THE USE OF RADAR BACKSCATTERING SIGNALS FOR MEASURING SOIL-MOISTURE AND SURFACE-ROUGHNESS

In this article, the possible we of synthetic-aperture radar (SAR), fo r measuring the moisture content and surface roughness is analyzed. Da ta have been acquired on the Melarchez subwatershed during the Orgeval '89 campaign from March to December 1989. Radar backscattering measur ements were provided by the French airborne scatterometer ERASME. Simu ltaneous ground measurements of soil moisture and roughness, leaf-area index, and water content of the canopy were conducted on 12 test fiel ds. Using radar configurations close that of the ERS-1 SAR, the result s clearly indicate that radar data cannot be easily converted into moi sture estimates at field scale over a variety of bare sell, essentiall y 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 O ctober. The same feature is clearly obtained by averaged radar measure ments over all bare-soil fields, indicating that ERS-1 could be used f or 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 sa me date of measurement over bare soil, suggests that, under natural fi eld conditions, backscattering from a soil surface may not be sufficie nt to explain and understand the radar response and that volume scatte ring may have to be accounted for as well. Finally, concerning soil ro ughness, the standard deviation of surface-height slope discriminates the roughness states, which apparently is a pertinent parameter for an alyzing the variation of the backscattering coefficient with incidence angle.