Evapotranspiration from Nonuniform Surfaces: A First Approach for Short-Term Numerical Weather Prediction

Natural land surfaces are rarely homogeneous over the resolvable scales of numerical weather prediction models. Therefore, these models must somehow account for the subgrid variability in processes that are nonlinealy dependent on surface characteristics. Because of its complex dependence on soil moisture and vegetation, the flux of latent heat is an acutely nonlinear process, involving variables which can vary widely over very short distances. Thus, if the effects of subgrid-scale surface variability are significant, they should appear most prominently in the prediction of evapotranspiration. 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. Sensitivity tests are also conducted with the model using the field observations and using a one-dimensional atmospheric boundary layer model which allows the surface and atmosphere to adjust interactively to changes in evapotranspiration. Results comparing point evapotranspiration computations with grid-area-averaged values show a significant impact due to the unresolved variability. In fact, in some case, the size of the grid unit can be a dominant factor in determining the mean rate of evapotranspiration. This is because of the observed increased variability of soil moisture with grid size, and because of the highly nonlinear relationships between soil moisture and evapotranspiration at a point. 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.