Millimeter-Scale Monitoring of Near-Surface Soil Water Content During Column Evaporation Using a Multi-TDR Probe

Water-content changes in the upper few centimeters control the evaporation response, but this layer is difficult to measure at millimeter-scale spacing. We evaluated a printed-circuit-board-based multi-TDR probe (MTP) in a homogeneous sand column. The probe has eight three-wire measurement sections at depths of 0.28–2.52 cm. Electric-field simulation showed that the response of each section was localized around the embedded conductors, so the measured permittivity had to be treated as a probe-specific response. Calibration against gravimetrically determined water content confirmed that the εMTP–θ relationship differed from the standard Topp equation and required section-specific calibration. The calibrated MTP estimates agreed with the gravimetric reference values, with R2 = 0.996 and root mean square error (RMSE) = 0.0075 m3 m−3. During evaporation, MTP-derived profiles in the upper 0–30 mm were compared with HYDRUS-1D simulations constrained by pressure-head, mass-loss, and water-retention data. The MTP–HYDRUS comparison gave a pooled RMSE of 0.0376 m3 m−3 and a weighted mean absolute error (MAE) of 0.0282 m3 m−3, with the largest differences in the upper 0–20 mm. The results indicate that the MTP can provide independent millimeter-scale water-content profiles for checking near-surface water redistribution and for identifying depths where simulated surface-layer θ profiles depart from direct observations.