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<br />June 3rd in the Gunnison to Montrose region. He indicated that meteorologic conditions were present to
<br />potentially produce 2-hour duration, convective rainfall in westem Colorado during the aftemoon of June 3,
<br />1952. With the available meteorological data, Henz assumed the maximum surtace temperature in
<br />Cimarron reached about 60 degrees Fahrenheit with a local valley dewpoint of 52 Fahrenheit to support a
<br />9O-minute duration stonn of about 5 inches. Henz also used his model to retrodict that the local
<br />temperature and dew point had reached only 53 Fahrenheit and 49 Fahrenheit, respectively. This
<br />analysis indicated that rainfall amounts would have been onlr. about 0.55 inches in 30 minutes. These
<br />temperature and dew-point temperature differences are smal , thus, it is not unrealistic to believe that
<br />closer to 0.5 inches of rainfall occurred. Henz detennined that the stonn-genesis areas could have been in
<br />one of two areas located south and southwest of Cimarron depending on the time of day (figure 1). The
<br />stonn-genesis areas probably could not have occurred further upstream in the basin because the air at
<br />that elevation would have been too dry for substantial convective rainfall amounts. The stonn tracks,
<br />which are primarily based on prevailing upper-level winds that shifted from the southwest to south, also
<br />are shown on figure 1. Both tracks, which had small rainfall-areal extents (figure 1), would have resulted in
<br />rainfall on the Cimarron 3SE gage. However, because the raingage observer reported the stonn occurred
<br />after 3 pm (MDT), it is most likely the stonn-genesis area was the one located to the south of Cimarron.
<br />Henz (Henz Meteorological Services, Denver, Colorado, personal commun., 1996) indicated that if there is
<br />no paleoflood or streamflow evidence either in the Cimarron or Gunnison River streamflow gages under
<br />the stonn tracks, then the June 3rd Cimarron rainstonn probably did not occur with the intensity reported.
<br />
<br />A RAINFALL-RUNOFF RELATION OF FLOODING IN COLORADO
<br />
<br />Developing a relation between rainfall and rainfall runoff can provide another check of the reliability of
<br />the June 3, 1952 rainstonn at Cimarron. A compilation of readily available rainfall and rainfall-runoff data for
<br />convective rainstonns that occurred on mountainous basins less than about 10 square miles in Colorado
<br />was done (table 1). A simple relation between rainfall and net peak rainfall runoff was developed (figure
<br />10). Certainly, this relation does not take into account the many hydroclimatologic and hydraulic factors
<br />that affect rainfall runoff. However, in mountainous basins subject to extreme rainfall, infiltration and other
<br />factors have substantially less effect on runoff than rainfall amount and intensity (Costa, 1987b; Jarrett
<br />and Waythomas, in press). Thus, it is not surprising that the rainfall-runoff relation (figure 10) is fairly well
<br />defined. Superimposed on this relation are the 1952 rainfall-runoff event data for the Cimarron River
<br />below Squaw Creek at Cimarron (09127000) denoted by solid squares on figure 10. Although this is a
<br />simple rainfall-runoff relation, clearly, the Cimarron rainfall data have substantially less peak-rainfall runoff
<br />than other small basins in Colorado. For example, if the Cimarron River were to respond to rainfall as the
<br />other small basins in Colorado, a 5.25-inch convective rainstonn, would be expected to result in a peak
<br />discharge of about 7,000 cubic feet per second. Using the envelope curves (figure 10) of other flood data
<br />for Colorado, the peak discharge would range from about 5,000 to 9,000 cubic feet per second. That only
<br />70 cubic feet per second of net peak rainfall runoff at the Cimarron gage occurred on June 3rd indicates a
<br />potential problem in either the recorded 5.25-inch rainfall or the resulting streamflow hydrograph. However,
<br />the previous discussion indicates the streamflow records are correct and that there is no paleoflood
<br />evidence in the Cimarron River basin. Therefore, we conclude that rainfall-data measurements or
<br />recordings were in error.
<br />
<br />DISCUSSION
<br />
<br />Because of the spatial variability of convective rainfall, it is difficult to definitively say that 5.25 inches
<br />of rain could not have occurred very locally. Although the onsite paleoflood investigations and analyses
<br />of hydrometeorologic data cannot conclusively rule out that the 5.25-inch rainfall of June 3rd, 1952
<br />occurred, it suggests that the heavy precipitation would have been extremely localized (a few acres or
<br />less) because it did not leave identifiable paleoflood evidence. Could a 5.25-inch very localized rainstonn
<br />have occurred? Over the years, the senior investigator has had numerous discussions about extreme
<br />rainfall, and specifically for the June 1952 Cimarron rainstonn, with meteorologists, climatologists, and
<br />hydrologists. One issue concems the possibility of a localized rainstonn greater than about 1.5 to 2
<br />inches occurring in 1 to 2 hours in mountainous basins of Colorado, yet having little rainfall runoff.
<br />Although steep-rainfall gradients (spatial variability) of significant convective rainstonns have been
<br />observed in Colorado, these storms have more substantial areal coverage of heavy rainfall, producing
<br />associated flood evidence (table 1 and figure 10). Exceptions where little flood runoff occurs during heavy
<br />rains include areas having rapidly drained and extensive sand soils such as the sandhill areas in parts of
<br />eastern Colorado and at Great Sand Dunes National Monument (Uvingston and Minges, 1987) and
<br />densely-vegetated basins (RD. Jarrett, unpublished data for Virginia Canyon, 1991).
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