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Three-Level Framework, Level 1: Comprehensive hydrologic desktop methods

A Level-1 approach is based on hydrological “desktop” methods, which generally rely upon hydrological data (e.g., daily flow levels). This approach is appropriate for regional planning and preliminary standard setting or for organizing and pre-analyzing information for a Level 2 approach. Hydrologic desktop methods are commonly used due to their primary advantage of being relatively quick and inexpensive. However, their primary drawback is that they generally provide overly simplistic flow levels that do not fully encompass current understanding of river functions and processes. We suggest that hydrologic desktop methods are available that, when combined with a review of available information for a given river system and augmented by basic understanding of river functions, can produce credible and comprehensive initial flow recommendations. 

A comprehensive hydrologic desktop approach synthesizes two primary sources of information: (1) a hydrological analysis tool that is capable of assessing a range of flow levels; and (2) a literature review of the linkages between the flow regime and key riverine resources. This review should incorporate all the available relevant information for the specific river or basin augmented by broader literature on riverine processes. In the absence of specific information on the focal river, practitioners can draw on broader literature with an emphasis on information relevant to similar river types (e.g., in terms of geomorphology, drainage area, valley characteristics) and ecosystems. This integration of a literature review to the hydrological analysis is what advances the comprehensive hydrologic desktop approach beyond simple “rules of thumb.”

The Nature Conservancy developed the Indicators of Hydrologic Alteration (IHA) software to support a comprehensive desktop approach. IHA uses daily flow data to calculate 67 ecologically relevant flow statistics, including environmental flow components (EFCs). The U.S. Geological Survey’s Sustainable Yield Estimator is a relatively rapid new method for estimating daily flow data for ungaged sites, which can be used as input to IHA.

For regional-scale assessments, a team of leading river scientists representing 10 international scientific organizations developed the Ecological Limits of Hydrologic Alteration (ELOHA), a flexible, scientific framework for assessing and managing environmental flows across large regions, when limited time and resources preclude evaluating individual rivers. ELOHA is a Level 1 framework that combines desktop hydrologic analysis with a review of existing ecological databases and literature.

Continue to Level 2.