Evapotranspiration is a collective term that includes all processes by which water at the earth’s surface is converted to water vapor. It includes evaporation from the plant canopy, transpiration, sublimation and evaporation from the soil. Evapotranspiration is the primary mechanism by which water is removed from a watershed. Roughly 62% of the precipitation that falls on the continents is evapotranspired. Evapotranspiration exceeds runoff in most river basins and on all continents except Antarctica (Dingman, 1994). The difference between precipitation and evapotranspiration is the water available for human use and management. An accurate estimation of evapotranspiration is critical in the assessment of water sources and the impact of climate and land use change on those resources A Watershed is considered as a unitary land unit in development and management strategies for the sustainable development programme. Ever increasing pressure of population coupled with poor management of land and water resources is responsible for the tremendous stress on our fragile environment. Hence, accurate and timely mapping, monitoring and assessment of conditions in the watershed area are essential for assessing land and water resources and their optimum utilization for sustainable development.