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 the Limpopo 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-river basin) aquifer
Snow melt
Inter-basin transfer - water transferred into the basin from an adjacent river basin
Extractions include:
Evaporation
Evapotranspiration
Extraction for consumptive use from streams and rivers - water for industrial or domestic 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
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
THE ROLE OF GROUNDWATER IN THE HYDROLOGIC CYCLE. SOURCE: CSIR 2004
