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<br />. NWS 11 May 2001 <br /> <br />16. NWS HYDRO 45, dated January 1995, states on page 75, Finding 15 that "By logical <br />deduction and extension, the conclusion is that a reasonable and prudent antecedent <br />precipitation associated with a 3 to 5-day PMP event in the region of study would be 10-20% of <br />the PMP within a 31-day period centered on the day of maximum precipitation in the PMP <br />storm for the region of study." Explicitly what caused the difference between this 10-20% and <br />32% provided in the Cherry Creek Antecedent Precipitation Study? (Tomlinson) <br /> <br />USACE will investigate the possibility of revising the Antecedent Precipitation Study and <br />its potential impacts. <br /> <br />Answer 16. <br /> <br />As seen in Table 1 of The Antecedent Study, as central event precipitation depths <br />increase, th" antecedent precipitation ratios (APR) decrease into the same 10 to <br />20% range mentioned in HYDRO 45. However, as seen in The Antecedent <br />Study, Table 5, the APR for the 1965 Plum Creek storm, both at the central <br />location and for the "total" storm area, indicated larger APR values. The stated <br />requirement for The Antecedent Study was to "determine the LARGESTlargest <br />rainfall" (emphasis by NWS) antecedent to the PMP storm, rather than a typical <br />value. In that light, it was believed that an APR larger than 20% was warranted. <br />The selected value of 32% at Cherry Creek represented a reasonable compromise <br />between 20%, and the 48% APR from the "total" Plum Creek storm. <br /> <br />. <br /> <br />Our answer to the previous question recognizes that the smaller values from <br />HYDRO 45 should be used for the Cherry Creek drainage, since the Cherry Creek <br />drainage is entirely east of the 105th meridian. The largest "reasonable and <br />prudent" APR recommended in HYDRO 45 is 20%. <br /> <br />. <br /> <br />14 <br />