River Ecology and Restoration - River Dynamics and Integrated River Management - page 684

10.3.4.2 Habitat Evaluation Procedure
The Habitat Evaluation Procedures (HEP) can be used for several different types of habitat studies,
including impact assessment, mitigation, and habitat management. The HEP provides information for
two general types of habitat comparisons—the relative value of different areas at the same point in time
and the relative value of the same area at different points in time.
The HEP is based on two fundamental ecological principles—habitat has a definable carrying capacity
to support wildlife populations, and the suitability of habitat for a given wildlife species can be estimated
using measurements of vegetative, physical, and chemical characteristics of the habitat. The suitability of
a habitat for a given species is described by a habitat suitability index (HSI) constrained between 0
(unsuitable habitat) and 1 (optimum habitat). HSI models have been developed and published (Schamberger
et al., 1982), the U.S. Fish and Wildlife Service (USFWS, 1981) also provides guidelines for use in
developing HSI models for specific projects. HSI models can be developed for many of the previously
described metrics, including species, guilds, and communities (Schroeder and Haire, 1993).
The fundamental unit of measure in The HEP is the Habitat Unit, computed as follows:
HU AREA u HIS (10.13)
where HU is the number of habitat units (units of area), AREA is the areal extent of the habitat being
described (in km 2 ), and HSI is the index of suitability of the habitat (dimensionless). Conceptually, an
HU integrates the quantity and quality of habitat into a single measure, and one HU is equivalent to one
unit of optimal habitat. The HEP provides an assessment of the net change in the number of HUs
attributable to a proposed future action, such as a stream restoration initiative. A HEP application is
essentially a two-step process—calculating future HUs for a particular project alternative and calculating
the net change as compared to a base condition.
10.3.4.3 Habitat Modeling
Many habitat evaluation models have been developed. The Physical Habitat Simulation Model was
designed by the U.S. Fish and Wildlife Service primarily for instream flow analysis (Bovee, 1982). The
model allows evaluation of available habitat within a study reach for various life stages of different fish
species. The first component of the model is hydraulic simulation for predicting water surface elevations
and velocities at unmeasured discharges (e.g., stage vs. discharge relations, Manning's equation, step-
backwater computations). The second component of the model, habitat simulation, integrates species and
lifestage—specific habitat suitability curves for water depth, velocity, and substrate with the hydraulic
data. Output is a plot of weighted usable area against discharge for the species and life stages of interest.
Riverine Community Habitat Assessment and Restoration Concept Model is based on the assumption
that aquatic habitat in a restored stream reach will best mimic natural conditions if the frequency
distribution of depth and velocity in the subject channel is similar to a reference reach with good aquatic
habitat. Study site and reference site data can be measured or calculated using a computer model. The
similarity of the proposed design and reference reach is expressed with three-dimensional graphs and
statistics (Nestler et al., 1993; Abt, 1995). The model has been used as the primary tool for environmental
analysis on studies of flow management for the Missouri River and the Alabama Basin.
SALMOD (Salmonid Population Model) is a conceptual and mathematical model for the salmonid
population for Chinook salmon in concert with a 12-year flow evaluation study in the Trinity River of
California using experts on the local river system and fish species in workshop settings (Williamson et al.,
1993; Bartholow et al., 1993). The structure of the model is a middle ground between a highly aggregated
classical population model that tracks cohorts/size groups for a generally large area without spatial
resolution, and an individual-based model that tracks individuals at a great level of detail for a generally
small area. The conceptual model states that fish growth, movement, and mortality are directly related to