A daily water balance, expressed in terms of depletion at the end of the day, is:
Dr,i = Dr, i-1 - Pi + ROi - Ii - CRi + ETa,i + DPi
where
Dr,i
= root zone depletion at the end of day i [mm]
Dr, i-1 = depletion in the root
zone at the end of the previous day, i-1 [mm]
Pi = effective precipitation
on day i [mm], limited by maximum daily infiltration rate [mm]
ROi = surface runoff from the
soil surface on day i [mm]
Ii = net irrigation depth (averaged
over the entire area) on day i that infiltrates the soil [mm]
CRi = capillary rise from the
groundwater table on day i [mm]
ETa,i = actual crop evapotranspiration
on day i [mm]
DPi = water flux out of the root
zone by deep percolation on day i [mm]
The capillary rise, CRi, can normally be assumed to be zero when the water table is more than about 1 m below the bottom of the root zone. Following heavy rain or irrigation, the soil water content in the root zone may temporarily exceed field capacity or even saturation. The amount above saturation goes to surface runoff, whereas the amount between field capacity and saturation goes to deep percolation (limited by maximum percolation rate -- any excess will remain in the soil) within the same day of a wetting event, and depletion Dr,i becomes zero. As long as the soil water content in the root zone is below field capacity (i.e., Dr,i > 0), the soil is assumed to not drain. The root zone depletion will gradually increase as a result of ETa. In the absence of a wetting event, the root zone depletion will reach the value TAW. At that moment no water is left for ETa, and Ks and ETa become zero. The DPi term is not to be confused with the DPe,i term used for the evaporation layer. Both terms can be calculated at the same time, but are independent of one another.