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Fact Sheet 2012–3038

Calculating Weighted Estimates of Peak Streamflow Statistics

By Timothy A. Cohn, Charles Berenbrock, Julie E. Kiang, and Robert R. Mason, Jr.

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According to the Federal guidelines for flood-frequency estimation, the uncertainty of peak streamflow statistics, such as the 1-percent annual exceedance probability (AEP) flow at a streamgage, can be reduced by combining the at-site estimate with the regional regression estimate to obtain a weighted estimate of the flow statistic. The procedure assumes the estimates are independent, which is reasonable in most practical situations. The purpose of this publication is to describe and make available a method for calculating a weighted estimate from the uncertainty or variance of the two independent estimates.

First posted March 23, 2012

For additional information contact:
Timothy A. Cohn
U.S. Geological Survey
415 National Center
12201 Sunrise Valley Drive
Reston, VA 20192
Telephone: 703–648–5711
Fax: 703–648–6693

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Suggested citation:

Cohn, T.A., Berenbrock, Charles, Kiang, J.E., and Mason, R.R., Jr., 2012, Calculating weighted estimates of peak streamflow statistics: U.S. Geological Survey Fact Sheet 2012–3038, 4 p., available at

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