Responses of streamflow and sediment load to vegetation restoration in catchments on the Loess Plateau

Abstract: To reduce the severe soil erosion on the Loess Plateau, a great number of soil and water conservation measures was implemented in 1970s-1980s, and after that, Grain for Green project was applied since 1999. The great anthropogenic forces resulted in the decreasing trend of stream flow and sediment load on the Loess Plateau. However, the relationship between streamflow and sediment load wasn't clear. Understanding the change of streamflow, sediment load, and their relation was helpful to enable sustainable catchment management in the region. The monthly streamflow and sediment load data were used in the study and summed annually to check the trends in the past five decades. To compare the impacts between catchments, the data were standardized with precipitation at a corresponding temporal scale and catchment area. The results showed that all the 5 catchments in the study had a statistically significant negative trend in annual streamflow and sediment load given by the M-K trend test. Change points given by the Pettitt test in streamflow of the catchments occurred were basically consistent with that of sediment load. The extent of reduction in the sediment load was much greater than that of streamflow with the periods altered, and an increasing number of zero-sediment load days was observed. The average annual reduction both in streamflow and sediment load was much greater in the low-vegetation-coverage catchments than that in the high-vegetation-coverage catchments, and the change point was also earlier. The scattered distribution for monthly streamflow and sediment load for the wet season indicated that the low-vegetation-coverage catchments have wider domain by one order of magnitude in sediment load than that of the high-vegetation-coverage catchments. The sediment load was simply regressed with the streamflow, and the regression coefficient was considered as the sediment production coefficient here. The sediment production coefficient of the 5 catchments was observed to decrease by periods during the past 50 years. Implementation of soil and water conservation measures, and "Grain for Green" application in latter periods, led to an average reduction in the sediment production coefficient of 24.5% and 70.6%, respectively. It was also detected that the absolute value of the constant in the linear regression and the streamflow volume when the catchments reach scour and silting balance decreased by periods. The impacts of soil and water conservation implementation and Grain for Green policy application on streamflow, sediment load and their relation was further illustrated.