Application of a coupled microwave, energy and water transfer model to relate passive microwave emission from bare soils to near-surface water content and evaporation

The paper examines the stability of the relation between microwave emission from the soil and the average near-surface water content in the case of re latively smooth, bare soils, and then considers the extent to which microwa ve radiometry can be used to estimate the effective surface resistance to v apour transfer, which is also related to the near-surface water status. The analysis is based on the use of a model (MICRO-SWEAT) which couples a micr owave radiative transfer model with a SVAT scheme that describes the exchan ges of water vapour, energy and sensible heat at the land surface. Verifica tion of MICRO-SWEAT showed good agreement (about 3K RMSE) between predicted L band (1.4 GHz) brightness temperature over soils a with contrasting text ure during a multi-day drydown, and those measured using a truck-mounted ra diometer. There was good agreement between the measured and predicted relat ions between the average water content of the upper 2 cm of the soil profil e and the brightness temperature normalised with respect to the radiometric surface temperature. Some of the scatter in this relationship was attribut able to diurnal variation in the magnitude of near-surface gradients in tem perature and water content, and could be accounted for by using the physica lly-based simulation model. The influence of soil texture on this relations hip was well-simulated using MICRO-SWEAT. The paper concludes by demonstrat ing how MICRO-SWEAT can be used to establish a relationship between the nor malised brightness temperature and the surface resistance for use in the pr ediction of evaporation using the Penman-Monteith equation.