Evapotranspiration (ET) is a major component of the energy and water balances over land. Many studies of long-term averages have shown that more than half of the net solar energy and two thirds of precipitation go to ET over land. Evaporation of water from soil surfaces (ES) is a major component in the soil water balance for field crops with incomplete cover and for bare soil conditions. Most crop, hydrology, and water quality models require the simulation of evaporation from the soil surface. Quantification of ES is necessary in evaluating the water balance of soils for use in environmental and hydrologic studies and for crop management.