WEIGHT OF EVIDENCE APPROACH TO ASSIGN FLOW REGIMES TO ARIZONA STREAMS

The arid western United States has many surface waters with a non-perennial flow regime and that do not flow year-round. However, the guidance for determining Clean Water Act regulated surface waters puts an emphasis on flow permanency. The lack of understanding between flow regime and flow permanency creates regulatory uncertainty. To provide regulatory clarity, the Arizona Department of Environmental Quality (ADEQ) created a process to use existing data and information to assign and verify flow regimes.

Currently, only 18% of Arizona’s surface waters have been assigned a flow regime, leaving more than 3,000 waterbodies with undetermined flow regimes. ADEQ previously utilized a flow regime algorithm to assign flow regimes incorporating priority and scope into flow regime assignments. To achieve more definitive results, ADEQ developed a weight of evidence approach by revamping the previously used algorithm. This approach utilizes data from multiple sources, such as stream gauges, biological surveys, and water quality sampling data, to determine a single flow regime result for a waterbody reach. Each data source is assigned a flow regime of either perennial, at least intermittent, intermittent, or ephemeral; and is assigned a numeric score based on the quality of data. The numeric scores within each flow regime category are combined, and the flow regime category with the greatest score is assigned to the waterbody.

ADEQ validated the weight of evidence approach by comparing the calculated weight of evidence flow regime of 51 waterbodies with their known flow regimes that were either perennial, intermittent, or ephemeral. The overall accuracy was 92% when ADEQ compared the weight of evidence flow regimes to their known validation flow regimes. Using the weight of evidence approach, all perennial and intermittent validation reaches were assigned correct flow regimes and 75% of ephemeral validation reaches were assigned correctly. ADEQ is now utilizing the weight of evidence approach to assign or verify flow regimes in the state.