Characteristics Of Sedement Trapped Behind Check Dams After ’GFG’ Project In The Hill-gully Loess Plateau Region

Check dams are one of the most effective engineering measures to control gully erosion in Loess Plateau.Because vegetation has been restored and succeeded with the continuing of Grain for Green Project that changed the erosion environment,the effect of sediment trapped by check dams is unclear under such changes.In order to understand the effect,we conducted the experiment by estimating the amount of sediment trapped in 17 typical check dams located in Majiagou Watershed,Ansai County,Hilly-gully Loess Plateau in 2004-2015,combining with analyzing changes of land use and vegetation distribution in these check dam controlled watersheds.The results and conclusions are as follows:1)The implement of the Grain for Green Project could significantly reduce soil erosion in watershed and sediment trapped in check dams.Intensity of soil erosion in check dam controlled area before the Project was severe and destructive.The erosion modulus has no significant difference among 3 periods,which was 19342.23 t/km~2 before 1968,11554.46 t/km~2 in 1968-1983,and 14946.32 t/km~2 in 1983-1999,while there was significant difference after the Project and the erosion modulus was 2765.28 t/km~2.Although the intensity of erosion became slight,there was severe eroded area existed and the erosion modulus was still over the limit of soil loss tolerance in Loess Plateau.2)The range of altitude in 17 check dam controlled watersheds was 1095-1489 m and the range of slope in these watersheds was 15-60°.Main land use types are shrub land and grassland in 2004.The vegetation coverage in these watersheds was 12.18-30.05%,and shrub land had the greatest vegetation coverage.In 2015,in most of these 17 watersheds forest land increased significantly and became the dominated type of land use,while the area of shrub land and grassland decreased.The average of vegetation coverage was increased to 44.70%,and the growth rate of vegetation coverage in forest land was the fastest while that in shrub land and grassland was slower.Terrace became the main type of farmland in all watersheds.The comprehensive dynamic of land use in 17 watersheds was between 1.29-7.50%.3)The relation between trapped sediment and changes of forestland,shrub land and unused land was significantly correlated.The change of forestland was able to control the erosion in check dam controlled area more effectively.Since vegetation coverage increased in all of these watersheds,it was not the main cause of the difference in amount of trapped sediment.4)After the Grain for Green Project,the landscape patterns in all watersheds was beneficial to soil conservation in 2004 and 2015,which has the greatest effect on horizontal direction,while the ability of soil conservation was weakened due to the construction of terrace between 2004 and 2015.In 2004 the distribution of vegetation was grassland-shrub land from the top to the bottom of the slope.The landscape patterns were better in severely eroded watershed,yet there was little vegetation in the head of the gully.In 2015,vegetation in watersheds eroded more slightly was dominated by forestland and followed with grassland and shrub land.Unused land and farmland were the minor types.In severely eroded watersheds,there was a lower vegetation coverage rate,and only in the bottom of the slope forestland was the main land use type.In the top of the slope there was no dominated type of vegetation.The advantages of slope cropland and terrace increased in area near the dam.Thus proper land use pattern could reduce the effect of erosion and the amount of sediment trapped in check dams.