Laboratory and field characterization of preferential flow under consideration of geometric fracture features in a karst region in southwest China

As a result of the widespread distribution of soil fractures in karst regions, preferential flow is a well-known occurrence there, and understanding how it moves is crucial for the cognition of the local hydrological process. This study used indoor column experiments in conjunction with dye tracing to simulate the widely dispersed soil fractures in the karst region, and analysed the movement of preferential flow under various fractures. On the basis of indoor test results, field experiments were designed to measure the real-time infiltration process of the preferential flow under two typical forests. Results revealed that preferential flow only occurred when the fracture was connected to the soil surface. The connectivity of fractures and their angle affected the movement of preferential flow, as well as the magnitude of wetting adjacent to the preferential flow paths. The magnitude of flow imbibition into the matrix system is ranked as follows: soil with fractures connected to the surface > soil with fractures not connected to the surface > homogeneous soil without fracture. In addition, the wetting area in the soil was larger when two adjacent fractures were inwardly inclined than when two fractures were vertical or outwardly inclined, but the degree of preferential flow development was relatively lower. The cumulative infiltration volume and steady infiltration rate of the preferential flow under the mixed forest were higher than those of the matrix flow, whereas the opposite was true for the Pinus massoniana pure forest. The cumulative infiltration volume and steady infiltration rate of both matrix flow and the preferential flow under the mixed forest were greater than those of the P. massoniana pure forest. The response of soil fracture characteristics on the preferential flow could not be ignored, and it can provide directions for the hydrological processes in the karst region.