Differentiation of Wetland Areas in the West Siberian Lowland Using NOAA/AVHRR Imagery
3. Differentiation of Wetland Areas
3.1 Spatial Profiles of AVHRR Data
Fig. 2 shows the spatial profiles of AVHRR data along the transect line (white dotted line) indicated in Fig. 1(b). The abscissa denotes distance in a pixel unit; the ordinate denotes reflectance (%) for visible and near infrared bands (bands 1 and 2), and radiometric brightness temperature (°C) for thermal bands (bands 3-5). The interval between 6 to 22 pixel distance corresponds to a wetland area; the rest parts correspond to forest areas. We see that from the spatial profit characteristics, AVHRR spectral band data can be classified into three groups, i.e. band 1, band 2. and bands 3-5. Band 1 reflectance is higher in wetlands than in forests. It can be explained by the fact that majority of sphagnum mosses found in this region take on brownish color, having higher reflectance in red band than forest green leaves. Band 2 data have higher values in forests than in wetlands, which demonstrates that green leave have higher reflectance than sphagnum mosses in near infrared band. The dip from 18 to 21 pixel distance correspond to the place where wetlands have been dried by digging trenches. The bands 3-5 have more or less the same spatial profiles, having higher brightness temperature in wetlands than in forests. This temperature difference may be explained by two mechanisms. First, forests may have a greater transpiration rate than wetlands; thus, forests have lower surface temperature in midday due to greater latent heat transfer. Second, forest leaves may be cooled by air until leaf temperatures are in close adjustment with air temperatues
2; on the other hand wetland plants represented by sphagnum mosses may be less cooled by air because they grow in dense clusters near or at the ground surface. We note that surface temperatures have highest in dried parts of wetlands.
Fig. 2. Spatial profiles of AVHRR data along the transect line in Fig. 1 (b).
From the observation of the spatial profiles of AVHRR data, we have chosen two indices for delineation cover type in the study area. One is Normalized Difference Vegetation Index (NDVI).
Where RED and NIR are reflectance values of bands 1 and 2 of the AVHRR. The other is surface temperature (T
s ) computed by the split-window method
3;
Ts = 1.764 T4 -0.764 T5+0.78
Where T
4 and T
5 are radiometric brightness temperatures derived from AVHRR bands 4 and 5 respectively. Fig. 3 shows the spatial profiles of NDVI and T
s along the same transect line as in Fig. 2. We can see there is a negative correlation between NDVI and T
s, which has been observed by several researchers for various types of ecosystems
4,5,6,7. NDVI expresses spectral reflectance properties and density of vegetation; surface temperature, on the other hand, is related to water and energy balance determined by moisture availability, amount of evapotranspiration and local climate factors. Since wetland vegetation has different features from other land-cover types in vegetation properties and moisture status, we can expect that that wetland areas can be differentiated from other land-cover types by using NDVI and surface temperature as feature-extraction axes.
Fig. 3. Spatial profiles of NDVI and surface temperature along the transect line in Fig. 1(b).
