DIFFICULTIES OF ESTIMATING EVAPOTRANSPIRATION FROM THE WATER-BALANCE EQUATION

The effect of soil spatial variability on the estimation of evapotranspiration, using the water balance equation, is evaluated using data from 25 experimental plots, distributed along a transect of 125 m, on a dark red tropical latosol, The variability of soil water storage, total hydraulic gradients, soil hydraulic conductivity and soil water flux densities, and their influence on the calculation of evapotranspiration, are discussed. The variability of these parameters confers a coefficient of variation of the order of 40% to evapotranspiration estimates, indicating that aerodynamic and empiric approaches are a better choice for evapotranspiration estimation of extensive field areas, in which spatial variability of soil hydraulic characteristics is relevant.