Influence of local splitting behavior at intersections on infiltration in an unsaturated fracture network

Understanding and predicting the macro-scale flow characteristics in the fractured vadose zone is of great importance for subsurface hydrological applications. Here we develop a network model to study infiltration in unsaturated fracture networks. We consider an idealized honeycomb-like fracture network composed of a series of Y-shaped and inverted Y-shaped intersections. At the scale of intersections, liquid storage/release and splitting/convergence behaviors are modeled according to local splitting relationships obtained from detailed laboratory work and numerical simulations. By varying the splitting relationships, the influence of local flow behaviors on large scale flow structures is systematically investigated. We find that when the water split tends to split equally at the intersection, a divergent flow structure forms in the network. Conversely, unequal splitting leads to preferential pathways. We also find that an avalanche infiltration mode, i.e., sudden release of a large amount of water from the network, emerges spontaneously, and is modulated by the local splitting behavior. The pathways of preferential flow is controlled by the liquid volume triggered by avalanches and the network structure. The improved understanding from this study may shed new light on the complex flow dynamics for unsaturated flow in fractured media.