The notion of Weighted Usable (habitat) Area (WUA) in aquatic environments stems from Ken Bovee's technical report on the Development and evaluation of Habitat Suitability Criteria for use in the instream flow incremental methodology from 1986. The method builds on the evaluation of a Habitat Suitability Index (HSI) that ranges from 0 to 1, but is not a probability. Because of the value range of HSI, it is sometimes confusingly used as a multiplier to calculate usable habitat area from two-dimensional (2d) numerical model output. The outcome of this approach is a notion of m²-Index and not m², which is strictly speaking a non-sense unit. To overcome the use of confusing units when HSI is used with spatially explicit 2d models, we use in our River Architect software (https://riverarchitect.github.io) a threshold value as on-off trigger to define if a pixel's area counts as usable habitat or not. Other commercial software, such as CASiMiR, use fuzzy sets to overcome some challenges, but eventually come back to a per-pixel association of habitat area. To this end, habitat assessment is complex and should account for multiple target fish species and their lifestages. For instance, the preferred habitat of juvenile trout differs significantly from that of adult trout. In a nutshell, there are many options and nearby every country has defined own standards for snapshot-like habitat assessment in public projects.
This is why I am addressing the following questions for an open discussion and experience reporting to the community : How do you assess habitat in rivers? How do you account for changing habitat preferences among target fish species and/or lifestages? Ultimately, how do you deal with competing habitat requirements in river restoration design, when the blurry overall goal is to "improve habitat conditions for indigenous species"?
Dear Sebastian Schwindt,
I invite you to read this document. It is a helpful paper on modelization.
This is a good question we deal with quite a bit. Given maps of 2D maps of habitat quality, what value(s) do you use to summarize changes in habitat? There is no agreed upon approach I am aware of. For a bit, people summarized amounts in different quality bins, e.g. "there was a 50% increase in areas with HSI > 0.5". However, I have found this not be too helpful to stakeholders who need simpler answers. Generally you have to assume a "cut-off" value. If bio-verification is available that helps in that the preference and avoidance values could be a defensible cut-off. For example, if bio verification says there is avoidance below HSC of 0.25 and preference above you could use 0.25 as the cut-off. Ultimately it goes back to why this is being calculated. For comparing river restoration designs the relative change is what is important. However, this is very different than instream flow studies that are trying to manage populations.
Thanks Rocko Anthony Brown for the valuable input. An additional degree that we often have to deal with are dynamics and spatial variability that can hardly be reproduced with 2d numerical models. The latter are currently hard to overcome at a representative river reach scale, but I am wondering if there are viable approaches around to deal with temporal variability to account for seasonal discharge variability combined with fish lifestages. Does anyone have relevant experience with that?
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