Geography Reference
In-Depth Information
7.2. Assigning Rank to Different Water Quality Variables
The ND
index
maps are rated between 1 and 10 to generate a ‗rank map'.
The rank 1 indicates minimum impact on water quality, while the rank 10
indicates maximum impact. The minimum ND
index
value (-1) is set equal to 1,
the median value (0) is set equal to 5 and the maximum value (1) is set equal
to 10. The following polynomial equation can be used to rank the
contamination level (or ND
index
) of every pixel between 1 and 10:
5
2
R
0
(
ND
)
4
(
ND
)
index
index
(35)
where R
= rank value of every pixel corresponding to its ND
index
value.
7.3. Developing Water Quality Index Map
Water Quality Index (WQI) is calculated as follows (Babiker et al., 2007):
WQI
100
(
R
w
R
w
R
w
)
/
N
1
1
2
2
n
n
(36)
where R = rate of the rank map (1-10), w = relative weight of the parameter
which corresponds to the ‗mean' rating value (R) of each rank map (1-10),
and N = total number of parameters used in the suitability analysis.
The definition of WQI [Equation (35)] is similar to the weighted linear
combination technique. The weight (w) assigned to each parameter indicates
its relative importance to water quality and corresponds to the mean rating
value of its ‗rank map'. The total number of parameters (N) involved in the
expression of WQI averages and limits the index values between 1 and 100.
The ‗100' in the first part of the formula is incorporated to directly project
the WQI value such that high index values close to 100 reflect ‗high water
quality' and the index values far below 100 (close to 1) indicate ‗low water
quality'. The entire steps of developing a water quality index map are depicted
in a flowchart shown in Figure 9.
