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<br />GAUGED SITES <br /> <br />The basic approach for analysis at a gauged site is briefly outlined <br />below. The period of record should be at least ten years and include only <br />unregulated peak discharge. The analysis should follow the guidelines <br />published by the U.S, Water Resources Council. <br /> <br />- Analyze original dail~ record and separate annual rainfall and <br />snowmelt peak discharges. _ <br /> <br />Compile an annual series for each type event. <br /> <br />- Adjust historical data and outliers. <br /> <br />- Fit each series to a log-Pearson type III distribution; that is, com- <br />pute separate station statistics for snowmelt and rainfall records. <br /> <br />- Adjust short period statistics by considering regional snowmelt and <br />rainfall mean, standard deviation, and skew, if available. A gen- <br />eralized skew based on mixed population is not applicable. <br /> <br />- Statistically combine the two frequencies. <br /> <br />Separation of the gauged data must be based on some accepted hydrologic <br />criteria. Separation on the basis of calendar periods is not necessarily <br />valid hydrologically, unless the events in each series are seasonally related. <br />The shape of the stream flow record is the primary basis in deciding if an <br />event was a result of snowmelt or. rainfall. Weather data collected at nearby <br />stations should also be consulted for regional temperatures and precipitation <br />in the basin. A comparative example of a typical recorded hydrograph for a <br />snowmelt and a rainfall flood event is shown in Figure 3. <br />The two frequency curves are statistically combined utilizing set theory. <br />The composite probability is given by the equation: <br /> <br />P(RUS) = peR) + pes) - P(RnS) <br /> <br />(2) <br /> <br />where, <br /> <br />P(RUS) = the combined exceedance probability of a selected peak discharge, <br /> <br />peR) = the exceedance probability of a rain event at the selected peak <br />discharge, <br /> <br />peS) = the exceedance probability of a snowmelt event at the selected peak <br />discharge, <br /> <br />P(RnS) = the exceedance probability of the interception of rain and snowmelt <br />events at the selected peak discharge, <br /> <br />If rain and snowmelt events are statistically independent, Equation 2 becomes: <br /> <br />P(RUS) = peR) + pes) - peR) pes) <br /> <br />(3) <br /> <br />An example showing the separate frequency-discharge curves for rain and <br />snowmelt events, compiled from the gauge record shown in Table 2 from a site <br /> <br />7 <br />