Scientific Investigations Report 2006-5306
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Estimates of the magnitude and frequency of peak discharges are necessary for the reliable flood-hazard mapping in the Navajo Nation in Arizona, Utah, Colorado, and New Mexico. The Bureau of Indian Affairs, U.S. Army Corps of Engineers, and Navajo Nation requested that the U.S. Geological Survey update estimates of peak discharge magnitude for gaging stations in the region and update regional equations for estimation of peak discharge and frequency at ungaged sites.
Equations were developed for estimating the magnitude of peak discharges for recurrence intervals of 2, 5, 10, 25, 50, 100, and 500 years at ungaged sites using data collected through 1999 at 146 gaging stations, an additional 13 years of peak-discharge data since a 1997 investigation, which used gaging-station data through 1986. The equations for estimation of peak discharges at ungaged sites were developed for flood regions 8, 11, high elevation, and 6 and are delineated on the basis of the hydrologic codes from the 1997 investigation.
Peak discharges for selected recurrence intervals were determined at gaging stations by fitting observed data to a log-Pearson Type III distribution with adjustments for a low-discharge threshold and a zero skew coefficient. A low-discharge threshold was applied to frequency analysis of 82 of the 146 gaging stations. This application provides an improved fit of the log-Pearson Type III frequency distribution. Use of the low-discharge threshold generally eliminated the peak discharge having a recurrence interval of less than 1.4 years in the probability-density function.
Within each region, logarithms of the peak discharges for selected recurrence intervals were related to logarithms of basin and climatic characteristics using stepwise ordinary least-squares regression techniques for exploratory data analysis. Generalized least-squares regression techniques, an improved regression procedure that accounts for time and spatial sampling errors, then was applied to the same data used in the ordinary least-squares regression analyses. The average standard error of prediction for a peak discharge have a recurrence interval of 100-years for region 8 was 53 percent (average) for the 100-year flood. The average standard of prediction, which includes average sampling error and average standard error of regression, ranged from 45 to 83 percent for the 100-year flood. Estimated standard error of prediction for a hybrid method for region 11 was large in the 1997 investigation. No distinction of floods produced from a high-elevation region was presented in the 1997 investigation. Overall, the equations based on generalized least-squares regression techniques are considered to be more reliable than those in the 1997 report because of the increased length of record and improved GIS method.
Techniques for transferring flood-frequency relations to ungaged sites on the same stream can be estimated at an ungaged site by a direct application of the regional regression equation or at an ungaged site on a stream that has a gaging station upstream or downstream by using the drainage-area ratio and the drainage-area exponent from the regional regression equation of the respective region.
Waltemeyer, S.D., 2006, Analysis of the magnitude and frequency of peak discharges for the Navajo Nation in Arizona, Utah, Colorado, and New Mexico: U.S. Geological Survey Scientific Investigations Report 2006–5306, 42 p.
Purpose and Scope
Description of Study Area
Analysis of Magnitude and Frequency of Peak Discharges
Station Flood-Frequency Analysis
Regional Flood-Frequency Analysis
Determination of Basin and Climatic Characteristics
Geographical Information System
Basin and Climatic Characteristics
Development of Generalized Least-Squares Regression Equations
Improvement of Estimates at Gaging Stations
Limitations and Accuracy of Regression Equations
Estimate of Magnitude and Frequency of Peak Discharges at Ungaged Sites
Ungaged Site on an Ungaged Stream
Ungaged Site on a Stream Having a Nearby Gaging Station
Appendix 1—Selected Basin, Climatic, and Flood Characteristics, Maximum Peak Discharge Recorded, and Low-Discharge Threshold for Gaging Stations in Flood Regions 8, 11, High Elevation, and 6
Appendix 2—Basin and Climatic Characteristics Determined Using Geographical Information System Weasel
|For additional information contact:
Director, Texas Water Science Center
U.S. Geological Survey
8027 Exchange Drive
Austin, Texas 78754-4733
World Wide Web: http://tx.usgs.gov/
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