STAGES OF SOIL DRYING

The evapotranspiration process over the catchment considered in this work, can be subdivided into two stages. If there is enough water at the surface of the soil, the evapotranspiration proceeds at the potential rate and the process is determined by the atmospheric conditions above the soil surface and the evapotranspiration rate does not depend on the state of the soil. Once the soil moisture at the surface layer is below some value prescribed by soil and vegetation conditions, it is these conditions, that control the evapotranspiration rate, independently of the atmospheric conditions. The controlling factor is usually taken as a threshold average root zone water contents below which the transport of water to the plant leaves limits the transpiration process (Gardner et al., 1975; Cordova & Bras, 1981). In the literature, these two stages of evapotranspiration were tackled using switching boundary conditions (e.g. Entekhabi & Eagleson, 1989; and Kuhnel, 1989). In the first stage the condition of constant moisture flux at the surface was used and in the second stage this condition was replaced by the condition of the constant moisture contents at the surface.