Sediment Yield In Dam-controlled Watersheds In The Pisha Sandstone Region And Effects Of Check Dams On Variation Of Sediment Load

The regional soil erosion environment has changed significantly,and the sediment entering the Yellow River has been significantly reduced due to the implementation of large-scale soil and water conservation measures on the Loess Plateau.The average annual sediment load of the Yellow River has been decreased from 1.6 billion t in the mid-last century to 200-300 million t during the latest time,but soil erosion is quite serious in the Pisha sandstone region,which is the main source area of coarse sediment into the Yellow River.Since the implementation of the “Grain-for-Green” project in1999,the vegetation cover of the Pisha sandstone region has been significantly improved,and sediment load has been significantly decreased with the continuous development of afforestation and check dam construction.However,the relationship between the effect of check dams and reduction in sediment input to the Yellow River is not clear at present.Therefore,this study is carried out the typical dam-controlled watersheds in the Pisha sandstone region,and the erosion and sediment yield of the dam-controlled watersheds is estimated through unmanned aerial photography and sedimentation routine generalization method and is combined with historical characteristics of erosion and sediment yield in dam-controlled watersheds of the Pisha sandstone region to clarify the erosion and sediment yield characteristics of the damcontrolled watersheds before and after the “Grain-for-Green” project.Statistical methods such as correlation analysis and multiple regression were used to explore the key factors affecting erosion and sediment yield in the dam-controlled watershed.the amount of sediment retention from check dams was estimated in the Huangfuchuan and Hantaichuan watersheds,to quantify the contribution of sediment retention by check dams to sediment load variation in the typical tributaries.The main conclusions are as follows:(1)Since the implementation of the “Grain-for-Green” project,the situation of water and soil loss in the Pisha sandstone region has been improved significantly.The average sediment yield intensity in the dam-controlled watershed has decreased by41.53%,compared with that before the “Grain-for-Green” project;and the degree of erosion in the dam-controlled watershed was reduced from very intensive erosion to intensive erosion.Although the intensity of sediment yield in the dam-controlled watersheds has decreased significantly,it is still much higher than the tolerance of soil loss in the Loess Plateau.(2)There are clear discrepancies in spatial distribution between sediment yield intensity of dam-controlled watersheds in the Pisha sandstone region.The average sediment yield intensity of the dam-controlled watersheds in the sand-covered area,loess-covered area,and the exposed area are moderate,very intensive,and very intensive erosion respectively.The relationship between the magnitude of erosion and sediment yield in the dam-controlled watersheds of the different zones is exposed area(104.17 t/(ha?a))> loess-covered area(93.05 t/(ha?a))> sand-covered area(49.98t/(ha?a)).(3)Watershed erosion and sediment yield characteristics are attributed to the combination effect of factors.The results for multiple regression analysis indicated erosion and sediment yield characteristics in dam-controlled watersheds were most closely related to watershed area,average slope,proportion of badland area(exposed weathered sandstone)and erosive precipitation.(4)Large amounts of sediment produced from watershed erosion processes were effectively retained by check dams during extreme rainstorm events.And the average annual sediment retention in the Huangfuchuan and Hantaichuan watersheds has increased by approximately 80% during 2010-2018,compared to 2000-2009 due to the frequency of extreme rainstorm events.(5)Human activities are the dominant factor influencing the reduction of sediment load in the Huangfuchuan and Hantaichuan watersheds.The reduction in sediment load in the Huangfuchuan and Hantaichuan watersheds was as high as 81% or more,and the effect of check dams was largely consistent,accounting for 37.91% and 35.46% of the anthropogenic contribution to sediment reduction,respectively,indicating that check dam construction is still an important part of comprehensive soil loss control in the Pisha sandstone area.