In order to calculate precipitation recharge, the annual recharge at a set of locations around the basin is kriged to produce a cell by cell distribution of precipitation for every year. Precipitation recharge is then calculated using precipitation recharge curves which translate precipitation to recharge from precipitation. For each cell a specific soil type is identified, with each soil type having a different precipitation recharge curve. There is also a different curve for irrigated lands and non-irrigated lands. Within each cell, that fraction of irrigated lands is used to apportion the recharge between the irrigated and non-irrigated recharge curves. The precipitation recharge is further adjusted using the spatial multipliers to yield the annual recharge from precipitation. The annual recharge from precipitation is then distributed to months using a fixed monthly distribution.
Groundwater return flows are calculated as a fraction of groundwater pumping. Surface water return flows are calculated as a fraction of surface water deliveries. Both of these return flows are are added to the precipitation recharge in the month that the pumping or deliveries occur.
Canal leakage is calculated based on records kept by the canal owners. Canal leakage is added to the recharge array in the month that the leakage occurs.
The result is the total recharge