Robotic mapping of soil volumetric water content with geospatial soil apparent electrical conductivity in micro-irrigated citrus orchards in California

Soil moisture plays a crucial role in irrigation management and in understanding key hydrological and agronomic processes. This study investigated an innovative approach to estimate soil volumetric water content (VWC) using apparent electrical conductivity (EC a ) measured by a portable electromagnetic induction (EMI) sensor mounted on a semi-autonomous ground robot. The investigation was conducted between October 2024 and March 2025, in two California citrus orchards, each surveyed four times. Geospatial EMI data were acquired across the entire fields. The VWC ground-truth measurements (0–0.12 m) were collected using a time domain reflectometry sensor (i.e., TDR-VWC) at twenty 0.5 m × 0.5 m footprints per orchard. The EC a data were calibrated to estimate TDR-VWC with analysis of covariance regression. Different model formulations were used to investigate the TDR-VWC prediction errors due to varying model inputs and size of the ground-truth sample (N = 2, 3, 4, 5, 6, 8, 10, and 12). All models were calibrated and evaluated (3 datapoints per field) 10,000 times using randomly selected calibration and evaluation datapoints. With N = 12 per field, most model formulations had median evaluation root mean square errors (RMSEs) ∼ 0.040 m 3 m −3 . When minimal ground-truth (N = 2) was used to calibrate the relationship between EC a and TDR-VWC, the best model had a median evaluation RMSE = 0.048 m 3 m −3 . Accuracy improved (RMSEs = 0.040 m 3 m −3 at N = 6) with more calibration points. With N > 6 benefits became marginal. This research advanced the field of VWC sensing in precision agriculture by combining robotic EC a measurements and data driven modeling using minimal ground truth to derive accurate VWC estimations. Researchers, growers, and practitioners may employ this approach to obtain VWC maps to improve irrigation management at the orchard scale.