The instream flow incremental methodology (IFIM) and one-dimensional (1D) physical habitat simulation (PHABSIM) modeling are standard approaches to assessing instream flow needs for aquatic species (Figure 5). The first step is to perform a detailed stream or aerial survey, or map habitats, in the study area to determine the extent and distribution of habitat types. Following habitat mapping, individual study reaches and instream flow transect locations are established. Hydraulic and habitat data are collected at each transect using standard techniques employed in instream flow studies. The transect data are then processed through hydraulic simulation submodels within the PHABSIM model, generating simulations of depth and velocity distributions over a broad range of flows. Literature-derived or site-specific habitat suitability criteria (HSC) are applied to the predicted hydraulic parameters to produce functional relationships between flow and aquatic habitat (expressed as weighted usable area, or WUA). Lastly, a habitat time series analysis is performed to integrate WUA results across spatial (reach-long) and temporal (over the hydrologic period of record) scales.
4.1.2.1 Advantages and disadvantages of approach
Use of IFIM and one-dimensional (1D) PHABSIM to assess instream flows has advantages and disadvantages relative to the other approaches evaluated, as discussed below:
Advantages
- Measurement approach at transects is standardized, widely used, and repeatable.
- The level of precision in the results can be adjusted by altering sampling intensity.
- Other studies can utilize data collected at transects, including studies on potential fish stranding, hydrology, or fluvial geomorphic processes.
- Multiple habitat values (depth, velocity, substrate, and cover) can be simultaneously accounted for.
- Limited professional judgment is required for input data, with the exception of selecting habitat suitability criteria.
- Habitat criteria values, or analysis species, can be updated or added to the analysis as new information becomes available, without collecting additional field data.
- Incremental approach allows habitat assessments to be conducted at any simulated flow (although it is most accurate within the range of measured flows).
Disadvantages
- There are sampling issues associated with using data collected at transects to represent river reaches. There is no ability to account for conditions upstream or downstream of transects, and therefore transect location heavily influences results. Unless the transects are representative of the remainder of the river, small biases (e.g., particularly low or high amount of habitat at one location) in the results at one transect are multiplied during the extrapolation. The more complexity in a river system, the greater the risk of bias. This is typically addressed by increasing the number of transects in complex (e.g. high gradient) systems.
- There is limited ability to address hydraulic conditions where the water surface elevations vary across a transect (e.g., split channels, and high gradient riffles).
- Can only simultaneously account for a limited set of habitat values (depth, velocity, and a channel index such as substrate or cover).
- Hydraulic modeling typically occurs at a coarser scale than that at which organisms respond to their hydraulic environment; a mis-match in scale occurs when combining results from hydraulic models with habitat suitability or preference data collected on a finer scale. Suitability criteria are also often biased because they are based on site-specific habitat conditions and use of vertically-averaged velocities, and the criteria fail to account for habitat preferences that vary based on the scale that is considered. For example, the focal point (or "nose") water velocities used by a fish are often much lower than the average water velocity at the same location.
- There is a weak tie to population response, and interpretation of results almost always assumes that minimum flows limit the abundance and/or distribution of populations. There is a limited ability to integrate results with a limiting factors analysis, so discerning which species or life-stage should drive flow selection is unreliable. Does not provide total amount of preferred or usable habitat available at varying flows, so the data can not be used in population modeling (see Section 4.6).
- Integrating WUA results with other analyses (e.g. limiting factors analysis) is problematic, since the metric serves mostly as a relative index to compare various flows.
- Application of the model requires professional judgment regarding how well the habitat portion of the model actually fits what is being modeled.
- Does not allow calculations of statistical error bounds on the predicted habitat-flow relationships, so the reliability of the results cannot be estimated and considered when making management decisions.
4.1.2.2 Site-specific considerations and applicability
Many of the advantages and disadvantages of this approach will depend on site-specific considerations, as discussed below:
- 1D PHABSIM models rely on the assumption of a simple channel with either gradual variation in flow or uniform flow, which is atypical for some habitat types.
