Temporal behavior of major crops with SWOT Ka-band SAR observations

Accurate monitoring of crop development is critical for understanding production drivers and implementing effective agricultural policies. In this study, we propose a novel application of the surface water and ocean topography (SWOT) Ka-band synthetic aperture radar (SAR) sensor by exploring the temporal evolution of its backscattering signature across various winter and summer crops. The analysis assessed the incidence angle dependency of SWOT backscatter for each crop type, the relationship between backscatter dynamics and soil moisture variation, and the influence of the parcel slope on near-nadir SWOT backscattering. Finally, the study provides a comprehensive evaluation of the temporal backscatter behavior of SWOT at key phenological stages of each crop compared to that obtained from the C-band Sentinel-1 time series. The results revealed a typical SAR incidence-angle dependency in SWOT for each crop type despite having a small range of incidence angles between 0.7° and 4°. With near vertical looking, SWOT backscattering dynamics followed the soil moisture variation regardless of vegetation attenuation. For near-nadir geometric acquisition, steeper slopes generally resulted in lower backscatters, except for wheat, barley, and grassland, which showed increased backscatters for steeper slopes during peak vegetation stages. Concerning SWOT temporal behavior, gramineous crops exhibited similar Ka- and C-band patterns from sowing to postgermination until the heading phase. The temporal profiles of maize and rapeseed in the Ka-band aligned with previously observed X-band behaviors, whereas sunflower showed a Ka-band backscattering pattern comparable to that of the C-band. With its 21-day revisit cycle, the SWOT Ka-band time series demonstrates promising potential for operational crop growth monitoring.