Response Of Sediment Yield To Rainfall And Land Use Pattern In Catchments In The Hill And Gully Region On The Loess Plateau

The hill and gully region on the Loess Plateau is one of the most serious areas of soil and water loss in China.Unreasonable land use pattern is the main reason for triggering and aggravating sediment yield.Since the Grain for Green project launched in 1999,the land use pattern in this region has changed significantly.Relevant scholars have carried out some studies on the relationship between land use pattern and sediment yield.However,the research mainly focuses on the plot scale,whereas there is the insufficient research on the change of response mechanism of sediment yield to land use pattern under different rainfall conditions on the catchment scale.Therefore,this study selects 12 dam-controlled catchments in Yan’an and Zizhou areas as the research objects,to clarify the current situations of vegetation restoration and land use in these catchments.Based on that,the land use type and topography are superimposed to construct the Land Use Type Pattern Index of small catchments(W)and Land Use Pattern Integrated Index of Catchment(LUPIC).Based on rainfall data and sediment deposition information in the dam land,the sediment yield characteristics of catchments under the existing ecological environment are inverted.Furthermore,the impact of land use structure characteristics on sediment yield in catchments was analyzed.The response mechanism of sediment yield to land use type pattern in catchments was explored.The main research results are as follows:(1)Two decades after the implementation of the Grain for Green project,forests and grasses are widely distributed in catchments in Yan’an and Zizhou areas,accounting for more than 74%of the total catchment areas.However,there is still a large area of slope cropland in Zizhou area,with an average area of 9.8%and the maximum proportion being 26.5%.Sediment yield in catchments still exceeds the permissible amount of soil and water loss on the Loess Plateau.During 2018-2020,the annual sediment yield modulus of catchments in Zizhou area is 3.0 times that of Yan’an area,with annual sediment yield modulus of 4942.4 t/km~2 and 1644.9 t/km~2.(2)Downpour and heavy downpour events can increase the annual sediment yield modulus of catchments by 2.02-22.88 times.Affected by the continuous downpour events in July 2013,the average annual sediment yield modulus of catchments in Yan’an area is 14733 t/km~2,which can reach 3.51-22.88 times of the average annual sediment yield modulus of the same catchment from 2014 to 2020,with an average of8.7 times.The average annual sediment yield modulus of the catchments in Zizhou area in 2017 was 24058 t/km~2 due to the heavy downpour on 26 July 2017,which was 2.02-22.70 times that of the same catchments from 2018 to 2020(4942 t/km~2),with an average of 6.30 times.(3)The responses of sediment yield to rainfall features vary among small catchments with different land use structure characteristics.Generally,the sediment yield modulus of small catchments increases with the increase of rainfall amount,and the catchments with large terraces are not sensitive to railfall features.In Yan’an area,sediment yield in the catchments with forest land accounting for more than 60%is not sensitive to the changes of maximum 2-hour rainfall and average rainfall intensity;sediment yield in the catchments with terrace accounting for more than 25%is less sensitive to the changes of maximum 1-hour rainfall and rainfall duration.In Zihzou area,the sediment yield modulus of the catchments with grassland as the dominated land use structure,sloping farmland area<8%and terrace area>5%was extremely insensitive to the change of rainfall duration;the sediment yield of the catchments with sloping farmland area>8%and terrace area<5%increases the most with the increase of maximum 1-hour rainfall,maximum 2-hour rainfall,maximum 3-hour rainfall and average rainfall intensity.(4)The characteristics of land use structure in small catchments have a significant impact on sediment yield.In Yan’an area,the annual average sediment yield modulus of small catchments with forest land accounting for>60%(397.69 t/km~2)is within the permissible amount of soil and water loss on the Loess Plateau;the annual average sediment yield modulus of the other two land use structures catchments with forest land<60%(terrace<25%and terrace>25%)are 2010.91 t/km~2 and 2281.21 t/km~2respectively.In Zizhou area,the annual sediment yield modulus of catchments with slope cropland accounting for more than 8%is significantly higher than that of the catchments with slope cropland accounting for less than 8%,which are 6277.11 t/km~2and 3103.97 t/km~2,respectively.(5)Under different rainfall conditions,there are differences in the most important land use type and their pattern that lead to the difference of sediment yield among catchments.In addition,under the same pattern,the role of a land use type in“enhancing”and“weakening”sediment yield will change.The slope cropland below the gully line and the unpaved road above the gully line are the most vulnerable to rainfall erosion.In the process of sediment yield caused by concentrated rainfall,the lower slope cropland is the main source of sediment,during the“7.26”heavy downpour event in 2017,the slope cropland below the gully line contributed 32%of the sediment with 1.62%of the total catchment area;in the process of sediment yield triggered by uniform rainfall,the unpaved road above the gully line is the main sediment source.As a non-major source of sediment yield,the terraces(newly built and old-type terraces)above the gully line can reduce the intensity of sediment yield in catchments under downpours,compared with slope-cropland and unpaved road;under non-downpour conditions,the newly built terraces above the gully line are one of the main sediment sources in catchments.