Validation of a dry-down event using UAV-based passive L-band microwave radiometer

Although satellite-based soil moisture products are useful at regional and global scales, they may not fully meet the needs of users who require high resolution information for applications such as precision agriculture or catchment hydrologic modeling. The advent of uncrewed aerial systems (UAS) has opened new opportunities for bridging this need. We offer one of the very first validation studies on a dry-down event captured by a novel L-band radiometer onboard Black Swift Technologies' S2 fixed-wing drone. A fallow cornfield at the Beltsville Agricultural Research Center (BARC) was selected as the study area to validate volumetric soil moisture (VSM) estimates in mid-April 2023. By leveraging a comparable radiative transfer model to that of SMAP and SMOS derived products, brightness temperature retrievals from this sensor are shown to successfully capture a week of dry-down post-storm event, as validated by HydraGO probe estimates (calibrated with gravimetric samples) and a nearby SCAN site. However, with a reported bias of -11% (compared to calibrated, fitted VSM values) and an ubRMSE of 5.2% potential concerns remain regarding the calibration, vegetation, surface roughness corrections, and georeferencing. Nevertheless, given the UAS' great potential for on-demand, high-resolution soil moisture retrievals, this study is critical for informing future applications and improvements in the field of passive microwave remote sensing.