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

Abstract

National Oceanic and Atmospheric Administration Technical Paper-29, published in the late 1950s, remains the most com- monly 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 km 2 , 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 corre- sponding 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.