Environmental Engineering Reference
In-Depth Information
represent the 80 % con
dence limits. Other rating curves for other parameters can
be found in Ott ( 1978 ). The participants were also requested to give a relative
signi
five point scale to each parameter. This enables weighting
for the parameters to be obtained.
The investigators sought to derive a set of weights for the index which would
sum to 1.0, but more importantly, re
cance grading on a
cance rating of the parameters by
the panelist. The following three step procedure was used:
ect the signi
The arithmetic means of signi
cant rating were calculated from the parameters
￿
Temporary Weight was then calculated by the following formula:
Temporary Weight = Signi
￿
cant rating of each parameter/Highest signi
cant
rating
Final weights as sub index weights were calculated by:
Final Weight = Temporary Weight/Sum of Temporary weights
￿
The NSFWQI is calculated based on arithmetic and geometric means as given
below:
i ¼ n
i ¼ 1 W i I i
NFWQI ¼
ð 3 : 1 Þ
W
i ¼ 1 I i
NFWQI ¼
To calculate the index, the sub-index value I is read from the appropriate rating
curves for pollutant parameter I, and then is multiplied by the sub-index weights
calculated for each parameter, and summed over all the parameters. Tables 3.1 and
3.2 indicate the river classi
cation using this method.
Table 3.1 Stream
classi cation system
suggested by NSFWQI (Ott
1978 )
Representing color spectrum
Numerical range
Class
Red
0
25
Very bad
-
Orange
26
50
Bad
-
Yellow
51 - 70
Medium
Green
71
90
Good
-
Blue
91
100
Excellent
-
Red
0 - 25
Very bad
Table 3.2 Subdivisions of
the WQI scale (House and
Ellis 1980)
NWC Class
WQI range
1A
91
-
100
1B
71
90
-
2
41
70
-
3
21 - 40
4
10
20
-
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