Changes Of River Runoff And Sediment Load And Sediment Yield In The Tableland Area Of The Loess Plateau

The Loess Tableland area,a unique geomorphic area of the Loess Plateau,suffers severe soil erosion and is one of the main sources of sediment entering the Yellow River.For a long time,the gully has been scoured by the runoff from surface of the tableland frequently,resulting in the moving forward of the gully head,the widening and deepening of the gully,and the breaking and shrinking of the tableland surface.Soil erosion threatens regional ecological security,food security and people’s life and property security.Social development,economic growth,and the construction of the ecological barriers in the Loess Tableland area are severely restricted.However,the changes of the runoff and sediment and the evolution law and the response mechanism of soil erosion sediment yield are still unclear.A profound understanding of the evolution process and law of soil erosion in the Loess Tableland area attaches great importance to the“Gully Stabilization and Tableland Protection”project and the comprehensive control of soil and water loss.In this study,three-level basins in the Long Dong Loess Tableland area,namely Malian River and Pu River basins（the area is19086 km² and 7443 km² respectively）,Yanwachuan River Basin（the area is 376.69km²）,and Yangjiagou and dongzhuanggou River Basins（the area is 0.87 km² and 1.12km² respectively）were selected.There were few dams and reservoirs,and thus the sediment transport ratio of the basin was considered to be approximately 1.Collecting the data of hydrometeorology and ecological construction of soil and water conservation of the basin,and using the methods of hydrostatistics and numerical simulation,the study analyzed the temporal and spatial evolution characteristics of runoff and sediment,and revealed the evolution law of runoff and sediment yield pattern in multi-scale basins.It quantified the contribution rate of climate change and human activities to the runoff and sediment reduction of the basin,and clarified the regulating effect of comprehensive basin management on the process of runoff and sediment.The main conclusions are as follows:（1）The Malian River and Pu River flow through the eastern and western regions of the Long Dong Loess Tableland area,respectively.The runoff and sediment load in the middle and lower reaches of the basin could reflect the characteristics of soil erosion in the Loess Tableland area.The annual average special runoff yield and special sediment yield of Malian River Basin in the eastern region of Long Dong Loess Tableland area showed a significant decreasing trend（P<0.1）during 1960-2016 at the rates of 143.17 m³ km^-2 yr^-1 and 53.50 t km^-2 yr^-1,respectively.The runoff center of the Malian River Basin was located in the lower reaches of the basin,with the runoff increasing from north to south gradually.The sediment center was the area above Hongde Station,namely the upper reaches of the Malian River Basin.The special runoff yield and special sediment yield in the Malian River Basin changed significantly in2003（P<0.5）,and the annual special runoff yield and special sediment yield decreased by 27.28%and 49.07%respectively after 2003.The contribution rates of rainfall to the decrease of special runoff yield and special sediment yield were 9.28%and 21.29%,respectively,and the contribution rates of human activities were 90.72%and 78.71%,respectively.Human activities were the main reason for the decrease of runoff and sediment in the basin.The average special runoff yield and special sediment yield of the Pu River Basin were 37549 m³ km^-2 yr^-1 and 8196 t km^-2 yr^-1 during 1951-1996,showing no significant trend changes.（2）The Yanwanchuan River Basin was located in the hinterland of the Long Dong Loess Tableland area.The special runoff yield and special sediment yield in the basin basically represented the characteristics of Loess Tableland area.The annual average special runoff yield and special sediment yield decreased significantly during 1976-2017 with the rates of 198.01 m³ km^-2 yr^-1 and 97.01 t km^-2 yr^-1,respectively.There were significant changes in special runoff yield and special sediment yield in 1997（P<0.5）.Compared with those before 1997,the annual average special runoff yield and special sediment yield in the Yanwachuan River Basin decreased by 23.87%and75.79%,respectively.The monthly runoff depth and special sediment yield in Yanwachuan River Basin in the flood season from 1997 to 2017 decreased significantly compared with that from 1976 to 1996,and the largest reduction in water and sediment occurred in July and August.The monthly average runoff depth in July and Augest decreased by 1.80 mm and 1.78 mm,respectively,and the monthly average special sediment yield decreased by 886.41 t km^-2 and 717.41 t km^-2,respectively.According to the study on the relationship between daily runoff and sediment,the sediment concentration in Yanwachuan River basin,when reaching a steady state,was significantly higher during the period 1976 to 1996(500 kg m^-3)than that during the period 1997 to 2017(370 kg m^-3),representing a 26%reduction.Flow and sediment duration curves showed that the high runoff and sediment decreased 69.31%and 94.93%respectively after 1997,and the medium and low runoff and sediment increased significantly.The variation of annual daily runoff and sediment tended to be stable.Before 1997,the annual average maximum daily runoff depth and special sediment yield of the basin were 3.10 mm and 1509 t km^-2,and the annual average maximum 10-day runoff depth and special sediment yield of the basin were 2.98 mm and 454 t km^-2after 1997,respectively.The results showed that the implementation of soil and water conservation measures reduced the runoff and sediment rapidly.