Extreme precipitation frequency in the Semiarid Southwest

Rainfall frequency analyses and depth-area-duration curves to determine design storms and storm water runoff, are critical to the design of hydraulic structures and management of water resources. In the development of new precipitation frequencies for the Semiarid Southwest by the Hydrometeorological Design Studies Center of the National Weather Service, various characteristics of extreme precipitation were documented, including predominant duration, timing, and seasonality. Some examples of the results and applications will be shown here. The complete study will be published as NOAA ATLAS 14, Precipitation-frequency Atlas of the United States, 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 Study will also have a web-based version. It will supersede the previous atlases, NOAA Atlas 2 (1973) and Technical Paper 49 (1964), for these western States. Up to 30 percent more stations and an additional 30 or more years of record were analyzed, compared to NOAA Atlas 2. L-moment statistics were used for frequency analysis, permitting more objective quality control, regionalization of data, and objective curve-fitting techniques. Precipitation estimates for events from 2- to 100-year return periods and for durations from 5 minutes to 60 days were developed. In most of the Southwest short-duration extreme events, usually less than 12 hours, were prevalent. And within these events, the rain most often fell within 3 hours. To evaluate the temporal distribution of the rainfall, 12-, 24-, and 72-hour events were divided into 4 equal periods. The results showed that more of the precipitation occurred in the first quartile than in succeeding quartiles in 70 to 80 percent of the cases for all 3 durations. With regard to seasonality, the area was divided into warm and cool seasons, according to the prevailing extreme rainfall type: Warm - convective storms, or Cool -general storm rainfall. Although the seasonality varies geographically, much of the area has a definite characteristic extreme-precipitation season. For example, in New Mexico, nearly 100 percent of the extremes - for all durations - occur in the Warm season (May through October). In contrast, in the California desert, the extremes occur in the Cool season (November through April or May) and very few or none in the Warm season. In Utah, although the type of rainfall varies between seasons, the extremes are more evenly distributed through the year.