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Vegetation restoration and its effects on runoff and sediment yield in Hekouzhen-Longmen Section of the middle reaches of Yellow River

XIN Zhong-bao1;YU Xin-xiao1;GAN Jing2;WANG Xiao-ping2;LI Jin-hai2.1 Key Laboratory of Soil and Water Conservation & Desertification Combating of Ministry of Education,Beijing Forestry University,100083,P.R.China;2 Beijing Municipal Bureau of Landscape and Forestry,100029,P.R.China  
Sediment yield of the Yellow River is mainly originated from the drainage area between Hekouzhen and Longmen area(Helong Section).Spatio-temporal variations of vegetation cover in this area during 1981--2007 were investigated using GIMMS and SPOT VGT NDVI data.The authors also analyzed the interannual variations of vegetation cover,annual runoff and sediment yield,which were resulted from precipitation changes and the Project of Conversion of Cropland to Forestland(PCCF).The results showed that vegetation cover of Helong area had experienced an increasing trend during 1981--2007.The northwestern Helong,where the flat sandy land was covered by grass,was the mostly significant increasing region.The region where vegetation cover showing a decreasing trend mainly appeared in southern and southeastern Helong area,which was gullied hilly area or forested area.During 1999--2007,although precipitation was relatively low,vegetation cover showed a significant increasing trend in the Helong area due to the implementation of PCCF.During 1999--2007,the most significant improvement of vegetation cover mainly appeared on the loess gullied hilly area,such as the drainage basins of the Kuye River,Tuwei River and the middle and lower reaches of Wuding River and Yanhe River.Comparison of the average annual maximum NDVI between the earlier(1998--2002)stage and the later stage(2003--2007)of PCCF indicated that,the areas with increases of 10% and 20% in NDVI accounted for 72.5% and 36.4% of the total area,respectively.Interannual variability of annual runoff and sediment yield showed a decreasing trend,especially since the 1980s,the decreasing trend was very obvious.Compared with 1950--1969,the average runoff and sediment yield of 1980--2007 decreased by 34.8×108 m3 and 6.4×108 t,accounting for 49.4% and 64.9% to those of 1950--1969,respectively.There was a positive correlation between the annual maximum NDVI and annual runoff as well as sediment yield.But the positive correlation had been reversed after the implementation of PCCF since 1999,i.e.an increasing NDVI trend of the Helong area was associated with the decreased runoff and sediment yield.Although less precipitation was an important driving factor for the decreasing runoff and sediment yield during 1999--2007,the restoration and improvement of vegetation cover might exacerbate this decreasing trend by enhancing evapotranspiration and alleviating soil erosion.
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