Use of a C-Band Ground-Based Scatterometer to Monitor Surface Roughness and Soil Moisture Changes

Eight experiments on remote sensing of soil moisture and surface roughness were carried out over bare fields with a microwave C-band scatterometer from 1998 until 2001. This device is able to provide backscattering coefficients in the range of +10 dB and −40 dB for incidence angles between 10° and 60°. The objective is to assess the conditions (of roughness and incidence angle) in which it is possible to separate the effects of roughness and soil moisture, thus allowing a reliable estimation of soil moisture from backscattering coefficients. In particular, starting with measurements carried out at different incidence angles, we experimented with an approach to normalize the backscattering coefficients to a reference angle with the principal aim of comparing data sets acquired in different conditions and putting in evidence the radar response to soil moisture variations. A sensitivity analysis, performed over the whole acquired data set, confirmed that the dependence of backscattering coefficients on the incidence angle is influenced more by surface roughness than by soil moisture, as indicated also by theoretical models and other similar data sets. A significant result is that a simple model for comparing data acquired with different incidence angles works better for rough surfaces: only in this case an acceptable correlation between backscattering coefficients and soil moisture is retained. In order to better understand this behavior, the experiments carried out in 2001 were designed to acquire radar measurements on a test site where soil moisture was controlled with artificial irrigation and constantly monitored during the dry-down phase. This allowed a direct estimation of the relationship between radar responses and soil moisture, a quantitative evaluation of the sensitivity of our device and a test for the model developed using the previous acquisitions.