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30 BIOLOGICAL REPORT 29 -----""1 Calibration I parameter : adjustment : z error work and reduce the imprecision of small samples that are used to represent a dynamic stream sys- tem. Site-specific data also foster communication among the diverse disciplines of engineering, law, ecology, and economics. Just as grab sample meas- urements of temperature, water quality, depths, and velocities are routinely used to calibrate physi- cal and chemical models, samples of the aquatic organisms and their habitat use must be used to 'calibrate' the habitat simulations used in IFIM alternatives analyses. Properly completed, phase three results in reli- able estimates of the relation between flow and total habitat, as well as good measures of the amount of habitat available under the chosen base- line conditions and the various with-project alter- natives. This habitat quantification leads naturally Macrohabitat Microhabitat Discharge Temperat~~~..J \ Water Channel quality structure ~DePth Velocity Substrate Cover Miles of usable stream x Habitat area per mile of stream = Total Habitat Fig. 5.2. Total habitat combines elements of macro habitat and microhabitat. Fig. 5.1. Calibration process. into the next phase, which will compare and evalu- ate the alternatives. Before discussing the next phase, however, it would be best to make specific mentionofPHABSIM. PHABSIM Many people confuse IFIM with the Physical HABitat SIMulation System (PHABSIM). Whereas IFIM is a general problem-solving ap- proach employing systems analysis techniques, PHABSIM is a specific model designed to calculate an index to the amount of microhabitat available for different life stages at different flow levels. PHABSIM has two major analytical components: stream hydraulics and life stage-specific habitat requirements (Figs. 5.3a and 5.3b). The stream hydraulic component predicts depths and water velocities at specific locations on a cross section of a stream. Field measurements of depth, velocity, substrate material, and cover at specific sampling points on a cross section are taken at different flows. The sampling points are called verticals and describe conditions for some distance around them (cells) judged to be relatively homogeneous. Hydraulic measurements, such as water surface elevations, are also collected during the field inventory. These data are used to cali- brate the hydraulic models. The models are then used to predict depths and velocities at flows dif- ferent from those measured. It is usually assumed that the substrate material and cover do not change at different flow levels, but this assumption is not required.