Comparative Simulation And Analysis Of The Influence Of Key Soil Hydraulic Properties On The Runoff Process In The Qilian Mountains

Soil hydraulic properties refer to the physical and chemical properties of soil that affect hydrological processes.Key soil hydraulic properties refer to soil hydraulic properties that play a key role in different hydrological processes.Accurately identifying the key soil hydraulic properties that affect runoff processes and revealing their influence mechanisms can help clarify the relationship between soil hydraulic properties and hydrological processes,understand the impact of changes in soil hydraulic properties on runoff processes,and provide a theoretical basis for the development of hydrology and improvement of model simulation performances.In the northwest arid and semi-arid mountainous areas,the outflow from mountain is almost the total water resources of the entire river basin,which is extremely important for the production,life,and ecology of people in the middle and lower reaches.This study takes two small watersheds,Pailugou and Shangzhangfanggou in Qilian Mountains as research areas,uses classical statistical methods to study the heterogeneity of soil hydraulic properties of soils with different textures in the watersheds,and identifies key soil hydraulic properties that have a significant impact on runoff processes based on Soil and Water Assessment Tool（SWAT）model.Setting up different scenarios with different soil hydraulic properties,this study analyzes the impact and mechanism of changes in key soil hydraulic properties on runoff processes.It is expected to provide data support for the development of mountainous hydrology.The main findings are as follows:（1）The soil hydraulic properties of Silt and Silt Loam in the study area show significant vertical distribution differences.With the increase of soil depth,soil bulk density（BD）tends to increase,while soil saturated hydraulic conductivity（Ks）,soil available water capacity（AWC）,and soil organic carbon（SOC）tend to decrease.The maximum difference between soil layers almost always appears between Layer 1 and Layer 2 or between Layer 2 and Layer 3.The mean values of BD for each soil layer are between 0.67 and 1.16g·cm^-3,the mean values of Ks are between 23.47 and 112.68mm·hour^-1,the mean values of AWC are between 0.14 and 0.20 mm,and the mean values of SOC are between 1.42 and 14.68g·100g^-1.Ks has the most obvious vertical change,while AWC has the least obvious change.The values of soil hydraulic property heterogeneity from large to small is Ks,SOC,BD,and AWC,and there is a significant change in heterogeneity with increasing soil depth.The coefficient of variation range of Ks for each soil layer is between 0.52 and 1.04;the coefficient of variation range of SOC is between 0.44 and 1.03;the coefficient of variation range of BD is between 0.07and 0.44;the coefficient of variation range of AWC is between 0.07 and 0.26.With the increase of soil depth,the heterogeneity of Ks and SOC increases,while that of BD and AWC decreases.（2）In the arid and semi-arid mountainous areas,the key soil hydraulic properties that affect the runoff process are Ks,BD,and AWC.The sensitivity ranking of the three parameters in the Pailugou is AWC>BD>Ks,while that in the Xiying River area is Ks>AWC>BD.The difference in sensitivity is mainly affected by the difference in evapotranspiration and runoff generation mechanism.The evapotranspiration in Pailugou accounts for 67.7%of the basin precipitation,and almost all runoff comes from interflow;while in Xiying River,evapotranspiration accounts for 60.6%of basin precipitation,surface runoff accounts for 41.8%,groundwater runoff accounts for12.2%,and interflow accounts for 46.0%of the runoff.Among them,AWC and BD can affect the evapotranspiration under soil water stress and interflow conditions,while Ks affects infiltration,surface runoff,and interflow,resulting in AWC and BD being more sensitive than Ks in exposed ditches;in Xiying River,Ks is more sensitive than AWC and BD.（3）Keeping vegetation and climate conditions constant,changes in soil hydraulic properties can significantly affect runoff,with an impact range of-10.21%to 11.58%.When explaining the change in runoff,the key soil hydraulic properties can explain 53%to 126%of the impact of all soil hydraulic property changes on runoff,with the part exceeding 100%being the case where the impact of replacing all soil hydraulic properties is smaller than that of replacing only key soil hydraulic properties.The main impact is on runoff from April to October;texture changes also have a significant impact on the runoff process,and non-key soil hydraulic properties that include texture changes（proportions of clay,gravel,silt,soil organic carbon,and soil conductivity）can explain 47%of the impact of replacing all soil hydraulic properties on runoff in the texture classification change scenario.After changes in soil hydraulic properties,runoff processes are mainly affected by changes in evapotranspiration,deep percolation,and interflow.Among them,deep percolation refers to the amount of water supplied to the aquifer that can affect the groundwater flow;interflow refers to the lateral flow produced in the soil area above the aquifer below the surface.（4）The order of the impact of the three key soil hydraulic properties on runoff is Ks>BD>AWC.When BD increases,Ks increases,and AWC decreases,the amount of runoff in the watershed increases;when BD decreases,Ks decreases,and AWC increases,the amount of runoff in the watershed decreases.Changes in BD mainly affect changes in interflow production conditions,infiltration conditions,and evapotranspiration,thereby affecting the runoff process;the ranges of changes in the three main hydrological components of interflow,evapotranspiration,and deep percolation are-10.75～4.98 mm,-5.45～4.76 mm,and-9.6～-0.08 mm respectively.Changes in Ks cause changes in interflow production environment,changes in water supply to the recharge zone by infiltration,and changes in groundwater supply,thereby affecting the runoff process;the ranges of changes in the four main hydrological components of interflow,groundwater flow,deep percolation and evapotranspiration are-79.65～21.01 mm,0.18～21.67 mm,-0.02～69.87 mm and-18.54～9.59 mm respectively;when Ks changes significantly,the main components of watershed runoff may change.Changes in AWC lead to changes in field capacity,changes in interflow production conditions,and changes in evapotranspiration under soil water stress,thereby affecting the runoff process.The ranges of changes in the three main hydrological components of interflow,evapotranspiration and deep percolation are-9.55～-1.88 mm,-0.86～14.36 mm and-15.54～8.79 mm respectively.