The Water Quality Index (WQI) is a tool developed to better communicate water quality information in a concise and understandable way. The WQI generates a single standardized number reported on a scale from 0 to 100, with 100 representing high water quality. To achieve a top score the samples must have uniform attainment of water quality standards.
How is it scored?
The general index considers the percentage chemical parameters exhibiting exceedances of water quality standards, the percentage of samples exhibiting exceedances of water quality standards, and the magnitude exceedences over the water quality standard. For more details see: McCarty, D. 2018. The Arizona Water Quality Index: A Communications Tool for Water Quality Summaries. Arizona Department of Environmental Quality.
The methodology for water quality index changed in 2025 from those used in 2020. In 2020, the score was based on a calculation that was determined by the Arizona Department of Environmental Quality (ADEQ) and ADEQ has since discontinued that calculation. In response, Verde Watershed stakeholders worked to determine a comparable score with input from ADEQ.
Water quality index is scored based on the percentage of stream miles that have been assessed by ADEQ that are meeting state water quality standards and supporting designated uses.
Each stream has “designated uses” that are determined by the State of Arizona. Some of the major uses in our area include aquatic and wildlife habitat, swimming, fish consumption and more. For each use, the stream must be within a set limit of various pollutants. If a stream is labeled “supporting all uses” that means that
- A sufficient amount of samples have been collected in this area, and;
- Samples have been compared to state standards, and;
- Each of the results are within the set limit.