Analysis of short-term soil moisture effects on the ASCAT backscatter-incidence angle dependence

Abstract

The incidence angle dependence of C-band backscatter is strongly affected by the presence of vegetation in the sensor footprint. Many studies have shown the suitability of this dependence for studying and monitoring vegetation dynamics. However, short-term dynamics in the backscatter-incidence angle dependence remain unexplained and indicate that secondary effects might be superimposed on the vegetation component. In this study, we hypothesize that the observed short-term dynamics are caused by soil moisture. We investigate the effect by exploring relationships between the slope of the backscatter-incidence angle dependence (σ′) from the Advanced Scatterometer (ASCAT) and soil moisture, rainfall, temperature, and leaf area index. We carry out the analysis over six study regions in Portugal, Austria, and Russia with different climate, land cover, and vegetation cycles. Our results indicate that soil moisture has an effect on σ′. Spearman correlations of σ′ anomalies with soil moisture anomalies are stronger than with any other variable in most study regions and range from −0.38 to −0.70. Even when accounting for effects of water on canopy, correlations between σ′ and soil moisture remain relatively strong, ranging from −0.14 to −0.46. These results confirm the presence of secondary effects in the dynamic σ′, which need to be corrected for when applying σ′ in studies of vegetation dynamics. A correction may be achieved by the application of a suitable smoothing on σ′ (i.e., removing high frequency signal components), by masking observations taken under wet conditions, or by the use of models that explicitly account for the effect of soil moisture on σ′.