Observing irrigation using SWOT SAR Ka-band data from daily calibration and validation acquisitions

While primarily designed for ocean and inland water monitoring through Interferometric SAR (InSAR) technology, the Surface Water and Ocean Topography (SWOT) Ka-band SAR sensor also presents a novel potential for agricultural applications. This study explores the sensitivity of SWOT's Ka-band backscatter to soil moisture variations, focusing on detecting irrigation events using daily observations collected during the calibration/validation (Cal/Val) phase. Daily backscatter variations from the SWOT Level 1B High-Rate Single-look Complex product were examined over an experimental irrigated grassland site, in response to irrigation events and rainfall. The analysis included first evaluating the stability of SWOT Ka-band backscatter signal, the temporal responses to both irrigation and rainfall, and the influence of vegetation density on Ka-band SAR signal penetration. Main findings showed that the Ka-band SAR data was sensitive to soil moisture variation due to irrigation, inducing an increased backscattering by an average of 4.3 dB on the same day of irrigation. For some cases of flooded vegetation persisting after irrigation, specular reflection and/or double-bounce scattering mechanisms were observed, causing an extreme increase in the Ka-band backscattering. Following complete infiltration, irrigation events induced an average increase of about 2 dB one day after irrigation which dropped back to previous levels two days later due to natural soil drying. Despite the Ka-band's short wavelength, typically limiting canopy penetration, SWOT's near-vertical incidence angle appears to enhance its ability to penetrate dense vegetation cover reaching the soil surface and detecting soil moisture dynamics. These findings open new perspectives for leveraging the daily CAL/VAL SWOT acquisitions to map irrigated areas and support agricultural water management.