Improvement of the hillslope‐storage Boussinesq model by considering lateral flow in the unsaturated zone

Abstract

AbstractUnsaturated flow is an important factor that affects groundwater motion. Among various drainage models, the nonlinear Hillslope‐storage Boussinesq (HSB) model has been commonly used to predict water flux along a slope. In this study, we improved this model by considering lateral flow in the unsaturated zone. Using modified van Genuchten functions, we analytically expressed the concept of equivalent propagation thickness in the vadose zone. This analytical expression was then incorporated into the HSB model to reflect two different stages of the drainage process and to simulate the hillslope drainage process more accurately. The model results indicated that lateral flow has significant effects in the unsaturated zone during the hillslope drainage process. Even in sandy aquifers, the amount of water contributed by the unsaturated zone is a key factor that enables a decrease in the water table during the middle and late stages of the process. A comparison between the measured and simulated results based on both convergent‐type and divergent‐type hillslope drainage processes revealed that the thickness of the saturated zone decreases as the unsaturated flow increases. This study emphasizes the necessity of considering unsaturated flow in the HSB model to improve the accuracy of predicting groundwater outflow rates and develop more accurate hydrographs. The concept of equivalent propagation thickness also provides a criterion for assessing the importance of unsaturated lateral flow for future drainage research.