Three-Hour and Twenty-Four-Hour Rainstorm Ratios across the Southern United States

Atlases of extreme rainfall were produced over the past 50 years (both nationally and regionally), for various applications. Quantile estimates—rainfall amounts for a given duration and return period—are often used in stormwater drainage and impoundment design, flood control, agricultural engineering, as well as in many other applications. The foundation for these atlases rests upon daily rainfall data. Past researchers determined relationships between daily rainfall frequency/magnitude relationships and 24-hour storm frequency/magnitude relationships, and then used ratios of 24-hour to shorter $n$ -hour durations to determine storm magnitudes for durations of less than 24 hours. However, these ratios can vary geographically and the application of one averaged ratio over a large region may produce significant error into the quantile estimate maps. This research examines 14 hourly rainfall sites across the southern United States, thereby forming a transect from San Diego, to Jacksonville, Fla. Results show that the ratios between 3-hour and 24-hour rainstorms vary considerably from one climatic regime to another.