Accounting for climate change in future flow predictions of collection system and watershed modeling

Global warming is the long-term rise in the average temperature of the Earth's climate system. It is a major aspect of climate change and has been demonstrated by direct temperature measurements and by measurements of various effects of the warming (Intergovernmental Panel on Climate Change, IPCC). Ninety-seven percent of climate scientists agree that human activities are likely the cause of the climate-warming trends over the past century. This presentation will discuss the modeling techniques to account for future climate changes impacts on collection system and watershed modeling to predict the future flows.

• Temperature - The planet's average surface temperature has risen about 1.62 degrees Fahrenheit (0.9 degrees Celsius) since the late 19th century. Most of the warming occurred in the past 35 years, with the five warmest years on record taking place since 2010 (IPCC).
• Sea Level Rise (SLR) - Global sea level rose about 8 inches in the last century. In the last two decades, the rate is nearly double and is accelerating slightly every year (IPCC).
• Rainfall – Some regions have experienced increasing frequency and intensity of rainfall events (IPCC).
• Lakes - Long-term water levels in the Great Lakes have fallen since reaching record highs in the 1980s. While most models project continued, long-term declines in lake levels, shorter-term variations will remain large, and periods of high lake levels are happening. (Climate Change in the Great Lakes Region).
• Rivers – The Ohio River may have higher water levels and flooding in springtime due to heavier rains and snow melts. Summer droughts are likely to be more severe with higher evaporation and lower summer rainfall which are likely to periodically reduce river flows (USEPA).

The following summarizes the flow components typically modeled in collection system and watershed models and their corresponding methods to account for the future climate change impacts.

• Base Wastewater Flow (BWF). Under drought conditions, utilities and localities may adopt stringent conservation measures to control water consumptions, which results in lower BWF. On the other hand, BWF may increase if the climate is getting wetter and the conservation measures are less stringent. The future BWF adjustment can be based on the historic observed water consumption data during both dry and wet seasons. The future BWF can also be based on the targeted water consumption goal under conservation measures and other related locality rules.  Future population movements due to climate change may also increase or reduce BWF. 
• Groundwater Infiltration (GWI). GWI is a function of the groundwater table impacted by precipitation, SLR and lake/river levels. It is ideal to build a mathematical relationship between the observed DWF/GWI seasonal variations and the groundwater table seasonal variations. Then the relationship can be applied to future precipitation, SLR and lake/river level changes to predict the future GWI.
• Rainfall-driven Inflow and Infiltration (RDII) for the Sanitary Sewer System. Climate change will affect precipitation patterns, frequency, intensities, volumes, etc.  The Future design storms and typical year rainfalls will have to be re-evaluated using the historical data and future precipitation prediction. If the model uses design storm, it can be scaled up to represent the future design storm with higher intensity and total volume but with the same return period for the future design year. However, the adjustment of a design typical year rainfall is more complicated. Climate change may not have a uniform impact on an annual rainfall. The seasonal variations may be more severe with lower intensities in dry season/months and higher intensities in wet season/months. Then, the future annual rainfall adjustment may be scaled between rainfall decrease in dry season and rainfall increase in wet season.
• Wet Weather Runoff for the Combined Sewer System and Stormwater Runoff. Like RDII, the primary task is to come up with the future design storm and typical year rainfall with climate change impacts using the above methods. The same method can be applied to the temperature data input to account for future global warming. The temperature impacts the modeled snow melt and evapotranspiration.
• Outfall and Overflow Boundary Level. Some models have boundary levels at the system outfalls or CSO outfalls to represent the typical water level of a water body. The boundary level impacts the flows in the connecting pipes and hence the upstream systems. The boundary level may be a constant value representing the average or peak level of the water body. The future SLR and lake/river level increase/decrease factor can be directly applied to the constant boundary levels. The boundary level may also be an observed time series or the tidal data, typically for a year when using a design typical year rainfall to design the system. The future SLR increase may be applied as a constant adjusting factor to the existing tidal time series. However, the river/lake water level may have more severe seasonal variations. Then, the future river/lake water level adjustment can be scaled between the maximum level decrease in dry season and the maximum level increase in wet season.

The presentation will use real-life project examples to illustrate how to adjust the flow components to account for the future climate change impacts in collection system and watershed models.

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