Semiarid Precipitation Frequency Project Contrasted with Previously Published Rainfall Atlases

Abstract

Design of hydraulic structures and management of water resources requires rainfall frequency analyses and depth-area-duration curves to determine design storms and stormwater runoff. For the western United States, two atlases provide rainfall information for return frequencies from 2 to 100 years: NOAA Atlas 2 and Technical Paper 49. The Semiarid Precipitation Frequency Study (Semiarid Study) was begun several years ago to revise these atlases for Arizona, Nevada, New Mexico, Utah, and Southeast California. The Semiarid Study will supersede the previous atlases for these regions. Precipitation frequencies will be provided for events as frequent as six times a year up to 1,000-year return periods and for durations from 5 min to 60 days. New depth-area-duration curves will be provided. The Semiarid Study differs from the earlier studies in (a) 30 percent more daily stations and 230 supplemental stations; (b) new statistical methods permitting more objective quality control, regionalization of data, and objective curve-fitting techniques; and (c) direct use of partial-duration data series. With Utah as an example, comparisons with NOAA Atlas 2 of 100-year, 24-hr return frequency values are also shown. Semiarid Study values are lower in central and southern Utah but higher in the Uinta Mountains and the northern Wasatch Range. Results for the rest of the Semiarid area are similar; that is, values are generally higher than NOAA Atlas 2 in the mountains and lower in the desert regions. Seasonality is illustrated with 45-day events in central and western Utah. The maximum 45-day rainfalls occur in this area in March and April and in August and September, a bimodal distribution.