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<br />Comparisons and adjustment of a frequency curve based upon flood <br />experience in nearby hydrologically similar watersheds can improve most <br />flood frequency determinations. Comparisons of statistical parameters <br />of the distribution or of flows with selected exceedance probabilities <br />can be made using prediction equations [e.g., (13), (14), (15), (16)], <br />the index flood method (17), or simple drainage area plots. As these <br />estimates are independent of the station analysis, a weighted average of <br />the two estimates will be more accurate than either alone. The weight <br />given to each estimate should be inversely proportional to its variance <br />as described in Appendix 8. Recommendations of specific procedures for <br />regional comparisons or for appraising the accuracy of such estimates <br />are beyond the scope of this guide. In the absence of an accuracy <br />appraisal, the accuracy of a regional estimate of a flood with 0.01 <br />exceedance probability can be assumed equivalent to that from an analysis <br />of a 10-year station record. <br />3. Flood Estimates From Precipitation--F100ds estimated from <br />observed or estimated precipitation (rainfall and/or snowmelt) can be <br />used in several ways to improve definition of watershed flood potential. <br />Such estimates, however, require a procedure (e.g., calibrated watershed <br />model, unit hydrograph, rain-runoff relationships) for converting <br />precipitation to discharge. Unless such procedures are available, <br />considerable effort may be required to make these flood estimates. <br />Whether or not such effort is warranted depends upon the procedures and <br />data available and on the use to be made of the estimate. <br />Observed watershed precipitation can sometimes be used to estimate <br />a missing maximum event in an incomplete flood record. <br />Observed watershed precipitation or precipitation observed at <br />nearby stations in a meteorologically homogeneous region can be used to <br />generate a synthetic record of floods for as many years as adequate <br />precipitation records are available. Appraisal of the technique is <br />outside the scope of this guide. Consequently, alternative procedures <br />for making such studies, or criteria for deciding when available flood <br />records should be extended by such procedures have not been eva1uted. <br /> <br />19 <br />