Hydrology:

Water Balance

Water balance is a concept used to understand the availability and the overall 'state' of water resources in a hydrological system. A hydrological system is usually a standard surface water unit such as a quaternary catchment or, in the case of theLimpopo River, a River basin. This concept is also sometimes referred to as a Water Budget.

This holistic approach takes into account all of the water inputs into the system and the extractions take out of the system or out of circulation.

Inputs include:

Precipitation - rain or snow
Groundwater influx from an adjacent aquifer or a transboundary (trans-riverbasin) aquifer
Snow melt
Inter-basin transfer - water transferred into the basin from an adjacent riverbasin

Extractions include:

Evaporation
Evapotranspiration
Extraction for consumptive use from streams and rivers - water for industrial ordomestic use and irrigation
Extraction for consumptive use from groundwater aquifers
Inter-basin transfer - water transferred out of the basin to adjacent river basin

A simple approach to a water balance equation could be considered as:

P + R + B - F - E -T = ΔS

Wanielista et al. 1997

Annotated Hydrologic Cycle.

Source:(adapted from) FISRWG 1998

( click to enlarge )

Where:

P = Precipitation
R = Runoff or excess rainfall
B = Subsurface flow
F = Infiltration
E = Evapotranspiration
T = Transpiration
S = Change in storage in the saturated zone - soil or groundwater