The required storage capacity of a reservoir is normally determined by a water balance analysis based on a representative sequence of historic inflows and all anticipated water demands, including evaporation losses and ice formation.

Starting with an observed value from the surrounding wells of the modeled basin, the temporal changes in the average ground water depth is based on the water balance analysis for the whole catchment, and the rate of change of the average ground water table is assumed to be the rate of change of local water table.