Multi-annual analysis of ERS surface soil moisture measurements of different land uses

Abstract

This paper describes a new approach to convert ERS backscatter signals from different test sites within Germany into surface soil moisture values over several years. Because the radar backscatter signal depend mainly on soil type, vegetation development, vegetation structure and water content of the soil, vegetation and soil dependencies were eliminated to receive the backscatter signal only influenced by the amount of soil moisture. To retrieve a soil type independent relationship between surface soil moisture and the radar backscattering signal, all in situ measured surface soil moisture values were converted into dielectric constant values using the relationship of dielectric constant and volumetric soil moisture established by Hallikainen et al. (1985). The influence on the backscattering signal of different agricultural use of the test fields were corrected by using a roughness criteria for each crop type under investigation. The correction of this influence was done by including roughness parameter into this approach, based on investigations by Ulaby (1992). The approach was successfully used for corn, barley, oat and harvested fields. A common nonlinear regression curve is now available for these crops. For meadows, a very common land use in the Bavarian Alpine Foreland, an additional vegetation parameter (biomass) had to be included, to compensate the attenuation effect of strong radar effective vegetation structure changes within the period of observation. On the basis of these results, an attempt was made to determine the temporal development and spatial distribution of the course of the top soil moisture, using the corrected ERS backscattering coefficients of each field in the whole test site.