Evaluating the Impact of Climate Change on Water Quality and Quantity in an Urban Watershed Using an Ensemble Approach

Considerable efforts are underway to restore watersheds and estuaries downstream impacted by urban development; however, climate change (CC) may be undermining them. Current methods are limited in their ability to predict hydrology and water quality with CC and assess its effect on the efficiency of stormwater control measures (SCMs). We developed a method using downscaled global climate models (GCMs) from the Coupled Model Intercomparison Project Phase 5 (CMIP5) to project precipitation and temperatures; these were used to force a Storm Water Management Model (SWMM). Three scenarios, a historical and two Representative Concentration Pathways (RCP 4.5 and 8.5) with five GCMs, were used to produce ensemble results. All GCMs in both RCP scenarios projected increases in precipitation and temperature compared to historical conditions. Both RCPs exhibited their largest increases in precipitation, streamflow, total suspended solids (TSS), total nitrogen (TN), and total phosphorous (TP) loads in the winter, summer exhibited the largest increase in temperature. Median loads of TSS, TN, and TP increased 3.1%, 2.5%, and 9.9%, respectively, for RCP 4.5, and increased 3.8%, 3.1%, and 10.4%, respectively, for RCP 8.5. Median reductions in TSS, TN, and TP SCM efficiency for RCP 4.5 were projected to be 6%, 7%, and 11%, respectively; and 11%, 12%, and 17% for RCP 8.5, respectively. Thus, it is likely that additional efforts will be needed to meet water quality goals in the future. Methods such as these can help create climate resilient watershed improvement strategies and guide urban stormwater planning against likely future changes as a result of CC.