Development of flow model for partly and fully saturated soils using water balance and water table depth fluctuation analysis

Accurate estimation of soil water content and water table depth is important for making irrigation measures and rational groundwater use in arid agricultural areas with shallow water table depth. Soil water balance models are widely used due to their high computational efficiency, while they are less applicable in areas with shallow water table depths because they usually ignore the matric potential. In addition, the effect of capillary rise on soil water state is also ignored. To address these problems, a flow model for partly and fully saturated soil, referred to as modified UBMOD, was developed in this study based on UBMOD and the water table depth fluctuation analysis. UBMOD is a soil water balance model that differs from other water balance models by considering soil water movement driven by matric potential. In the modified UBMOD, soil water flow and groundwater recharge rate are calculated using the original UBMOD algorithm, and water table depth is calculated using the water table depth fluctuation analysis. The field capacity used in the modified UBMOD was considered as a variable related to the water table depth to reflect the impact of capillary rise, which helps to obtain accurate water flux across the saturated-unsaturated interface. Two synthetic cases, simulated with HYDRUS, were used to test the sen-sitivities of the model parameters and specifications. Two real-world cases were used to show the fidelity of the model to the observed data. The results demonstrated that the model could solve the unsaturated-saturated problems based on more readily available parameters, without stringent requirements on spatial and time steps, with a full guarantee of water balance and low computational cost. This study provides an effective tool for hydrodynamic studies in areas with shallow water table depths.