Characterization of surface parameters over arid and semi-arid areas by use of ERS-1 Windscatterometer

This study was performed within the Hapex-Sahel (Hydrologic Atmospheric Pilot Experiment in the Sahel) framework. One of Hapex Sahel objectives is to improve the parameterization of surface hydrology over semi-arid areas. To assess surface parameters (soil moisture, surface roughness, biomass) we have studied ERS-1 windscatterometer (WSC) and the Advanced very High Resolution Radiometer on board the National Oceanic and Atmospheric Administration (NOAA/AVHRR) series of polar orbiters. The ERS-1 WSC (5.3 GHz) operates in VV polarization. It has 3 antennas (fore, mid, aft) which observe the target with incidence angles ranging from 18 to 59°. Its spatial resolution is 50km. It was primarily designed to assess wind characteristics (speed and direction) over ocean through its antennas which measure the backscattering coefficient (σ0) with different viewing angles. In this study we will present the opportunity to use the multiangular observations of backscattering coefficients to quantify soil and vegetation characteristics. For this purpose, the angular behavior of σ0 will be used. The angular dependence being quantified by the slope and intercept of the linear regression of the σ0 values as a function of view angle. In order to separate the different contributions from the surface on the signal, it is useful to optimize the WSC parameters (incidence angles). We will thus consider different ranges of view angle, depending upon the mid beam values. The slope and the intercept of the σ0= f(θ) will be analysed in a first step to assess roughness and biomass. Then ERS-1 WSC data will be coupled with NOAA/AVHRR (visible and near infrared) data. A first result of this study shows the high correlation between the two biomass estimates. Temporal evolution of angular signatures of WSC data was correlated to surface features. But surface roughness seemed difficult to retrieve with such a method. Consequently, we inverted an empirical model to extract soil moisture and surface roughness whose effects will be studied.