Monitoring soil moisture over the Canadian Prairies with the ERS scatterometer

The capability of the scatterometers onboard the European Remote Sensing Sa tellites (ERS-1 and ERS-2) for soil moisture retrieval is investigated. The ERS scatterometer consists of three antennas that illuminate the earth's s urface from three different viewing directions, This allows us to study the dependence of the backscattering coefficient sigma(0) on the azimuth and t he incidence angle, An analysis of ERS scatterometer data over the Canadian Prairie region shows that land surfaces are slightly anisotropic with resp ect to the azimuth angle. It is proposed to consider the azimuthal anisotro py as an additional error source to sigma(0). The variation of sigma(0) wit h the incidence angle was found to be linked to vegetation, but independent of soil moisture. Based on these observations, a method for the normalizat ion of the backscattering coefficient with respect to the incidence angle i s proposed. The normalized backscattering coefficient at an incidence angle of 40 degrees, sigma(0)(40), is sensitive to vegetation and, in the case o f moderate vegetation (grassland to sparsely forested areas), to the soil m oisture content. Soil moisture maps derived from ERS-1 scatterometer measur ements are compared to maps representing conditions on annually cropped lan d showing agreement. Results suggest that, over the Canadian Prairies, esti mates of the total water content in the soil profile might be possible with an accuracy of about 10% of field capacity if little or no rainfall has oc curred for three days before radar image acquisition.