The Habitat Suitability Criteria (HSC) models provide an objective, quantifiable method of assessing the existing habitat conditions for a given aquatic species within a study area by measuring how well each habitat variable meets the habitat requirements of the species by life stage. This serves as an objective basis for predicting probable project impacts, documenting post-project impacts, and guiding habitat protection, mitigation, enhancement, and management decision.
Instream Flow Incremental Method (IFIM) is a method of determining the HSC, and is one of the most widely used instruments in the world for assessing the effect of flow manipulation on river habitats. Originally developed by the US Fish and Wildlife Service (Bovee, 1982), USA EPA describes IFIM as integrating “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” (EPA, 2018).
At this stage, however, the PHABSIM models to not simulate water quality. PHABSIM concentrates on simulation of physical habitat based on depth, velocity, and a channel index. In their manual on PHABSIM, Milhous and Waddle (2012) refer users to other sets of models, such as the QUAL-2 model for approaches to simulate water quality constituents. Furthermore, PHABSIM, River2D and similar IFIM models are restricted to 2D simulation. With EEMS now enhanced to perform the IFIM analysis, it serves as a fully highly advanced 3D PHABSIM, incorporating water quality, sediment, toxics and many other highly relevant sub-models.
Habitat availability with IFIM is measured using an index called the weighted useable area (WUA). This is the wetted area of a stream weighted by its suitability for use by an organism. In EE WUA is calculated using the product of the habitat suitability indices, which are further described below:
Figure 1 Model Analysis: Habitat Suitability Criteria form.
Figure 2 Editing tool for suitability index curves.
Figure 3 Suitability Criteria: Secondary processing.