Accuracy of soil water content measurements using ground penetrating radar: comparison of ground penetrating radar and lysimeter data

The ground penetration radar (GPR) can be used for soil water content measurements. To prove the accuracy of the GPR, measurements were carried out on four lysimeters (surface 1 m2, depth 1.5 m) during one vegetation period. The lysimeters were planted with lucerne and filled with three different soils: a loamy sand, a sandy loam and a silt loam. The ground water tables were at 1.35 and 2.1 m. The lysimeters were weighed, so that it was possible to calculate the changes in water content with very high accuracy. For the GPR-measurements a 1 GHz-antenna was used. Only in the sandy loam reflected signal from the bottom of the lysimeter could be obtained. The standard deviation between the GPR and the lysimeter data was 0.01 m3/m3 using all data and 0.0026 m3/m3 using only measurements in the wetter range. For this material an experimental calibration curve between the soil water content (θ) and the relative dielectric constant (ε) was calculated by comparing GPR-results with lysimeter data. The data results in a linear function θ(ε) for the measured range of water contents. Compared to published calibration curves used for TDR-technique, the Topp-function gives the best results.