Influence of Liquid Splitting Behavior at Intersections on Infiltration Dynamics in an Unsaturated Fracture Network

Abstract

AbstractUnderstanding the macro‐scale flow characteristics in the fractured vadose zone is of great importance for subsurface hydrological and environmental applications. Here we develop an idealized fracture network model composed of a series of linked intersections, aiming to reveal the roles of local fluid flow, storage and splitting behaviors at intersections in controlling macroscopic unsaturated flow in a fracture network. By setting different local flow rules, unsaturated flow dynamics and structure in networks are systematically investigated. Numerical simulations suggest that avalanche infiltration mode, that is, sudden release of a large amount of water from the network, emerges spontaneously, and is modulated by the local splitting behavior. In terms of water infiltration structure, we find that uneven liquid splitting at intersections inevitably leads to preferential flow in networks. The preferential paths strongly rely on the competition between gravitational and capillary forces, relating to network structure and water saturation. When flow is dominated by gravity, preferential paths extend along large‐aperture routes; conversely, when flow is dominated by capillarity, the small‐aperture paths are preferred. In terms of infiltration dynamics, we show that the power spectral density of the water saturation time series in the network follows a power law with an exponent of −2 for all simulations with different structural parameters and local flow rules, suggesting a universal self‐organized criticality behavior for unsaturated flow in fractured rocks. The improved understanding from this study may shed new light on the complex flow dynamics for unsaturated flow in fractured media.