Field calibrations of a Diviner 2000 capacitive soil water content probe on a shallow groundwater site and the application in a weighable groundwater lysimeter

The determination of the volumetric soil water content θᵥ by means of capacitive profile probes is often applied to investigate the soil water storage change ΔS that serves as basis for decisions in agricultural water management. The soil properties have a big effect on the accuracy of the θᵥ measurements. The use of only one calibration function, often provided by the manufacturers of the probes, cannot fulfil the requirements of all site conditions. Therefore, many individual calibration functions have in the past been determined for different soils and sensors. A literature review of existing calibration functions of the capacitive profile probe Diviner 2000 shows the broad range of available functions. The review makes it clear that there is a lack of functions for organic soils. These soils are typical soils of wet sites with shallow groundwater tables. The soil moisture is of big importance for many ecological processes of these sites and therefore an exact determination of θᵥ is important. A Diviner 2000 profile probe was calibrated on such a shallow groundwater site in a classic field calibration procedure and the determined functions were applied to the soil profile of a weighable groundwater lysimeter. The soil water storage change ΔS was estimated with the measured θᵥ values and compared with the measured mass change Δm of the lysimeter. The mean error (bias) between ΔS with the field calibration function and Δm was 7.8 kg and the root mean squared error (RMSE) 19.9 kg. An iterative adaptation of the calibration functions to the measured Δm values of the lysimeter reduced the bias to 0.9 kg and the RMSE to 14.0 kg. The investigations illustrate the problems of a classic field calibration under the conditions of a shallow groundwater site with low θᵥ changes in deeper soil horizons and soils with high Cₒᵣg contents as well as the inaccuracy in the determination of ΔS based on θᵥ measurements with capacitive profile probes.