Water Balance

Water can enter from...	Water can exit from...
Surface irrigation Underground irrigation Horizontal sub-surface water movements Rising water table and capillary action	Surface evaporation Horizontal water movements Vertical movements (through-drainage) Evapotranspiration by the crop

Crop Water Use

If we assume that there is no horizontal losses (which will not be true for every site...), and ignore for now surface evaporation and through-drainage, then...

Plant uptake = rate of change of soil water

That is, if we measure the...

Daily change in the amount of water in the profile

then we know...

The daily water use of the crop

DWU = (Water_1 - Water_2 + Irrigation + Rain ) / Delta_days

But we will have to allow for losses from through-drainage and from surface evaporation.