Sampling water quality in a watershed as large and diverse as the Verde River Watershed is an enormous challenge. This indicator measures whether water quality samples in a given region were adequate to provide reliable assessments of water quality.
How is it scored?
The “Index Stability Score” (ISS), used here to represent Water Quality Certainty, was developed as tool to assist in interpreting Water Quality Index scores. The ISS mathematically scores adequacy of a given set of samples to provide reliable assessments of water quality (scored from 0-100, with 100 representing a perfectly adequate sample) by determining the samples meet three criteria: 1.) Mathematical/statistical sufficiency (e.g., sample size > 30), 2.) Natural variability (e.g., maximum coefficient of the dataset equals the square root of the number of values in the set), and 3.) Data representiveness (e.g., does the sample: include stormflows, include baseflows, cover the all seasons, have a reasonable percentage of events not exhibiting “greater than” vales, and at least a one-year duration of the sample period). For more details see McCarty, D. 2019. Index Stability Score: An Adjunct tool for Water Quality Index Reporting. Arizona Department of Environmental Quality.
The methodology for water quality certainty changed between 2020 and 2025. In 2020, the score was based on a calculation that was determined by the Arizona Department of Environmental Quality (ADEQ) and has since discontinued that calculation. In response, Verde Watershed stakeholders worked to determine a comparable score with input from ADEQ.
The water quality certainty score is based on the proportion of perennial and intermittent streams within the watershed that are categorized as assessed for water quality by ADEQ. This score does not grade the water quality itself, but rather how confident we are in our understanding of water quality in the watershed.