millimeter per day

Evapotranspiration is a depth of water leaving the field each day — evaporated from the soil and transpired through the crop — and it is the demand side of every irrigation decision.

Reference ET for a well-watered grass surface runs 2–4 mm/day in spring and 6–9 mm/day at midsummer peak in a continental climate. Crop ET is that reference multiplied by a crop coefficient, low at emergence and above 1.0 at full canopy.

The number turns into a schedule through the soil's storage. If the root zone holds 100 mm of readily available water and the crop is using 7 mm/day, the interval between irrigations is about 14 days — and every day of that countdown is what a scheduling program is actually tracking.