Instream Flow Incremental Methodology (IFIM)
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Numerous methods can be used to determine instream-flow needs for fish and wildlife, but one of the most widely used is the Instream Flow Incremental Methodology (IFIM), developed in the 1970s by physical and biological scientists in the U.S. Fish and Wildlife Service. IFIM integrates concepts of water-supply planning, analytical hydraulic engineering models, and empirically derived habitat-versus-flow functions to address water-use and instream-flow issues and questions concerning life-stage-specific effects on selected species and the general well being of aquatic biological populations. The method has undergone continual refinement by the Fort Collins Science Center scientists and has remained a state-of-the-art tool.
A major component of IFIM is a collection of computer models called the Physical Habitat Simulation Model (PHABSIM). This model incorporates hydrology, stream morphology, and microhabitat preferences to determine relations between streamflow and habitat availability. Habitat availability is measured by an index called the weighted useable area (WUA), which is the wetted area of a stream weighted by its suitability for use by an organism. PHABSIM simulates streamflow habitat relations for various species and life stages and allows quantitative habitat comparisons at different streamflows.