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<br />The Modified Puis method applied to reservoirs consists of a repetitive <br />solution of the continuity equation. It is assumed that the reservoir water surface <br />remains horizontal, and therefore, outflow is a unique function of reservoir storage. <br />Equation (1) can be manipulated to get both of the unknown variables on the left hand <br />side of the equation as follows: <br /> <br />1.s..2 + O2) = (.s..1 + 0, ) - 0 1 + (L1..:!::.l.2 ) (2) <br />& 2 & 2 2 <br /> <br />Since the inflow, I, is known for all time steps, and 0, and S, are known <br />for the first time step, the right hand side of the equation can be calculated. The left <br />hand side of the equation can then be solved by trial and error. Thisis accomplished <br />by assuming a value for either S 2 or 0 2' obtaining the corresponding value from the <br />storage-outflow relationship, and then iterate until left equation (2) is solved within a <br />specific tolerance. <br /> <br />Rather than resort to an iterative procedure, a value of ~t is selected and <br />points on the storage-outflow curve are replotted as a "Storage-Indication" curve. The <br />Storage-Indication curve, which is a relationship between the left side of equation (2) <br />and outflow, allows for a direct determination of the outflow (0 2) once a value of the <br />Storage-Indication has been computed from the right side of equation (2). <br /> <br />The procedure for applying the Modified Puis method to reservoirs can <br />be summarized as follows: <br /> <br />(1) Determine a composite elevation-discharge rating curve for all of the <br />reservoir outlet structures (i.e. outlet works, spillway, top- <br />of-dam). <br /> <br />(2) Determine the elevation-storage relationship for the reservoir. <br /> <br />(3) Select a time-step and construct a storage-indication vs outflow curve <br />[ (S/~t) + (0/2) ] vs 0 <br /> <br />(4) Determine the initial conditions for reservoir storage and outflow at <br />the start of the routing. <br /> <br />(5) Route the inflow hydrograph through the reservoir using equation (2) <br />and the storage indication curve. <br /> <br />Procedures for developing the elevation storage relationship, the elevation <br />outflow relationship and the initial conditions are discussed in the following <br />paragraphs. <br /> <br />7-71 <br />