Effects Of Land Use/Cover Change On Soil Erosion In A Loess Watershed

Soil erosion is one of the global eco-environmental problems at present.Consequently,soil and water conservation is a hot topic in agronomy,hydrology,soil sciences and geography,et al.Soil erosion is categorized into wind erosion and water erosion,gravitational erosion and freeze-thaw erosion.Climate,topography and geomorphology characteristics,land use/cover structure,etc.,are main natural factors affecting soil erosion.Human driven forces for soil erosion include excessive grazing,excessive reclamation of land resources,deforestation,mining,etc.Among them,deforestation is the strongest one.Loess Plateau is one of the main soil erosion area in China,especially in the loess hilly-gully region due to loose structure of loessial soil,hillgully topography,heavy storms and extensive anthropogenic activity related to land cultivation.In this study,Yang Juangou watershed,which is located in Liquzhen,Baota District,Yanan,Shannxi province,was selected as a target area for reveal the effects of revegetation on soil erosion and sediment delivery.The period for the study ranges from 1984 to 2014.The land use / land cover of 1984,1996,2006,2012 and 2014 were used.RUSLE model driven by rainfall data(R),soil erodibility factor(K),topographical factor(LS),vegetation cover factor(C)and soil erosion control practice factor(P)was applied as a fundamental tool.The LS was derived from a DEM of 5 m resolution,which is produced using topographical map with a scale of 1:5000 by using Arc GIS 10.2.Data analysis was conducted by using Excel2013.Besides of the modelling of the soil erosion rate,the sedimentological connectivity that indicates the risk of sediment delivery to the river and evaluated and mapped.A sediment transport risk equation was developed.This study come to following results and conclusions:Firstly,the land use / cover structure in the Yangjuangou watershed changed dramatically in the past decades,from a cultivated land dominated into forest and grassland dominated watershed.In 1984,the forest accounted for 11.16%,grassland accounted for 40.54%,and slope farmland accounted for 29.09%.While in 1996,the total area of forest and grassland area expanded,accounted for 42.61% of the total watershed acreage.Howevere,slope farmland acreage shrunk to 17.81% of the watershed area.After the launch of Green for Grain project,land use/cover came a new stage.In 2006,the forest acreage rose to 26.09% of the watershed area.In the same period,shrub grassland increase from 38.83% to 8.73%.By 2015,most of the watershed was covered by forest,grassland and shrub,accounted for 32.86%,43.96% and 6.61% of the total area,respectively.The acreage of terrace and orchard were basically stable after returning farmland to vegetation.Secondly,the effect of land use/cover change on soil erosion in the watershed was assessed.The average annual rainfall erosivity(R)before and after the conversion of cropland to forest was 1755.83(MJ·mm)/(hm2·h·a)and 2155.71(MJ·mm)/(hm2·h·a).From 1984 to 2014,the erosion modulus change from 15327.57 t/ basin t/(km2 · a)to 3270.19 t/(km2·a).At the watershed level,soil erosion intensity was maily strength and severe level.However,after returning farmland to revegetation,areas with slight,light and moderate erosion intensity dominated the watershed.Finally,the sediment connectivity that indicates the sediment delivery risk was assessed by using a connectivity index.Five land use / cover scenarios were established to assess the future sediment connectivity.The sediment connectivity and sediment risk in different slope and space is mainly at the level of-3 and-2.The sediment transport risk is the lowest in the scenario 1 scenario 2,which is mainly on the order of 0.005.But the scenario 2 the area with high sediment transport risk was greate.Scenario 3 and scenario 4 generates very high sediment transportation risk.They are not suitable for soil and water loss prevention in the region.In scenario 5 and the current land use / cover,the grade of sediment transport risk differed.In the summary,the land use / cover structure under scenario 1 and scenario 2 is suitable and is a choice in the future.It is concluded that sediment connectivity and sediment delivery risk are closely related to the topographic features.Greater slope gradient implies a greater sediment connectivity and a higher sediment risk.For summary,Land use / cover plays an important role in soil erosion and sediment transport.It change is significantly reshaped the soil erosion and sediment connectivity of Yangjuangou watershed.