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<br />study area on each map, one can select the 2-year rainfall depth for each of the three <br />durations. Since durations of one hour and less are also needed, one must use the <br />HYDRO-35 maps of 15- and 50-minute rainfall depth and repeat the extraction of the <br />desired 2-year rainfall depths. A value of the 30-minute rainfall depth is also obtained <br />by applying the equation given in HYDRO-35. At this point, all available rainfall-depth <br />information for a 6-hour-duration 15-minute-interval storm has been extracted from the <br />NWS publications. <br /> <br />3.3.2. WESTERN UNITED STATES. Procedures for extracting basic <br />rainfall data from NOAA Atlas 2 for the Western states are significantly different from <br />procedures for the rest of the United States. Maps are available for only the 6- and <br />24-hour-duration for various return periods. To determine a 6-hour duration 15- <br />minute-interval 2-year-return-period storm, one would use the appropriate volume for <br />the state in which the study area lies and extract the 6-hour and 24-hour rainfall depth <br />at the location of the study area from the two maps. Although the total storm <br />duration is only 6 hours, the 24-hour depth is usually needed to solve for the 1-hour- <br />duration depths using the appropriate ratio and equation. Equations for the 2- and 3- <br />hour-duration depths can be solved once the 1- and 6-hour depths are known. <br />Durations of less than one hour are determined by multiplying the one hour-depth by , <br />various ratios given in Atlas 2. For the example, the maps and equations in Atlas 2 <br />would be used to extract the 15- and 30-minute, 1-, 2-, 3- and 6-hour rainfall depths. <br /> <br />3.4. AREAL ADJUSTMENT. Regardless of which publications were <br />used, one now has six rainfall-depth values that must be further modified by one or <br />more adjustment factors. The first adjustment factor is applied to the rainfall data <br />taken from the publications. These depths are "point rainfall depths"; that is, as <br />measured at a rain gage, i.e., a single point. The hypothetical storm will be applied <br />to a specific watershed having a defined drainage area. For example, the amount of <br />rainfall from a particular return-period event over a "say" 50-square-mile area would <br />not be the same as that at a point, but would be less. A storm cannot be as intense <br />when spread over a large area as it can be over a single point. Although the rainfall <br />depth for any finite drainage area will be smaller than the value at a point, the <br />adjustment is often not made unless the study area is more than 10 square miles. The <br />application of the adjustment factor for small areas results in rainfall values that are <br />Iinle different from the point values. When the drainage area is larger than 1 Q.square <br />miles, the adjustment becomes significant, particularly for the 30- and 60-minute <br />durations. It should be noted that the shape of the storm may not necessarily be the <br />same as the shape of the drainage area of the basin. In the absence of site-specific <br />information on the aerial extent of the storm, it is recommended that an elleptial storm <br />panern with a major to minor axis ratio of 2.5 be used. This shape can be centered <br />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 <br />is subject to questions. <br /> <br />7-15 <br />