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:
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Precipitation - rain or snow
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Groundwater influx from an adjacent aquifer or a transboundary (trans-river basin) aquifer
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Snow melt
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Inter-basin transfer - water transferred into the basin from an adjacent river basin
Extractions include:
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Evaporation
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Evapotranspiration
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Extraction for consumptive use from streams and rivers - water for industrial or domestic use and irrigation
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Extraction for consumptive use from groundwater aquifers
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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
Annotated Hydrologic Cycle with abbreviations inserted.
Source: (adapted from) FISRWG 1998
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