Effects of climate and human activities on vegetation cover changes inDanjiangkou Water Source Areas

Abstract：Vegetation coverage in catchment has a direct impact on its ecological environment, and monitoring vegetationcoverage change and its determinants is important to reconstruct effectively ecological engineering projects to sustaineconomic development without compromising ecological environment. Danjiangkou Reservoir is a waterhead at the middleroute of the South-North Water Transfer Project in China, and its ecological environment has a direct impact on quality andquantity of the water in the transferring cannel. Available study on vegetation coverage change in this area is problematic,including short experiment duration and inadequate evaluation of the impact of the human activities. We address thisknowledge gap in this paper using data extracted from GIMMS NDVI and MODIS NDVI. The empirical orthogonalteleconnection (EOT) algorithm was used to reconstruct the normalized difference vegetation index (NDVI) from 1982 to2018, from which we analyzed the spatiotemporal change in the vegetation coverage and individual contribution of climatefactors and human activities to the vegetation coverage changes. The results revealed that NDVI showed an oscillatingincrease from1982 to 2018 at an average rate of 0.002 9 /a (P<0.05). Spatially, 89.93% of the studied areas saw a first-yearincrease in NDVI, and 10.06% witnessed a first-year decrease. Temperature impacted on NDVI most with a correlationcoefficient of 0.792 (P<0.05), followed by precipitation with a correlation coefficient of - 0.43 (P<0.05). The influence ofclimate factors on vegetation coverage varied spatially. In mountainous areas, vegetation growth was affected by precipitationand temperature combined, while in basins and river valleys characterized by low and flat terrain, NDVI was positivelycorrelated with temperature and negatively with precipitation, both at significant level. Human activities impacted onvegetation coverage in two opposite ways. On the one hand, human activity in 67.74% of the studied areas had a positiveeffect on vegetation coverage due to engineering measures being taken to restore and improve ecological functions of thesearea. On the other hand, human activity in 32.26% of the studies areas had a detrimental impact on vegetation coverage due tounsustainable activities used in social-economic development. The impacts of climate and human activities on regionalvegetation coverage were separated by the residual method, and the results indicated that overall contribution of the climatefactors to the vegetation coverage change was 92.14%, compared to the 7.86% from the human activities. Spatially, in areaswith more human activities, such as areas that had taken ecological measures or in the proximity of central city, humanactivities have been becoming the determinants of the NDVI change.