U.S. annual precipitation totals

Introduction

In 1989 the U.S. Army Corps of Engineers was charged with the responsibility of conducting a national study of water management during periods of drought. One of the results of the study is the National Drought Atlas (Willeke et al., 1995), which contains analyses of data on monthly precipitation, streamflow, reservoir levels, and the Palmer Drought Index for over 1,000 measuring sites in the continental United States. Analysis of the precipitation data used regional frequency analysis and was based on L-moments. Precipitation data were available as totals, in inches, for durations of 1, 2, 3, 6, 12, 24, 36 and 60 months starting in each calendar month January through December. Though regions could in principle have been defined separately for each combination of duration and starting month, this would have led to an atlas that would have been excessively large and difficult to use. It was therefore decided to construct a single set of regions, based on the data for annual precipitation totals, and to use these regions when fitting regional frequency distributions to the data for all durations and starting months.

Here we describe the analysis of the data for annual precipitation totals (though data for other durations and starting months affect some parts of the analysis). The analysis illustrates the steps involved in a large-scale regional frequency analysis exercise and shows how some of the commonly occurring problems in regional frequency analysis may be overcome.