There is strong scientific consensus that precipitation intensities will increase in the near future as a result of climate change and a warmer atmosphere. NextGen's project is to examine this for Tucson and Eastern Pima County (EPC) to account for the projected increase in rainfall intensity and temperatures and their implications for engineering practices.
EPC encompasses 3,021 square miles of County extending from northern to southern County boundaries and from the eastern County boundary to 55 miles west. Structures in Pima County, as in many places in the United States, are mainly designed based on the historical rainfall values provided by NOAA Atlas 14 in the Intensity Duration Frequency (IDF) curves.
NextGen uses the most recent series of climate model projections for the near horizon of 2020 to 2100 to incorporate the impact of climate change in IDF curves. The result of this project can be used by the County to adapt engineering practices and increase the resiliency of the community toward climate change.
Read our our paper, "Climate changes impact on stormwater infrastructure design in Tucson Arizona," published in Volume 72 of Sustainable Cities and Society.
Although the public is mostly exposed to global warming as the first impression of climate change, research shows that climate change impacts on temperature and rainfall are regionally based. This means climate change can prolong the drought period in one region while increasing flood frequency in another. As a general pattern, a semi-arid region like Tucson is expected to experience more prolonged dry seasons and more severe rain events, especially in the monsoon season. It is expected that future rainfall events will be more extreme due to climate change impacts.
How do the odds look in my neighborhood in Tucson?
In Tucson, like most of the U.S., stormwater infrastructure is designed based on historical extreme rainfall events and excludes the impact of climate change. Simply said, the stormwater infrastructure systems were not designed and are not being designed to handle the intensified rainfall events in the future due to climate change. Consequently, under-designed stormwater infrastructures will become overwhelmed more often in the future and the public will be more at risk because of climate change.
Greater Impacts of Extreme Rainfall Events for Tucson in the Future
NextGen has conducted research to demonstrate the impacts of climate change on intensifying extreme rainfall events in Tucson for the next 30 years from 2020 to 2051. For this purpose, four future prospective scenarios (SSP1, SSP2, SSP3, SSP5) were considered for the eight latest climate models of Coupled Model Intercomparison Project Phase 6 (CMIP6). A total of 32 prospective future horizons were considered.
The prospective future scenarios are called Shared Socioeconomic Pathways (SSP) recognized by the Intergovernmental Panel on Climate Change (IPCC), which is part of the United Nations (UN). Each of these scenarios defines the road toward challenges of mitigation and adaption to climate change in the future. SSP1 projects a future world focused on sustainable growth and equality, SSP2 projections follow the historical pattern, SSP3 is a world of ever-increasing inequality and SSP5 is a world of a rapid increase in energy use and economic growth.
Regardless of the underlying assumptions, each scenario is one probable future projection that tries to reflect the climate change impacts.
NextGen examined the next 30 years (2020-2051) with the climate models for extreme rainfall events (24-hr rainfall for the 100-year storm - equivalent to a 1% chance of occurrence during each year). The rainfall is expected to increase in magnitude, shown by the majority of climate models, up to 350% for some projections.
NextGen also found that an extreme rainfall event with the historical 100-year return period is expected to happen more frequently in the future considering climate change. In other words, as shown in the figure below, the occurrence of the 100-year extreme rainfall event, based on historical data, is not 1% per year anymore and has a higher chance of occurrence annually.
The figure below shows how a 100-year event is expected to happen more frequently in less than 40 years instead of the expected 100 years considering the three projected scenarios of SSP1, SSP3, and SSP5 for the next 30 years (2020-2051). More information is available Here.
In Tucson, stormwater infrastructure is designed based on so-called Intensity-Duration-Frequency (IDF) curves provided by NOAA in Atlas 14 (NA14). These IDF curves were developed based on historical precipitation data and do not incorporate the predictive nature of climate change.
NextGen updated IDF curves for Tucson based on the outputs of eight climate models and four scenarios considering the next 30 years (2020-2051). An illustrative culvert design example was selected to picture the impacts of climate change. For a hypothetical watershed, a culvert was designed to convey the stormwater of 100-year rainfall events. The culvert was first designed based on NA14 and the required culvert size was estimated as 24-inch diameter. The culvert design was repeated for the same hypothetical watershed, yet this time using the updated IDF curves based on climate models.
The figure above shows that based on almost all future projections (97%), a larger culvert size, bigger than 24-inch diameter is required to handle the intensified extreme rainfall events due to climate change in the future. The largest culvert size is a 48-inch diameter predicted by only 6% of future projections. And only 3% of future projections support the same size of 24 inches, such as NOAA in this hypothetical example. The above figure highlights the importance of updating IDF curves under climate change for the design and engineering of stormwater infrastructures. The consensus of the predictive models is that 33-inch diameter culvert would take the place of the 24-inch diameter culvert that was designed on historical data.
We need to change our planning and design of stormwater infrastructure as our climate changes. To protect the community, limit flood health and safety risks, and have a more sustainable city we recommend to revise the design approach to stormwater infrastructure and come up with more adaptable ideas to incorporate climate change impacts. Detailed information on the NextGen research for Tucson on climate change's impact on stormwater infrastructure design, and recommendations for public works officials can be found Here.