Projected Average Summer Air Temperature Increases and the Implications for Philadelphia's Surface Drinking Water Supply

Abstract

Water managers are faced with numerous uncertainties that need to be addressed in the development of long-term planning initiatives and large-scale investment decisions. One of the primary and perhaps most far-reaching of these uncertainties is climate change. The objective of this project is to utilize one aspect of projected climate change impacts, increasing average summer air temperature, to understand potential impacts to surface drinking water supply temperatures in the Schuylkill River at Philadelphia, PA. The project consists of three major components. As an initial step, climate model output from the Coupled Model Intercomparison Project Phase 5 (CMIP5) was evaluated for the Northeast US and Philadelphia by mapping and analyzing Network Common Data Form (NetCDF) files for near-surface air temperatures in Matlab. The evaluation of climate model output included model validation for six selected CMIP5 Global Climate Models (GCMs), as well as future projections using the Representative Concentration Pathway 8.5 (RCP8.5) climate scenario. Secondly, this project aimed to develop a statistical relationship between air and surface water temperatures in Philadelphia using publicly available data from the United States Geological Survey (USGS) and Daymet. Following the aforementioned empirical analyses, research was performed to provide insight regarding the impact of increased surface water supply temperatures on the formation of disinfection byproducts (DBPs) during drinking water treatment. The three most accurate GCMs for the Northeast US and Philadelphia indicate that the average air temperature over June, July, August and September (JJAS) will increase approximately 2°C by mid-century. Assuming the RCP8.5 climate scenario prevails beyond mid-century, the results indicate that the average JJAS near-surface air temperature may increase by as much as 5.7°C in the Northeast US and 5.3°C in Philadelphia by 2100. For the Schuylkill River at Philadelphia, statistical analyses reveal that air temperature explains the majority of variation in water temperature over the time period of analysis, from 1999-2001 and 2011- 2013 for the months of JJAS. Projected increases in average JJAS air temperature are expected to increase average JJAS surface water temperature in the Schuylkill River at Philadelphia by approximately 1.69°C (3.05°F) and 4.23°C (7.62°F) by 2050 and 2100, respectively. Current climate science needs to be directly related to actionable adaptation initiatives. The outcome of this study directly links one aspect of climate change to a potential drinking water impact, with the goal of providing actionable information to inform future operational and supply management strategies.