Sediment Retention By Check Dams And Theirs Effects On River Sediment Reduction In The Hekou–Longmen Region Of The Middle Reaches Of The Yellow River

The synchronous development of biological measures and check dam construction has greatly improved the regional natural ecological environment,curbed serious soil loss,and reduced sediment entering the YR.It is of great significance to identify the future development direction of soil and water conservation on the Loess Plateau by analyzing the contribution of of slope erosion reduction and check dam retention to the sediment reduction of the YR.However,as the most effective engineering measures to block sediment entering the YR,what is the response mechanism of the sedimentation rate and the change of water and sediment of the existing check dams to the evolution of the erosion environment of the watershed above?and what is the contribution of the check dams to the sediment reduction of the YR?These are important links to understand the process and mechanism of sediment sudden reduction in the YR,which is still unclear and need to be further studied.Therefore,this study took the Hekou–Longmen region in the middle reaches of the YR as the research object,selected 28 dam-controlled watersheds in four typical watersheds（Kuye River,Yanhe,Zhujiachuan and Xinshui River watersheds）for field investigation and sampling,and conducted analysis and model simulation in combination with regional check dam,remote sensing,meteorological and hydrological data.Then,the response of soil erodibility to erosion environment was studied,the sediment yield of dam-controlled watersheds and its response to the evolution of erosion environment were analyzed,the influence of sediment retention by check dam on sediment transport of regional tributaries was discussed,and the contribution of different control factors to sediment dynamic change at the regional scale was quantified.The main conclusions are as follows:（1）There was a significant difference in soil erodibility factors between the north and south watersheds in the Hekou–Longmen region.Vegetation restoration can improve soil erosion resistance by improving soil characteristics.The soil erodibility factors in the southern watersheds（Yanhe and Xinshui River watersheds）of the Hekou–Longmen region were significantly higher than that in the northern watersheds（Kuye River and Zhujiachuan watersheds）,indicating that the soil in the Yanhe and Xinshui River watersheds was more vulnerable to hydraulic erosion.The differences of soil erodibility factors among grassland,shrubland and woodland were not significant,but they were significantly smaller than those of cultivated land.（2）The annual sediment yield of dam-controlled watershed showed obvious spatial heterogeneity in four typical watersheds.The annual sediment yield of the watersheds was not completely controlled by erosive rainfall,but also related to the different underlying surface conditions of the watersheds.The sediment yield of the eight dam-controlled watersheds of the Kuye River indicated a fluctuating and decreasing trend from the upstream to the downstream.The annual sediment yield of the seven dam-controlled watersheds in the Zhujiachuan showed a slight increasing trend from the upstream to the downstream.From the upstream to the downstream,the annual sediment yield of the seven sub-watersheds of the Yanhe showed a W-shaped distribution.The annual sediment yield of the six dam-controlled watersheds in the Xinshui River showed a slight upward trend from the upstream to the downstream.The annual average sediment yield of the Kuye River,Zhujiachuan,Yanhe and Xinshui River watersheds were 10415.86,4001.80,5984.95 and 6279.37t·km^-2·a^-1 respectively,which were greater than the allowable erosion rate of these region.With the increase of annual erosive rainfall,the variation range of annual sediment yield and the annual sediment yield of the watershed were increasing.The results indicated that erosive rainfall,soil erodibility,slope,surface roughness,soil and water conservation measures and vegetation conditions all affected the sediment yield of the watersheds.（3）under the joint action of ecological restoration and soil and water conservation measures such as check dams,the sediment discharge of regional tributaries decreased significantly.However,with the appearance of vegetation restoration effect,the relative contribution of check dam to sediment reduction was weakening.The NDVI value of the Hekou–Longmen region fluctuated before 1999and increased after 1999.According to the survey,3703 key dams were counted in the Hekou–Longmen region,of which the Wuding River watershed had the largest number of key dams（1139）,and the sediment deposition ratio of Wuding River,Jialu River and Weifen River watersheds reached more than 60%.The annual sediment load of all tributaries showed a significant decreasing trend（P<0.01）over the past six decades.Compared with the reference period（1955～1979）,the annual average sediment discharge of the 12 tributaries decreased by 57.5%in period 1（1980～1999）and 89.2%in period 2（2000～2018）,respectively.This is mainly affected by human activities.Although the dam-controlled area of the key dams in the Hekou–Longmen region was increasing,the annual sediment retention capacity of key dams was decreasing.The average sediment reduction efficiency of key dams for the selected watersheds decreased from 24.4%of period 1 to 11.2%of period 2.（4）The land cover and water and sediment regimes had changed significantly in the Hekou–Longmen region,but the current situation of regional soil erosion was still severe.Since the implementation of the‘Grain for Green’project,the overall land use changes in the Hekou–Longmen region had been a continuous transformation from cultivated land and bareland to woodland,grassland and construction land.From 1985to 2019,the runoff and sediment transport in the Hekou–Longmen region showed a decreasing trend,decreasing at the rate of 15.5 million t and 89 million m³ each year.According to the Chinese Soil Erosion Classification and Grading Standards（SL190–2007）,although the area proportion of micro erosion in the Hekou–Longmen region was the largest,the area proportion exceeding the allowable erosion intensity in 1990 and 2018 was still greater than 50%.With the increase of rainfall erosivity,the soil erosion modulus in the Hekou–Longmen region and four typical wtersheds rebounded in 2018 compared with that in 2015.（5）By simulating the variation of sediment load in the Hekou–Longmen region under different scenarios,the contribution of different control factors to the reduction of regional sediment load was quantified.Compared with 1990,the regional sediment load decreased by 3.57×10⁸ t in 2015,and the relative contributions of precipitation variation,soil conservation measures and land use changes to the sediment load reduction were 48.77%,27.69%and 23.54%respectively.Although the rainfall erosivity increased in 2018,the regional sediment load still decreased by 3.10×10⁸ t.The relative contribution rates of the three controlling factors to the decrease in sediment load were-44.66%,31.56%,and 131.10%,respectively.The results indicated that when rainfall erosivity decreases significantly,the impact of climate change on regional sediment load reduction was very obvious,and the relative contribution of soil conservation measures and land use changes to sediment reduction was close.Under the condition of high rainfall intensity and frequency,compared with soil conservation measures,land use changes（ecological restoration）played a more prominent role in reducing sediment load.To sum up,remarkable achievements have been made in the ecological restoration of the Hekou–Longmen region,and the soil conservation measures（check dam,etc.）and slope vegetation restoration have played a good role in preventing and controlling regional soil and water loss.However,due to the frequent occurrence of extreme rainstorm events in recent years,the current situation of soil erosion in the Hekou–Longmen region was still severe.In the future,check dam construction should focus on areas with severe soil erosion,improve the construction standard,maximize their benefits,and reduce human and material consumption.Furthermore,ecological restoration plays an increasingly important role in water and soil conservation in the region.Hence,the management strategy and construction planning of the Hekou–Longmen region in the middle reaches of the YR should not only consider the appropriate density and distribution of check dams and other measures,improve the construction standards,but also explore how to maintain the sustainability of the vegetation ecosystem.