Phabsim Software Page

This extensive set of programs is designed to predict the micro-habitat (depth, velocities, channel indices) conditions in rivers as a function of streamflow, and the relative suitability of those conditions to aquatic life. A user's manual and a set of tutorial exercises are provided. As of April 2001 the software has been tested with Windows 95, 98 and NT 4.0. Phabsim for Windows supercedes the previous DOS-based version (PHABSIM V 2.0). Version 2.0 is also available on our web site, but is no longer supported.

There are four major components of a stream system that determine the productivity of the fishery. These are:

The complex interaction of these components determines the primary production, secondary production, and fish population of the stream reach. PHABSIM deals with the physical habitat results of various flow regimes. The other components are briefly discussed at the end of this introduction.

In water allocation decisions, it is desirable to know the relationship between benefits and streamflow for the various uses. In general, the benefit of an out-of-stream use can be quantified in terms of some type of production function that relates the quantity of water used to the benefits produced. Unfortunately, little work has been done to relate the quantity of flow instream to the benefits produced by that flow. Prior to about 1973, instream flow assessment typically arrived at a single streamflow value--a "minimum flow" above which all flows were considered available for out-of-stream use. These flow recommendations were determined from analyses of hydrologic records and/or fish population studies. Because of their inherent threshold concepts, these approaches provided only limited opportunity for negotiation and compromise.

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.

PHABSIM is exercised in four major steps. The first is to simulate water surface elevations, the second is to simulate velocities, the third is to simulate the physical habitat versus streamflow relationship, and the fourth is to simulate the physical habitat when combinations of flows are involved.

The results from PHABSIM can be used alone; however, for a complete application of IFIM generation of habitat time series to produce monthly, weekly or daily habitat results from proposed alternative flow regimes is required. Habitat time series are created by obtaining a habitat value from the Weighted Usable Area versus streamflow table resulting from the habitat simulation programs for each discharge in a streamflow time series. Monthly and daily streamflow time series may be obtained from USGS gages near the study site or as the output of river system management models. Weekly flow time series can easily be aggregated from daily values using spreadsheets or other tools.

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.

Physical Habitat Simulation (PHABSIM) Software

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