Runoff And Sediment Variation In The Yanwachuan Basin And Their Response To Precipitation And Land Use Cover Change

The Loess Plateau is the region with the most serious soil erosion in my country.In recent decades,climate change has been intensifying,and large-scale water and soil conservation measures and ecological restoration projects have been implemented in the Loess Plateau.Under this background,how to assess the impact of climate change and land use change The impact of river basin runoff and sediment has always been a hot and difficult point of current research.This study takes the Yanwachuan basin,a typical small basin in the loess plateau gully area,as the research object.Based on the annual rainfall and water and sediment data of the basin,time series statistics are used to analyze the evolution of rainfall,runoff and sediment transport at different time scales in the basin.Based on the three factors of flood runoff depth,flood duration,and peak flow,combined with k-means,the contribution rate of annual rainfall and vegetation cover changes to water and sediment changes in the basin was quantitatively evaluated;a total of 266 flood data from 1981 to 2016 were screened,based on the three factors of flood runoff depth,flood duration,and peak discharge The cluster analysis and discriminant analysis method divided the 266 flood events into 4 types,explored the sand transport laws of different flood types and the characteristics of water-sediment relationship,and proposed the similar flood event method to quantitatively analyze the sub-flood scale drop water and vegetation.The contribution rate of coverage to water-sediment changes in the basin reveals the response mechanism of water-sediment to precipitation and vegetation coverage at annual scales and during floods,in order to provide a scientific basis for water and soil erosion control and land use adjustment in the basin.The main research conclusions of the thesis are as follows:(1)Vegetation cover change is the leading factor of water and sediment change in Yanwachuan Basin,and its influence is much higher than precipitation.The annual precipitation in the Yanwachuan Basin showed an insignificant increasing trend,while the runoff and annual sediment transport showed a significant decreasing trend;There are big differences in annual precipitation in the basin,with a single-peak curve in the distribution during the year,while a double-peak curve in the annual distribution of runoff and sediment;The abrupt year of runoff and sediment transport in Yanwachuan watershed were both in 1998.The contribution rates of precipitation and vegetation cover changes to the runoff changes in the Yanwachuan Basin were-11.7%and 111.7%,respectively,and the contribution rates to the changes in sediment transport were-6.2%and 106.2%,respectively.(2)Based on the three factors of flood runoff depth,flood duration,and peak flow,K-means cluster analysis and discriminant analysis methods are used to classify 266 flood events into 4 types.The main controlling factors affecting the sediment transport of different types of floods are different.The leading factor affecting the sediment transport of the F2 flood is the depth of runoff,and the leading factor affecting the sediment transport of the F3 and F14 floods is the peak discharge.The main type of hysteresis in the basin is compound hysteresis.Flood events with this type of hysteresis account for 43.8%of the total flood events,followed by the figure-eight hysteresis relationship,and the flow and content of flood events with compound hysteresis relationships.The amount of sand is relatively large,and the counterclockwise hysteresis relationship is more common in smaller types of flood events;the figure-eight and clockwise hysteresis relationships are mainly medium-type flood events.(3)The main land use types in the Yanwachuan basin are cultivated land and grassland.Among the land use types,woodland and grassland have the most dramatic changes.The most dramatic changes in the area of each land type in the Yanwachuan basin are from 2000 to 2010.During this period,the forest and grass vegetation change index was the largest,as high as 33.13%,which was related to the implementation of water and soil conservation measures such as returning farmland to forest in the basin in recent years.(4)Based on the definition of the similar flood event method,this chapter has screened out 7 groups of flood events with similar rainfall conditions for comparative analysis.Compared with the base period,the flood characteristic values of all similar flood event groups decreased during the change period.The results show that vegetation coverage can significantly reduce the magnitude of flood events and flood variability.The regression analysis of the relationship between runoff depth and sediment transport in the Yanwachuan Basin during the base period and the change period shows that compared with the change period,the unit runoff deep sediment transport capacity and the average sediment concentration of the flood events in the base period are higher,indicating that During the change period,vegetation cover has a significant regulating effect on flood and sand.(5)Based on the analysis of similar flood events selected in this paper,the results show that the contribution rate of vegetation coverage to runoff reduction is 23%-81%,the average contribution rate is 56%,and the average contribution rate of precipitation to runoff reduction is 44%.Correspondingly,the contribution of vegetation cover to the reduction of sediment transport is 71%-99%,with an average of 92%,and the average contribution of precipitation to the reduction of sediment transport is 8%.Quantitative analysis of the impact of rainfall and vegetation coverage on floods and sediments at the sub-flood scale further confirms that vegetation coverage changes are the main influencing factors of water and sediment changes in the Yanwachuan Basin.