Active microwave remote sensing for soil moisture measurement: a field evaluation using ERS-2

Active microwave remote sensing observations of backscattering, such as C-band vertically polarized synthetic aperture radar (SAR) observations from the second European remote sensing (ERS-2) satellite, have the potential to measure moisture content in a near-surface layer of soil. However, SAR backscattering observations are highly dependent on topography, soil texture, surface roughness and soil moisture, meaning that soil moisture inversion from single frequency and polarization SAR observations is difficult. In this paper, the potential for measuring near-surface soil moisture with the ERS-2 satellite is explored by comparing model estimates of backscattering with ERS-2 SAR observations. This comparison was made for two ERS-2 overpasses coincident with near-surface soil moisture measurements in a 6 ha catchment using 15-cm time domain reflectometry probes on a 20 m grid. In addition, 1-cm soil moisture data were obtained from a calibrated soil moisture model. Using state-of-the-art theoretical, semi-empirical and empirical backscattering models, it was found that using measured soil moisture and roughness data there were root mean square (RMS) errors from 3Ð5 to 8Ð5 dB and r2 values from 0Ð00 to 0Ð25, depending on the backscattering model and degree of filtering. Using model soil moisture in place of measured soil moisture reduced RMS errors slightly (0Ð5 to 2 dB) but did not improve r 2 values. Likewise, using the first day of ERS-2 backscattering and soil moisture data to solve for RMS surface roughness reduced RMS errors in backscattering for the second day to between 0Ð9 and 2Ð8 dB, but did not improve r2 values. Moreover, RMS differences were as large as 3Ð7 dB and r2 values as low as 0Ð53 between the various backscattering models, even when using the same data as input. These results suggest that more research is required to improve the agreement between backscattering models, and that ERS-2 SAR data may be useful for estimating fields-scale average soil moisture but not variations at the hillslope scale. Copyright  2004 John Wiley & Sons, Ltd.