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<br />.~ <br /> <br />;e <br /> <br />,e <br /> <br />Ie <br /> <br />The precipitation-frequency maps given in the NWS and NOAA publications <br />show point.precipitation values that can be assumed to apply to areas up tD <br />10 sq mi. For larger areas, the average precipitation over the area is less <br />than the maximum value at a point, and thus adjustments are required. In <br />general, Qepth-area-duration relations vary with storm type and intensity and <br />can also vary with the region, <br /> <br />A hypothetical storm developed from l'fflS data is sometimes called a <br />"balanced" storm, because a consistent depth-frequency relation is used for <br />each duration interval of the total storm. For exarlllle, for the lOG-year <br />return perioo, 48-hour duration storm, rainfall depths for a given period <br />(say the 3D-minute, I-hour, 6-hour, or 24-hour periods) would each be equal <br />to the lOO-year rainfall depth for that duration interval. This consistent <br />frequency-depth-duration relationship throughout a storm does not occur in <br />nature, of course, because of the random nature of rainfall events. Use of a <br />balanced storm, however, permits the construction and arrangement of a storm <br />event such that an average precipitation intensity of a specified frequency <br />is provided for all durations -- including one that matches the time-response <br />characteristics of the particular watershed being analyzed. <br /> <br />4,2 Types of Hypothetical Storms <br /> <br />The storms determined from National Weather Service criteria as <br />described above give hypothetical storm data for specific frequencies. For <br />example, a storm with an X-year return period has a 100lX percent probability <br />of being equaled or exceeded in any given year. Thus, a 50-year storm has a <br />2 percent chance of being equaled or exceeded in any given year and would be <br />equaled or exceeded, on the average, once every 50 years, <br /> <br />The other types of storms that are frequently used are the Standard <br />Project Storm (SPS) and the Probable Maxinum StDrm (PMS). The 5PS represents <br />the most severe meteorologic conditions considered reasonably characteristic <br />of the geographic reQion involved, eXCluding extremely rare events, The <br />flood resulting from the SPS gives a "standard" against which the performance <br />of a system can be compared with similar systems in other locations. <br /> <br />19 <br />