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<br />a result, the parameters K and X are expressed as follows: <br /> <br />(34) <br /> <br />(35) <br /> <br />6.2. SOLUTION OF THE EQUATIONS. The method is non-linear in that the flow <br />hydraulics (Q, B, cl. and therefore the routing coefficients (C ,C 2' C 3' and C4) are <br />re-calculated for every 6.X distance step and 6.t time step. An iterative four-point <br />averaging scheme is used to solve for c, Band Q. This process has been described <br />in detail by Ponce. <br /> <br />Values for At and AX are chosen internally by the model for accuracy and <br />stability. First, 6.t is evaluated by looking at the following 3 criteria and selecting the <br />smallest value: <br /> <br />1. The user defined computation interval, NMIN, form the first field of <br />the IT record. <br /> <br />2. The time of rise of the inflow hydrograph divided by 20 ( "/20)' <br /> <br />3. The travel time of the channel reach. <br /> <br />Once At is chosen, AX is defined as follows: <br /> <br />6.X = C 6.t <br /> <br />(36) <br /> <br />but 6.X must also meet the following criteria to preserve consistency in the method: <br /> <br />2 <br /> <br />6. X ,;; ~ (c 6.t + ~) <br />BS oC <br /> <br />(37) <br /> <br />where Q is the reference flow and Q B is the baseflow taken from the inflow <br />o <br />hydrograph as: <br /> <br />Q = Q B + 0.50 (Q peak - Q B) <br />o <br /> <br />(38) <br /> <br />7-53 <br /> <br />__,..J"~__!_ <br />