## 4 Results
### Mean Annual VI and Variability  
#### 4.2.1 Mean Annual VI  
The annual mean of the NDVI, SAVI, and EVI for the XUAR in 2001-2012 are presented in Figure 4. The annual mean VI images exhibit similar spatial patterns for vegetation conditions, but with varying magnitudes. All VIs depict a spatial pattern with northern and central Xinjiang having high VI values and the eastern and southern areas having low values. The spatial variation of annual mean VI across the northern and southern XUAR corresponds with regional climate conditions (Figure 2). With higher rainfall in northern XUAR, the forests, grasslands, and agricultural land have better growing conditions and longer growing seasons. In contrast, relatively short growing seasons are found along the Kunlun Mountains in the southern XUAR. The growth of vegetation is limited by low precipitation throughout the year in this zone. The NDVI image shows higher values than SAVI and EVI images in most areas. Over dense forests and grasslands distributed around the Tianshan Mountains and northern Altay Mountains, NDVI exhibits high mean annual values comparing with SAVI and EVI images. In order to illustrate the spatial differences between the NDVI, SAVI and EVI in distinctive ecoregions, three transects were sampled within the annual mean VI images (marked in Figure 4a,b). Figure 4: Spatial distribution of average annual (**a**) NDVI; (**b**) SAVI; and (**c**) EVI derived from MODIS 8-day surface reflectance data for 2001–2012 for the XUAR. The three transects are marked in the figure. Each transect crossed sparsely to densely vegetated regions in order to encompass a wide range of VI values. The land cover types along the transects included cropland, grassland, forest, and barren desert. Comparing the profiles, similar trends can be found between the VI profiles (Figure 5). However, larger data ranges were observed in the NDVI profiles than the SAVI and EVI profiles. Transects a and b start in the northwest and end in the southeast and are about 300 km long. Transect a starts in the agricultural land near the Ebinur Lake in the north. It extends to the sparse grassland and crosses the forests of the Tianshan Mountains, which are areas with very high VI values. The transect ends in sparsely vegetated areas. Transect b starts from the crop areas of an oasis in the north, crosses a mixture of sparse vegetation and agricultural land located in the desert. The transect ends in the foothills of the Kunlun Mountains, where the mixture of natural vegetation types in small patches causes strong oscillation of the mean annual VI. The transect c follows the 87\({}^{\circ}\)E longitude line from north to south and is about 1460 km long. The northern part of the transect is characterized by a mixture of grassland and forest, where EVI and SAVI show variation while NDVI has little variation. In the zone between 47\({}^{\circ}\)N and 45\({}^{\circ}\)N the VI values are lower and show a rather smooth behaviour. This region is characterized by homogeneous land cover that is mainly grassland with small areas of bare grounds and open shrublands. The higher values between 45\({}^{\circ}\)N and 41\({}^{\circ}\)N correspond to the agricultural land, forests, and steppe along the Tianshan Mountains. The VI of the grassland between 41\({}^{\circ}\)N and 39\({}^{\circ}\)N, located in the Taklamakan Desert, is significantly lower than that of the grassland between 47\({}^{\circ}\)N and 45\({}^{\circ}\)N, since this area has an extremely arid desert climate and the vegetation is very sparse. The transect line crosses a transitional zone with open shrubland, cropland, and grassland and reaches the Kunlun Mountains south of 39\({}^{\circ}\)N. In this zone, the climate is extremely cold and dry with primary land covers of gravel desert, alpine desert and alpine desert steppe. Figure 5: Profiles of mean annual VIs for 2001–2012 along the three transects. (**a**) Ecoregion gradient passing several oases; (**b**) elevation gradient; (**c**) variation along 87\({}^{\circ}\)E longitude.