Learn More: Water Quality Index (WQI) – Sarasota County Water Atlas – Sarasota.WaterAtlas.org

The Water Quality Index (WQI)1 is similar to the Trophic State Index (TSI) in that both
were developed for the statewide assessment of surface waters. While both the are useful as thumbnail
indicators of water quality, they are non-regulatory measurements; they may be used by the Florida
Department of Environmental Protection to identify water bodies that may need further study, but not
to determine attainment of water quality standards under the Impaired Waters Rule of the Clean Water Act.

WQI is used for streams, black waters (natural tea- and coffee-colored waters), and springs, while TSI is used for lakes and estuaries.

The Water Atlas presents WQIs for three-month intervals (Jan–Mar, Apr–Jun, Jul–Sep, Oct–Dec). The quarterly WQI for a waterbody is determined by averaging the index values for selected water quality parameters for the interval. Ranges of WQI values have been established to provide a general ranking of the waterbody (Figure 1.) The index is accompanied by a ‘confidence value’ that expresses the degree of completeness of the index (Figure 2). For more information, see the Calculations section, below.

WQI
Rating

0-45
Good

45-60
Fair

>60
Poor

Figure 1. Water Quality Index (WQI) ranges and their designations.

WQI
Rating
Confidence
Season

30
Good
5/5
Winter (2000)

40
Good
3/5
Fall (2000)

30
Good
2/5
Summer (2000)

50
Fair
3/5
Summer (2000)

Figure 2. WQI rankings are provided with examples of Confidence values.

1The acronym WQI also stands for “Water Quality Inspection” in much of the DEP literature.