American Society of Civil Engineers

Areal Reduction Factors for Two Eastern United States Regions with High Rain-Gauge Density

by Robert J. Allen, (Grad. Student, Dept. of Geology and Geophysics Yale Univ., New Haven, CT 06520) and Arthur T. DeGaetano, (corresponding author), (Assoc. Prof., Northeast Regional Climate Ctr., 1119 Bradfield Hall, Cornell Univ., Ithaca, NY 14853 E-mail:

Journal of Hydrologic Engineering, Vol. 10, No. 4, July/August 2005, pp. 327-335, (doi:

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Document type: Journal Paper
Abstract: National Oceanic and Atmospheric Administration Technical Paper-29, published in the late 1950s, remains the most commonly used reference for estimating extreme areal precipitation from station data in the United States. Although a number of alternative methods have been proposed over the intervening years, a rigorous evaluation of the assumptions used in the compilation of TP-29 has not been presented. Overall, TP-29 areal reduction factors provide a conservative means of relating station precipitation extremes to basin average values. For watershed areas less than 1000 km², reevaluated areal reduction factors, are in close agreement with the TP-29 values. For larger watersheds, which TP-29 does not address, the areal reduction factors continue to decay exponentially. The areal reduction factors were found to be particularly sensitive to return period and season, with less extreme areal precipitation relative to the corresponding station precipitation at longer return periods and during the warm season. The reevaluated factors exhibit modest differences between study areas in North Carolina and New Jersey. The influence of station density, interpolation method, and topographical rainfall biases appears insignificant.

ASCE Subject Headings:
Data analysis
Rainfall intensity
Spatial analysis
United States