Response Of Erosion And Sediment Yield To Change Of Land Use And Cover In The Yanhe Watershed

Since the implementation of the integrated management of small watershed and the conversion of farmland to forest or grass project,the land-use pattern,runoff and sediment yield has changed significantly in the Loess Plateau.This paper takes the Yanhe watershed in the hilly and gully region of the Loess Plateau as the study area,and together with remote images data from 1990/2000/2005/2010,the runoff and sediment yield data from 1960 to 2012 at Ganguyi hydrology station and Mathematical statistics as the basis to study the changes of the land surface and its driving factors in the Yanhe watershed from 1990 to 2010,analyzing the characteristics of runoff and sediment changes in the Yanhe watershed,and exploring the impact of environmental changes on the land surface to hydrological connectivity and the temporal and spatial changes of sediment delivery ratio,combined with the RUSLE model simulating the amount of soil erosion in the four periods(1990/2000/2005/2010)of the Yanhe watershed,the sediment delivery ratio was used in the RUSLE model to simulate the sediment yield under different land use scenarios of the Yanhe watershed,and compared the simulated sediment yield with the measured sediment yield at the Ganguyi hydrology station to validate the accuracy of the model.The research results will help to understand the response mechanism of erosion and sediment production on the environmental changes of the land surface,provide scientific basis for protection and rational use of water and soil resources,and improve the regional ecological environment,and effectively promote the water and soil conservation and watershed comprehensive management in the hilly and gully region of the Loess Plateau.The main conclusions are as follows:(1)The Mann-Kendall trend test showed that the runoff and sediment had a significant decline over the past 50 years in the Yanhe watershed.Prior to the implementation of the“conversion of farmland to forest or grass”project in 1998,the annual average runoff and sediment discharge were 2.195×10~8m~3 and 0.468×10~8t,respectively.The annual average runoff and sediment discharge were 1.408×10~8m~3 and 0.141×10~8t during 1999 to 2012.The runoff and sediment discharge decreased by 35.85%and 69.87%,respectively.Pettitt mutation test results show that the 1996 was the runoff and sediment yield points.Environmental changes of the land surface is the main reason that caused changes in runoff and sediment yield.(2)The cumulative measured runoff and sediment yield compared to the simulated cumulative runoff and sediment yield were reduced by 9.579×10~8m~3 and 3.106×10~8t in the Yanhe watershed from 1970 to 2012.The contribution rate of precipitation changes to the reduction of runoff and sediment yield during 1999 to 2012 is 10.91%and 18.96%;the contribution rate of human activities to the reduction of runoff and sediment yield is 89.09%and 81.04%.The results show that human activities was more significant than precipitation changes for reduction of runoff and sediment yield during 1999 to 2012.(3)The area of cultivated land decreased by 40.97%,the grassland increased by 20.26%,the forest land increased by 8.18%,the construction land area increased by 1.5 times from 1900 to2010 in the Yanhe watershed.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)The index of connectivity and sediment delivery ratio presented a spatial distribution feature of“large valley and small slope”as a whole in the Yanhe watershed from 1990 to 2010,with the range of value change being-11.37~0.34 and 0~0.50 respectively.The index of connectivity in the vicinity of the main channel has increased and in other regions has shown a decreasing trend from 1990 to 2000 in the Yanhe watershed.The index of connectivity at the whole watershed has decreased from 2000 to 2010.The sediment delivery ratio of the main channel has increased and away from channel regions has decreased from 1990 to 2010.The index of connectivity and sediment delivery ratio showed a decreasing trend over time from 1990to 2010 in the Yanhe watershed.The mean values of index of connectivity from 2000/2005/2010decreased by 9.15%,8.39%and 10.89%compared to 1990,and the mean values of sediment delivery ratio from 2000/2005/2010 decreased by 11.11%,12.50%and 17.14%compared to 1990.(5)The erosion modulus and soil loss area have shown a decreasing trend over time from1990 to 2010 in the Yanhe watershed.The increasing of vegetation coverage on the land surface and the deployment of soil and water conservation measures such as terraces are the main reasons for the reduction of sediment yield in the Yanhe watershed.Under the change of land scenario,the simulated value of sediment yield was well consistent with the monitored values at Ganguyi hydrology station.The NSE coefficient and correlation coefficient were 0.73 and 0.79 respectively.The simulated and monitored values of NSE and correlation coefficient for the sediment yield at the Ganguyi hydrology station under the terrace configuration scenario were 0.78 and 0.81respectively.The simulation results of the model are high accuracy and good agreement with the actual situation.