Water Quality Index Calculator Formula

Understand the math behind the water quality index calculator. Each variable explained with a worked example.

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Formulas Used

Water Quality Index

wqi = weighted_sum

DO Contribution

rating = do_score * 0.30

Turbidity Contribution

turbidity_contrib = turbidity_score * 0.20

Nutrient Contribution

nutrient_contrib = nutrient_score * 0.20

Variables

VariableDescriptionDefault
do_scoreDissolved Oxygen Score(0-100)75
ph_scorepH Score(0-100)80
turbidity_scoreTurbidity Score(0-100)70
temp_scoreTemperature Score(0-100)85
nutrient_scoreNutrient Score(0-100)65
weighted_sumDerived value= do_score * 0.30 + ph_score * 0.15 + turbidity_score * 0.20 + temp_score * 0.15 + nutrient_score * 0.20calculated

How It Works

How to Calculate Water Quality Index

The WQI combines multiple water quality parameters into a single score.

Formula

WQI = DO(0.30) + pH(0.15) + Turbidity(0.20) + Temp(0.15) + Nutrients(0.20)

Rating Scale

  • 90-100: Excellent
  • 70-89: Good
  • 50-69: Medium
  • 25-49: Bad
  • 0-24: Very Bad

    • Each parameter is first converted to a 0-100 sub-index score based on its measured value.

    Worked Example

    A water body with DO=75, pH=80, Turbidity=70, Temperature=85, Nutrients=65.

    do_score = 75ph_score = 80turbidity_score = 70temp_score = 85nutrient_score = 65
    1. 01DO: 75 * 0.30 = 22.5
    2. 02pH: 80 * 0.15 = 12.0
    3. 03Turbidity: 70 * 0.20 = 14.0
    4. 04Temperature: 85 * 0.15 = 12.75
    5. 05Nutrients: 65 * 0.20 = 13.0
    6. 06WQI = 22.5 + 12.0 + 14.0 + 12.75 + 13.0 = 74.3 (Good)

    Frequently Asked Questions

    What does the WQI measure?

    The Water Quality Index consolidates multiple measurements into a single number indicating overall water quality. Higher scores indicate better quality for aquatic life and human use.

    Why is dissolved oxygen weighted most heavily?

    Dissolved oxygen is the most critical parameter for aquatic life. Most fish need DO above 5 mg/L. Low DO indicates pollution and eutrophication.

    What causes low water quality?

    Common causes include nutrient runoff (fertilizers), sewage discharge, industrial pollution, sediment erosion, thermal pollution, and acid rain.

    Ready to run the numbers?

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