Effect Of Ecological Construction On Runoff And Sediment Connectivity In A Typical Small Watershed On Loess Plateau

In recent years,the significantly decrease of water and sediment in the Yellow River has attracted wide attention from domestic and foreign scholars.The Loess Plateau is the main source of sediment in the Yellow River,which ecological environment changes caused by large-scale ecological construction measures is considered as one of the main factors affecting the water and sediment changes in the Yellow River.In this study,the Wangmaogou small watershed in Loess Plateau was taken as the study area.On the basis of summing up the process of ecological construction in Wangmaogou watershed,and restoring the topography before ecological construction by topographic map,we set up four scenarios of ecological construction to analyzed the characteristics of runoff and sediment connectivity under different ecological construction scenarios and the effects of ecological construction on hydrological and sediment connectivity,which are before ecological construction,only slope measures are built,only channel measures are constructed,and at the same time slope measures and channel measures are constructed.The main work and achievements of this paper are as follows:(1)A high-precision grid digital elevation model(DEM)for Wangmaogou watershed is constructed by using topographic maps.The assess results showed Hutchinson method is superior to the irregular triangular network(TIN)method in modeling effect.The contour line is directly used when using TIN method to construct DEM,which mean the description of the surface morphology was insufficient,cause the ridge and the dam land were treated as flat ground.When calculating the DEM established by TIN method,it is found that the gradient change is large,the terrain continuity is poor,the extracted runoff path is discontinuous,and multiple parallel comb basins are prone to occur.The Hutchinson method strengthens the topographic information which the topographic map can not be expressed directly,which DEM surface is smooth and continuous,and the extracted runoff path is more realistic.The parameter calibration of grid DEM constructed by Hutchinson method based on 5m contour is carried out.The smaller the grid,the higher the accuracy of DEM constructed.The grid size of 5m contour data should be no more than 2.5m.There is a threshold for the influence of profile curvature roughness coefficient on the accuracy of DEM,and the number of iterations should be set to no less than 20 times according to the actual data requirements.(2)Under the same ecological construction scenario,the flow length index(FL)of Wangmaogou watershed decreases from ridge to gully,which mean the hydrological connectivity increases accordingly.Under different ecological construction scenarios,the runoff length index of Wangmaogou small watershed obeys the positive skewness distribution from peak to left and long tail to right.The flow length index of only slope measures,only channel measures,two measures admixed in Wangmaogou small watershed is 2.05 times,1.65 times and 2.69 times the condition of before ecological construction,respectively.Flow length index increased significantly(p<0.01)and the hydrological connectivity of the basin decreased after ecological construction.Meanwhile,the obstructing ability of slope measures and channel measures to hydrological connectivity was greater than that of single measures,and the obstructing ability of slope measures to hydrological connectivity was greater than that of channel measures.(3)Under the same ecological construction scenario,the index of runoff and sediment connectivity(IC)of the basin shows a decreasing trend from ridge to gully,which mean the connectivity of the runoff and sediment at the ridge is less than that at the gully,and the gully are more prone to occur soil erosion than ridge.The distributed of large amount of construction land in the middle and lower reaches at the main gully of Wangmaogou small watershed reduces the connectivity of their surrounding sediment,and the region is prone to occur sediment deposition.Eco-construction measures have decreased significantly the runoff and sediment connectivity index(p<0.01)of Wangmaogou small watershed,and reduced the occurrence of soil erosion.Laying ecological measures lessened the possibility of local soil erosion,and increased the resistance of runoff and sediment in the transport process.Compared with the situation without ecological control,the mean of Dup index decreased by 75.27%by laying slope and gully measures,while the mean of Dup index decreased by only 6.45%by laying gully measures.