| Hydrological connectivity refers to the degree of transfer and exchanges of sediment,nutrients and other organisms by water-mediated within and across landscape（as source or sink）of the catchment,which influenced by rainfall,topography,vegetation and human activities.Human activities,such as the construction of ditches,roads and terraces,have changed the natural underlying surface conditions and the spatial-temporal distribution of catchment landscape elements,leading to the changes of hydrological processes from natural terrain features to dual natural-artificial features.This affects the hydrological connectivity of catchment.Previous studies have carried out a lot of work on the impact of single human activity on hydrological processes.Due to the diversity of human activities and the complexity of hydrological processes,the actual hydrological connectivity is regulated by a variety of human activities.Therefore,it is necessary to further study the influence mechanism of human activities on hydrological connectivity.Clarifying the impact of different human activities on hydrological connectivity is of great significance to understand the hydrological dynamics,and provides an important scientific basis for sustainable management and environmental construction of the catchment.To fill these knowledge gaps,we conducted this study in Wulongchi catchment,located in Danjiangkou reservoir,which is the water source area of the middle route of South-to-North Water Diversion Project.We constructed three confluence situations:natural confluence network（N1）,ditch-road confluence network（N2）and comprehensive confluence network（N3）by using graph theory and geospatial analysis.The effects of different human activities on the flow paths,the structural characteristics of network and hydrological connectivity were quantitatively evaluated,and the key connectivity areas were identified.Moreover,two strategies were proposed to modify the confluence network structure and optimize the key connectivity areas.This study provided a scientific basis for the formulation of future catchment management countermeasures.The main results were as follows:（1）Human activities had significantly changed the natural flow paths and confluence network structure.Compared with N1 network,the change rates of flow direction of nodes in N2 network and N3 network were 22.01%and 61.90%,respectively.The results of accessibility index（Shi）and potential flow index（Fi）showed that the number of nodes with Shi>2.35×10-2 in N2 network and N3 network decreased by 56.25%and 50.00%,respectively,compared with N1 network.The node with Fi>0.73×10-2 in the N2 and N3network reduced by 8.70%and 36.96%,respectively,compared with N1 network.The construction of ditches and roads changed the flow paths and improved the nodes accessibility and the material transfer efficiency from upstream to downstream.The construction of terraces,ponds and cisterns further enhanced the hillsides and hydrological linkages between rivers and the complexity of catchment confluence network structure.Human activities enhanced the coupling degree between landscape elements and watershed outlet,and improved the structural stability of confluence network.（2）The complexity of human activities caused obvious spatial variation of hydrological connectivity.The comprehensive evaluation system of hydrological connectivity was constructed by Network Structural Connectivity index（NSC）,Residual Flow index（RF）,Integral index of connectivity（IIC）and Node importance index（d IIC）.The results showed that,compared with N1 network,the area changes of different connectivity levels in N2 and N3 network had the similar pattern,which showed that low connectivity(NSC<0.32,RF<-0.10×10-2,d IIC<1)and the high connectivity(NSC>4.08,RF>0.10×10-2,d IIC>15)decreased,and the moderate connectivity(0.32≤NSC<4.08,-0.10×10-2≤RF<0.10×10-2,1<d IIC<15)increased.Taking d IIC as an example,compared with the N2 network,the area of the low connectivity area of the N3 network reduced by 1.28%,and the area of the medium connectivity area increased by 5.77%.Ditches and roads were the key factors to control the changes of the overall hydrological connectivity.However,the terraces,ponds and cisterns had relatively small impacts.The key connectivity areas of the N2 network and the N3 network reduced by 92.94%and95.29%,respectively,compared with the N1 network,indicating that human activities had significantly reduced the hydrological connectivity.（3）Modifying the catchment network structure and optimizing the key connectivity areas had significantly changed the hydrological connectivity.The catchment network structure was modified by increasing the number of ditches.The results showed that the area of low connectivity reduced by 1.18%and high connectivity areas increased by 3.33%,indicating that reasonable planning and layout of ditches could effectively change the hydrological connectivity.In addition,the key connectivity area was optimized by increasing the number of main and tributary intersections.The results showed that the low connectivity area of the optimized catchment decreased by 0.86%;the area of the key connectivity（d IIC>15）increased by 66.67%.The d IIC increased by 7.79%,indicating that the area of key connectivity areas and the overall hydrological connectivity could be improved by increasing the intersection of main and tributary.We proposed the future management countermeasures based on the previous results,such as reasonable planning of the confluence network structure,improving the spatial distribution of roads and ditches,regular dredging ditches,carrying out hydrological monitoring.These suggestions help to maintain the dynamic stability of hydrological connectivity.In summary,the research results showed that the increasing in the intensity of human activities changed the network structure of watershed confluence and reduced the hydrological connectivity.The influence of ditches and roads on hydrological connectivity in agricultural small watersheds was significantly greater than that of terraces,ponds and reservoirs.This study provided a framework for evaluating the interaction between hydrological connectivity and human activities,and was crucial for understanding the role of human activities in hydrological processes,formulating management strategies,and controlling non-point source pollution at watershed scale. |
