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A ratio outside of this will result in WSE predictions that are either too <br />high or low. Using the WSEs and discharge values from the WSEI4S program <br />the IFG4 program can predict velocities. The accuracy of the velocity <br />predictions is then evaluated by the ratio between the measured and the <br />predicted discharge. <br />The IFG4 program is typically used to predict stream velocities at <br />0.6 depth, the mean column velocity. It can also be used to simulate <br />velocities at other depths, such as the nose velocity of the fish <br />(Milhous et.al 1984}. In this analysis, it is assumed that Colorado <br />squawfish and razorback suckers use depths closer to the streambed than <br />0.6 depth and that a simulation of velocities at 0.8 depth 10.2 depth <br />above the streambed) might give a more accurate indication of optimum <br />flows. It must be noted, however, that the SI curves were developed from <br />velocity measurements at 0.6 depth and may not be adequate for predicting <br />habitat use at 0.8 depth. <br />Both models are based on the assumption that steady flow conditions <br />exist within a rigid stream channel. A rigid stream channel does not <br />change shape during the period of time over which the calibration data are <br />collected. However, the stream may change course seasonally or as a <br />result of conveying peak flows (Trihey 1980). <br />6. Calculation of Weighted-Useable-Area (WUA) <br />The ultimate goal of the PHABSIM analysis is to determine the flow <br />which provides an optimum amount of habitat, as defined by depth, velocity <br />and substrate, for particular fish species and life stages. In the <br />previous section, measurements of physical microhabitat, such as depth, <br />18 <br />