In arid regions, almost all agricultural crop water requirements come from irrigation. Irrigation water is applied to replace water lost from the crop rooting zone by transpiration through the crop leaf canopy, evaporation from the soil surface, and percolation below the rooting zone, before physiological stress occurs. The combined loss of water from the rooting zone by transpiration and evaporation is crop consumptive use or crop evapotranspiration (ETc).