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<br />J2.14 PAlEOHYDROLOGIC ESTIMATES OF CONVECTIVE RAINFAll IN THE ROCKY MOUNTAINS
<br />
<br />Robert O. Jarrett .
<br />U.S. Geological Survey, Lakewood. Colorado
<br />
<br />1. INTRODUCTION
<br />
<br />Increased emphasis on extreme rainstorm and flood
<br />documentation will maximize benefits from emerging
<br />technologies, will help improve hydrologic modeling, and
<br />Improve flash-flood lorecasting, Convective rainlall is
<br />characteristically localized and can have large
<br />gradients in both rain rates and rainfall amounts over
<br />very short distances, often a lew kilometers or less. In
<br />remote areas, systematic precipitation networks may be
<br />sparse or nonexistent. Umlled resources oIlen
<br />preclude extensive, instrumented data collection
<br />efforts, and there also is a likelihood rainstorms and
<br />floods will not occur in instrumented basins,
<br />Two paleohydrologic (geomorphic and hydrologic)
<br />techniques to estimate rainfall amounts 01 convective
<br />rainstorms are being developed and evaluated lor
<br />mountainous areas. Paleohydrology Includes the study
<br />olllood-transported sediments and botanic Inlormation
<br />from past floods preserved in river basins (Jarrett,
<br />1991; Jarrett and Tomlinson, 2000), Paleohydrology,
<br />whiCh can be viewed as lorensic hydrology, uses this
<br />physical evidence to make inlerences 01
<br />hydrometeorologic Information, Elements of this
<br />presentation Include dl"",,,ions of the: (1) approach,
<br />results, and benefits lor recent, historic, and prehistoric
<br />rainstorm reconstructions in the Rocky Mountain region;
<br />(2) IImilallons and uncertainties of rainfall estimates.
<br />and; (3) transfer to other hydroclimatlc regions.
<br />In this paper, the approach is discussed lor the
<br />community of Bullalo Creek, located about 50 kin
<br />southwest of Denver, Colorado (fig. 1a). On May 18,
<br />1996, an intense wildfire (Bullalo Creek fire) bumed
<br />about 50 km2 of forest, produced hydrophobic (waler-
<br />repellent) soils, and making the are more susceptible to
<br />flooding, Subsequently, two people were killed and
<br />several miUIon dollars in public and private property
<br />damage were caused by a flood on the evening of July
<br />12,1996 (Colorado WaterConserv8lion Board, 1997),
<br />Maximum water depths as much as 4 m occurred within
<br />about 30 to 4S minutes of the storm's onset In Buffalo
<br />Creek, Spring Creek, and the North Fork South Platte
<br />and South Platte RIvers. The paleohydrologic rainfall
<br />estimate for the July 12th storm was alleast 110 mm In
<br />about an about an hour, and the storm lootprint (within
<br />the 25 nvn isohyet) was about 120 km2.
<br />For comparison, Henz (1998) estimated a maximum
<br />raInfaI1 of about 130 mm, and FuIlon (1999) estimated a
<br />
<br />.Corresponding author address: Robert 0, Jarrett,
<br />U.S. Geological Survey, P.O. Box 25046, MS 412,
<br />Lakewood, CO 80225; e-mail: rjarrett@usgs.gov.
<br />
<br />J40
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<br />AMERICAN MmOROlOGICAl SOCIID
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<br />maximum of 72 mm; however, their storm footprints were
<br />located slightly different and were much larger, The
<br />paleohydrologic results were obtsined by July 16. 1996
<br />(two days of field and office work) and subsequently
<br />were used for emergency flood response, The
<br />paleohydrologic methodology Is a llexible, 'storm-
<br />chasing" approach that provides independent, cosI-
<br />effective rainfall estimates, and can be used to
<br />complement conventional Instrumented monRoring.
<br />
<br />2. STUDY AREA
<br />
<br />The community of Buffalo Creek Is located In the
<br />foothills 01 the Colorado Rocky Mountains (fig, 1 a). Ths
<br />community, at an elevation of about 2,012 m, consists
<br />of several hundrad homes within a montane forest
<br />(predominantly ponderosa pine, Iodgapole pine, 00ugIas
<br />fir, and aspen). Accumulation 01 organic litter (duff) In
<br />forested areas prim8ly pine needles has an average
<br />depth 01 about 75 mm, Topography is rugged (slopes
<br />range from 5to 60 petaH1t) and soiis are shallow (- 1 m
<br />to bedrock wRh numerous outcrops), moderately well
<br />drained, and composed of coarse sandy gravel (Sphinx.
<br />Legault-Rock granlle complex). The climate Is semiaJid
<br />and mean annual precipitation is aboul400 mm, The
<br />1OQ-yr, 1-hr rainfall is about 56 mm for the Buffalo Crsek
<br />area (MiUer et aI., 1973). Most streams In the s\1Jdyarsa
<br />are ephemeral, These streams I10w into Buffalo Creek
<br />and the North FDfk South Platte and South Platte
<br />Rivers, which primanly are fed by melting snowpsck and
<br />trans-basin flow olVersions, SIteam graclenls typlcsl
<br />range from about 0.005 to 0.06 mlm, F100d lIows in Il1t
<br />Colorado foothiUs can result from generalized
<br />rainstorms, spring snowmeR, but primarily resuR from
<br />Intense, localized thunderstorms (Jarrett, 1990).
<br />
<br />3. METHODS
<br />
<br />In the first or geomorphic method, ralnlaU amounts
<br />can be inferred from the amount of hlllslope erosion.
<br />maximum size of sediments transported, and deposition
<br />Characteristics, preferably on sparsely vegetated
<br />hillslopes, The hUisIopes used should have as similar
<br />characterisltcs as possible. The dimensions of fresh
<br />rills, gullies, and headcuts as well as maximum size of
<br />sediments transported and their deposition
<br />characteristics are obtalnsd and locsted on topograpl1iC
<br />maps. Looal residents can often provide valuable
<br />information about the rainstorm including ralniaJI 'bucket
<br />data," storm duration, and hail (which also can be
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