Simple Water Quality Index Calculator
Simple Water Quality Index Calculator
An index value is calculated for each of five water quality parameters, temperature, biological oxygen demand (BOD), total suspended sediment (TSS), dissolved oxygen (DO), and conductivity. A higher value of each index indicates better water quality. The ISQA is calculated as the temperature index times the sum of the other four index values.
Click here to learn more about the ISQA and the five water quality parameters used to compute it.
Water Quality Index Calculation
The water quality index is calculated based on measured values for each of five parameters: Temperature, Biological Oxygen Demand, Total Suspended Solids, Dissolved Oxygen and Conductivity. Here we describe what each of these parameters means in terms of local water quality and how they are used in the index.
Simple Water Quality Index (ISQA)
ISQA is calculated as: ISQA = ITEMP * (IBOD + ITSS + IDO + ICOND). Where ITEMP, IBOD, ITSS, IDO, and ICOND represent individual index terms with different weighting factors for each parameter.
ISQA varies from 0 to 100, with 100 indicating excellent water quality, as shown to the left.
Water Temperature Index (ITEMP)
Fish and other organisms all have a preferred temperature range, and as temperature falls above or below this range, the number of individuals decreases. Water temperature is also important for its influence on water chemistry; warmer water can hold less dissolved oxygen.
The water temperature index varies from 0 to 1. The temperature index decreases from 1 for every degree that water temperature is greater than 20°C.
Biological Oxygen Demand Index (IBOD)
Biological oxygen demand indicates how much oxygen is needed by bacteria to break down organic matter in the water. A high BOD indicates a high amount of organic pollution in water, which may be an indication of contamination by sewage or other waste.
IBOD reaches a high of 30 for BOD = 0 mg/L. For BOD values > 12 mg/L, IBOD = 0.
Total Suspended Solids Index (ITSS)
Total suspended solids are a measure of the mass of particles suspended in water, which can come from erosion of land surfaces and be washed in from urban areas. These particles can interfere with drinking water treatment and irrigation infrastructure and may contain contaminants that pose a health risk.
ITSS reaches a high of 25 for TSS = 0 mg/L. For TSS values > 250 mg/L, ITSS =0.
Conductivity Index (ICOND)
Conductivity is a measure of the ability of water to conduct electricity. It is related to the amount of dissolved salt that is present in water. Water with a lot of salt can be dangerous to crops.
ICOND reaches a high of 20 when conductivity is 200 μS/cm (conductivity of drinking water). For conductivity values greater than 4000 μS/cm, ICOND = 0.