Planting and Observation Geometry Effects on L-Band SAR for Water Management Monitoring in Paddy Fields

Methane is the second most significant greenhouse gas after carbon dioxide contributing to climate change. In recent years, methane emissions from paddy fields have attracted much attention, and Alternate Wetting and Drying (AWD) is a water management technique that contributes to both water-saving and the reduction of methane emissions. Although monitoring rice growth using satellite data has progressed, research on methane emissions from rice paddies is still limited. In this study, we used L-band SAR data to explore how to identify inundated and non-inundated paddy fields to monitor AWD implementation. We analyzed data from field surveys and data from over 15,000 fields in Miyagi Prefecture, Japan where rice is regularly planted, and evaluated the impact of geometric characteristics such as satellite observation direction and planting direction on backscatter coefficients. Our results confirmed that when the observation direction and planting direction were aligned, strong backscattering occurred for surface scattering and double bounce regardless of the incidence angle. Furthermore, we found that it is necessary to develop algorithms that consider both planting direction and planting time/rice growth stage for inundated/non inundated identification using L-band SAR data.