What method is a better predictor of excursions from the aquatic life criteria?
EPA used both the hydrologically-based design flow method and the biologically-based design flow method on approximately 60 rivers to compare the 1Q10 with the 1B3 and the 7Q10 with the 4B3. For most of the rivers, the hydrologically-based design flows (i.e., 1Q10, 7Q10) resulted in more than the allowed excursions. For some of the rivers the 1Q10 and 7Q10 allowed substantially more or fewer excursions than the intended number of excursions. Since the biologically-based method calculates the design flows directly from the national or site-specific duration and frequency, it always provides the maximum allowed number of excursions (and never provides more or fewer excursions than allowed).
