Monitoring soil water content and erosion-induced surface lowering using time domain reflectometry

Accurate quantification of erosion-induced surface lowering (Δz) is essential for assessing tillage sustainability yet remains challenging. Time domain reflectometry (TDR), a promising technique for Δz measurement, relies on the contrast in dielectric properties at the soil-air interface. Existing TDR approaches require soil-specific calibration in unsaturated conditions, limiting their broad applicability. This study proposes a universal Δz monitoring method based on TDR electromagnetic waveforms in distilled water, eliminating the need for soil-specific calibration. A general relationship was established between the probe length exposed to air ( L air ) and the TDR waveform, specifically the shift in the first reflection point (Δ L 1 ), enabling simultaneous monitoring of Δz and volumetric water content (θ). Laboratory repacked soil columns and simulated rainfall experiments were conducted to evaluate the performance of the L air -Δ L 1 relationship (i.e., the Exposure-Shift model). Results show that the Exposure-Shift model accurately estimated L air and θ values, with average root mean square errors ( RMSE s) of 0.0035 m for L air and 0.0102 m 3 m −3 for θ, compared with measured values. Under simulated erosion conditions, the method achieved high accuracy, with an RMSE of 0.0027 m and a bias of −0.0003 m. The proposed approach enables reliable, simultaneous, and dynamic monitoring of erosion-induced surface lowering and θ during soil scouring. Its advantages of simplified calibration, applicability across diverse soil types, and compatibility with automated multi-point monitoring make this TDR-based approach a promising method for monitoring tillage soil erosion.