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<br />4 <br /> <br />divided by the number of such locations. Calculations of basin <br />average depths employed GRASS software (GRASS Version 4.0, "Users <br />Reference Manual," U.S. Army Corps of Engineers, Construction <br />Engineering Research Laboratory, Champaign, Illinois, 1991). <br />GRASS is a geographical information system. In this mode, the <br />Cherry Creek Drainage was represented by approximately 13,568 <br />grid locations where the difference in latitude between grid <br />points was approximately .19 miles and the longitudinal <br />difference was approximately .15 miles. The area represented by <br />a grid point (.03 mi2) varies insignificantly across the latitude <br />range of the Cherry Creek Drainage. <br /> <br />( <br /> <br />The key assumption made in developing values for within- <br />storm K-factors was that such factors were a function of the <br />return period of the level of within-storm non-orographic <br />precipitation centered at a point. For within-storm <br />non-orographic depths at or above the 100-year return period <br />level, the K-factor should be developed from 100-year frequency <br />analyses, and for lesser levels of non-orographic precipitation, <br />from correspondingly shorter return period frequency analyses. <br />The tendency is for K-factors based on shorter return period <br />frequency analyses to be larger than those based on longer return <br />period frequency analyses. This may happen because atmospheric <br />conditions are better for orographic production of precipitation <br />when they are less unstable than those required to operate at or <br />above the 100-year level. <br /> <br />It was found in developing the point values of within-storm <br />non-orographic PMP (still to be described in this section) that <br />for durations at or greater than 6-hours, the lowest value was <br />three times larqer than the level of 100-year non-orographic <br />precipitation, whereas at the l-hour duration, the lowest level <br />was approximately 95 percent of the 100-year level. It was <br />decided, nonetheless, to use among-storm, 100-year return period <br />based K-factors for all durations from 1 to 72 hours to produce <br />within-storm, basin averaged complete or total PMP estimates. <br /> <br />To develop the average depths in Tables 4 and 5, the <br />available 6- to 24-hour, 1- to 6-hour and 72- to 24-hour ratios <br />in the non-orographic areas east of the Cherry Creek Drainage <br />were employed to prepare among-storm non-orographic PMP index <br />maps at 1, 6, and 72 hours to accompany the 24-hour index map <br />used in the preparation of Plate IIIb of HMR 55A. These four <br />index maps were digitized and "raster fields" consisting of <br />approximately 13,568 values for each duration were then created <br />for the domain of the Cherry Creek Drainage. The values of total <br />PMP as shown on Plates Ib through IVb from HMR 55A were then <br />divided by the corresponding non-orographic values to create <br />isolines of K-factors for each of the four durations. These <br />K-factor maps were also digitized and raster fields of K-factor <br />created. Values at six to eight critically positioned locations <br />in the Cherry Creek Drainage were checked to ensure that the <br />