The runoff from an area can be expressed in the following water balance equation:
R = P – ΔS – ΔM – E
where R = runoff
P = precipitation in the form of rain
ΔS = change in storage due to snow cover
ΔM = other change in storage (ground, lakes and groundwater)
E = evaporation
These parameters are calculated with the aid of several sub-models described below.