Influence of agricultural crop type on UAV-based passive microwave soil moisture retrieval at L-band

Soil moisture is a critical variable for hydrological and agricultural processes, yet its accurate characterization remains challenging due to its high spatial and temporal variability. Passive microwave-based soil moisture observations are routinely available from satellite platforms, but their spatial resolution limits their applicability for many field-scale applications, creating a need for intermediate-scale solutions. Recent advances in unmanned aerial vehicle (UAV) platforms provide new opportunities to observe soil moisture at relatively high spatial resolution offering a promising approach to bridge this spatial gap.

This study evaluates a new UAV-mounted L-band (1.4 GHz) polarimeter developed by Skaha Remote Sensing for monitoring soil moisture at high spatial resolutions. Weekly UAV flights were carried out from mid-April to early November 2025 over a long-term crop rotation and tillage experiment at the University of Guelph’s Elora Research Station. Over these plots, the long-term management practices have resulted in measurable differences in soil structure and hydrological properties across experimental plots. Passive microwave L-band brightness temperature observations were collected across 56 plots representing several crop types (soybeans, corn, winter wheat, barley, and alfalfa) and tillage treatments. The brightness temperatures were compared to in situ soil moisture measurements obtained from each of the plots.

Over the growing season, strong, statistically significant correlation relationships were observed between the brightness temperatures and measured soil moisture (r -0.38 – r -0.9). These relationships suggest that the UAV-based L-band polarimeter can measure moisture variability at relatively high spatial resolutions (~7m; dependent on flying height) and during all time periods of the growing season analyzed. Comparisons by crop type and phenological stage suggests the performance of the sensor for soil moisture retrieval varied by crop type and phenological stage. Crops such as corn and soybeans with relatively larger row spacing generally showed stronger (negative) correlations between the measured brightness temperatures and soil moisture (r < -0.7) while denser crops with more ground coverage (alfalfa, barley) exhibited weaker correlations (r between -0.6 and -0.38). Overall, the results demonstrate the potential of UAV-mounted L-band radiometry to potentially bridge the scale gap between point-based in situ measurements and coarse-resolution satellite observations.