From sewer overflows to basement back-ups and street flooding, this practical method can be used to simulate an array of future conditions. In turn, this can greatly inform stormwater planning and modeling. This framework can also be applied to other disciplines and applications where high-resolution precipitation time series are required, such as riverine flood modeling, drought planning, and even water quality modeling – which can then inform climate resilient design guidelines for infrastructure projects.
Step 1: Define problem and requirements
Figure out what specific challenges does your stormwater system need to address—is it combined sewer overflows, surface water quality issues, street flooding? You also need to assess your current stormwater management practices to determine what results are needed, such as IDF curves for design, sub-hourly precipitation time series for continuous modeling, hyetograph generation, and so forth.
Step 2: Collect and pre-process data
Gather observed rainfall records near the area of interest, ideally hourly or sub-hourly data points, to evaluate and aggregate extreme rainfall conditions. It is important to have at least 20 to 30 years of observed records to capture natural variability in rainfall. Then use a resource such as the U.S. Bureau of Reclamation’s Downscaled CMIP5 Climate Projections to obtain downscaled GCM output on a daily time scale.
Step 3: Determine global climate model ensemble
There are a few different approaches to selecting the ensemble of GCMs for your needs. It is recommended to use a minimum of 10 GCMs to capture a diverse set of models. One of the most common approaches is the “envelope approach” which capture a range of plausible projected precipitation changes. The method used in Philadelphia combined the envelope approach with an approach based on past performance.
Step 4: Choose planning horizons
Decide a baseline period of at least 20 years to represent a comparison of “current conditions”. From there, choose one or more future time periods where projections are needed based on your system’s assets or relevant regulatory requirements, such as MS4 compliance.
Step 5: Develop future projections
Develop future projections using the delta change factor method, using existing rain gauge precipitation data and downscaling the GCM ensemble to sub-daily time steps. This will allow you to create a future hourly precipitation time series.
Step 6: Apply projections to planning and design
Using the time series from step 5, develop or update the tools needed for urban drainage planning and design application (e.g. IDF curves, hyetographs, "Typical Years", etc.).