Improvement of the Kovács water retention model

Abstract

In the unsaturated soil zone (vadose zone), the relationship between saturation and suction potential plays a crucial role in describing the connection, known as the water retention curve (WRC). Various models for the WRC exist, with the most widely used being the van Genuchten model. Determining the parameters of the van Genuchten model requires laboratory investigations. The advantage of the Kovács water retention model lies in its physically interpretable parameters that can be derived through independent measurements. However, a drawback is its tendency to overestimate soil saturation in lower suction ranges. This paper focuses on addressing this issue by improving the Kovács model and comparing it with the van Genuchten model using the 3D Soil Hydraulic Database of Europe v1.0. For comparison, the root mean square error (RMSE) was calculated between the results of the two methods, with parameter optimization during calibration. The improvement was achieved by a suction correction function. The average RMSE value between the van Genuchten and the original Kovács model was 0.0613, while between the van Genuchten and the modified Kovács model, the average RMSE value was 0.0145. Therefore, the modification resulted in a 4.3-fold improvement. Polynomial correlation was established between the values of the suction correction factor and the equivalent capillary rise, simplifying the computation of the modified function. With this simplification, the average RMSE value reduced to 0.0150. This represents a 3% degradation, but the improvement remains significant compared to the original Kovács function. The computations were performed for over 600 monitoring wells during optimization. Comparative analysis was conducted for 30 randomly selected monitoring wells, comparing the equivalent capillary rise derived from borehole data with the equivalent capillary rise derived from the parameters of the van Genuchten model from the 3D Soil Hydraulic Database of Europe v1.0. The correlation between the equivalent capillary rise determined by the two methods proved weak, with a correlation coefficient of R² < 0.25. This comparison highlighted the need for further refinement of the soil hydraulic database, for which the new and improved Kovács water retention model proves suitable as a pedotransfer function.