Sensitivity of Sentinel-1 Backscatter to Vegetation Dynamics: An Austrian Case Study

Abstract

Crop monitoring is of great importance for e.g., yield prediction and increasing water use efficiency. The Copernicus Sentinel-1 mission operated by the European Space Agency provides the opportunity to monitor Earth’s surface using radar at high spatial and temporal resolution. Sentinel-1’s Synthetic Aperture Radar provides co- and cross-polarized backscatter, enabling the calculation of microwave indices. In this study, we assess the potential of Sentinel-1 VV and VH backscatter and their ratio VH/VV, the cross ratio (CR), to monitor crop conditions. A quantitative assessment is provided based on in situ reference data of vegetation variables for different crops under varying meteorological conditions. Vegetation Water Content (VWC), biomass, Leaf Area Index (LAI) and height are measured in situ for oilseed-rape, corn and winter cereals at different fields during two growing seasons. To quantify the sensitivity of backscatter and microwave indices to vegetation dynamics, linear and exponential models and machine learning methods have been applied to the Sentinel-1 data and in situ measurements. Using an exponential model, the CR can account for 87% and 63% of the variability in VWC for corn and winter cereals. In oilseed-rape, the coefficient of determination ( R 2 ) is lower ( R 2 = 0.34) due to the large difference in VWC between the two growing seasons and changes in vegetation structure that affect backscatter. Findings from the Random Forest analysis, which uses backscatter, microwave indices and soil moisture as input variables, show that CR is by and large the most important variable to estimate VWC. This study demonstrates, based on a quantitative analysis, the large potential of microwave indices for vegetation monitoring of VWC and phenology.

Crop Monitoring Using Sentinel-1 Data: A Case Study from The Netherlands

“…The VV and VH backscatter start to plateau as the maize reaches its maximum height and the grain begins to develop (BBCH 71). This is consistent with previous studies showing that radar backscatter from maize parcels tends to saturate around the tasseling stage (BBCH 51) as the LAI reaches values of 2-3 [25,51,52].…”

“…At the same time, the VV backscatter increased slightly from −9.5 to −7.5 dB. The combined effect is a decrease in VH/VV ratio consistent with the conclusion of Vreugdenhil et al that the dynamics VH/VV reflect changes in vegetation water content [25].…”

“…VV backscatter is dominated by the direct ground and canopy contributions. The decrease in VV during the stem elongation is therefore attributed to the increasing attenuation due to the predominantly vertical structure of the wheat [24,25,[55][56][57][58]. The decrease observed in VH is less severe than in VV.…”

“…During the tillering and stem elongation periods, the increased attenuation of the double-bounce is countered by an increase in volume scattering from the new biomass [24,48]. The net effect is an increase in the VH/VV ratio, affirming its suitability as a measure of fresh biomass [25]. From mid-May to 1 June, during the booting and heading periods, VH backscatter remains stable while the VV backscatter starts to increase as the flag leaf opens and the inflorescences emerges.…”

“…It is noteworthy that these events have little or no influence on the VH/VV ratio. This reflects its reduced sensitivity to soil moisture variations compared to the individual VV and VH backscatter [25].…”