耦合基质区与裂隙网络的土壤优先流模型及验证

该文旨在通过分析土壤裂隙存在下的土壤优先流运动模式与特点,建立耦合基质区与裂隙网络的土壤优先流模型(crack preferential flow model,CP模型)。用控制体积有限元法求解两区土壤水分运动的基本方程,采用整体法将土壤基质水流运动方程和三维土壤裂隙网络区水流运动方程耦合。模拟与试验结果表明,增加降雨或灌溉强度能促进激活裂隙通道作为优先流通道,有利于优先流形成。CP模型及耦合方法较好地描述出了优先流运动的特点,模拟与实测的土壤饱和度均方根误差为0.016~0.025,决定系数均在0.78以上(P0.05),偏差为0.020~0.026,一致性指标为0.918~0.938。与等效连续体模型(Richards模型)相比较,CP模型模拟裂隙存在情况下土壤水流非均匀性规律更有效。研究丰富了土壤优先流理论,对提高农田水分利用率具有重要的意义。

Preferential flow model coupling soil matrix with fracture network and its validation

Abstract: Preferential flow refers to such a flow transport pattern that water and solute flow round soil matrix, which accelerates soil water movement. Soil cracks are a part of reasons that lead to preferential flow. This study aimed to analyze soil preferential flow pattern in order to establish a model of preferential model based on soil fracture network. A field color tracer experiment was carried out in Ningxia University from July to September in 2015 under precipitation intensity of 20 and 50 mm/h, and rainfall duration of 20-60 min. The results showed that when the rainfall intensity was 20 mm/h, the water infiltration depth increased with the increase of rainfall duration and the preferential flow began to develop, but it was still overall-matrix flow and the preferential flow was only locally developed. When precipitation intensity was 50mm/h and the duration of rainfall was 20min, water flows bypassed the soil matrix and preferential flow phenomena was observed obviously. The infiltration depth exceeded the fracture depth after duration of rainfall increased to 50min, soil fracture had not play a major role in the soil water movement, and the flow patterns were overall-matrix flow. The results showed that the flow patterns were mainly matrix flow under lower rainfall or irrigation intensity, and preferential flow under higher rainfall or irrigation intensity. A coupling model of soil matrix and fracture network for simulating preferential flow was established (CP model). In this model, the soil medium was divided into matrix and fracture networks formed by flow channels, each having its own saturated hydraulic conductivity and water movement equation. These equations were solved by using the control volume finite element method, and the nonlinear equations were linearized by Newton-Raphson methods to improve the convergence, and shorten the operation time. Equation of water movement in matrix and fracture network flow equation were coupled using global approach which solved the problems of the flow mutual influence caused by the cross-fracture network, and improved the computational efficiency. The validity of the proposed model was tested by the analysis of the soil moisture content between the simulated and the observed values. The results showed that the root mean squared error (RMSE) of soil moisture was 0.016-0.025, R2 of soil moisture was above 0.78, bias was 0.020-0.026, index of agreement (IA) was 0.918-0.938, indicating good simulation by the CP model. In the end, different crack structure but statistically equivalent networks was generated to validate the model simulation and produced well simulation results. These numerous fractures had stable effect on the variances of soil moisture. The results of this paper can provide valuable information for analyzing preferential flow and the coupling mechanism of the water content and solute transport between soil dry shrinkage crack and soil matrix.