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<br />_' ,- , ,>'" ,,-'-'^'P,'""-' ,,_, ',,,' :,,'," '-'''-''''-.''l?>~
<br />.. - ---. - - ~_U~'._~.'_~'_:;Ii~~:t"'~.it-:-_-:-:-:-:...~~~ ~~-
<br />~,/ oOllS1'6" ';"f"',-_"':,;", ,','/- ~<"'\:~:"'" .' ~'C
<br />
<br />
<br />/evalualed maximum annu~ pr~~:Pilalion--~v:nl~ ~or d~~-' F~~~';~:'s;io~~;:~:'~~H~~-~:~ the Great Basin section': ~~2i~,~'::-~::~~if',
<br />
<br />lions of I, 2, 3. 6, 12, and 24 hours. Figure 2 illustrates ploued were chosen to represent the climatic events that
<br />the study area, boundaries created, and number of station may occur when severe thunderstorm activity is prevalent
<br />years of data per region evaluated. Their analysi. indicaled in the area of analysis. It should be noted that the Hme of
<br />that only in the Pacific Northwest (West) do 70 percent or year and climatic conditions contribute to the way precipi-
<br />more of the climatic events result in measurable precipita- tat ion is distributed with respect to time.
<br />lion in all of the 24 hours of a 24-hour evenL When the
<br />time duration is reduced to 6 hours, then all regions had
<br />storms with measurable precipitation in each hour at least
<br />40 pcrcent of the lime, Frederick also indicated that for all
<br />sections except the Pacific Northwest (West) the beginning
<br />hour of shorter duration storms or rainfall within a longer
<br />duration of rainfall or storm usually commenced within the
<br />first hour.
<br />
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<br />SOl I
<br />~ _ l
<br />~J
<br />'b
<br />
<br />I
<br />YPE I
<br />
<br />/'
<br />
<br />10. I~ 14 \l II iC i~ ~4
<br />TIME 1:>. HOlJRS
<br />
<br />Figure I. SCS Precipitation Distribution
<br />for the 24-Hour Synthetic Storm.
<br />
<br />To examine the differences between large and sl11all
<br />storms, Frederick ranked data according to magnitude and
<br />separated the data sets into quartiles where Quartile I re-
<br />presented the top 25 percent of lhe storms analyzed with
<br />regard 10 magnitude and Quartile IV representing the lower
<br />"5 percent of the storms. A technique was developed by
<br />Frederick such that the lOO-year 24-hour precipitation
<br />e'vent can be distributed over 24 hours. Figure 3 illustrates
<br />lhe lOa-year 24-hour distribution for 4.0 inches of precipi-
<br />tation at Gillette, Wyoming, based upon Frederick's method
<br />and compared to values presented in the U.S. Weather
<br />Bureau NOAA 2 Atlas for the same time periods. Figure 3
<br />indicates that substantial differences exist concerning the
<br />distribution of precipitation which in turn have an impact in
<br />evaluating hydrologic characteristics of nood nows from
<br />ungaged watersheds. Similar distribution differences exist
<br />for each of the regions illustrated in Figure 2. Figure 4
<br />compares Fredericks' method to actuaJ conditions observed
<br />for '24.how climatic events of various stations in the South.
<br />
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<br />........ '\
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<br />... .';
<br />,
<br />-i i. ,'"'_
<br />"....:.: ~.~ - .......
<br />
<br />Figure 2. Study Area Sho....ing Station Years of Data
<br />Pcr Repon (from Ftederick, et Qt.. 1981).
<br />
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<br />1T1-i-'''- -r--C--'
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<br />I 1__.1---/'---/
<br />y-1 I
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<br />Iii I
<br />! II
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<br />''"I
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<br />:" J/f--, FR~DE ICK
<br />"'. I I I
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<br />i~ I ll,-Y:~
<br />
<br />~. i ~,"INOA~ 2 !
<br />~ I /--- , i
<br />~=
<br />1
<br />j
<br />
<br />.
<br />"0
<br />
<br />i
<br />
<br />10 \1. l~
<br />TlMe 1;0;: HOURS
<br />
<br />..
<br />
<br />..
<br />
<br />'0
<br />
<br />"
<br />
<br />"
<br />
<br />Figure 3. 10Q-Year 24-Hour Precipitation
<br />Distribution for Gillette, Wyoming.
<br />
<br />532
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