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<br />The following sections outline procedures to use for (a) computing <br />confidence limits which can be used to evaluate the uncertainties inherent <br />in the frequency determination, (b) calculating risks for specific time <br />periods, and (c) adjusting the frequency curve to obtain the expected <br />probability estimate. The recommendations given are guides as to how <br />the procedures should be applied rather than instruction on when to <br />apply them. Decisions on when to use each of the methods depend on the <br />purpose of the estimate. <br /> <br />A. Confidence Limits <br />The user of frequency curves should be aware that the curve is only <br />an estimate of the population curve; it is not an exact representation. <br />A streamflow record is only a sample. How well this sample will predict <br />the total flood experience (population) depends upon the sample size, <br />its accuracy, and whether or not the underlying distribution is known. <br />Confidence limits provide either a measure of the uncertainty of the <br />estimated exceedance probability of a selected discharge or a measure of <br />the uncertainty of the discharge at a selected exceedance probability. <br />Confidence limits on the discharge can be computed by the procedure <br />described in Appendix 9. <br />Application of confidence limits in reaching water resource planning <br />decisions depends upon the needs of the user. This discussion is presented <br />to emphasize that the frequency curve developed using this guide is only <br />today's best estimate of the flood frequency distribution. As more data <br />become available, the estimate will normally be improved and the confidence <br />limits narrowed. <br /> <br />B. Ri sk <br />As used in this guide, risk is defined as the probability that one <br />or more events will exceed a given flood magnitude within a specified <br />period of years. Accepting the flow frequency curve as accurately <br />representing the flood exceedance probability, an estimate of risk may <br />be computed for any selected time period. For a 1-year period the <br />probability of exceedance, which is the reciprocal of the recurrence <br /> <br />21 <br />