The general water balance equation states that total runoff is equal to precipitation minus real evaporation. Evaporation is the critical parameter in this calculation. It is influenced under natural conditions primarily by vegetation, climate, and soil. The water balance in one of its most basic forms can be expressed as:
dw/dt = Pr + M – ET – R
Where:
dw/dt = rate of change of soil water content over time;
Pr = precipitation rate;
M = rate of snowmelt;
ET = rate of evapotranspiration; and
R = rate of runoff.
For experimental watersheds, the simplified annual water balance equation can be written as:
ET = P – R ± S
Where:
ET = evapotranspiration;
P = precipitation;
R = runoff; and
±S = change in groundwater storage.
For long-term experiments, it has been found that the groundwater storage change term is generally insignificant compared to the other terms, and hence:
ET = P – R
The precipitation and runoff are sample values with a corresponding standard error of estimate.
Writing a water balance equation requires listing the important process and deciding whether they are primarily inputs, primarily outputs, or both.
Water balance is based on the law of conservation of mass: any change in the water content of a given soil volume during a specified period must equal the difference between the amount of water added to the soil volume and the amount of water withdrawn from it. In other words, the water content of the soil volume will increase when additional water from outside is added by infiltration or capillary rise, and decrease when water is withdrawn by evapotranspiration or deep drainage. The control soil volume for which the water balance is computed is often determined arbitrarily. In principle, a water balance can be computed for any soil volume, ranging from a....
Click here to buy this essay.
