Runoff And Sediment Variation In The Yanhe Watershed And Their Response To Land Use Cover Change

In recent decades,due to the impact of climate change and human activities,the runoff and sediment of the Yellow River have been significantly reduced.The evolution law of soil and water resources and its relationship with climate and human activities in the changing environment under the influence of climate change and human activities are the hot and difficult points of international hydrological science research.Therefore,using the Yanhe Watershed in the Loess Hilly-gully Region,a tributary to the middle reaches of Yellow River,was selected as the research area in this study.Based on the observed data of major meteorological stations and hydrological data of the last control station(Ganguyi)form 1956-2016,and combined with four period of land use data(1990,2000,2005 and 2010),the Mann-Kendall trend test,Pettitt mutation test,cumulative distance equality method were employed to detect the long-term trends of hydro-climatic variables and its driving factors in the Yanhe Watershed for the last past 61 years.This paper also analyzed the land use structure and its dynamic change of Yanhe Watershed during 1990-2000,and the influence of land use change on runoff and sediment process was discussed.Moreover,based on the parameter rate of Soil and Water Assessment Tool(SWAT)model and the verification of models,the simulation of the water sand process in different periods of land use scenarios is carried out;quantitative analysis of the impact of land use change on the water and sand process in the basin in different periods reveals the regulation mechanism of land use change caused by large-scale return of farmland to forest and grass in the loess plateau.The main conclusions were as follows:(1)The Mann-Kendall trend,Pettitt mutation test and double mass curve method were used to analyze the change characteristics of runoff and sediment and the response of precipitation and human activities in Yanhe Watershed form 1956-2016,the results showed that the runoff and sediment had a significant decline over the past 61 years in the Yanhe Watershed(p<0.01),the annual average values of runoff and sediment were 2.012×10~8m~3and 0.387×10~8t.Decrease at the rate of 0.018×10~8m~3 and 0.010×10~8t per year;The 1996 was the runoff and sediment yield points in the Yanhe Watershed.Rainfall change and human activities are main factors affecting the decrease of runoff and sediment in Yanhe watershed,which the contribution rate of rainfall change to decerase of runoff and sediment is 10.62%and 23.06%.The contribution rate of human activeties to decerase of runoff and sediment is 89.38%and 76.94,and the effect of human activities on runoff and sediment reduction was more significant.(2)Based on DEM data,meteorological and hydrological data,land use data and soil data of Yanhe watershed,the SWAT model of Yanhe watershed was established,and applicability of SWAT model in Yanhe watershed was verified.The results show that the NS and R~2of runoff and sediment during the calibration period(1961-1967)and the validation period(1968-1972)are all greater than 0.55,which meets the evaluation criteria of the model.Therefore,it is concluded that the SWAT model constructed in this study is suitable for the Yanhe watershed and can well reflect the change process of runoff and sediment in Yanhe watershed from month to month.(3)From 1990 to 2010,the change trend of land use type in Yanhe watershed was roughly that the cultivated land area decreased significantly,the grassland area increased significantly,and the area of forest land and construction land increased slightly.The main transformation mode of land use type was the conversion of cultivated land into grassland and forest land,and the urbanization construction gradually improved.From 1990 to 2010,the cultivated land area decreased by 46.54%,the grassland area increased by 25.08%,the forestland area increased by 16.08%,and the construction land area increased by about 1.2times.It shows that the sloping farmland was mainly converted to forest land and grassland and the ecological restoration project and small watershed comprehensive are the main reasons for the transformation of land use patterns in the Yanhe watershed.(4)Based on the land use data in 1990,2000 and 2010,the SWAT model was used to simulate the changes of runoff and sediment in the Yanhe watershed in 1994-1999 under different land-use conditions.The result showed that the change of land use type has a significant influence on the runoff and sediment quantity of the outlet river.Compared to land-use conditions in 1990(scenario S1),the 2000(scenario S2)and 2010(scenario S3)runoff and sediment discharge simulation values are less than 1990 simulation values,runoff decreased by 12.11%and 23.68%,and the sediment discharge decreased by 21.46%and 36.91%.When land use type is converted into grass land and forest land,it has significant effect on water and sediment reduction,and the capacity of sediment reduction is obviously greater than of water reduction.