The Impact Of Vegetation Restoration On Soil Moisture On The Chinese Loess Plateau

The Chinese Loess Plateau suffers from severe soil erosion,and degraded eco-environment.In order to control soil erosion and restore eco-environment,a large scale of ‘Grain-for-Green' project has been implementing on the Loess Plateau since 1999,significantly increasing vegetation cover in the region.However,increased vegetation cover may impact the spatiotemporal pattern of soil moisture across the plateau.Based on monthly precipitation,NDVI and SWI,trend analysis and mean value method were employed to explore the changing trend of annual and monthly NDVI and SWI between 1992 and 2013,and to examine the impact of vegetation restoration on soil moisture content through eliminating the impact of precipitation on soil moisture with the concept of similar precipitation conditions.The main findings are as follow:(1)Based on the trend analysis of the annual and monthly NDVI and SWI at a pixel level,it was found that vegetation cover and soil moisture content on the Loess Plateau decreased from the southeast to northwest.Average NDVI and SWI in 2013 were greater than those in 1992,with the increase of NDVI being higher than that of SWI.NDVI increased by 53% from 1992 to 2013.Annual NDVI significantly increased during the study period.Areas with increasing NDVI occupied 91.35 % of the plateau,and 69.62% of the Loess Plateau experienced an extremely significant increase in NDVI.Seasonal NDVI also increased during the study period,and areas with increased seasonal NDVI were found to be 70% of the Loess Plateau.Annual SWI increased by only 2.5% from 1992 to 2013.This is much smaller than that of NDVI.The change in annual SWI was insignificant,and areas with significantly increased SWI were only 0.8% of the plateau.The increase of spring and summer SWI was more significant than that of autumn SWI.The increase in autumn SWI was not significant and areas with insignificant increase was found to be 86.52% of the Loess Plateau.(2)According to combinations of the changing trend of annual NDVI and SWI,the Loess Plateau was grouped into several sub-regions.The 9 regions,which are over 2% of the Loess Plateau,were selected to study the relationship between the change in NDVI and SWI under the similar precipitation conditions.Results showed that there were four types of relationships between NDVI and SWI including increased NDVI and SWI,increased NDVIand decreased SWI,decreased NDVI and increased SWI and decreased NDVI and SWI.Areas with decreased NDVI were found to be limited,and they(decreased NDVI and increased SWI,decreased NDVI and SWI)were therefore not further discussed.Both NDVI and SWI increased in west Liupanshan mountainous areas and the part of Hetao Plain in Ningxia province.Increased NDVI and decreased SWI mainly occurred in hilly-gully and tabeland areas of middle-eastern Loess Plateau,Fenwei Plain and Taihang mountainous areas.It is suggested by the results that there was an apparent spatial difference in the vegetation cover and soil moisture on the Loess Plateau.In some places,vegetation restoration exacerbated soil water shortage.Therefore,it is necessary to further understand the relationship between vegetation restoration and soil moisture change,and this should be beneficial for the future ecological restoration of the Loess Plateau.