Diagnostics of land surface spatial variability and water vapor flux

To assess the spatial variability of the water vapor (i.e., latent heat) flux LE, it is convenient to scale it with its equilibrium analog LE(e). From an analysis of the data from the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment, or FIFE, the spatial distribution of daily values of this dimensionless evaporation, alpha = LE/LE(e), was found to be strongly related to the distributions of soil moisture and of the state of the vegetation. However, the relative strengths of these relationships depended on the soil moisture content and on its distribution. When the mean soil moisture content SM was high, in excess of about 27%, the distribution of evaporation was quite uniform regardless of the vegetation uniformity. In the intermediate range, with 20 < SM < 27%, both soil moisture and vegetation contributed to the spatial distribution of alpha. This distribution was controlled by the vegetation, when the soil moisture was uniform; under nonuniform soil moisture conditions, however, soil moisture exerted the major control. For SM < 20%, soil moisture normally was nonuniform, and it was found to hold the primary control of the spatial variation of alpha. The daily distribution of soil moisture content was largely independent from that of the state of the vegetation, due to the different timescales involved.