（3）The main influencing factors of soil erosion in Yanwachuan River Basin were revealed.The main types of land use in the basin were cultivated land,grassland and forest,covering 94.68%of the total area of the basin in 2020.From 1990 to 2020,grassland,water and building land increased by 8.41 km²,0.13 km² and 12.49 km²,respectively,while cultivated land and forest decreased by 14.10 km² and 6.93 km²,respectively.During 1990-2000,there was no mutual conversion between different types,except that 0.08 km² of cultivated land was converted to building land.During2000-2010,cultivated land decreased by 10.88 km²,which was converted into grassland,forest and building land,and building land increased by 5.77 km².During2010-2020,a large amount of cultivated land（11.93 km²）and a small amount of grassland（0.18 km²）and forest（0.05 km²）were converted into building land,and building land played a leading role in the changes of land use types in the basin.From1981 to 2020,The NDVI index in Yanwachuan River Basin showed a significant increasing trend with an annual rate of 0.0046（P<0.01）,and the average NDVI was0.56 during 2000-2020,an increase of 27.3%compared with that before 2000.Besides,the terraces in Yanwachuan River Basin had been rebuilt to 6 km² by 2021.The average annual rainfall of Yanwachuan River Basin was 520.0 mm,with an increasing rate of1.49 mm yr^-1,and it showed by high rainfall in the west and low rainfall in the east.The rainfall in flood season（From May to September）accounted for 77.4%of the annual rainfall.The contribution rates of rainfall to runoff and sediment reduction were-19.67%and-8.79%,respectively.The contribution rates of human activities to runoff and sediment reduction were 119.67%and 108.79%,respectively,indicating that human activities were the leading causes for runoff and sediment reduction of Loess Tableland area.（4）Yangjiagou and Dongzhuanggou River Basins were typical basins for comparative study of comprehensive basin management in the Long Dong Loess Tableland area,which can reveal the impact of different management modes on runoff and sediment yield of the basin.There were 524 flood events in Yangjiagou and Dongzhuanggou River basins,and the flood events were were classified into four flood patterns（pattern A,B,C and D）based on runoff depth,flood duration,peak discharge and average special sediment yield.Pattern B,with short duration,large runoff and sediment yield,occurred 5 times in Yangjiagou River Basin and 6 times in Dongzhuanggou River Basin.Pattern C,with the characteristics of long duration,few runoff and sediment yield,occurred 23 and 28 times respectively.Pattern D,with short duration,few runoff and sediment yield,occurred 304 and 161 times respectively.Pattern A,which was an extreme flood event,occurred only once.The analysis of runoff and sediment yield relationship shows that the flood suspended sediment concentration in Dongzhuanggou River Basin was higher than that in Yangjiagou River Basin,which demonstrated better results of the comprehensive management in Yangjiagou River Basin.The main types of hysteresis loops in Yangjiagou River Basin were counter-clockwise hysteretic loops（33%）,followed by clockwise hysteretic loops（27%）and eight hysteretic loops（26%）,and the main types of hysteresis loops in Dongzhuanggou River Basin were complex hysteresis loops（39%）,followed by counter-clockwise hysteretic loops（24%）and eight hysteretic loops（24%）.（5）After soil and water conservation measures,the benefits of runoff storage and sediment containment in Yangjiagou River basin were higher than those in Dongzhuang River Basin.Under different comprehensive management patterns,the annual average special runoff yield of Yangjiagou and Dongzhuanggou River Basins were 4303.50 m³km^-2 yr^-1 and 8741.83 m³ km^-2 yr^-1 respectively,and the annual average special sediment yield were 1131.93 t km^-2yr^-1 and 4190.69 t km^-2 yr^-1 respectively.The benefits of runoff storage and sediment containment in Yangjiagou River Basin were 54.1%and72.99%,respectively.The annual average special runoff yield of Dongzhuang River Basin was 8.1 times,and that of Yangjiagou River Basin was 5.5 times,which is 32.1%lower than that of Dongzhuanggou River Basin.The maximum rainfall events in the year showed that the average special runoff yield of the annual maximum flood process in Yangjiagou and Dongzhuanggou River Basins were 2662.21 m³ km^-2and 5375.10 m³km^-2 respectively,andthe benefit of runoff storage was 50.47%.Besides,the average special sediment yield were 989.11 t km^-2 and 2915.86 t km^-2 respectively,and the benefit of sediment containment was 66.08%.The results indicated that the implementation of water and soil conservation measures increased the benefits of runoff storage and sediment containment of the basin.Those measures reduced the frequency of runoff effectively,and decreased the special runoff yield,special sediment yield and peak discharge.（6）The LISEM model（Limburg Soil Erosion Model）could simulate the processes of runoff and sediment well,and the Nash coefficient（NSE）and R² were both higher than 0.7 in the basin of the Loess Tableland area.The rainfall event of July 29,1982 in Yangjiagou River Basin of Loess Tableland area was selected to simulate runoff and sediment processes based on LISEM model.The Nash coefficients（NSE）and R² were above 0.7,which indicated that the model results met the demand of precision.The simulated runoff and sediment yield were both higher than the measured values,with the increase of 10.9%and 17.9%respectively.The simulated peak discharge and peak sediment delivery are lower than the measured values,and the simulated values peaked later than the measured values.Further research is needed to improve the simulation efficiency of the model,and carry out simulation of water and sediment processes in different scenarios,so as to provide technical methods for comprehensive watershed management.