Technical Notes on Climate Adaptation in Civil Engineering
ASCE Committee on Adaptation to a Changing Climate (CACC)
Technical Note 24.2 Predicting Future Precipitation Intensity, Duration and Frequency
The intensity, duration and frequency (IDF) of expected precipitation at a given location is a critical design input in many aspects of civil engineering. As an example, hydraulic structures are typically designed for the rainfall amount expected during an event of a given duration (in hours) and of a given probability expressed as an average recurrence interval in years (i.e. 25 or 50 years for simple projects where overflowing roadways is acceptable or 100 or 500 years for major bridge structures). There are many other civil engineering applications of rainfall IDF estimates, including floodplain management.
Most civil engineering applications have, until recently, assumed that climate conditions have been assumed to be statistically “stationary,” i.e. a constant, unchanged over time. It is now recognized that climate conditions exhibit “non-stationarity” on time scales relevant to engineering design and have been changing on a trend that will continue or in some cases, accelerate into the foreseeable future. In a nonstationary climate, precipitation intensities will change – most in the United States will likely increase while some will decrease. Accounting for such changes in civil engineering projects that have service lives of 50 to 100 years is critical going forward, requiring new estimates that account for projected changes in climate that are in development by the Office of Water Prediction of the National Atmospheric and Oceanic Administration’s (NOAA) National Weather Service.
The primary source of precipitation frequency estimates in the United States over the past 25 years has been NOAA Atlas 14 (PF Data Server-PFDS/HDSC/OWP (noaa.gov)) . Atlas 14, which is based solely on historical data up to the date of publication, provides tables of precipitation amounts at durations from 5 minutes to 60 days at corresponding average recurrence intervals of 1 year to 1,000 years. As an example of the use of Atlas 14, go to the website in parentheses, click on Kentucky on the map, under “select station” scroll down to “Louisville WSO airport”, scroll down to the data table, and find that the 50-year, 2 hour storm for the Louisville airport vicinity is 3.16 inches and the 50-year, 24 hour storm is 6.15 inches. Atlas 14 can be similarly queried for most locales in the United States, but it is only based to date on historical data, reflecting a stationary climate.
NOAA/NWS Office of Water Prediction is currently preparing Atlas 15 (expected in 2026 and 2027), which will bring the historical estimates up to date and provide spatially continuous future projections of rainfall IDF for stations across the United States and its territories ((NOAA_Atlas_15_Flyer.pdf (weather.gov) Atlas 15, Volume 1, will provide an update for each station using the latest historical precipitation observations while Volume 2 will provide future precipitation IDF estimates through the year 2100, based on the output of global climate models such as reported by the Coupled Model Intercomparison Project Phase 6 (CMIP6), with varying assumptions for the rate at which global greenhouse gas emissions occur in the future. This will yield future curves showing ranges of rainfall amounts over time for any given duration and recurrence interval nationwide, similar to the graphic presented below. Using Volume 2, the design engineer will be presented not with a number for a selected rainfall IDF, but rather a range reflecting the uncertainty of the rate at which global greenhouse gas emissions will change and of the projections themselves. For more information on the proposed methods for Atlas 15, see this NOAA/NWS Public Notification Statement.
Atlas 15, Volume 2, will represent the best information available as of the mid 2020’s regarding precipitation IDF over the remainder of this century. In the future, as climate data continues to be gathered and as global and regional climate models are updated, these projections will need to be revisited and revised periodically.
The development of Atlas 15 was funded by the 2022 Bipartisan Infrastructure Law and is currently underway. In 2024, NOAA will release an Atlas 15 Pilot covering Montana for both Volumes 1 and 2. Preliminary estimates are anticipated for the lower 48 states in 2025 with those available in final form in 2026 after undergoing a public peer review process. Adding Alaska, Hawaii, Puerto Rico, the US Virgin Islands and Guam is expected in 2027.
How will these Atlas 15 projections be incorporated into applicable regulations and civil engineering design practice? That is a question that remains to be answered. Since the future projections will vary over time with projection assumptions, creating numerical design input will require a standardized procedure. For some circumstances, this may involve selection of the “middle” projection of greenhouse gas emission scenarios (Representative Concentration Pathways, RCP 4.5 to 8.5) as a design parameter while considering the range of projections in design. Another alternative approach might entail the selection of an overall global temperature increase assumption of perhaps +3 degrees C since the late 1800’s for use in determining design parameters.
ASCE standards that are commonly used in the engineering profession and which reference rainfall IDF as critical parameters will be updated once Atlas 15 is available. From a regulatory standpoint, design standards are commonly set by state, regional, and local governmental agencies and each agency will likely adopt updates on their own schedule. It remains to be seen whether each applicable agency regulation will track Atlas 15 with the historical updates, or the Atlas 15 version with projections of future precipitation IDF.
Technical Notes on Climate Change in Civil Engineering are prepared by the American Society of Civil Engineers Committee on Adaptation to Climate Change, Dan Walker, Ph.D., and Craig N. Musselman, P.E., editors. NOTE: These Technical Notes are intended to alert the reader to emerging topics related to climate change and civil engineering practice and are NOT intended to act as a substitute for any sources cited herein.