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38 <br />predict conditions at flows other than those observed. The habitat <br />quality index used with this method is referred to as Net Preferred ti <br />Habitat (NPH) and is calculated as: <br />4 <br />n <br />NPH = I vi - di • Aai, (2-6) <br />i=1 <br />where vi = velocity weighting factor between 0 and 1, <br />di = depth weighting factor between 0 and 1, <br />Aai = bottom surface area of ith element of stream reach, and <br />n = number of elements in stream reach. <br />2.8 IFG Incremental Methodology <br />The Cooperative Instream Flow Service Group (IFG) of the U.S. <br />Fish and Wildlife Service developed a set of habitat evaluation <br />procedures known as the Incremental Methodology (Stalnaker 1978, <br />1979b; Trihey 1979). A package of computer programs, collectively <br />called PHABSIM (Physical HABitat SIMulation system), is used to <br />implement this analysis of instream flow needs (Fig. 2-5). The <br />overall approach combines (1) multiple-transect field data from a <br />representative and/or critical river reach, (2) hydraulic simulation <br />models to predict physical habitat parameters such as mean velocity <br />(v), depth (d), and substrate (s), and (3) species-specific <br />suitability functions (Sv, Sd, Ss) similar to those first described by <br />Waters (1976). Suitability functions are used to calculate weighting <br />coefficients representing the habitat preferences of various life <br />stages of target fish species. Finally, measures of habitat <br />suitability and availability (as wetted surface area, a i ) are used in <br />the computation of Weighted Usable Area (WUA), an index of habitat <br />condition. This index is computed for each life stage [e.g., spawning <br />(S), fry (F), juvenile (J), and adult (A)] and can be plotted against <br />discharge (Fig. 2-5). The intent in developing the Incremental