中文摘要：

利用16天合成的250m分辨率的MODIS数据，对三江源区NDVI和EVI植被指数时空分布特征进行了系统性的研究。结果表明：NDVI和EVI空间分布呈现出由东南向西北逐步下降的趋势。植被指数随季节呈现出规律变化，最高值出现在8月。不同海拔高度两种植被指数对植被覆盖情况的指示作用比较一致。植被类型的NDVI与EVI的差异随植被指数值的增大而增大，而EVI比NDVI对植被变化的表征更为稳定。对植被指数与气象要素的关系研究表明，月平均气温和月平均降水量与植被指数呈现明显的相关关系，植被指数对降水量存在滞后性而对月平均气温不存在滞后现象。降水量在植被生长中的作用要大于气温。

英文摘要：

This article conducts a systematic research on the spatial and temporal characteristics in the source region of three rivers on the Qinghai-Tibet Plateau, using 16-day 250 m NDVI and EVI of MODIS data in 2007. The results indicated that the spatial distribution of the NDVI and the EVI had very obvious regional characteristics, which presented a gradual reducing tendency o{ the NDVI and EVI values from southeast to northwest in the study area. The regular seasonal variations of vegetation index were very distinct, with the maximum value in August. The variation trends of two kinds of vegetation indices were consistent at different altitudes, which showed that the functions of NDVI and EVI were quite consistent. The difference value between NDVI and EVI gradually increased with the increase of vegetation index value, while EVI was more stable than NDVI as an indicator for vegetation variation. The research on the relationship between vegetation index and meteorological elements showed that the monthly mean temperature, precipitation and the vegetation index presented significant correlation dependence. In the study area, the correlation coefficient value between the current month precipitation and the next month vegetation index was much greater than that between the current month precipitation and the current month vegetation index, indicating that there was a time lag relation between the vegetation index and the precipitation. However, there was no relation between the vegetation index and the monthly average temperature. The effect of the precipitation on the vegetation growth was greater than that of the temperature.