Evapotranspiration

ET is the foundation of scientific irrigation scheduling. The FAO-56 Penman-Monteith equation combines solar radiation, air temperature, humidity, and wind speed to produce a daily ET₀ value (mm/day) for a reference grass surface. Crop-specific water demand (ETc) is then ETc = ET₀ × Kc, where Kc is the crop coefficient — a number between 0.3 (early growth) and 1.25 (peak canopy) that represents how much water the crop uses relative to reference grass.

For a corn crop at peak tasseling with ET₀ = 6 mm/day and Kc = 1.2, daily water demand is 7.2 mm. Over a 10-hectare field, that equals 720,000 liters per day. Irrigation replaces this demand minus effective rainfall.

Accurate ET-based scheduling typically reduces water use 20–40% compared to calendar-based irrigation, while holding or improving yields (IWMI, 2021). Modern farm platforms pull daily ET₀ from local weather stations or satellite-derived datasets (e.g., OpenET, ECMWF) and apply published Kc values from FAO-56 tables for 80+ crops. WiseYield's irrigation scheduler computes ETc daily per crop and recommends irrigation volumes that match the actual soil-water deficit, not a fixed schedule.