Equilibrium evaporation and positive evaporation-surface temperature relationships across a grassland

Sensible (H) and latent (LE) heat fluxes, soil moisture (SM) and surface te mperatures (T-s) were analyzed from seven sites at FIFE to evaluate relatio nships among the spatial variability of evaporative fraction, EF, SM, and t he diurnal surface temperature range (T-dr) Intersite correlations between EF and T-dr were significantly negative for regional average soil moisture SMr < 20 percent, insignificant for 20 < SMr< 27 percent, and slightly posi tive for SM(r)r > 27 percent. Statistical analysis of the pooled correlatio n coefficient between EF and T-dr for SMr < 20 percent indicates that it is less than zero at a very high level of significance, while the pooled corr elation coefficient for regional SMr > 27 percent is greater than zero at t he 10 percent level. The positive EF:T-dr correlations are attributed to in creased surface vapor pressure at warmer sites under nearly potential condi tions. These results suggest that to characterize the spatial variability o f the energy budget partitioning, a variable representing the thermal respo nse of the site should be included. An important application of these findi ngs relates to modeling the subgrid variability of a region by subdividing the region into a few classes within which surface variables and parameters are assumed invariant. The thermal response of the surface should be inclu ded as a variable in defining these classes.