The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B7. Beijing 2008 
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2. STUDY AREA 
Located in the upstream of the Heihe River basin, the Qilian 
Mountain area has a steep topography with an elevation range 
from 1680 m to 5100 m (Figure 1). The intermountain basin 
and longitudinal valley are widely developed in the area. The 
northern part of the Qilian Mountain surrounded by tributaries 
of Heihe River to the east and west was selected to be the study 
area (the area outlined with the bold black line in Figure 1) 
because this area represents a typical mountain range and best 
reflects the vegetation change with elevation. The total study 
area is 2968 km2. The climate in this area is characterized by 
typical high plateau continental climate. The average annual 
temperature is 0.6 °C and the amount of precipitation increases 
with the elevation. Due to complex topography, the climate is 
diverse and has distinct vertical characteristics. These vertical 
climate characteristics have important impacts on the soil 
development and vegetation growth in the areas as they do in 
many other mountains. 
The vegetation distribution in this area exhibits an obvious 
vertical gradient due to the climatic changes with elevation. The 
vegetation types from the low altitude to high altitude are: 
desert-grassland vegetation (1800-2100 m), dry 
shrub-grassland vegetation (2100-2400 m), mountain 
forest-grassland vegetation (2400-3400 m), sub-alpine 
shrub-grassland vegetation (3400-3900 m), and cold-desert 
alpine meadow vegetation (>3900 m). The mountain 
forest-grassland vegetation is the main vegetation type and the 
main component of the Qilian Mountains ecosystem. The range 
of elevations (1800-5100 m) in study area was divided into a 
total of 31 intervals with 100m in each of the intervals and the 
aspect angle was divided into a total of 72 intervals with 5° in 
each of the interval. 
The vegetation in the Qilian Mountain area plays an important 
role in the local water cycle by affecting hydrological processes, 
e.g., evapotranspiration and runoff, and is an important 
ecological storage for water resources. Qilian Mountain 
supplies water for Hexi Corridor which is the most important 
agricultural region and settlement in northwest China. The 
vegetation in the Qilian Mountain area significantly affects the 
oasis system in the region and protects the middle and 
downstream area of Heihe River against desertification. 
3, DATASET 
The MODIS NDVI data, the vegetation index maps depicting 
spatial and temporal variations in vegetation activities, is 
derived by precisely monitoring the Earth’s vegetation. These 
vegetation index maps have been corrected for molecular 
scattering, ozone absorption, and aerosols. The MODIS NDVI 
data is based on 16-day composites and its spatial resolution is 
250 m. Currently, the MODIS NDVI products have been used 
throughout a wide range of disciplines, such as inter- and 
intra-annual global vegetation monitoring climate and 
hydrologic modeling, and agricultural activities and drought 
studies (Zhan et al. 2000; Jin and Sader 2005; Sakamoto et al. 
2005; Knight et al. 2006; Lunetta et al. 2006). In this study the 
NDVI values from 28 MODIS NDVI images of the 16-day 
composites of June, July, August and September in seven years 
from 2000 to 2006 were used to study the spatial distribution of 
vegetation in the northern part of the Qilian mountain area 
because June, July, August and September are the most 
productive months of vegetation growth during a year and thus 
the NDVI values of these four months can best reflect the 
pattern of the vegetation cover in the region. 
The Digital Elevation Model (DEM) data was downloaded 
from the Digital River Basin website (http://heihe.westgis.ac.cn) 
and its spatial resolution is 100 m. The MODIS NDVI was 
resampled and interpolated to have the same spatial resolution 
as the DEM data in this study. 
4. RESULTS AND DISCUSSION 
It is well known that spatial distribution of vegetation cover is 
usually affected by elevation and aspect. Most vegetation in the 
northern part of Qilian Mountain area is distributed between the 
elevations of 1800 m and 4500 m. To the best of our knowledge, 
however, the obvious spatial distribution and patterns have not 
been studied quantitatively. We show in this study that the 
readily available NDVI data can be used to quantify the spatial 
distribution of vegetation. The range of elevations from 1800 m 
and 4500 m was divided into a total of 270 intervals with 10m 
in each intervals. The aspect angle of 360° were divided into a 
total of 72 intervals with 5° in each intervals. These divisions 
result a total of 19360 cells among which 19060 cells with the 
NDVI values larger than zero. In each cell the NDVI values 
from year 2000 to 2006 were averaged. The mean values 
represent the general conditions of vegetation growth in 
different elevations and aspects. A contour map of the mean 
NDVI values with elevation and aspect in the northern part of 
Qilian Mountain was plotted in Figure 2. A Gaussian smooth 
filter was used and a low pass convolution was performed on 
the gridded data to obtain the more consistent and smooth map 
in Figure 2. 
Several observations can be made in Figure 2 regarding the 
effects of elevation and aspect on the vegetation growth in the 
mountain area. First of all, it is clearly seen that the elevation is 
the main controlling factor in the vegetation growth. The NDVI 
value increases with the elevation and reaches its maximum 
value around 3400 m and then decreases as the elevation 
increases beyond 3400 m. The NDVI value is mostly larger 
than 0.50 (the dark green region in Figure 2) when the elevation 
is between 3200 m and 3600 m which is the best vertical zone 
in terms of vegetation growth. The NDVI values are less than 
0.50 when the elevation is lower than 3200 m and higher than 
3600 m or the vegetation growth is poorer in these elevations 
that in the zone of 3200 m and 3600 m. 
Secondly, the vegetation growth in the Qilian Mountain area is 
significantly affected by aspect. The impact of aspect on the 
vegetation growth is most significant in the vertical zone of 
3200 m and 3600 m. The best vegetation in this zone is 
distributed between NW340 0 and NE70° (the darkest green area 
in Figure 2 with the NDVI value larger than 0.56). In other 
words, the best vegetation growth is on the shady side of the 
mountain where much less evapotranspiration (ET) is expected. 
The reduced ET on the shaded side is important for the 
vegetation growth in the Qilian Mountain area since it is 
located in a semi-arid region. It is also observed in Figure 2 that 
a better vegetation growth occurs over a larger elevation range 
on the side facing north and northeast. At the aspect of N0°, for 
example, the NDVI value of 0.50 or larger are observed over 
the vertical zone of 600 m between the elevation range of 3100 
m~3700 m while at the aspect of SI 80° the same NDVI values 
are observed in a smaller range 400 m between 3200 m and 
3600 m. The much wider vertical zone with better vegetation