1. PHABSIM: A TOOL FOR EVALUATING FLOW REQUIREMENTS IN FLORIDA STREAMS?
Florida Fish and Wildlife Conservation Commission
Eric Nagid and Gary Warren
PHABSIM:
A TOOL FOR EVALUATING
FLOW REQUIREMENTS
IN FLORIDA STREAMS?

2. Physical HABitat SIMulation System
Combines hydrological and biological models
to predict changes in habitat area over a range
of discharges.
Is a component of the Instream Flow
Incremental Methodology (IFIM).
Is the most widely used model for deductive
estimation of biological effects of stream flow
alteration.
Amount of habitat is the decision variable, not abundance, biomass, production, or predation.
PHABSIM is the scientific modelling component of the IFIM, developed by USGS fisheries biologists in the late 70s – early 80s as a system to equitably manage streamflows for the good of all involved parties. IFIM combines the modeling component with social science components, such as stakeholder inclusion and negotiation.
Has been successfully defended in court.

3. Primary Components of PHABSIM
Hydrological
Field measurements of physical habitat conditions
along transects.
velocity
depth
substrate / cover
2. Hydrological modeling of flow / stage relationships.
Biological
Habitat Preference Criteria – for each life stage.
velocity
depth
substrate / cover

4. PHABSIM Flowchart

5. PHABSIM Output Weighted Usable Area
The usable area of habitat available, in a
stream reach, to a given species at
a given discharge.
The product of velocity, depth, and
substrate/cover preferences multiplied
by cell size.
Output of the PHABSIM model is the weighted usable area of habitat for a selected species.
The model expressed as a simple equation looks more complicated than it really is.
Weighted usable area is simply the sum of the surface areas of the identified cells at a given flow rate (which is the hydrological portion of the model) times the composite suitability of the identified cells at the same given flow (which is the biological component of the model).
WUAs are summed over an entire reach
and used as an index of habitat quality
and availability.

6. Biological Suspects for the Suwannee River Basin
Suwannee bass
(Micropterus notius)
Net-spinning caddisflies
(Hydropsychidae),
Mayflies (Baetidae)
Limited range – SSC
Recreationally important
Easily sampled
Flow sensitive
Sessile
Easily sampled

7. Criticisms of PHABSIM respond to changes in weighted usable area.
General:
Little evidence that fish populations actually
respond to changes in weighted usable area.
Does not perform well in soft-bottomed,
shifting sediment streams.
PHABSIM is unidimensional.
Locally:
Fish in peninsular Florida, particularly
Centrarchids, are ubiquitous. Therefore, HSC
may be vague and uninformative.

8. Evaluation of Stream Flow Models
Selecting and Modifying the Most Appropriate Model for Florida Streams
PHABSIM / IFIM
River 2D
Channel Units / Channel Types (CU / CT)
Indicators of Hydrologic Alteration (IHA)
A model that we are strongly considering is PHABSIM, which is a component of a decision support system called the instream flow incremental methodology, developed by the USGS at their Ft. Collins Science Center.
River 2D newest of models; incorporates a second, lateral, flow dimension.
CU/CT an approach under development by USGS Co-op unit for GA
IHA developed by Nature Conservancy; focuses on impact of high water events.
SIAM integrates water quantity, water quality, and biological models.
System Impact Assessment Model (SIAM)