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3.4. AREAL ADJUSTMENT. Re9ardles:3 of which publications were used one now has six rainfall-depth values thai must Ile fumler modified by one or mom adjustment factors. The first adjustment factor IS applied to the rainfall data taken from the publications. These depths are "point rainfall deptlls"; that is. as rneasured at a rain 9age. i.e.. a single point. The hypothetical storm will be applied to a specific watershed having a defined drainage area. For example. the amount of rainfall from a pan:icular return- period event over a "say" 50-square-rnile area would not be the same as that at a point. but would be less. A storm cannot be as intense when spread over a large area as it Can be over a single pOint. Although tile rainfall depth for any finite drainage area will be smaller than the value at a point. the adjustment is often not made unless tile study area is more than 10 square miles. The application 01 the acljustment factor for small areas results in rainfall values that are littlE' different from the point values. Wilen tile drainage area is larger than 10 square miles. the adjustment becomes significant, particularly for the 30- and 60-minute clurations. It should be noted that the sllape of the storm may not necessarily be the same as the shape of tile drainage area of the basin. In the absence of site-specific information on the a,,,'ial extent of the storm, it is recommended that an elliptical storm pattern with a major to minor axis ratio of 2..5 be used. This shapEl can be centered over the basin and the a.erial coverage determined as the length of the major axis squared and divided by 3.18. Since thE' NWS publications provide no guidance for adjustments for durations of less than 30 minutes. extrapolation to shortel' ,jurations is subject to question>. The adjustment factor found for each duration is used to modify the corresponding rainfall depth for that duration by multiplying these two quantities. Once this step has been completed, all rainfall values have been adjusted forlhe particular drainage area being studied. 3.5. PARTIAL-TO-ANNUAL SERIES AD,JUSTMIENT. The previously described rainfall amounts are for partial-duration series of rainfall values. Conversion 01 rainfall values to an annual series may be needecl. This adjustment is applicable only to return periods more frequent than 10 years. The rainfall depth. frequency curve is converled from one for which one or more events per year were usecl (called a partial-duration series, which is one in whiGh all events abovB some selected base value are used) to one using only a single event each year (callecl an annual sBries, in which the single Ili!lhest event each year is used, even though the seGOnd Ilighest in som'3 years may be wealer than the highest in other years). For economic analysis of agricultural areas, it IS possible that damages are susta ned by flood events thai have a probability of occurrBnce of more than once per year. WhBre several floods per year are causing significant damag9, it would be desirable to use a partial-duration Sel'iBS anc' not make the adjustment. In many economic analyses, especially in urbanizing areaS. tllese multiple iloods (once or twice-per-year events) do not cause significant damage. Therefore. the adjustment faclor would be applied to prevent the use of biased (lligh) rainfall depths to determine the low end of the frequency curve. Tle adjustment from partial-to-annual series is penormed by multiplying the rainfall depths for each duration by tile appropriate conversion factor. The conversion factors are as follows: Colorado Flood Hydrology Manual DRAfT 7.5