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Crop water use is ETc = Kc x ETo. Dividing the readily available water held in the root zone by ETc gives the number of days before the crop reaches its allowable depletion, which is the irrigation interval. The depth applied must be grossed up by the application efficiency because part of every irrigation is lost to runoff, wind drift and deep percolation.
Crop water use
ETc = Kc x ETo
Interval and depth
Interval (days) = readily available water / ETc; gross depth = net depth / efficiency
ETo comes from a nearby weather station or the FAO Penman-Monteith estimate. Kc depends on crop and growth stage — roughly 0.3-0.5 at emergence, 1.0-1.2 at mid-season and 0.6-0.9 at maturity.
Surface and furrow systems are typically 50-65%, sprinklers 70-80% and well-managed drip 85-95%. Lower efficiency means more water must be pumped for the same crop benefit.