Evapotranspiration

Efficient use of water resources in semiarid and arid agroecosystems of the World has become increasingly important because of rapid depletion of water resources, industrial development and population increase, drought conditions, and degradation of ground and surface water quality in many regions. In many cases, evapotranspiration (ET), which is the sum of transpiration through plant canopy and evaporation from soil, plant, and open water surface, can be the largest component of the hydrologic cycle. Improved techniques are needed for accurate quantification of ET on a field, watershed, and regional scale to enhance efficient use of water resources and protect the environment and water quality. Accurate quantification of ET is crucial in water allocation, irrigation management, evaluating the effects of changing land use on water yield, environmental assessment, and development of best management practices to protect surface and groundwater quantity and quality. The objective of this article is to define and discuss several ET terms, including potential, reference, and actual (crop) ET, and present information on one of the methods of using crop coefficient and reference ET approach to quantify actual ET for agroecosystems.