Flows and Transport in Rivers - Hydro-Environmental Analysis: Freshwater Environments - page 118

3. “Models of the river are constructed and calibrated. IFIM distinguishes between micro-
habitat, commonly modeled using an approach such as PHABSIM, and macro-habitat,
which includes water chemistry/quality and physio-chemical elements such as water tem-
perature. There is a structure for specifying channel and loodplain maintenance lows but
there is little guidance on speciic methods. Hydrological models of alternative scenarios,
including a baseline of either naturalized or historical conditions drive the habitat models.
The models are integrated, using habitat as a common currency.”

4. “Alternative scenarios of dam releases or abstraction restrictions are formulated and tested
using the models to determine the impact of different levels of low alteration on individual
species, communities or whole ecosystems.”

5. “The technical outputs are used in negotiations between different parties to resolve the
issues set out in step one.”

The strength of the IFIM is in its prediction of environmental impacts and its assessment of
tradeoffs. For example, the IFIM might result in monthly or weekly low envelopes within which
the low might vary depending on other uses (Schroeter et al. 2005).
The IFIM incorporates the PHABSIM as a problem-solving outline for decision making. The
PHABSIM was originally developed in the late 1970s by the U.S. Fish and Wildlife Service (FWS),
through the U.S. EPA-funded Cooperative Instream Flow Service Group, organized to develop
methods for quantifying the impacts of altered streamlows. The Instream Flow group developed
the IFIM (Stalnaker et al. 1995; Bovee et al. 1998), of which the PHABSIM was a major component
( Wadd le 20 01).
The PHABSIM is based on the assumption that aquatic organisms respond to changes in the
hydraulic environment. The methodology irst requires some relationship between the hydraulic
conditions and habitat suitability. The relationship is usually expressed as univariate suitability
curves for particular species and/or life stages of interest, with the suitability ranging from 0 to
unity (Figure 4.30). Models are then used to determine the hydraulic (e.g., depth, velocity, and cover
as a function of depth) and water quality characteristics (e.g., temperature) as a function of low
and other environmental conditions. The product of the surface area for a section of stream and the
univariate suitability curve values result in a habitat index called the weighted usable area (WUA).
The PHABSIM method does not explicitly include the speciication of some low characteristic,
such as magnitude, frequency, duration, timing, or rate of change. Instead, it allows for an assess-
ment of the impact of those low characteristics on the quality of the available habitat. For exam-
ple, the WUA under one low magnitude could be compared to another (such as perhaps due to a

B

FIGURE 4.30 An example of wetted perimeter computation for a rectangular channel (P w = B + 2Y).