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<br />. The interdisciplinary research cited above has provided definitive infonnation that substantial rainfall
<br />flooding has not occurred above an elevation limit that is latitude dependent in the Rocky Mountains, In
<br />Colorado, the maximum elevation limit for substantial rainfall flooding is about 7,500 ft (Jarrett, 1987, 1990a,
<br />1990b, 1993; Jarrett and Costa, 1983, 1988), but varies somewhat because of locaVregional hydroclimatic
<br />variations. The limit is somewhat lower in selected basins east of the Continental Divide (Jarrett, 1990b)
<br />and most of the basins west of the Divide (Jarrett, 1993, in review). Ukewise, Jarrett (1987, 1990b)
<br />indicated that substantial rainfall above 8,000 ft in Colorado is about an order of magnitude less (a
<br />maximum of about 3 in. in 6 hours) than at lower elevations on the eastern plains of Colorado where up to
<br />24 in. of rain was estimated to have occurred in 3,5 hours in May 1935 near Hale, Colorado (Follansbee
<br />and Spiegel, 1937). The 1935 maximum rainfall estimate has been questioned as being substantially
<br />overestimated and may have only been 14 in. or less (Jarrett and others, in review),
<br />
<br />A relation of the largest rainfall versus elevation for Colorado is presented in figure 3a, which was
<br />compiled from documented rainstonn data (gaged and "rainfall-bucket survey data") during about the last
<br />100 years by the National Weather Service, U.S. Anny Corps of Engineers, Bureau or Reclamation, and
<br />others in Colorado (Jarrett, 1987, 1990b) and updated for stonns that have occurred through 1995.
<br />Maximum precipitation for northwestem Colorado is shown in figure 3b. The Opal, Wyoming rainstonn
<br />was estimated to range from about 5.75 to 7 in. in about 2 hr, which occurred on August 16,1990, also is
<br />shown on figure 3b because it is one of the largest candidate stonns in the site-specific PMP study for the
<br />. Elkhead Reservoir study, Opal is located about 175 mi NW of Craig, thus,validity of its transposition to
<br />the Elkhead Creek Basin may be questionable. Several topographic barriers occur between Opal and
<br />Craig, which may preclude such moisture from entering the upper Yampa River Basin including Elkhead
<br />Creek Basin. Because there have been few intense flood-producing rainstonns documented in western
<br />Colorado, particularly northwestem Colorado (probably because they occur so infrequently), maximum 24-
<br />hr precipitation amounts for western Colorado to extend the data base also are shown on figure 3b. The
<br />enveloping curve of maximum rainfall amounts is substantially less in westem Colorado, particularly in
<br />northwestem Colorado, Note that the gage located on Wolf Creek Pass has recorded the maximum 24-hr
<br />precipitation amounts for westem Colorado, which primarily occurred as snowfall during the winter. The
<br />data shown on figures 3a and 3b provide indirect evidence that there is substantially less precipitable
<br />moisture available for generating extreme rainfall amounts at higher elevations and in westem Colorado
<br />than eastem Colorado below 7,500 ft, which can receive abundant amounts of extreme-stonn generating
<br />moisture (Collins and others, 1991).
<br />
<br />Hansen and Schwarz (1981) and Hansen and others (1988), as justification for transposing extreme
<br />rainstonns from lower elevations of Colorado or other parts of the country into the Rocky Mountains, cite
<br />the lack of large rainstonn data at higher elevations in the mountains. They suggest that such stonns were
<br />not recorded due to a sparse precipitation-gage network and a small human population in the mountains
<br />that precludes gaged or bucket surveys. The plains east of the Colorado Rockies is sparsely populated
<br />. and has few precipitation gages. However, this sparsely-populated area has had many extreme
<br />rainstonns documented (usually by rainfall-bucket surveys), most notably the 1921 Penrose stonn, the
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