Technical Bulletins on Climate Adaptation in Civil and Environmental Engineering
ASCE Committee on Adaptation to a Changing Climate (CACC)
Climate Adaptation Engineering Bulletin No. 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 annual exceedance probability (AEP) or recurrence interval in years (e.g., 0.04 AEP or 25 years for simple projects where overflowing roadways is acceptable or 0.002 AEP or 500 years for major buildings or bridges). There are many other civil engineering applications of rainfall IDF estimates, including both high intensity events and drought occurrence.
Most civil engineering applications have, until recently, assumed climate conditions to be statistically “stationary” (i.e. a constant, unchanged over time). It is now recognized that climate conditions exhibit “non-stationarity” reflecting long-term (30 years or longer) statistical changes associated with a warming world, and those trends are expected to continue, or in some cases accelerate, into the foreseeable future. In a nonstationary climate, rainfall intensities will change - most in the United States will increase. 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 NOAA's Office of Water Prediction (a part of the National Weather Service [NWS]).
The primary source of precipitation frequency estimates in the United States over the past 25 years has been Atlas 14 provided by NWS. Atlas 14 (PF Data Server-PFDS/HDSC/OWP (noaa.gov)), which is based solely on historical data to date, 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. And in some states, Atlas 14 has not been updated in twenty years.
NOAA is currently preparing Atlas 15 (expected in late 2026), which will bring the historical data base up to date and provide future projections of rainfall IDF for stations across the United States and its territories (Atlas 15 info page). Atlas 15 will consist of two volumes. NOAA Atlas 15 Volume 1 will provide a snapshot of current estimates that account for temporal changes in historical observations. When published, NBOAA Atlas 15 Volume 1 will supersede the current NOAA Atlas 14 precipitation frequency estimates.
NOAA Atlas 15 Volume 2 will provide model-based precipitation frequency estimates projected into the future. Volume 2 estimates will be developed by applying adjustment factors to Volume 1 estimates (i.e. future relative changes obtained from downscaled climate model data).
This will yield future curves showing ranges of rainfall amounts over time for any given duration and recurrence interval for all stations in the continental United States 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.
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 revised periodically, critically requiring future congressional appropriations as on-going data and projections will change.
The format for the presentation of Atlas 15 Volumes 1 and 2 from the NOAA Office of Water Prediction is available with data for the State of Montana (NOAA Atlas 15 Pilot). Similar estimates are anticipated for the continental US in late 2026. 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 may require project specific judgment. For some circumstances, this may involve selection of a “middle” projection of greenhouse gas emission scenarios (Representative Concentration Pathways, such as RCP 4.5) as a design parameter while considering the range of projections in design. An alternative approach may involve selecting a global warming target such as 3 degrees C (global average temperature change since the 19th century) which sets a design target with the rate of approaching that target to be determined.
ASCE standards that are commonly used in the engineering profession and which reference rainfall IDF as critical parameters will need to 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 and in their own format. Civil engineers will have a professional responsibility to meet evolving design and regulatory standards, to take changing conditions into account in design particularly with respect to timing of resilience construction cost investments, and to protect public health, safety and welfare.
The availability of Atlas 15 Volume 2 will likely be transformative in the practice of civil engineering.
Engineering Bulletins on Climate Change in Civil and Environmental Engineering are prepared by the American Society of Civil Engineers Committee on Adaptation to Climate Change, Dan Walker, Ph.D., and Craig Musselman, P.E., editors. NOTE: These Engineering Bulletins 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.