TN-29 : Estimation of evapotranspiration under variable soil moisture situation

Abstract

Evapotranspiration (ET) is the amount of water transpired by the plant and evaporated from the soil surface. ET from vegetated surface is a function of several process like radiation exchanges, vapour transport and biological growth operating within a system involving the atmosphere, plants and soils. Models reported by Saxton et al (1974 a,b), Ritchie (1972). Van Keulen(1975), Hanks et al.(1969) etc. are typical examples of this integrated process.

The upper zone of the soil, i.e. the unsaturated zone constitutes the medium between the atmosphere and the saturated ground water system. Soil moisture evaporation occurs under unsteady conditions and results in a net loss of water from the soil i.e. it results in drying. This process involves considerable loss of water, especially in arid regions where these losses can amount of 50 percent or more of total precipitation. A technical note has been prepared for estimation of evaporation rate under quasi steady state conditions. For a given relationship between suction head(0 and volumetric soil moisture content(9) and relationship between hydraulic conductivity K(9) and 9, the amount of evaporation loss in a soil and the soil moisture variation at different time steps due to evaporation losses has been estimated.