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Validation of rapid assessment methods to determine streamflow duration classes in the Pacific Northwest, USA
Citation:
Nadeau, Tracie-Lynn, Scott G. Leibowitz, P. Wigington, Jr., Joseph L. Ebersole, Ken M. Fritz, R. Coulombe, Randy L. Comeleo, AND Karen A. Blocksom. Validation of rapid assessment methods to determine streamflow duration classes in the Pacific Northwest, USA. ENVIRONMENTAL MANAGEMENT. Springer-Verlag, New York, NY, 56:34-53, (2015).
Impact/Purpose:
U.S. Supreme Court rulings have created uncertainty regarding federal Clean Water Act (CWA) authority over certain waters, including ephemeral and intermittent streams, and established new data and analytical requirements for determining whether a water body is covered under the CWA. As a result, rapid-assessment methods are needed that can differentiate between ephemeral, intermittent, and perennial streams. We completed data analyses from a multi-year, three-state study validating a rapid assessment method to distinguish between ephemeral, intermittent and perennial streams. Our objectives were to assess the accuracy of an interim method, developed through best professional judgment, in determining streamflow duration classes, and to compare this with three other methods. These included an alternative method that was developed through statistical analysis of the field data. The result was a revised version of the method that was simpler and more accurate in Oregon*, and was found to be equally as accurate in Idaho and Washington. This manuscript reports on the development and field validation of the empirically based method, which correctly determined known streamflow duration 83.9% of the time, versus 62.3% accuracy of the method developed through best professional judgment alone. The validation study described herein supports a consistent, repeatable (i.e., less than 12% variation between duplicate assessments) method that is applicable across the diverse hydrological landscapes of the Pacific Northwest, and thus useful where knowledge of streamflow duration improves environmental assessment, management, and decision-making. *Nadeau, T.-L. 2011. Streamflow Duration Assessment Method for Oregon. http://www.epa.gov/region10/pdf/water/sdam/final_sdam_oregon_nov2011.pdf (downloaded over 1000 times since posted in 2011)
Description:
U.S. Supreme Court rulings have created uncertainty regarding federal Clean Water Act (CWA) authority over certain waters, including ephemeral and intermittent streams, and established new data and analytical requirements for determining whether a water body is covered under the CWA. As a result, rapid-assessment methods are needed that can differentiate between ephemeral, intermittent, and perennial streams. We report on the validation of several methods within Oregon, Washington, and Idaho. The first (Interim Method) was developed primarily through best professional judgment (BPJ); an alternative (Revised Method) resulted from statistical analysis of Oregon field data. In the first phase of the study, we tested the Interim Method on 178 study reaches in Oregon, and constructed the Revised Method based on statistical analysis of the Oregon data. In a second phase, we evaluated the regional applicability of the methods on 86 study reaches across a variety of hydrologic landscapes in Washington and Idaho. During the second phase, we also compared the Revised Method, developed from Oregon data alone, with a similar approach (Combined Method) based on combined field data from Oregon, Washington and Idaho. We further compared these field-based methods with a GIS-based approach (GIS Method) that used the National Hydrography Dataset and a synthetic stream network. Our evaluations compared results of all methods with actual streamflow duration classes determined by independently and objectively defined field criteria. For all observations, the Revised Method correctly determined known streamflow duration 83.9% of the time, versus 62.3% accuracy of the Interim Method and 43.6% accuracy for the GIS-based approach. The Combined Method did not significantly outperform the Revised Method developed using Oregon data alone. Analysis of empirical data showed biological indicators most accurately discriminate streamflow duration classes in the Pacific Northwest. While BPJ establishes a testable hypothesis, this study illustrates the importance of quantitative field testing of rapid assessment methods and identifying indicators with the strongest explanatory power in discriminating between classes. Results support a consistent, repeatable method that is applicable across a range of hydrologic landscapes in the Pacific Northwest.
