Abstract
Preferential flow is common in clay or expansive clay soils, involving water bypassing a large portion of the soil matrix. Dye tracer experiment and numerical modeling are used to simulate the surface runoff and subsurface preferential flow patterns influenced by the soil fracture network of a relatively steep hillslope system (slope angle equals to 10 degrees). The result of the experiments indicates that part of the water is infiltrated through cracks, leading to the delay of the initial runoff-yielding time and reduction of the discharge of the surface runoff. The soil water flow is mainly in the matrix when the intensity of precipitation is low. With the increasing of precipitation, soil water movement may become in the form of preferential flow through cracks. In addition, the nonuniformity of soil water infiltration and the depth of the average water infiltration increase as the precipitation intensity increases. To this end, the complete coupling model was established by using the surface-matrix-crack (SMC) model to simulate water flow within discrete fracture as well as to simulate water flow in the soil matrix based on the concept of dual permeability using the traditional Richards’ equation. In this model, the “cubic law” of fluid motion in cracks within smooth parallel plates and the two-dimensional diffusion wave approximation to Saint-Venant equations with momentum term ignored (two-dimensional shallow water equations) were used. The model divides soil water infiltration into two forms and uses the overall method to calculate the exchange of water between the crack networks and matrix regions as well as the exchange water between surface runoff and infiltration water. Results indicate that the SMC model has better performance compared with the traditional equivalent continuum model when those models are used to simulate the surface runoff movement and the soil water movement in the presence of cracks.
Highlights
Cracks are common in clay or expansive clay soils, which are recognized as one of the reasons to form preferential flow, involving water bypassing a large portion of the soil matrix and widely considered to be a common phenomenon in hydrology
When the precipitation intensity is 30 mm/h, the initial runoff-yielding times of experiments S1 and S4 are 3147 s and 1279 s and the discharges are 815 mL and 7960 mL, respectively. It shows that the existence of the fracture network delays the initial runoff yielding of slope flow and reduces the discharge
The average dye coverage of experiments S1 to S5 was 30.7%, 55.8%, 54.2%, 22.8%, and 19.0%, respectively. These results show that the existence of the fracture network increases the soil water infiltration when the precipitation intensity is the same
Summary
Cracks are common in clay or expansive clay soils, which are recognized as one of the reasons to form preferential flow, involving water bypassing a large portion of the soil matrix and widely considered to be a common phenomenon in hydrology. Singh and Bhallamudi [15] developed a mathematical model to simulate the slope runoff by combining surface and subsurface water flow In their model, the Saint-Venant equation and simple explicit numerical method without swing (ENO) were used for surface runoff, while two-dimensional Richards’ equation and fully implicit finite difference numerical method were used for subsurface water flow. Several models that simulate preferential flow have been developed in the past two decades [20, 23] Those models are generally based on either the dual-permeability, kinematic wave, or two-stage conceptual approaches, which all assume mobile and immobile regions in soil [24]. In conjunction with laboratory experiments, numerical modelling was built to simulate coupled water movement in soil crack networks, surface water, and groundwater flow under the experimental conditions
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