The Responses Of The Flood Runoff And Sediment Load To Vegetation Restoration In The Upper Reaches Of Beiluo River

The deterioration of the ecological environment and serious soil erosion in the Loess Plateau caused by the unreasonable land use patterns such as the denudation and slopeland reclamation. The ecological environment is improved since the implementation of the “grain for green project” in 1999. In this paper,we chose the upper catchment of Beiluo River which located in Wuqi County of Shanxi Province as study area that the effect of “Grain for Green Project” is very significant. Using the daily runoff and sediment, flood runoff and sediment data measured in Wuqi hydrological stations and hydrological statistics method, we discussed the behavior of runoff sediment response to the comprehensive management of soil and water conservation in hilly and gully area of Loess Plateau, especially to the large-scale restoration, to provide the necessary theoretical and practical basis for planning, ecological construction and water resources management of sand soil and water conservation in Loess Plateau. The main conclusions are as follows:(1) Over the past 50 years, the volume of runoff and sediment displayed obvious decreasing tendency with an average annual rate of-0.32 mm·a-1,-228.91 t·km-2·a-1. Contrasting with between 1963 and 1979, the volume of runoff and sediment during high flow and median flow of the three periods showed decreasing tendency and the volume of runoff and between 2002 and 2011 were abated about 54.6% and 27.1%, respectively. The decreasing fluctuations of the volume of sediment were stronger with 96.71% and 90.42%, respectively. However, the volume of base low increased gradually in low flow and the volumes of sediment of 2003~2011 were vanished completely.(2) Compared with the baseline period, there is not significant differences for the relationship between runoff and sediment in inter-annual scale, but the ecological restoration period has significantly change. The sediment / runoff ratio(C/Q) was decreased by the time, especially in 6-8 months of the ecological recovery period, the C/Q value reduced by the range of 50-81%. The frequency of flood events with high sediment concentration(average sediment concentration >300 kg·m-3) decreased by 82%. The frequency of low concentration(average sediment concentration <300 kg·m-3) has the opposite trend.(3) The volume of flood runoff and sediment displayed obvious decreasing tendency with an average annual rate of- 0.27 mm·a-1 and-221.96 t·km-2·a-1 with no significant change in precipitation condition. The flood occurrence frequency from 10.8 times per year, reduced to 6.7 times per year and 3.6 times per year. Average runoff and sediment peak decreased from the baseline period for the 1744.13 m3·s-1 and 1091.94 kg·m-3 reduced to 1279.65 m3·s-1 and 941.96 kg m-3, 368.93 m3·s-1 and 657.56 kg·m-3. However, as to the C-Q hysteresis loop, counterclockwise hysteresis accounted for the main part in each period, the baseline period is 79.65%, soil and water conservation period and ecological restoration period were 80.52%, 71.87%, respectively.(4) The coefficient(sediment dynamic coefficient) from 808.87 t·m-3 in the baseline period decreased to 764.07 t·m-3 and 536.63 t·m-3. There is a significant differences of the runoff and sediment relationship between the baseline period and soil and water conservation period(P=0.004<0.05), and the significant differences relationship between soil and water conservation period and ecological recovery period has enhanced(P=0.002<0.05). It showed that comprehensive management of soil and water conservation and ecological environment construction reduces the runoff and sediment dynamic in general(5) The classification of the flood return perioda according to P-III frequency curve wers analysised the flood events. The linear regression of runoff and sediment yield for the flood events for 1 to 2 year return period showed a greatly declined trend in sediment production coefficient. However, as for the flood events with more than 5 year return period, sediment production coefficient did not illustrate a decreasing trend and the equations did not show any difference. The results informed that vegetation restoration induced the decrease of runoff generation and sediment yield in the flood events, but it did not alter the runoff-sediment behavior, especially in the extreme events in the area.