Water Quality Certainty

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 measured?

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.