Estimation of areal soil water content during HAPEX-Sahel and EFEDA-Spain

In 1991 and 1992 two similar large field experiments took place: EFEDA-Spain and HAPEX-Sahel, respectively. The objectives of these experiments are (1) the improvement of the parameterisation of surface hydrology processes in semi-arid areas within the framework of global climate models and (2) to develop and apply methods for monitoring the surface hydrology at a large scale using remote sensing for climate prediction models. In this study the determination of areal soil water content by means of remote sensing is further explored and the implementation of such information in climate and hydrological models is discussed. The combined use of the various type of sensors is essential for retrieving this kind of information. Comparisons are made between the use of polarimetric SAR (P-, L-, C-band of JPL-AIRSAR) and a microwave radiometer operating at L-band for the determination of areal soil water content. The influence of vegetation can be significant and limit the applicability of both type of sensors within the framework of climate or hydrological modelling. The description of the path and interaction of electromagnetic radiation between the surface and the sensors is assessed by using radiative transfer models for as well the active microwave data and the passive microwave data. With this knowledge, simpler models will be applied to retrieve surface soil moisture and if necessary correct or avoid the problem of vegetation. For the active microwave data models will be used. For the passive data methods like empirical approaches using regression analysis and a more physically based approach will be utilised. It is anticipated that with the AIRSAR data estimates of soil moisture profiles can be retrieved over certain areas to a depth of approximately 0.5 m. The results of these microwave models will be validated against ground data and for certain agricultural fields against simulated data using a calibrated 1D hydrological model