Evapotranspiration from nonuniform surfaces: a first approach for short-term numerical weather prediction

Abstract

In this paper, a simple, explicit model for the computation of grid-cell average evapotranspiration is presented and tested. The model incorporates a statistical distribution of soil moisture on the subgrid scale, the variance of which is obtained from observed distributions of soil moisture and precipitation. Distributions are also assumed for other surface and vegetation characteristics. Water-stressed and potential evapotranspiration from both bare soil and vegetation are allowed to occur simultaneously within the grid element. Comparisons with estimated regional evapotranspiration from two extensive micrometeorological field programs verify the model under a wide range of conditions. Results comparing point evapotranspiration computations with grid-area-averaged values show a significant impact due to the unresolved variability. In fact, in some cases, the size of the grid unit can be a dominant factor in determining the mean rate of evapotranspiration. Other factors found important in regional evapotranspiration are radiation, surface water, soil moisture, amount of vegetation coverage, and vegetation internal resistance to moisture flow. Two factors found relatively unimportant in determining the rate of regional evapotranspiration are soil type and surface roughness length. -from Authors