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<br />mean watershed elevation, the frequency at which rain events become signifi-
<br />cant decreases (becomes rarer) as the drainage basin area increases.
<br />The examples presented herein for regionalized flood flow frequency esti-
<br />mates assume that both rain and snowmelt flood events, coming from two
<br />distinct and independent populations, follow a log-Pearson type III distribu-
<br />tion. The approach would also be applicable for other types of distributions
<br />except that. the statistical parameters may be different.
<br />It is imperative that precautions be taken when applying the resulting
<br />regression equations to ungauged sites where the drainage area and mean
<br />watershed elevation fall outside the data ranges. (40 to 2,000 square miles
<br />and 7,000 to 12,000 feet msl, respectively), Although the approach was for-
<br />mulated for two specific population types, it may apply as well to one or more
<br />different populations insofar as the distinct populations can be identified.
<br />Separation of gauged records into independent events, such as rainfall
<br />and snowmelt, although not unusually difficult, is a tedious and time con-
<br />suming task. On the other hand, the number of stations which have a suitable
<br />period of record are few, and once the record has been separated, it is only
<br />necessary to update the record annually, The time and effort needed to
<br />complete this task must always be weighed against the improved accuracy of the
<br />estimate expected from this approach. Knowledge of the desired frequency,
<br />basin size, and mean watershed elevation may be useful in deciding if the work
<br />is economically justifiable.
<br />
<br />ACKNOWLEDGEMENTS
<br />
<br />The writers would like to thank their colleagues for helpful comments on
<br />the manuscript, The stream flow gauging data used herein were collected
<br />during the course of flood insurance studies for the Federal Insurance
<br />Administration, Department of Housing and Urban Development.
<br />
<br />Appendix I - References
<br />
<br />Camp Dresser & McKee, Inc" "Flood Insurance Study, Town of Pagosa Springs and
<br />Unincorporated Areas, Archuleta County, Colorado," U.S, Deparment of Housing
<br />and Urban Development, Federal Insurance Administration, Denver, Colorado,
<br />August 1977,
<br />
<br />Camp Dresser & McKee Inc., "Flood Insurance Study, Town of Sil verton and
<br />Uni ncorporated Areas, San Juan County, Colorado," U. S, Department of Housi n9
<br />and Urban Development, Federal Insurance Administration, Denver, Colorado,
<br />May 1977.
<br />
<br />Co 1 orado Water Conservati on Board, "Manual for Estimating Flood
<br />Characteristics of Natural-Flow Streams in Colorado", 1976,
<br />
<br />U. S. Department of the Inter; or, Bureau of Recl amati on, "Water P1 an, Phase I,"
<br />appraisal report, February 1974.
<br />
<br />U,S. Water Resources Council, "Guidel ines for Determining Flood Flow
<br />Frequency," Bulletin No, 17 of the Hydrology Committee, June 1977.
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