The Influence Of Anisotropy Of Soil Hydraulic Conductivity On Rainfall-runoff Process In Mountainous Area Of Southwest China

Mountainous areas are characterized by steep terrain and frequent rainstorms,which are prone to form large floods within a short period of time.This may lead to flash floods,mud-rock flows and other geologic hazards,threatening the safety of human and property seriously.The hydrological response in mountainous catchment is the basis of flash flood and one of the core scientific issues in flash flood related research.Soil saturation hydraulic conductivity,which represents soil infiltration capacity,is an important physical quantity affecting rainfall-runoff process and is usually used as a key parameter in numerical simulation.The anisotropy of soil hydraulic conductivity is widespread in nature,and is generally expressed by anisotropy ratio(Kr):the ratio of horizontal(Kh)and vertical(Kv)saturated hydraulic conductivity(Kh/Kv).However,the influence of such soil characteristics on runoff generation has not been considered in most studies,making it difficult to reproduce the hydrograph of real flood event.In this paper,a physics-based model is applied to Jianpinggou,a mountainous catchment in China,to simulate the rainfall-runoff process under different scenarios of anisotropic hydraulic conductivity.The main conclusions are as follows:(1)The anisotropy of soil hydraulic conductivity could have significant impacts on runoff simulation.Peak discharge,total flow and peak time all vary with anisotropic ratio(Kr).With the increase in Kr,the peak discharge can be substantially elevated,when Kr is 10,it could be 70%higher than the isotropic base case.The time to peak discharge could also be shortened by larger Kr value.(2)During runoff generation,the overall surface saturation in the catchment increases with the decrease of Kr,and the saturation area from the river's periphery to the upstream source also expands gradually.With the increase of Kr,the surface water depth at the upland of the catchment gradually decreases.Although the surface saturated area is smaller,the channel water depth increases.(3)The anisotropy of soil hydraulic conductivity can effectively change velocity and direction of soil water movement.With the increase of Kr,the flow velocity increases significantly,especially the lateral movement of soil water,and the flow direction gradually switches to the horizontal direction.When subsurface stormflow reaches the channel,the larger Kr will also accelerate its seepage and recharges streamflow in the channel.At the same time,the increase of soil moisture movement speed also results in the shortened peak time.The research results suggest that the anisotropy of soil hydraulic conductivity plays an important role in rainfall-runoff process.Appropriate consideration of anisotropy could help improve the estimation of hydrological response,having important application value for flash flood early warning and prediction.