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r';' ~j The second issue is the use ofthe 1,000 foot elevation exemption, There is no discussion in the HMRs that provide justification for use of this 1,000 foot exemption. HMR 51 made no elevation adjustment for the mountainous terrain east ofthe Mississippi River. Discussion is HMR 55A, Section 8.4,2.2, page 132, states that in HMR 51, "Variations in elevations between storm and transposed locations were generally small, less than 1,000 feet.." HMR 55A further states in the same paragraph "In the present study, the same procedure was adopted. . . by making no adjustment for changes in elevation of 1,000 feet or less." A W A considers elevation and barrier adjustments to be calculations that can explicitly be made for elevation and barrier moisture depletion. It appears that the HMRs adopted this 1,000 foot exemption for no explicit reason(s) and have continued to use it because it had been used before. A W A uses explicit elevation and barrier adjustment calculations without the 1,000 foot exemption because this procedure provides adjustments based on vertical changes in atmospheric moisture and should be applied explicitly unless some physically sound basis can be provided to justify the 1,000 foot exemption. 5. Page 50. How was the value of 19-hours determined for the 1921 storm? Note: the narrative indicates the storm started about 6:00 pm and ended at noon the following day. Also, HMR 13 indicates the storm was 12-hours in duration. A W A response to Question 5, It appears that the 19-hour duration is a typo since the quotation is taken directly from HMR 55A as referenced (HMR 55A, Section 2.4.1.2, page 21). The value duration should be 18 hours, The mass rainfall curves attached to HMR 13 show the beginning of the storm period at 3 :OOpm and end of storm period at 3 :OOam but notes that some rainfall occurred previous to 3 :OOpm. The storm D-A-D table provided in HMR 55A, page 232 shows the 10-square mile rainfall values increasing after 12 hours with no additional rainfall after 18 hours. 6. Page 51, What data and methods were used to develop the isohyetal pattern of the 1921 storm? Note: The isohyetal pattern is significantly different from that drawn by the NWS and published in HMR 13. A W A response to Question 6. Point rainfall data for this storm were plotted and analyzed using ArcView GIS with many ofthe rainfall data taken from the HMR 13 isohyetal map, As noted, the isohyetal pattern produced using ArcView GIS shows larger areas of relatively large rainfall amounts than the NWS isohyetal analysis. Since there is some doubt as to the shape of the isohyetal pattern (e.g. how far north and east of Pueblo should the large rainfall values extend), A W A made the decision to use the GIS produced rainfall pattern since it provided the larger rainfall volumes. An alternative decision could have been to digitize the NWS isohyetal map and use it in the GIS analysis.