Vegetation Restoration Status And Its Relationship With Runoff And Sediment Variation In Typical Basins Of The Loess Plateau In Northern Shaanxi Province

This study took the Kuye River,Wuding River,Yanhe River and Beiluo River in the typical basins of the Loess Plateau in northern Shaanxi Provence as the study area.By analyzing the land use type data in 1995,2005 and 2015,the various land types and the time-space change of land use types(Farmland,Grassland,Forests,Water,Built-Up,Unused)in the four typical basins before and after the Returning Farmland to Forests(Grass)were determined.Using the normalized vegetation index(NDVI)data of the vegetation growth period in 2000,2010 and 2018,the change trends of the four basins’vegetation were analyzed.On this basis,the runoff and sediment transportation data were used to study the variation trend of runoff sediment transportation in four typical basins from 1960 to 2016.At the same time,the effects of precipitation and human activities on runoff sediment transportation,and the relationship between vegetation cover change and runoff sediment transportation were quantitatively studied.The main conclusions were as follows:(1)The land use types in the typical basins of the Loess Plateau in northern Shaanxi have changed significantly after the implementation of the project of Returning Farmland to Forests(Grass).The grasslands in the land use type of were mainly distributed in the central and northern parts of the Kuye River Basin,the middle and upper reaches of the Wuding River Basin,the entire Yanhe River Basin,and the northern part of the Beiluo River Basin.Moreover,the single dynamic degrees of farmland in the four typical basins were all negative,indicating that the farmland area in four basins were decreasing.The single dynamic degrees of forest land,urban and rural areas,industrial and mining residential land were positive,indicating that the two land use areas were increasing.From the analysis of the transfer matrix,the more farmland of four basins converted into grassland in the three periods of 1995-2005,2005-2015and 1995-2015,especially during the period of 1995-2005.The conversion area was 407.03km~2,1497.35 km~2,943.75 km~2,and 1631.94 km~2,respectively.(2)The spatial and temporal distribution of normalized vegetation index(NDVI)in the four typical basins of the Loess Plateau in northern Shaanxi have changed significantly.This study analyzed the NDVI in the period of vigorous vegetation growth in 2000,2010 and 2018.The NDVI of the basins were increasing,and the most obvious areas were the northern part of the Kuye River,the southeastern part of the Wuding River,the northeastern part of the Yanhe River and the northern part of the Beiluo River.The NDVI of the four basins area in the(0,0.3](the sparse vegetation coverage)were continuous reduction.The Kuye River,Wuding River,Yanhe River and Beiluo River basins was decreased by 58.71%,47.21%,2.74%and3.16%,respectively.The largest decrease was the Kuye River Basin.The NDVI of the four basins with the high vegetation coverage area in the(0.7,1]were increase,increased by 6.1%,2.43%,50.17%and 19.13%,respectively.The largest increase was the Yanhe River Basin.From the interannual variation,the annual average NDVI of the four typical basins in 2000-2016 values from large to small was:Beiluohe(0.43)>Yanhe(0.34)>Kuye River(0.23)>Wuding River(0.22).The Beiluo River had the best vegetation coverage.The NDVI fluctuation trend of typical basins were consistent with precipitation,but there was a certain lag.In the Kuye River and the Wuding River,the fluctutions were far more dramatic,indicating that the vegetation had sensitive response to precipitation.(3)The runoff and sediment transportation of the four basins showed a significantly decreasing trend from 1960 to 2016.Abrupt changes in runoff occurred in 1994,1981,2003,and 1998,while abrupt changes in sediment discharge occurred in 2002,2000,2006,and 2005.The changes in sediment discharge were influenced by precipitation and human activities.Compared with precipitation,human activities within the more northerly Kuye and Wuding River basins,such as soil and water conservation and environmental rehabilitation campaigns,have played a more prominent role in the changes in sediment regimes.For the more southerly Yanhe and Beiluo River basins,throughout the research period,the effect of a reduction in precipitation on the runoff and sediment discharge has been greater than the effect of human activities.The relationship between NDVI changes and runoff,sediment transportation in each of the four typical basins were relatively complex.In Yanhe River and Beiluo River,the NDVI were increasing,but the runoff in four basins were decreasing,the negative correlation between them were poor.The trend of average annual NDVI in the four typical basins were negative reletionship with the trend of annual average sediment transportation.The Pearson correlation coefficients of NDVI and sediment transportation was-0.664,-0.588,-0.457,-0.775,respectively.