When planning or designing infrastructure, Global Climate Models (GCM) are a powerful tool to help us understand future climate conditions such as sea level rise, temperature, and rainfall. Precipitation output, however, presents a particular challenge because these models typically focus on precipitation changes to large regional areas over months, years, or even decades. Although informative for many projects, GCM output is poorly suited for stormwater applications that require hourly data during storm events. Breaking down these global models to more granular data often requires highly specialized and complex processes that are not accessible to most municipalities or utilities and the results are still often poorly representations of local conditions.
Recently, a team of climate resiliency experts from CDM Smith and the Philadelphia Water Department discovered a practical approach to downscaling GCM output for local stormwater systems. As part of the Philadelphia Water Department’s Climate Change Adaptation Program, the team needed future precipitation projections to assess the impacts of changing storm size and intensity on stormwater and wastewater collection systems and infrastructure. By itself, the downscaled GCM data could only be broken down into daily outputs and couldn’t give accurate insight into individual storm events. Also, when compared to current local rain gauge numbers, the GCM outputs overestimated the number of consecutive wet days and underestimated the intensity of the rainfall.
To address the need for more realistic data, the team combined existing rain gauge information and the daily GCM outputs to create a future precipitation hourly time series. This time series was used as a basis for a stochastic (randomly determined) weather generator to explore potential variability in projected precipitation.
“We were able to align the GCM ensemble data to data from Philadelphia rain gauges from 1995-2015,” says CDM Smith senior vice president Mark Maimone. “This matching allowed us to take the 2080-2100 GCM predictions for the Philadelphia area and shape them into a more accurate estimate of hour by hour rainfall as it might occur at the end of the 21st century.”
Based on our work in Philadelphia, we came up with a six-step framework for others—especially municipalities and utilities—to develop the necessary tools to assess future impacts of climate change on urban stormwater systems.
Sebastian Malter, Water Resources Engineer
1. Tackle the easy stuff first
Start by reviewing your current policies and procedures to see what quick preventative measures you can take to decrease the generation of waste. Encourage your community to make daily changes that promote sustainability: taking five-minute showers, using recyclable shopping bags, carpooling, limiting overbuying at the grocery store, building compost piles for food waste, replacing leaky faucets, and other small changes that can add up to a big difference.
Transportation planning expert Davonna Moore adds that these don’t have to be huge lifts. “For decision-makers, small-to-medium-scale changes like curb management implementation and organizing delivery at non-peak times would allow your city to run more efficiently and better poised for future investments,” she says.
4. Re-examine your city's layout
“I’d encourage city mayors to look at bus routes, locations of libraries, city centers, houses of worship, schools, medical centers and businesses. Then, look at those routes to several residential sectors of the city—taking into consideration all levels of economic status. Leaders should examine transportation routes and the zoning of housing sectors in mixed-use business areas to ensure easy access to parks, communities, transit stations, and emergency centers,” says program management expert Karen Counes.
This method is both straightforward and very effective in providing the information needed to design and maintain stormwater collection systems.
Mark Maimone