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<br />the one hour-depth by various ratios given in Atlas 2. For the example, the maps and <br />equations in Atlas 2 would be used to extract the 15- and 30-minute, 1-,2-,3- and 6-hour <br />rainfall depths. <br /> <br />3,4. AREAL ADJUSTMENT. Regardless of which publications were used, one <br />now has six rainfall-depth values that must be further modified by one or more adjustment <br />factors. The first adjustment factor is applied to the rainfall data taken from the <br />publications. These depths are "point rainfall depths"; that is, as measured at a rain gage, <br />i.e., a single point. The hypothetical storm will be applied to a specific watershed having <br />a defined drainage area. For example, the amount of rainfall from a particular return- <br />period event over a "say" So-square-mile area would not be the same as that at a point, <br />but would be less. A storm cannot be as intense when spread over a large area as it can <br />be over a single point. Although the rainfall depth for any finite drainage area will be <br />smaller than the value at a point, the adjustment is often not made unless the study area <br />is more than 10 square miles. The application of the adjustment factor for small areas <br />results in rainfall values that are little different from the point values. When the drainage <br />area is larger than 10 square miles, the adjustment becomes significant, particularly for the <br />30- and 6O-minute durations. It should be noted that the shape of the storm may not <br />necessarily be the same as the shape of the drainage area of the basin. In the absence <br />of site-specific information on the aerial extent of the storm, it is recommended that an <br />elleptial storm pattern with a major to minor axis ratio of 2.5 be used. This shape can be <br />centered over the basin and the aerial coverage determined as the length of the major axis <br />squared and divided by 3.18. Since the NWS publications provide no guidance for <br />adjustments for durations of less than 30 minutes, extrapolation to shorter durations is <br />subject to questions. <br /> <br />The adjustment factor found for each duration is used to modify the corresponding <br />rainfall depth for that duration by multiplying these two quantities. Once this step has been <br />completed. all rainfall ,values have been'acfJustecHor;!he particular drainage area being <br />studied. <br /> <br />3.5. PARTlAL-To-ANNUAL SERIES ADJUSTMENT. The previously described <br />rainfall amounts are for partial-duration series of rainfall values. Conversion of rainfall <br />values to an annual series may be needed. This adjustment is applicable only to return <br />periods more frequent than 10 years. The rainfall depth-frequency curve is converted from <br />one for which one or more events per year were used (called a partial-duration series, <br />which is one in which all events above some selected base value are used) to one using <br />only a single event each year (called an annual series, in which the single highest event <br />each year is used, even though the second highest in some years may be greater than the <br />highest in other years). For economic analysis of agricultural areas, it is possible that <br />damages are sustained by flood events that have a probability of occurrence of more than <br />once per year. Where several floods per year are causing significant damage, it would be <br />desirable to use a partial-duration series and not make the adjustment. In many economic <br />analyses, especially in urbanizing areas, these multiple floods (once. or twice-per-year <br />events) do not cause significant damage. Therefore, the adjustment factor would be <br />applied to prevent the use of biased (high) rainfall depths to determine the low end of the <br /> <br />Cowrado Fwod <br />Hydrowgy Manual <br /> <br />DRAFI' <br /> <br />7.5 <br />