Evaporative fraction (EF; the ratio of latent heat flux to the sum of latent plus sensible heat flux) can be measured in the field to an accuracy of about 10%. To what accuracy must soil moisture be known in order to simulate surface energy fluxes to within this observational uncertainty? Is there a firm relationship between soil moisture and the variability of surface fluxes? A relationship would provide information for planning the future measurement of soil moisture, the design of field experiments, and points of focus for soil model development. We look for relationships in three different models (SSiB, Mosaic, and BATS) using GSWP results and ancillary integrations.
We find that the variation of evaporative fraction as a function of soil moisture is consistent between the models, within subsets of vegetation type. In forested areas, there is a fairly rapid increase of EF with soil moisture when soils are dry, and little sensitivity in moderate to wet soils. Where vegetation is more sparse, there is a more gradual increase of EF with soil moisture. Bare soil desert areas behave similar to sparsely vegetated areas but with lower peak EF. Tundra regions have a unique behavior, probably because evaporation is limited more by a lack of radiant energy at high latitudes. The results suggest that accuracy in the measurement or model simulation of soil moisture is most critical within the drier portion of the range of variability of soil moisture. It is also more important over sparsely vegetated areas, where evapotranspiration is dependent on moisture in a shallower column of soil