Water quality index (WQI)

A water quality index is a single composite score that combines several individual water-quality measurements, typically things like dissolved oxygen, pH, turbidity, nutrients and bacteria counts, into one number so water bodies or time periods can be compared without reading a full table of separate parameters. Higher usually means better water quality, though the scale and exact parameters vary by index.

Most indices work by converting each raw measurement into a sub-index score on a common scale, often 0-100, using a parameter-specific rating curve, then combining the sub-indices with a weighted average or a more conservative formula that penalizes any single badly failing parameter. There is no single universal WQI: national and regional environmental agencies define their own versions with different parameters and weights, so a score from one index is not directly comparable to a score from another without knowing the underlying formula, similar to how air quality index scales differ across countries.

Water quality indices support regulatory reporting, drinking-water and recreational-water safety communication, and trend tracking for rivers, lakes and coastal waters, often supplementing direct sensor readings from sensor calibration-checked monitoring stations and, for larger water bodies, ocean color remote sensing estimates. The main pitfall is treating the composite score as diagnostic: a moderate WQI can hide one severely failing parameter offset by others scoring well, so practitioners should check the underlying sub-index breakdown before acting on the summary number alone.