Water quality index calculation vs irrigation quality index calculation
Answers
Explanation:
It may be defined as a rating, reflecting the composite influence of different water quality parameters on the overall quality of water. The main objective of computing of water quality index (WQI) is to turn the complex water quality data into information which is easily understandable and usable.
Selection of parameters: First, study the Indian Standard (BIS 2004) for drinking water specification. Here, the physicochemical parameters along with the desirable limits and related health effects are given. A parameter has to be selected based on its impact in the overall quality of water and heath effects. This standard is freely available in net. However, I am enclosing the same for your convenience (see below).
Computation of WQI: The WQI is computed following the three steps.
First step – Assigning of weight (wi) to the selected water parameters (e.g., pH, TDS, TH, HCO3, Cl, SO4, NO3, Fe, ……) according to their relative importance in the overall quality of water for drinking purposes (weight may be from 1 to 5).
Second step – Computation of a relative weight (Wi) of the chemical parameter using the following equation:
Wi = wi / ∑ wi ( i = 1 to n)
where, Wi is the relative weight, wi is the weight of each parameter and 'n' is the number of parameters
Third step - Assigning of a quality rating scale (qi) for each parameter, as below:
qi = (Ci / Si ) x 100
where, qi is the quality rating, Ci is the concentration of each chemical parameter in each water sample in mg/l, and Si is the guide line value/desirable limit as given in Indian drinking water standard (BIS 2004).
For computation of WQI, the sub index (SI) is first determined for each chemical parameter, as given below:
SIi = Wi x qi
WQI = ∑ SIi 1 - n
where, SIi is the subindex of ith parameter; Wi is relative weight of ith parameter; qi is the rating based on concentration of ith parameter and 'n' is the number of chemical parameters.
Classification of water :The water may be classified into five types based on computed WQI as given below:
WQI range and water type:
< 50
Excellent water;
50 - 100
Good water;
100- 200
Poor water;
200 – 300
Very poor water;
> 300
Water unsuitable for drinking.