| Alxa League is located in the hinterland of the Asian continent, as the interior plateau with the typical continental climate characterized by dry, less rain, large evapotranspiration. The ecological environment of study area is quite fragile, and desertification has become increasingly serious. Desertification land not only has been imperiling the ecological environment of local and neighboring regions, but also threatened the live of more than20million people in Alxa League, Therefore, prevention and control of desertification is very important, which lays on the understanding of the status of land desertification and identification of the main driving forces for the desertification in study area. In this research paper, with MODIS and TRMM data for the period2000-2012, the degree of land desertification in recent years, the spatial distribution and change trends of precipitation, the spatial distribution and change trends of land surface temperature the spatial change trends of the vegetation growth, and the relative effect of two main drivers (nature elements change and human activities) in desertification process have been analyzed and discussed, the results are follow as:1. Vegetation coverage obtained from MODIS NDVI data was a good indicator to evaluate the desertification in the regional scale in Alxa League. The desertification problem in Alxa League from2000to2012was pretty serious, severe desertification accounted for45.57%-88.84%, the moderate desertification reached10.39%-45.44%, light desertification was0.65%-10.04%, non-desertification was only0.12%-0.6%. There are three counties in Alxa League, that is, Ejina Banner, Alxa Right Banner, and Alxa Left Banner. The desertification problem in Ejina Banner was the most serious, followed by Alxa Right Banner and Alxa Left Banner. The proportion of serve desertification in Alxa League had decreased in recent years, and the moderate desertification was on the contrary. The proportion of mild desertification increased slightly, and non-desertification was basically unchanged.2. By comparing the monthly precipitation data from weather station, we found that TRMM3B43could analyze the spatial distribution and change trends of precipitation in Alxa League. The spatial distribution of mean annual precipitation differed greatly ranging from58.2to272.3mm in study area with the mean precipitation108.1mm. The precipitation increased from the west to the right (i.e., from Ejina Banner to Alxa Right Banner, and then Alxa Left Banner). The spatial distribution of the mean summer precipitation was same as the mean annual precipitation, and the summer precipitation accouts for more than half (55.9%) of annual precipitation. The change rate of annual precipitation was between-0.8mm/a-6.3mm/a using Sen’s estimator analysis over the period (2000-2012), and the mean change rate was3.5mm/a which reflected the increase trend of annual precipitation. The area with increased trend of precipitation was more than that with the decreased trend of precipitation. The change rate of annual precipitation in Alxa Right Banner was the largest, followed by Alxa Left Banner, and then Ejina Banner. The change rate of summer precipitation was from-2.0mm/a to6.4mm/a, with the mean1.5mm/a. The change rate in Alxa Right Banner was the largest, followed by Ejina Banner, and then Alxa Left Banner.3. Because of data limitation, MODIS-LST data from Mar to Oct but not the whole year could be used in the LST study. The LST was very high in desert, such as Badain Jaran Desert, Tengger Desert and Ulan Buh Desert. LST of Gobi in Ejina was high as well. And the Low value of LST was mostly in the region near the Heihe River, and mountains, such as Mazong Mountain and Helan Mountain. The slope of mean LST from Mar to Oct was between-0.94℃/a-0.21℃/a, the regional average slope was about-0.02℃/a. The result shown that the overall LST in Alxa League was little changed.4. The temporal variation of NDVI could be used to monitor the condition of vegetation variation. The mean annual NDVI was between-0.118and0.605, with the mean NDVI0.085, That showed that vegetation growth in Alxa League was very poor. Comparing the growth state of vegetation among three counties, we found that vegetation growth in Alxa Left Banner was the best, followed by Alxa Right Banner, and then Ejina Banner. The annual mean NDVI increased with time, reached the highest value in August or September, and then decreased. The spatial distribution of mean annual NDVI is different in study area, ranging from-0.021to0.023with the mean NDVI0.0003. In general, the growth state of vegetation during the study period was changed a little. It was found that the correlation in areal distribution was positive between NDVImax (NDVI of August) and accumulated precipitation, and negative between NDVImax and accumulated LST, the regional mean correlation coefficients between NDVImax and accumulated precipitation, accumulated LST were0.548and-0.342, respectively.5. The heavy land degradation accounted for22.6%of study area by using the trends in annual sum NDVI. The desertification land is the largest in Ejina Banner, followed by Alxa Left Banner, and then Alxa Right Banner. The differences between the observed NDVImax and the predicted NDVImax were calculated for each pixel on degradation land and were referred to as residuals. The regional average slope of NDVI residual was negative value which indicated human being activities contribution to the desertificationm, that means human being activities improve the formation and development of desertification. To research the relative role of two main drivers (climate change and human activities) in desertification process, the study period was divided into three phases, the first time section (2000-2004), the second time section (2005-2008) and the third time section (2009-2012). From the first time section to the second and from the second to the third, human relative role was occupying a dominant position and the regional average value was between60.6%-62.1%(the regional average value of climate change role was39.4%-37.9%). Human influence in Ejina Banner was the largest, followed by Alxa Right Banner, and then Alxa Left Banner. |
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