DIFFICULTIES OF ESTIMATING EVAPOTRANSPIRATION FROM THE WATER-BALANCE EQUATION

The effect of soil spatial variability on the estimation of evapotrans piration, using the water balance equation, is evaluated using data fr om 25 experimental plots, distributed along a transect of 125 m, on a dark red tropical latosol, The variability of soil water storage, tota l hydraulic gradients, soil hydraulic conductivity and soil water flux densities, and their influence on the calculation of evapotranspirati on, are discussed. The variability of these parameters confers a coeff icient of variation of the order of 40% to evapotranspiration estimate s, indicating that aerodynamic and empiric approaches are a better cho ice for evapotranspiration estimation of extensive field areas, in whi ch spatial variability of soil hydraulic characteristics is relevant.