|
<br />
<br />Summary
<br />
<br />A regional flood-frequency study was completed
<br />for an area of the southwestern United States, including
<br />all of Arizona, Nevada, and Utah, and parts of
<br />California, Colorado, Idaho, New Mexico, Oregon,
<br />Texas, and Wyoming (fig. I). The study area was
<br />divided into 16 hydrologic flood regions as shown in
<br />Figure I. Region I comprises high-elevation areas
<br />throughout the study area. The regression equations
<br />developed for these regions are for estimating peak dis-
<br />charges (QT) having recurrence intervals T that range
<br />from 2 to 100 years. The explanatory basin variables
<br />used in the equations are drainage area (AREA), in
<br />square miles; mean basin elevation (ELEVIlOOO), in
<br />feet above sea level divided by 1000; mean annual pre-
<br />cipitation (PREC), in inches; mean annual free water
<br />surface evaporation (EVAP), in inches; latitude of the
<br />gaged site minus 28 divided by 10 ((LAT-28)/1O), in
<br />decimal degrees; and longitude of the gaged site minus
<br />99 divided by 10 ((LONG-99/1O)), in decimal degrees,
<br />The variables ELEV, LAT and LONG are modified by
<br />the given constants in the computer applications of the
<br />equations, The user should enter the actual values of
<br />ELEV, LAT and LONG. The variables AREA, ELEV,
<br />LAT and LONG can be measured from topographic
<br />maps. The variable PREC can be obtained from nor-
<br />mal-annual precipitation maps (I :500,000 scale) in
<br />U.S. Weather Bureau (1959-61, 1963). The variable
<br />EVAPcan be obtained from figures 2 and 3, The regres-
<br />sion equations were developed from peak-discharge
<br />records available as of 1986 at 1,162 stations in the 10-
<br />state study area. The equations are most applicable to
<br />unregulated streams with drainage areas less than 200
<br />square miles, In all regions some stations with drainage
<br />areas between 200 and 2000 square miles were used in
<br />developing the regression equations. Judicious use
<br />should be made of the equations for basins between
<br />200 square miles and the upper limit of the calibration
<br />data (this upper limit is provided in the NFF program).
<br />The average standard error of prediction of the regres-
<br />sion equations range from 45 to 135 percent.
<br />
<br />;",.:;1'\1
<br />")}+i
<br />
<br />Procedure
<br />
<br />Use topographic maps, mean annual precipita-
<br />tion maps in U.S. Weather Bureau (1959-61, 1963),
<br />mean annual free water surface evaporation maps in
<br />figures 2 and 3, and the following regression equations
<br />to estimate the needed peak discharges QT, in cubic
<br />feet per second, having selected recurrence intervals T.
<br />
<br />High-Elevation Region 1
<br />
<br />Q2 = 0.1 24AREAo,845PRECI.44
<br />Q5 = 0.629AREAO,807PREC1.l2
<br />QIO = 1.43AREA0786pRECO,958
<br />Q25 = 3,08AREAO,768PRECO,811
<br />Q50 = 4.75AREAO,758PRECo,732
<br />Qloo= 6,78AREAo.75OpRECo.668
<br />
<br />Northwest Region 2
<br />
<br />Q2 = 13.lAREAO,713
<br />Q5 = 22.4AREAo,723
<br />QIO = 55.7AREAO,727(ELEVII,OOOrO,353
<br />Q25 = 84.7AREAO,737(ELEV/l,OOOrO,438
<br />Q50 = I 13AREAO,746(ELEV/l ,OOOrO.5 I 1
<br />Qloo= 148AREAO,752(ELEV/I,OOOrO,584
<br />
<br />South-Central Idaho Region 3
<br />
<br />Q2 = O,444AREAo,649pRECI.l5
<br />Q5 = 1.21AREAo,639PRECo,995
<br />QIO = 1.99AREAo,633PRECo,924
<br />Q25 = 3.37AREAo,627PRECo,849
<br />Q50 = 4,70AREAo,625pRECo.802
<br />QtOO= 6.42AREAo,621PRECo,757
<br />
<br />180 Nellonwlde Summery 01 U.S. Geologlcel Survey Reglonel Regresolon Equetlonolor Eotlmatlng Megnltuds end Frequency 01
<br />Floodo lor Ungeged Slteo, 1993
<br />
|