Habitat Suitability Curves
Early forays into the evaluation of flow needs centered on changes in available habitat at differing flow levels. In depth identification of the habitat needs of various species, and subsequent stream surveying demonstrated that the amount of habitat available to a given species varies widely under differing flow conditions, and also varies widely in between species at a given flow rate. A habitat suitability analysis therefore centers on identification of the species in a given river of interest, their habitat needs, and direct measurement of the amount of habitat that occurs at select stream transects for these species under a variety of flow levels. The results of surveys are then combined with long term simulated flow conditions, with the habitat area at every flow rate normalized by the maximum value for each species. These normalized areas, or "Weighted Usable Area" values then provide an index between 0.0 and 1.0, representing the percentage of maximum habitat that is available for each species at every flow rate (see Figures 1 and 2).
By performing habitat suitability analyses and stream habitat mapping, in-stream flow practitioners attempt to craft withdrawal/flow-by/release rules that would provide the greatest availability of habitat to a wide variety of organisms. These studies have been popular, and also help to explain some of the reasons for consideration of the entire flow regime. However, they are also extremely data intensive, due to the vast differences in stream channel morphology and therefore habitat. These differences are not only prevalent between different streams, but also within a single stream, habitats can fluctuate substantially, requiring very extensive surveys to generate WUA curves that accurately depict the optimal range of flows.
Figure 1: IFIM Flow versus % of usable area for Lower James Redhorse Sucker.
Figure 2: IFIM Flow versus % of maximum usable area for multiple species.
In a Nutshell
- Different species have different optimal flow/habitat levels
- More water <> more habitat
- Is habitat everything?
- Visualizing Weighted Usable Area data