NOAA Atlas 14—New Precipitation Frequencies for the United States

As various rainfall frequency atlases for the United States are more than 20 years old, e.g., NOAA Atlas 2 (1973) and Technical Paper 49 (1964), and Weather Bureau Technical Paper No. 40 (TP 40) (1961), the National Weather Service is updating these studies. Rainfall frequency analyses and depth-area-duration curves are critical to determine design storms and storm water runoff, for design of hydraulic structures and management of water resources. As rainfall return-frequencies are used nationwide, it is important to have consistency across state lines, as well as in methodology, analysis, and data quality control. Improvements since earlier studies include an additional 20 to 30 years of data, high-speed computing, mapping and analysis using Geographic Information Systems, and state-of-the-art statistical techniques. L-moment statistics are used to estimate precipitation frequencies, and also provide objective quality control, regionalization of data, and curve-fitting techniques. Analysis of time series for trends or data shifts is part of each study. Tests are run for randomness, linear trends and shifts in mean and variance. The Update of TP 40 for the Ohio River basin shows that about 85 percent of nearly 2000 daily stations (60 years) have no linear trend in mean or variance. However, of stations that do have a significant trend, the trend is steadily upward in nearly all cases. Where the trend is downward, it is steadily downward. The shift test for pre-1958 data compared with 1959-1998 has a much stronger signal for extreme precipitation changes. Although fewer stations were available for the shift test, 15.2 percent showed a significant shift upward of the mean, and 54.9 percent showed a significant increase in variance. The final reports will include precipitation estimates for events for 2- to 1000-year return periods and for durations from 5 minutes to 60 days. Temporal distribution and seasonality of rainfall are also evaluated. Reports will be published as various volumes of NOAA ATLAS 14, Precipitation-frequency Atlas of the United States. The Semiarid study is contained in Volume 1: Semiarid Southwestern United States: 1.1 - Arizona, 1.2 - Nevada, 1.3 - New Mexico, 1.4 - Utah, and 1.5 - Southeastern California. The Ohio River Basin (midwest to eastern U.S.) study results are expected in mid-2001, and results for other studies in late 2001 and 2002. The atlases will be web-based and also available in hardcopy.