| Vegetation is the key node of a region to carry water system,connect soil and communicate atmosphere,vegetation cover is an important index to evaluate the ecological health of the surface ecosystem directly.The ecological environment is fragile,the vegetation cover is sparse,the human activity is frequent,the soil erosion is serious in the Loess Plateau,as the result,climate change and human activities make the vegetation cover and ecological environment facing serious challenges.In order to explore the temporal and spatial changes of vegetation cover and the driving force behind it,this paper is based on MODIS satellite remote sensing data,to obtain the spatial distribution characteristics of vegetation cover from 2000 to 2015,then analyze the spatiotemporal variation characteristics of vegetation cover in different periods.The relationship between vegetation cover and climatic factors(temperature,precipitation)and human activities(population,GDP,the policy of “Gain for Green” and urbanization process)was analyzed based on the meteorological and socio-economic data.The main conclusions of the paper include:(1)The spatial distribution of vegetation cover in Loess Plateau showing a obvious regional differences,vegetation cover decreased from southeast to northwest,average annual value of NDVI is between 0.48 to 0.58,the whole area long-time average annual value of NDVI is 0.53.During the period of 2000-2015,the vegetation cover was in the state of restoration,therefore the NDVI value increased about 0.8.(2)The temperature of the southeast is higher than the northwest in the Loess Plateau.,the average temperature is 9.6 ?.The distribution of precipitation decline from the southeast to the northwest,from the mountains to the ground,the average annual precipitation is 448.3 mm.The influence of temperature and precipitation on vegetation cover show different correlation in different regions,the area of positive correlation with temperature accounted for 62.35% of whole area,significant positive correlation area(P<0.05)accounted for 26.22%,the correlation coefficient was 0.476,which was at a significant level;and the area of precipitation and vegetation cover was significantly positive correlation accounted for 31.15% of whole area,the correlation coefficient was 0.834,which in state of extremely significant.From January to April and October to December in a year,the effect of temperature on vegetation cover is dominant,the effect of precipitation on vegetation cover is dominant from May to September.(3)Vegetation cover decreased with the increase of population density and GDP,compared to the area with small population density and low GDP,the NDVI will decreases more quickly with population density and GDP increased by a grade in the area with high population density and high GDP.The policy of Gain for Green has made the vegetation cover improved obviously,the result is the annual average NDVI was increased by 0.065.Urbanization process—A case study of Xi'an City,leads to a large number of cultivated land,green ecological land into construction land,so the vegetation cover decreased in the area.(4)The policy of “Gain for Green” large-scale revegetation program,which has been in operation since 1999,the contribution rate accounts for 48.793%,the contribution of precipitation rate to 26.602% rows in the second,temperature which finished third is similar to precipitation that all of them play an important role in vegetation cover in the whole study area,the total contribution rate of the first three parts reached 85.849%,the contribution rate of the other three factors influent vegetation cover in local area is very small,only 14.151%,and the sort between them is: population> urbanization process>GDP. |
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