The sensitivity of surface fluxes to soil water content in three land surface schemes

Evaporative fraction (EF; the ratio of latent heat flux to the sum of the l atent plus sensible heat fluxes) can be measured in the field to an accurac y of about 10%. In this modeling study, the authors try to determine to wha t accuracy soil moisture must be known in order to simulate surface energy fluxes within this observational uncertainty and whether there is a firm re lationship between the variabilities of soil moisture and surface turbulent energy fluxes. A relationship would provide information for planning the f uture measurement of soil moisture, the design of field experiments, and po ints of focus for soil model development The authors look for relationships in three different land surface schemes using results and ancillary integr ations in the Global Soil Wetness Project. It is found that the variation of evaporative fraction as a function of soi l moisture is consistent among the models and within subsets of vegetation type. In forested areas, there is high sensitivity of EF to soil moisture v ariations when soils are dry and there is little sensitivity in moderate to wet soils. Where vegetation is sparser, there is a more gradual decrease o f EF sensitivity with a decrease in soil moisture. Bare soil desert areas b ehave similarly to sparsely vegetated areas but with lower peak EE Tundra r egions have a unique behavior, probably because evaporation is limited more by a lack of radiant energy at high latitudes. The results suggest that ac curacy in the measurement or model simulation of soil moisture is most crit ical within the drier portion of the range of variability of soil moisture. It also is more important over sparsely vegetated areas, for which evapotr anspiration is dependent on moisture in a shallower column of soil.