From weather station or FAO PenmanโMonteith.
Pick stage-specific values from FAO-56 tables.
Use positive for rainfall/irrigation contribution.
Negative means drawing down root zone storage.
Drip โ0.9, sprinkler โ0.75, flood โ0.6.
Get total volume if you enter area.
Days between irrigations.
Helps estimate irrigation duration.
Grab your daily reference ET, multiply by Kc for the crop stage, subtract effective rainfall or soil moisture gain, then divide by system efficiency to schedule irrigation depth.
Kc ร EToETc โ Peff โ ฮSNet รท Efficiency