Empirical Study On The Impact Of Returning Farmland To Forest Project On Soil And Water Conservation Function Of Typical Watershed In The Loess Plateau

The Grain to Green Program (GTGP) is a great ecological project in China, the Loess Plateau is the key area where the GTGP is implemented in. But the ecological environment in the Loess Plateau is fragile, and the Loess Plateau is a relatively water resources shortage area.The implementation of GTGP have deep impacts on the basins vegetation ecology, watershed hydrology. In order to analyze GTGP’s impact on basins soil and water conservation function, the vegetation and hydrology of typical basins in the Loess Plateau are analyzed vertically and horizontally by time sequence and spatial sequence, respectively. The main contents and results are:(1) The vegetation changes were characterized by normalized difference vegetation index (NDVI) in this article, to research temporal and spatial variation of vegetation of typical basins in the Loess Plateau. The results show GTGP changes the basins vegetation,3 rivers NDVI have upward trends in 2000-2010, Beiluo river has the biggest growth, the maximum NDVI in the basins occurs in the years of 2007-2009; the NDVI in the 3 basins increased from Yanhe, Beiluo to Wuding river sequentially; (2) Analyze the change trend and extent of precipitation, runoff and sediment in these typical basins in the Loess Plateau vertically and horizontally, besides, the trend of the ratio of the total runoff of the research area basins to the Yellow River basin runoff is analyzed. The results show:precipitation change trend in the 5 basins was not significant, Kuye, Jinghe and Yanhe river show upward trend while Wuding and Beiluo river shows downward trend, Beiluo river precipitation are most fluctuant; the runoff in these 5 basins show downward trends which Kuye river is significant except Wuding river, which has insignificant upward trend, Jinghe and Beiluo river are more fluctuant while Wuding river is less fluctuant; the sediment in these 5 basins all show downward trends which Kuye and Beiluo river decreased significantly, all the basins are fluctuant; the ratio of the total runoff of the research area basins to the Yellow River basin runoff dropped from 13.1% to 8.2%, which shows insignificant downward trend. Precipitation, runoff and sediment in Yanhe river declined in 1980-2010 which runoff and sediment had obvious trend, the year of 2003 is identified as cut-off point of hydrological series by Mann-Kendall, the 30 years are divided into the earlier period (1980-2002) and the GTGP effect period(2003-2010); (3)By analyze annual variation of runoff coefficient since the implementation of the project, precipitation-runoff relationship in different periods in the same basin, precipitation-runoff relationship in different basins with different NDVI, to analyze GTGP effect on precipitation-runoff relationship in basins qualitatively. The result shows:the runoff coefficient in these 5 basins all show downward trends; In Yanhe river, precipitation had good correlation with runoff in the earlier period while the correlation was weak in the GTGP effect period, under the same amount of annual precipitation, the annual runoff drops by 0.87×108m3 in GTGP effect period, GTGP weaken precipitation-runoff relationship, reduce basins runoff; precipitation-runoff correlation in Wuding, Beiluo and Yanhe river decrease in turn as NDVI increasing, Wuding, Beiluo and Yanhe river runoff reduce in turn under similar precipitation conditions, means the growth of vegetation has weaken precipitation-runoff relationship, reduce basin runoff; (4)To quantify the GTGP impact on basins runoff and sediment, fitting relationship between NDVI with runoff coefficient, sediment in corresponding year from 2000. The results show both the runoff coefficient and the sediment have significant negative correlations with NDVI by correlation significant test(a=0.05).The research provide scientific basis for implementation of basins soil and water conservation and sustainable use of water resources in arid areas, improve the theory and methods of studies on human activities effect on basins runoff and sediment, in favor of policy follow-up evaluation.