the image by using unsupervised classification. In this way the water pixels were
differentiated from the land pixels. An unsupervised classification was applied on the
SPOT band 3 (NIR) image and resulted in a total of 150 categories. By assigning the
first class to water throughout the entire image, the water area of the reservoir can be
identified. The procedure developed a binary mask layer by renumbering the water
pixels as "1" and the rest of the pixels as "0". Afterward, the water body was extracted
from the other SPOT bands by multiplying by this mask layer.
With regard to the use of remote sensing in water quality monitoring and modeling,
the major concerns are: 1) the suitable spectral channel to match the various
characteristics of water quality variables, and 2) the appropriate and efficient image
processing to convert image brightness to traditional water quality indices. To cover
distinctive spectral characteristics and distinguish the brightness values reflected from
plankton and suspended solids, multispectral sensors would be a good choice
(Robinson, 1985). Therefore, a band ratio regression model was developed for
delineating the water quality conditions by remote sensing monitoring. A band ratio
between the near infrared (NIR) and red was suggested to detect chlorophyll in water,
due to a positive reflectivity of chlorophyll in the NIR and an inverse behavior in the
red (Rundquist et al., 1996). Concordant with the theoretical basis, the correlation
matrix shows that the ratio of the red-band to the infrared-band has the highest
correlation to chlorophyll. Also, a green/red (XS1/XS2) ratio shows a significant
correlation with Secchi depth and phosphorus concentration. These ratios were
adopted to derive these three water quality variables from satellite data. The
regression model was calibrated by in situ samples, which were collected from the
water below 1 m from surface. These samples were correlated to the remote sensing
data from SPOT using an average digital value from a 3x3 pixel window centered at
the sampling location. A natural logarithmic regression model was developed for
chlorophyll a (CHLA), Secchi depth (SD), and phosphorus (PO4) as: