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<br />D. Flood Estimates From Precipitation <br />Flood discharges estimated from climatic data (rainfall and/or <br />snowmelt) can be a useful adjunct to direct streamflow measurements. <br />Such estimates, however, require at least adequate climatic data and a <br />valid watershed model for converting precipitation to discharge. <br />Unless such models are already calibrated to the watershed, considerable <br />effort may be required to prepare such estimates. <br />Whether or not such studies are made will depend upon the availability <br />of the information, the adequacy of the existing records, and the exceedance <br />probability which is most important. <br /> <br />IV. Data Assumptions <br /> <br />Necessary assumptions for a statistical analysis are that the array <br />of flood information is a reliable and representative time sample of <br />random homogeneous events. Assessment of the adequacy and applicability <br />of flood records is therefore a necessary first step in flood frequency <br />analysis. This section discusses the effect of climatic trends, randomness <br />of events, watershed changes, mixed populations, and reliability of flow <br />estimates on flood frequency analysis. <br /> <br />A. Climatic Trends <br />There is much speculation about climatic changes. Available <br />evidence indicates that major changes occur in time scales involving <br />thousands of years. In hydrologic analysis it is conventional to <br />assume flood flows are not affected by climatic trends or cycles. <br />Climatic time invariance was assumed when developing this guide. <br /> <br />B. Randomness of Events <br />In general, an array of annual maximum peak flow rates may be <br />considered a sample of random and independent events. Even when statis- <br />tical tests of the serial correlation coefficients indicate a significant <br />deviation from this assumption. the annual peak data may define an unbiased <br />estimation of future flood activity if other assumptions are attained. <br />The nonrandomness of the peak series will. however, increase the degree <br /> <br />6 <br />