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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 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


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
( click to enlarge )