The purpose of the Physical Habitat Simulation System (PHABSIM) is to simulate a relationship between streamflow and physical habitat for various life stages of a species of fish or a recreational activity. The basic objective of physical habitat simulation is to obtain a representation of the physical stream so that the stream may be linked, through biological considerations, to the social, political, and economic world.
In order to improve the ability to make measurable tradeoffs between the various uses, the PHABSIM models were developed to analyze and display the relationship between streamflow and physical habitat, or between streamflow and recreational river space. This relationship is a function between the physical habitat and the streamflow. It can be used to examine the tradeoff between the value of water used instream with the water used out-of-stream. Therefore, tradeoffs can be made between alternative uses and mutually acceptable management criteria developed. The decision as to the "best" allocation of the available water is a matter of negotiation among various interest groups. The Instream Flow Incremental Methodology (IFIM) provides a framework for applying PHABSIM in a water resource decision setting. Download Stream Habitat Analysis Using the Instream Flow Incremental Methodology from the FORT web site for a comprehensive introduction to IFIM.
The two basic components of PHABSIM are the hydraulic and habitat simulations of a stream reach using defined hydraulic parameters and habitat suitability criteria. Hydraulic simulation is used to describe the area of a stream having various combinations of depth, velocity, and channel index as a function of flow. This information is used to calculate a habitat measure called Weighted Usable Area for the steam segment from suitability information based on field sampling of the various species of interest.
Simulation of physical habitat is accomplished using the physical structure of the stream and streamflow. The modification of physical habitat by temperature and water quality is analyzed separately from the physical habitat simulation contained in PHABSIM. Temperature in a stream varies with the seasons, local meteorological conditions, stream network configuration, and the flow regime; thus, the temperature influences on habitat must be analyzed on a stream system basis. Water quality under natural conditions is strongly influenced by climate and the geologic materials, with the result that there is considerable natural variation in water quality. When we add the activities of man, the possible range of water quality possibilities becomes rather large. Consequently, water quality must also be analyzed on a stream system basis. Such analysis is outside the scope of the PHABSIM manual, which concentrates on simulation of physical habitat based on depth, velocity, and a channel index. Users are referred to the Stream Segment and Stream Network Temperature models on the FORT web site and to generalized water quality models such as EPA's QUAL-2 model for description of other water quality constituents.
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