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<br />Floods developed from precipitation estimates can be used to adjust <br />frequency curves, including extrapolation beyond experienced values. <br />Because of the many variables, no specific procedure is recommended at <br />this time. Analysts making use of such procedures should first standardize <br />methods for computing the flood to be used and then evaluate its probability <br />of occurrence based upon flood and storm experience in a hydrologically <br />and lreteorologically lx:rrogeneous region. Plotting of the flocxl. at the <br />exceedance probability thus determined provides a guide for adjusting <br />and extrapolating the frequency curve. Any adjustments must recognize <br />the relative accuracy of the flood estimate and the other flood data. <br /> <br />VI. Reliability Application <br /> <br />The preceding sections have presented recommended procedures for <br />determination of the flood frequency curve at a gaged location. When <br />applying these curves to the solution of water resource problems, there <br />are certain additional considerations which must be kept in mind. These <br />are discussed in this section. <br />It is useful to make a distinction in hydrology between the concepts <br />of risk and uncertainty (18). <br />Risk is a permanent population property of any random phenomenon <br />such as floods. If the population distribution were known for floods, <br />then the risk would be exactly known. The risk is stated as the probability <br />that a specified flood magnitude will be exceeQed in a specified period <br />of years. Risk is inherent in the phenomenon itself and cannot be <br />avoided. <br />Because use is made of data which are deficient, or biased, and <br />because population properties must be estimated from these data by some <br />technique, various errors and information losses are introduced into the <br />flood frequency determination. Differences between the population <br />properties and estimates of these properties derived from sample data <br />constitute uncertainties. Risk can be decreased or minimized by various <br />water resources developments and measures, while uncertainties can be <br />decreased only by obtaining more or better data and by using better <br />statistical techniques. <br /> <br />20 <br />