The depletion of water from the soil as a result of crop transpiration and evaporation from the soil surface upon which the crop is grown. Evapotranspiration is commonly referred to as consumptive use. Potential evapotranspiration (ET P ) is that water which would be consumed by a short, well watered, vigorously growing crop that completely shades the ground surface. Actual evapotranspiration (ET) is the actual consumption of water by a specific crop considering that each crop has different growth characteristics with respect to root and foliage growth, planting dates, and water needs. ET and ET are generally expressed as volumes of water used in an irrigated area over a unit time increment (volume/area/time, e.g. in/hr, cm/day, etc. ).