Quantitative characterization and simulation of soil moisture distribution in heterogeneous vadose zone

Abstract:：[Objective] The vadose zone, as a crucial linkage between vegetation and groundwater, plays a significant role in influencing groundwater ecological functions. Among the various factors, lithological structure stands out as one of the primary factors affecting groundwater ecological functions. In order to investigate how the lithological structure of the vadose zone influences soil moisture distribution, [Methods] laboratory experiments were conducted on layered heterogeneous soil columns to investigate water release and infiltration. The distribution patterns of water movement during soil moisture infiltration, particularly the formation mechanism and distribution of capillary fringe above the fine-grained soil layer, were analyzed. Additionally, a comparative analysis was performed on the coupling of three commonly used Soil Water Characteristic Curve (SWCC) models with the Richards model. Based on this analysis, modifications were made to the simulation methods to better simulate water distribution in layered heterogeneous soils. [Results] During the experiment, the fine-grained soil interlayer structure causes water retention in the interlayer and at the upper and lower interfaces of the interlayer, forming a water accumulation area. Compared with column O, the water release duration of column A, column B and column C increased by 290h, 500h and 780h, respectively, and the water holding capacity increased by 6.20cm, 7.90cm and 7.83cm, respectively. The water retention in the interlayer of column A, column B and column C and at the interlayer interface accounted for 24.50%, 33.09%, 45.77% and 3.19%, 5.01% and 11.38% of the total water retention, respectively. The numerical simulation results show that the modified van Genuchten model can better simulate the layered soil moisture profile. [Conclusion] Fine-grained soil interlayer has a significant retarding effect on the underwater migration characteristics of infiltration, and water is mainly retained inside the interlayer and at the upper and lower interfaces of the interlayer. The increase in the thickness and number of interlayers will lead to more water retention inside the interlayer and at the interlayer interface. The corresponding relative permeability equation is divided into three stages, which can effectively improve the simulation accuracy of layered soil water content profile.