Dear Salem,
Once you select the parameters, their weights will be assigned based on the importance you want them to have for the overall water quality For example, pH may have a lower weight than fecal bacteria if the WQI is evaluated for for recreational activities suitability. The process is mostly 'expert-judgment' based I think..
The weight or importance of each parameter in the water quality index (WQI) depends on a variety of factors, such as the local regulations, environmental conditions, and intended use of the water. In general, the weight or importance of each parameter is determined by the impact that parameter has on the overall water quality and its potential effect on human health or the environment.
To determine the weight of each parameter in your specific case, you should follow these steps:
Sub-index = [(Measured value/Standard value) x 100]
Note that the weights or importance assigned to each parameter are subjective and may vary depending on the specific circumstances. Therefore, it is important to justify the selection of weights and document the methodology used to calculate the WQI.
We chose to assign equal weights to all parameters in each category in an attempt to remove some of the subjectivity over which parameters should be considered most important. The process of assigning and applying weights to parameters is not used in all WQIs (Abbasi and Abbasi, 2012). The relative importance of each parameter and its influence on water quality is subjective. When polled, experts often give different ranks to parameters (i.e., nitrate is more/ less important than phosphorus when determining water quality) or give different weights to parameters even if they have ranked them in the same order. Additionally, the overall score is in danger of being biased towards the most heavily weighted parameter if the score is weighted too high (Sutadian et al., 2016). While individual parameters are not actively weighted, the metals category is effectively weighted in the event that either iron or aluminum exceed aquatic life use standards through a “zeroing” out of that category score (Joanna et al., 2020).
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