- Not reliable in hydraulically complex areas due to limited ability to address spatial shifts in water velocity as flows change. IFIM and 1D approaches are best applied in systems without substantial bed roughness, or without hydraulic complexity.
- Has not been widely applied in spring-dominated streams, and may not adequately address aquatic habitats there.
- In study areas with long stream reaches, or a variety of stream channels (e.g., side channels) to be assessed, an extensive number of transects are needed to adequately characterize habitat-flow relationships.
- Works well in deep pools that have consistent average velocities, but if pools are hydraulically complex, the results are less reliable.
- Extensive data collection can be very expensive.
- Has been applied at many locations throughout the world, including high-gradient and multi-channel streams, as well as warmwater streams throughout the United States. The following link provides a list of contacts familiar with examples in their region, accessible by clicking on the state or province on the map.
4.1.2.3 Selected references
Bovee, K.D., B.L. Lamb, J.M. Bartholow, C.B. Stalnaker, J. Taylor, and J. Henriksen. 1998. Stream Habitat Analysis Using the Instream Flow Incremental Methodology. Fort Collins, CO: U.S. Geological Survey-BRD. Information and Technology Report USGS/BRD/ITR-1998-0004. 130 p.Parasiewicz, P. 2001. MesoHABSIM: a concept for application of instream flow models in river restoration planning. Fisheries 26: 6-13.
Castleberry, D. T., J. J. Cech, Jr., D. C. Erman, D. Hankin, M. Healey, G. M. Kondolf, M. Mangel, M. Mohr, P. B. Moyle, J. Nielsen, T. P. Speed, and J. G. Williams. 1996. Uncertainty and instream flow standards. Fisheries 21: 20-21.
Milhous, R.T. 1998. Application of the principles of IFIM to the analysis of environmental flow needs for substrate maintenance in the Trinity River, northern California. In: Hydroecological Modelling: Research, practice, legislation, and decision-making. Praha, Czech Republic: T.G. Masaryk Water Research Institute. p. 50-52.
Milhous, R. T., D. L. Wegner, and T. Waddle. 1984. User's guide to the Physical Habitat Simulation System (PHABSIM). Instream Flow Information Paper No. 11, FWS/OBS-81/43 Revised. Prepared by Instream Flow and Aquatic Systems Group, U. S. Fish and Wildlife Service, Fort Collins, Colorado and U. S. Bureau of Reclamation, Upper Colorado Region, Salt Lake City, Utah for U. S. Fish and Wildlife Service, Washington, D. C.
Nakamura, S., and T. J. Waddle. 1999. IFIM Nyuumon (Translation of two documents into Japanese: The Instream Flow Incremental Methodology - A Primer for IFIM and Stream Habitat Analysis Using the Instream Flow Incremental Methodology). Tokyo, Japan: Technology Center for Riverfront Development. 197 p.
Railsback, S. 1999. Reducing uncertainties in instream flow studies. Fisheries 24: 24-26.
Stalnaker, C.B. 1998. The Instream Flow Incremental Methodology. In: Hydroecological Modelling: Research, Practice, Legislation and Decision-Making. Prah, Czech Republic: T. G. Masaryk Water Research Institute. p. 9-11.
Stalnaker, C.B., B.L. Lamb, J. Henriksen, K. Bovee, and J. Bartholow. 1995. The Instream Flow Incremental Methodology: A Primer for IFIM. Washington, DC: U.S. Geological Survey. Biological Report 29. 45 p.
Van Winkle, W., C. C. Coutant, H. I. Jager, J. S. Mattice, D. J. Orth, R. G. Otto, S. F. Railsback, and M. J. Sale. 1997. Uncertainty and instream flow standards: perspectives based on hydropower research and assessment. Fisheries 22: 21-22.
WDFW and WDOE (Washington Department of Fish and Wildlife and Washington Department of Ecology). 2004. Instream flow study guidelines: technical and habitat suitability issues. WDFW and WDOE, Olympia, Washington.
