Attributional Comparative Analysis Of Runoff And Sediment Discharge Change In Typical Basins In Rich And Coarse Sediment Area In The Middle Reaches Of The Yellow River

The areas with high and coarse sediment yield of middle reaches of the Yellow River is not only the main source of coarse sediment in the Yellow River basin,but also the key area of ecological construction.In recent decades,there have been significant changes in runoff and sediment transport in this area.In order to explore the variation characteristics and causes of runoff and sediment transport in this area,this study first analyzed the characteristics of water and sediment in this area,and then selected the typical watersheds Wuding River and Huangfuchuan,and carried out Mann-Kendall trend test and Pettitt mutation test on the hydrological element series.The contribution rates of climatic factors and human activities to runoff and sediment transport change in typical watersheds were quantitatively analyzed by using the method of slope change ratio of cumulative quantity,hydrological method,elastic coefficient method and soil and water conservation method.Furthermore,the quantile attribution analysis method based on GAMLSS model is used to quantitatively analyze the effects of annual precipitation and annual potential evapotranspiration representing climate change,and various soil and water conservation measures representing human activities on the runoff and sediment transport in the river basin.The calculation results of different methods are compared and analyzed,and the differences between different methods are discussed.The main results are as follows:(1)The runoff and sediment transport in the section between Hekou and Longmen of the middle reaches of the Yellow River(SHL)decreased significantly,showing three stages:the average annual runoff was 6.65 billion m3 and the annual average sediment transport was 930 million t in the first stage(1952-1979),3.93 billion m3 and 420 million t in the second stage(1980-1999),and 2.09 billion m3 and 104 million t in the third stage(2000-2015).In addition to the change of mean value,the runoff and sediment transport sequence of the main stream and tributary of SHL also has significant changes.The interannual variation and reduction of sediment transport is greater than that of runoff.(2)The annual precipitation in Wuding River showed an insignificant increase;and the annual precipitation in Huangfuchuan showed an insignificant decrease.In terms of annual potential evapotranspiration,Wuding River basin and Huangfuchuan basin showed a significant downward trend,and the abrupt years were 1980 and 1991,respectively.In terms of runoff and sediment transport,Wuding River Basin and Huangfuchuan Basin showed a very significant downward trend,and nutation years of runoff and sediment transport were 1985,1996,1979 and 1989,respectively.(3)Under the condition that the hydrological series and time division of the basin are consistent,the driving factors of runoff and sediment change are analyzed by using the method of slope change ratio of cumulative quantity,hydrological method,elastic coefficient method and soil and water conservation method.The results show that human activities play a leading role in the study period and show an increasing trend,and the contribution rate of human activities to sediment transport reduction is greater than that to runoff reduction.Compared with the Huangfuchuan Basin,human activities in the Wuding River Basin have a greater contribution to runoff and sediment transport.The difference is that the contribution rate of runoff and sediment to human activities is different,and the results of hydrological method and elastic coefficient method are basically the same,which is between the cumulative slope method and the soil and water conservation method.(4)The functional relationship between different factors and runoff and sediment transport based on GAMLSS model shows that the fitting relationship between the area of soil and water conservation measures representing human activities in a single factor to runoff and sediment transport is better than that of annual precipitation and annual potential evapotranspiration to runoff and sediment transport.The overall fitting effect of multi-factor is better than that of single factor,and it has more practical significance for the better simulation of extreme value.On the whole,whether single factor or multi-factor,the functional relationship of Wuding River Basin is better than that of Huangfuchuan Basin.(5)The analysis results of the multivariable contribution rate of the Wuding River Basin show that,in terms of annual runoff,the contribution rate of human activities to the runoff of the basin as a whole continued to increase,and the contribution rate of annual precipitation decreased to 10.9%.Specifically,check dams and forest and grass measures have made a greater contribution,with a cumulative contribution rate of more than 80 per cent by 1997-2015.In terms of annual sediment transport,the contribution rate of terraced fields in human activities fluctuated at about 60%,while the contribution rate of other factors accelerated after 2002,and the effect of climate change declined.In the aspect of annual runoff in Huangfuchuan basin,the contribution rate of human activities to runoff reduction continues to rise,which mainly depends on afforestation in the early stage,and the check dams completed in the later stage strengthens the runoff and sediment storage of the basin.In terms of annual sediment transport,forest and check dams mainly contribute to the reduction of sediment transport.