Variation Of Hydrological Connectivity And Its Response To Runoff And Sediment Discharge In Typical Small Watersheds On The Loess Plateau

The study on the change of hydrological connectivity process under the influence of returning farmland to forest(grassland)on the Loess Plateau is of great significance to reveal the influence mechanisms of vegetation change on soil erosion process.Previous quantitative characterization of hydrological connectivity failed to reveal the resistance mechanism of hydrological connectivity in the process of runoff,confluence,sediment production and transport from the perspective of functional connectivity.Therefore,this paper analyzes the impact of returning farmland to forest and grassland on the runoff and sediment processes of four typical small watersheds in the middle reaches of the Yellow River.A new hydrological connectivity index(IC_f)is constructed by coupling the Darcy-Weissbach resistance(f),and the spatiotemporal variation of hydrological connectivity is evaluated.The comparative analysis of IC and IC_f in characterizing runoff and sediment transport processes is of great value for revealing soil erosion mechanism and hydrological process simulation.The main conclusions are as follows:(1)From 1980 to 2015,60%of farmland in Fenchuan River and Shiwangchuan,which were dominated by forest,was converted to forest;more than 40%of farmland in Qingjian River and Tuwei River,which were dominated by grassland,was converted to grassland.The runoff and sediment discharge of Fenchuan River,Shiwangchuan,Qingjian River and Tuwei River decreased significantly from 1980 to2015.The runoff of Fenchuan River and Shiwangchuan decreased by 63.7%,55.2%,and the sediment discharge decreased by 38.2%and 41.9%.The runoff of Qingjian River and Tuwei River decreased by 42.1%,38.0%,and the sediment discharge decreased by 57.4%and 84.0%.(2)From 1980 to 2015,IC and IC_f decreased significantly(P<0.05).The hydrological connectivity index(IC),with C as the weighted factor to indicate the possibility of erosion and transport,decreased by 71.63%,74.35%,81.46%and 87.17%in Fenchuan River,Shiwangchuan,Qingjian River and Tuwei River,respectively.The hydrological connectivity index IC_f,using f as the weighting factor to accurately represent the possibility of transport,decreased by 44.17%,41.32%,51.46%and47.38%,respectively.(3)The hydrological connectivity index IC and the hydrological connectivity index IC_f that coupling resistance have a significant positive linear correlation with the annual runoff and sediment discharge,and IC_f is better than IC in the characterization of hydrological connectivity process.