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<br />n'-" .~I "IJ 5 <br />u J. ,) - <br /> <br />Present Modified Flow and Quality Data <br /> <br />So-called present modified data were used as a basis in making runs <br />with the model. The basic philosophy involves an attempt to modify <br />historically observed hydrologic sequences to reflect the effects <br />of developments that subsequently occurred. Thus, if a known effect <br />occurs at a given point in time, previous data are modified to reflect <br />the development under the assumption that the project is in operation <br />during the entire hydrologic sequence. In essence, the technique <br />implies that should the same hydrologic sequence occur with the pres- <br />ent projects operating, the resultant flow and quality are given by <br />the present modified data. <br /> <br />The present modified concept works nicely where the effect of irri- <br />gation projects, transmountain diversions, and municipal and indus- <br />trial developments are considered. Changes ,in salinity and water <br />quantity are merely routed downstream. However, questions are posed <br />where reservoirs are involved, particularly when a new reservoir is <br />constructed and filled during the historic period. Two contrasting <br />approaches have been adopted to deal with this situation: regulated <br />and unregulated present modified data. <br /> <br />The regulated approach assumes that the historic reservoir releases, <br />modified to reflect basin development exclusive of water storage <br />projects, adequately reflects future operating conditions. The <br />unregulated method modifies historic data to represent conditions <br />of a constant content reservoir (zero fluctuation of level or con- <br />tent) with mean evaporation rates prevailing. Thus, historic flows <br />are routed through the reservoir, being reduced by evaporation losses <br />and modified by upstream developments. The technique is outlined as <br />follows: <br /> <br />(1) Start with the historic data and compute the historic evap- <br />oration using observed evaporation rates and reservoir surface <br />areas. <br /> <br />(2) Add the computed evaporation to the historic reservoir releases. <br /> <br />(3) Compute the change in storage and algebraically add it to the <br />releases of Step 2. <br /> <br />(4) Compute an expected mean evaporation using mean evaporation <br />rateS and the assumed constant reservoir content. <br /> <br />(5) Subtract the expected evaporation from the releases of Step 3 <br />to obtain the unregulated present modified flows. <br /> <br />5 <br />