ABSTRACT
This chapter discusses the factors that control evapotranspiration, some of the methods used for its estimation, and modeling evapotranspiration under conditions of limited soil moisture. Evapotranspiration is a major component of the water balance of a drainage basin. The factors that change the rate of evapotranspiration relative to the rate of evaporation are Soil-related factors, and Vegetation-related factors. Methods for determining evapotranspiration include a water balance, lysimeters and meteorologically-based equations. Numerous equations have been discussed here to estimate potential evapotranspiration: Thomthwaite, Blaney-Criddle, Modified Jensen-Haise, Penman, and Penman-Monteith. The concept of potential evapotranspiration has been replaced by the concept of reference crop evapotranspiration. Modeling actual evapotranspiration typically involves calculating potential evapotranspiration by equations and then modifying it by some function of soil moisture. Soil moisture alone is not sufficient to describe the availability of water to plants. This chapter briefly examines soil moisture, to help illustrate the factors that influence evapotranspiration.
