The use of radar backscattering signals for measuring soil moisture and surface roughness

Abstract

In this article, the possible use of synthetic-aperture radar (SAR), for measuring the moisture content and surface roughness is analyzed. Data have been acquired on the Melarchez subwatershed during the Orgeval '89 campaign from March to December 1989. Radar back-scattering measurements were provided by the French airborne scatterometer ERASME. Simultaneous ground measurements of soil moisture and roughness, leaf-area index, and water content of the canopy were conducted on 12 test fields. Using radar configurations close that of the ERS-1 SAR, the results clearly indicate that radar data cannot be easily converted into moisture estimates at field scale over a variety of bare soil, essentially due to the effect of roughness on radar response. Nevertheless, mean soil-moisture values measured within bare soil fields over the whole subwatershed, show a decrease in soil-surface moisture from March to October. The same feature is clearly obtained by averaged radar measurements over all bare-soil fields, indicating that ERS-1 could be used for monitoring the moisture state on a regional scale. Furthermore, the great variability of radar backscattering and radar-slope data, for a given soil practice, for example, ploughing or sowing, and for the same date of measurement over bare soil, suggests that, under natural field conditions, backscattering from a soil surface may not be sufficient to explain and understand the radar response and that volume scattering may have to be accounted for as well. Finally, concerning soil roughness, the standard deviation of surface-height slope discriminates the roughness states, which apparently is a pertinent parameter for analyzing the variation of the backscattering coefficient with incidence angle.