2000-2010年黄河流域植被覆盖的时空变化

黄河流域位于干旱、半干旱和半湿润地区，生态环境脆弱，近年来，在气候变化和人类活动影响下，植被覆盖状况发生了变化。因此需要对黄河流域植被覆盖的变化进行监测，进而掌握流域植被的动态变化特征。在此背景下，利用2000-2010年的250 m分辨率的MOD13Q1数据来研究黄河流域植被覆盖区域的NDVI时空变化特征。采用Theil-Sen Median趋势分析和Mann-Kendall检验来研究NDVI的变化趋势特征，通过对Theil-Sen Median趋势分析和Mann-Kendall检验的结果和Hurst指数的结果的叠加，来研究NDVI的可持续特征。研究表明：1）从空间分布上看，黄河流域NDVI呈现出西部和东南部高，北部低的特征；2）从时间变化特征上看，2000-2010年植被覆盖区域年均NDVI均值在0.3-0.4之间波动，其中2000-2004年NDVI波动较大，但自2005年以来NDVI呈现快速增长的趋势；3）从变化趋势上看，2000-2010年黄河流域植被改善的区域远远大于退化的区域，改善的区域占植被覆盖区域总面积的62.9%，退化的区域占27.7%，9.4%的区域NDVI稳定不变；4）从可持续性来看，86.0%的植被覆盖区域NDVI呈现正向可持续性，即NDVI的可持续性较强；由变化趋势与Hurst指数的耦合信息得出，持续改善的面积占植被覆盖区域总面积的53.7%，持续稳定不变的区域占7.8%，持续退化的区域占24.5%，另外14.0%的区域未来变化趋势无法确定，持续退化和未来变化趋势无法确定区域的植被变化状况需要研究人员继续关注。

The spatio-temporal variations of vegetation cover in the Yellow River Basin from 2000 to 2010

Vegetation cover is one of the primary indicators of changes to ecosystems. Large scale vegetation cover observations will indicate the effect of natural factors and human activities on the ecological environment, so it is one of the hot topics in global change research. The Yellow River Basin is located in an arid, semi-arid and semi-humid area and its ecological environment is vulnerable. In recent years, the vegetation in the Yellow River Basin has had a relatively large variance in spatial and temporal patterns, affected by changes in climatic conditions and perturbations from human activities. Therefore, improving the observation of vegetation will provide us with more information about vegetation status and make us better able to understand the variations of ecological environment characteristics. With this background, this paper provides an analysis of spatio-temporal variations of MODIS/NDVI in the vegetation covered area of the Yellow River Basin. The MODIS product, which is assumed to be suitable for large scale observations of vegetation variation, was used, and MOD13Q1 data with a spatial resolution of 250 m×250 m from 2000 to 2010 was utilized as the data source. NDVI was used as a proxy for vegetation cover and the Theil-Sen Median trend analysis with the Mann-Kendall test were both used to analyze NDVI trends as they are new methods in the long term series analysis of NDVI. Theil-Sen Median trend analysis was used to describe the trend of NDVI and the Mann-Kendall test was used to estimate the significance of trend. The combination of the result of Theil-Sen Median trend analysis and Mann-Kendall test, as well as the result of Hurst index were conducted to analyze the sustainability in NDVI trend. The results indicate that: 1) From the distribution of the annual NDVI in the Yellow River Basin, the NDVI values are high in the west and the southeast, but low in the north; 2) From the time variation of annual NDVI in the vegetation covered area in the Yellow River Basin, the values of NDVI fluctuate between 0.3 and 0.4, from 2000 to 2010, and NDVI from 2000-2004 shows high fluctuations, but rapid upward trend since 2005; 3) From the spatial trends of NDVI, the area where the values of NDVI increase is much larger than the area where the values of NDVI decrease and the area where the values of NDVI increase occupies 62.9% of the vegetation covered area, while the area where the values of NDVI decrease accounts for 27.7%. 9.4% of the vegetation covered area remained unchanged, during the period of 2000-2010; 4) From the spatial sustainability of NDVI trend, the overlaid result of the Theil-Sen Median trend analysis, Mann-Kendall test and Hurst index indicates that 86.0% of the area presents a positive sustainable development. From the overlaid result of the Theil-Sen Median trend analysis and Mann-Kendall map and Hurst index map, it can be concluded that 53.7% of the vegetation-covered area presents a sustainable-increased state, 7.8% of the vegetation-covered area presents a sustainable-unchanged state, 24.5% of the vegetation-covered area shows a sustainable-decreased state and 14.0% of vegetation-covered area cannot be identified in the future. These areas need to be focused upon and further researched.