The potential evapotranspiration during each time interval is calculated using a diurnal distribution of evapotranspiration together with inputted daily values. This potential evapotranspiration is applied as a first charge on the interception store. After it is depleted the balance of the atmospheric demand is applied to the depression storage. The remaining demand is applied to the upper and lower soil zones once the depression store has been depleted.

The potential quantities of water which may be supplied to the evaporating surfaces of the plants from the upper and lower soil zone stores, or root supply capacities, within a time interval are calculated as:

 

where:

FU = root supply capacity in time interval, or short interval equivalent to UH, for upper soil zone

FL = is analogous to FU, for lower soil zone US, LS, TF, USC, LSC have been defined previously.

UH = maximum evapotranspiration from upper soil zone (mm/day)

LH = maximum evapotranspiration from lower soil zone (mm/day)

ER = proportion of evapotranspiration from upper soil zone.

FU is compared with the remaining atmospheric demand. The smaller value is chosen and it is multiplied by ER to give the evapotranspiration from the upper soil zone. A similar comparison is made between FL and the remaining atmospheric demand. In this case the smaller value is multiplied by (1-ER) to give the evapotranspiration from the lower soil zone.