西北地区MODIS-NDVI指数饱和问题分析

Ananlyses on MODIS-NDVI Index Saturation in Northwest China

为了了解西北地区MODIS-NDVI和MODIS-EVI两种植被指数的特点，本文利用美国NASA LP DAAC（Land Process Distributed Active Archive Center）2004年1～12月的250m分辨率16天植被指数合成的MOD13 Q1数据集，对西北地区不同类型植被NDVI和EVI的特征进行分析，并对西北地区MODIS-NDVI饱和问题进行了初步研究。结果表明：NDVI和EVI对干旱-半干旱气候区植被覆盖度不高的植被类型描述能力相似，月际变化趋势一致。西北地区各种植被类型NDVI比EVI高，NDVI与EVI的差异总体上呈现从半荒漠、草原、农区到林区，随NDVI值的增加而增大的规律。对植被度覆盖度高的阔叶林和针叶林，在植被生长旺盛期，NDVI总在0．8附近波动，NDVI随植被的生长增加的很小，一直维持在一个高且平的范同内，不再能看出植被生长变化的现象，即饱和现象严重；而EVI表现良好，随着植被的生长而增加，能明显地反映出植被生长的季节变化。西北高寒草甸和陕西关中农业区NDVI也出现有不同程度的饱和，饱和时间因植被的不同从1～2月不等。0．8可作为NDVI饱和的阈值。NDVI饱和问题对卫星监测植被的研究和应用会产生误差，EVI能较好地解决NDVI的饱和问题。

We compared and evaluated the MODIS vegetation indices （NDVI EVI）, are produced at 250 m resolutions and 16-day compositing periods and chose typical vegetation types in 2004 including broadleaf forest, needleleaf forest, saline meadow, grassland, shrub, semi-deserst, irrigating-cultivated - agriculture, dry farming-cultivated agriculture as sample regions in Northwest China. The results show that the NDVI was higher than theEVIin most part of Northwest China. The difference between the two indices increased from deserts, steppes, cultivated vegetation, meadows to forest. Both NDVI and EVI were well indicated the distribution and the growth of various vegetations in arid and semi-arid areas. The NDVI saturated in these high biomass type vegetations, such as broadleaf forests, needleleaf forests, during the plants blooming period. TheNDVIdid not increase with the growth of these vegetations during the period. The EVI was different. It increased with the growth of vegetation. The NDVI saturation threshold was about 0.8. The NDVI saturated for 2 months in nothwest with high-cold meadows and part of cultivated vegetation. EVI played a good performance in high biomass regions and exhibited a more symmetrical seasonal profile during the plants flourished.