the surface water resources of South Africa (WRC, 1994), were used to simply simulate the virgin runoff from the catchment.

This approach was considered to be sufficiently accurate as the catchment area of Sterkfontein Dam is only 195 km2 and the runoff that it generates is relatively small compared to the overall hydrology. It also does not contain any catchment developments.

The results are given in Table 5.6.

Table 5.6:	Results of the simulation of virgin runoff for the Sterkfontein Dam subcatchment(1)

Description	Period	MAR

(106m3)	Standard deviation

(SD)	Seasonal index

(SI)	Coefficient of variation

(CV)
Simulated virgin runoff	1920-1994	18.12	14.25	32.96	0.79
1920-1989	18.80	14.38	33.03	0.76
1920-1983	18.91	14.86	33.98	0.79

Note:	(1)	Simulation using regional parameters from the WRC report (WRC, 1994), a catchment area of 195 km2 and an MAP of 735 mm.

Comparing the statistics of the various natural flow records given in Table 5.2 to Table 5.6 with those from other sources, indicate relatively small differences, as shown in Table 5.7.

Table 5.7:	Comparison between the natural MARs obtained from this study to previous estimates

Incremental subcatchment	Catchment area

(km2)	Period of record	Natural MAR
Other studies	WRSM90 simulation(1)	Current study
106m3	mm	106m3	mm	106m3	mm
Grootdraai	7 995	1920-1994	-	-	457.6	57.2	457.7	57.2
1920-1989	514.1(2)	64.3	468.3	58.6	476.4	59.6
1920-1983	495.4(3)	62.0	474.7	59.4	489.9	61.3
Delangesdrift	4 158	1920-1994	-	-	249.5	60.0	249.5	60.0
1920-1989	253.5(2)	61.0	256.7	61.7	259.6	62.4
1920-1983	?(4)	-	255.0	61.3	257.3	61.9
Frankfort incr.	15 498	1920-1994	-	-	733.2	47.3	733.3	47.3
1920-1989	808.6(2)	52.2	754.8	48.7	759.0	49.0
1920-1983	663.3(3)	42.8	732.0	47.2	754.3	48.7
Vaal incr.	10 792	1920-1994	-	-	518.6	48.1	518.7	48.1
1920-1989	476.6(2)	44.2	527.9	48.9	542.2	50.2
1920-1983	550.1(5)	51.0	530.0	49.1	550.6	51.0
Sterkfontein	195	1920-1994	-	-	18.1	92.9	18.1	92.9
1920-1989	16.3(2)	83.6	18.8	96.4	18.8	96.4
1920-1983	31.1(3)	159.5	18.9	97.0	18.9	97.0
TOTAL

VAAL DAM CATCHMENT	38 638	1920-1994	-	-	1977.0	51.2	1977.3	51.2
1920-1989	2069.1	53.6	2026.5	52.4	2056.0	53.2
1920-1983	1997.2	51.7	2010.6	52.0	2071.0	53.6

Note:	(1)	Simulation using calibrated Pitman parameters as given in Section 4.2 - Calibration results.

(2)	Simulated value from the WRC report (WRC, 1994).

(3)	From the previous BKS report on the hydrology of the Vaal Dam catchment, (BKS (Pty) Ltd, 1986).

(4)	This value was not available.

(5)	Best available estimate from the previous BKS report on the hydrology of the Vaal Dam catchment, (BKS (Pty) Ltd, 1986).

Based on this comparison, the naturalisation results of all five subcatchments upstream of Vaal Dam were accepted.

6.	CONCLUSIONS AND RECOMMENDATIONS

From the hydrological analysis of the Vaal Dam catchment, several conclusions have been drawn:

a)	Of the many streamflow gauges located within the Vaal Dam catchment, most are of little use for hydrological analyses, due to unfavourable locations or short periods of record. Several gauges cannot even be considered, due to large portions of unreliable data in the record.

b)	Only six gauges in the Vaal Dam catchment were selected for use in the study. All six are conveniently located and have exceptionally long periods of record, They are therefore considered to be key gauges, and of great importance in the hydrological analysis. They are:

Delangesdrift Weir (C1H002) in the Klip River,

Frankfort Weir (C8H001) in the Wilge River,

Standerton Weir (C1H001) in the Vaal River, supplemented by Grootdraai Dam (C1R002) and

Engelbrechtsdrift Weir (C2H003) in the Vaal River, supplemented by Vaal Dam (C1R001).

Two of these gauges, Delangesdrift and Frankfort, however, contain periods of missing or unreliable data, due to inaccuracy at high flows.

c)	The water balance at Sterkfontein Dam cannot be used to estimate natural inflow, since the surface area of Sterkfontein is unusually large compared to its catchment area (almost 35% at full supply level).

d)	Some catchment developments have a significant effect on the hydrology of the system. The most important of these are small dams, irrigation and various urban and industrial centres.

Small dams in the Vaal Dam catchment have a combined capacity of about 167 million m3 at 1994/5 development levels. Apart from the resulting reduction in runoff from the catchment, this has the more important implication that a notable amount of water is lost from the system through the process of evaporation - an estimated 60 million m3 per annum in the 1994 hydrological year (3.1% of the catchment natural annual runoff).

The total irrigation demand from the catchment for the same year is some 107 million m3 per annum (5.4% of the natural annual runoff), and demands from urban and industrial centres totalled about 33 million m3 per annum (1.7% of the natural annual runoff).

e)	Other developments were found to be of less importance. The effect of afforestation and increased runoff from paved areas in urbanised areas were considered to be of little significance to the overall hydrology and were not included in the analysis.

f)	The only transfer loss of some importance is on the Wilge River between Sterkfontein Dam and Vaal Dam. An estimated 103 million m3 (5.2% of the natural annual runoff from the catchment) of water was lost from the system during the 1994 hydrological year.

g)	Although there are two relatively large wetlands in the Vaal Dam catchment, they are shallow, and therefore have small capacities. Furthermore, since the wetlands are natural, it was concluded that any influence they may have on the hydrology is already reflected in the hydrological records and is therefore taken into account during the calibration process.

Based on the conclusions from this report, it is recommended that:

a)	The importance and reliability of gauges in the Vaal Dam catchment should be assessed. Gauges that are found to be of little importance should be closed and resources be used exclusively for the upgrading and maintaining of key gauges.

b)	The four gauges C1H001 (Standerton), C1H002 (Delangesdrift), C8H001 (Frankfort) and C2H003 (Engelbrechtsdrift) should be treated as key gauges. Existing flow data, particularly at gauges C1H001 (Standerton) and C2H003 (Engelbrechtsdrift) that are no longer functional, should be maintained or improved where possible.

c)	Gauges C1H002 (Delangesdrift) and C8H001 (Frankfort) should be upgraded to improve their accuracy at high flows.

d)	Suggested regional parameters (WRC, 1994) should be used to simulate virgin runoff from the Sterkfontein subcatchment. This is considered to be sufficiently accurate as the catchment area and the runoff that it generates are small.

7.	REFERENCES

BKS (Pty) Ltd, (1985).

BKS (Pty) Ltd, (1986).

BKS (Pty) Ltd, (1988).

BKS (Pty) Ltd, (1992).

BKS (Pty) Ltd, (1994).

BKS (Pty) Ltd, (1995).

COMMISSION OF ENQUIRY INTO WATER MATTERS, (1970).

CSS, (1968/9).

CSS, (1976).

CSS, (1988a).

CSS, (1988b).

DAWS, (1985).

DWAF, (1985).

DWAF, (1990a).

DWAF, (1990b).

DWAF, (1993/4).

DWAF, (1996).

SSI, (1993).

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