MICRO-SWEAT, a soil-vegetation-atmosphere transfer scheme (SWEAT) coupled with a microwave emission model, was used to predict the distributed microwave brightness temperatures measured in the El Reno area during the Southern Great Plains 1997 (SGP97) field experiment. MICRO-SWEAT was fitted using data from four sites, and produced excellent predictions of soil moisture and microwave brightness temperature at the other 11 available measurement sites. The distributed optical depth of the vegetation (a parameter describing the effects of vegetation on microwave emission) and an estimated error in that optical depth were calculated from an empirical relationship against the measured NDVI. Results of distributed modeling of microwave brightness temperatures show noticeable differences when compared with the measurements. These differences likely result from imperfect knowledge of the spatial distributions of soil properties, precipitation, and, not least, the estimated optical depth of the vegetation used in MICRO-SWEAT. The Assimilation Value Index, a statistical measure of the value of assimilating the observed soil moisture to correct for poorly specified soil properties and precipitation in the model was defined, and this was evaluated using likely observation errors and estimated errors in the value of optical depth. Analyses indicated that there is value in assimilating observations for significant portions of the modeled domain, but it is suggested that an enhanced value would result if the optical depth of the vegetation were also directly remotely sensed, as proposed in the AMSR and SMOS missions.