Study On Runoff And Sediment Variation And Spatial Distribution Of Erosion Energy In Wudinghe Watershed

Runoff-sediment amounts in Yellow River in China has decreased drastically in recent decades,which accounts for no more than 20%of historical multi-year average value.This phenomenon has raised concerns of scholars at home and abroad.It is generally believed that the reduction of runoff-sediment in the Yellow River is the result of the integrated effect of climate change and human activities.In particular,the implementing soil and water conservation measures in the Loess Plateau Region have led to severe change of the underlying surface conditions,further causing some variation of regional hydrological cycle.With such background,the Wudinghe watershed,a primary tributary of the Yellow River,is studied in this paper.Specifically,the characteristics of flow-sediment in the Wudinghe watershed and the contribution of climate change and human activities to the reduction of flow-sediment in this region are quantitatively explored.In this study,the concept of runoff erosion power is introduced and extended to the annual scale and its advantages are analyzed.The SWAT model is constructed in the Wudinghe watershed for applicability evaluation,which is used to explore spatial distribution characteristics of flow-sediment as well as distribution characteristics of runoff erosion power in this river basin.On this basis,the spatial threshold of runoff erosion power is calculated.In addition,through setting different scenarios in the SWAT model,the effect of warping-dam construction and measures of converting the slope-land for forestry and grass on flow-sediment and runoff erosion power of the Wudinghe watershed is quantitatively analyzed.The main conclusions are as follows:(1)The intra-annual and inter-annual variation characteristics of the Wudinghe watershed in the period of from 1963 to 2005 are respectively analyzed from three aspects:precipitation,runoff and sediment series.In terms of intra-annual distribution,they were uneven,which mainly concentrated in the flood season(from June to September).In terms of inter-annual distribution,the precipitation and runoff of the Wudinghe watershed changed smoothly,but its sediment series changed dramatically.The results of trend and change tests show that there is no significant change in trend or change point in terms of precipitation series from 1963 to 2005.But the runoff series(except the Qingyangcha Station)and sediment series(except the Suide Station)showed a significant downward trend and obvious change points at each station.Human activity is the main reason for reduction of flow-sediment in the Wudinghe watershed.The climate change is another factor leading to the reduction of runoff and sediment transport,whose contributions to them respectively account for 25%and 11%;but for human activity,its contributions to them accounts for 75%and 89%respectively.(2)The SWAT model of the Wudinghe watershed is established using its spatial data,meteorological data and hydrological data,and the applicability of the SWAT model to the Wudinghe watershed is verified.The period of from 1989 to 1991 is taken as the warm-up period of this model;the period of from 1992 to 1993 is taken as the calibration period of this model;the period of from 1994 to 1996 is taken as the verification period of this model.The percentage deviation between the calibration and verification periods,the Nash coefficient and the coefficient of determination all meet the accuracy requirements.Therefore,it is concluded that the SWAT model constructed in this study is suitable for the Wudinghe watershed and can better reflect true hydrological situation of this basin.(3)Compared with traditional equation of flow-sediment interaction,the correlation between runoff erosion power and sediment transport is higher,indicating that the runoff erosion power can more fully reflect the sediment transport conditions in the low,intermediate and abundant water periods.In addition,it shows the superiority of the runoff erosion power as a comprehensive factor in reflecting the actual sediment transport in the basin.In terms of runoff,sediment transport and runoff erosion power,the Wudinghe watershed has shown significant spatial characteristics:their values in the downstream area,east and south are respectively higher than those in the upstream area,west and north.The control area at the outlet section of the sub-basin is selected as a spatial scale factor.In the river source gully region and loess hilly-gully regions in the southern Wudinghe watershed,there exists a power function relationship between the runoff erosion power and spatial scale effect,and the threshold of spatial scale effect is 306km2.In the windy dust area in the northern part of the basin,the runoff erosion power is generally small,which is not affected by factors such as control area.(4)The measures of converting the slope-land for forestry and grass and the warping-dam construction significantly reduce the runoff and sediment formation and erosion power process in the Wudinghe watershed;moreover,their effect on sediment reduction is greater than that on water flow reduction.The measures of converting the slope-land for forestry and grass have taken great effect in drastically reducing the flow-sediment in the heavily eroded areas(the loess hilly and gully areas in the southeast of the basin,including main lower reaches of the Dali River,the Huaining River and of the Wuding River).But the construction of warping-dam only affects the downstream area of the warping dam,resulting in the reduction of flow-sediment in the downstream area by the same magnitude.