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wasteload allocations and total maximum daily loads. The U.S. Environmental Protection Agency (EPA) also supports the use of "biological" low-flow quantiles, complementing the familiar statistical low-flow estimates (USEPA, 1990, 1991). Estimation of biological low-flow quantiles has been developed to reflect the continuous recovery periods thought to be required by biological populations recovering from low-flow stress. Heuristic techniques have been developed that utilize the historical hydrologic record to identify flow levels that would empirically achieve nominal frequency duration criteria, based on historical streamflow records. Biological low flows for acute, 1-hour/3-year (1B3) and chronic 4-day/ 3-year (4B3) low-flow quantiles are recommended and have been utilized by EPA (CFR, 1992) in setting toxic discharge standards for aquatic life protection. The uncertain relationship between biological and hydrologic flows (e.g., 7Q10) represents an opportunity to clarify management criteria and permit decisions through the integration of Water Resources Division's expertise in hydrologic frequency analysis and Biological Resources Division's expertise in habitat requirements for aquatic species. The Survey's strengths in these areas suggest a natural opportunity to integrate research on low flow frequencies and habitat requirements with the science-based regulatory initiatives in instream flow maintenance and wetland restoration within the USEPA and USDA.
Role of the USGS
Drought-related interpretive studies expand the information base supporting research (Liu and Stedinger, 1991) and represent a valued mechanism to transfer research products to resource managers and cooperators (Ludwig and Tasker, 1993). The USGS continues to play an integral role in support of drought management, serving as a source for interdisciplinary hydrologic expertise.