Climate Change Impact on Critical Source Area Identification in a Maryland Watershed

Abstract. The potential impacts of climate change on critical source areas (CSAs) of surface runoff, sediments, nitrogen, and phosphorus were evaluated in an agricultural watershed of the Chesapeake Bay drainage basin, in the U.S. Northeast climate region. The SWAT model was calibrated for the study watershed and used to establish its baseline response and constituent CSAs under current climate (years 1990 to 2004). The calibrated model was then subjected to weather time series downscaled from the CMIP3 GFDL CM2.1 Atmosphere-Ocean Global Circulation Model (AOGCM) for IPCC SRES scenarios B1 (low emissions), A1B (medium emissions), and A2 (high emissions) to predict the watershed‘s response to climate change and identify how constituent CSAs may change under future climate (years 2046 to 2064 and 2081 to 2100). The utility of targeting best management practices (BMPs) to CSAs was assessed by computing advantage ratios that relate the fraction of watershed-generated constituents that emanate from CSAs to the fraction of watershed area occupied by these CSAs. Results indicated that, under current conditions, CSAs occupying 11% to 21% of the watershed area contribute 31% to 45% of constituents, corresponding to advantage ratios of 1.5:1 for runoff control and approximately 3:1 for other constituents. Under climate change scenario B1, constituent yields were predicted to increase by factors of 1.5 to 1.8 at the watershed outlet, from an increase in annual rainfall of 25% predicted by the AOGCM, over current conditions. Under scenarios A1B and A2, constituent yields were predicted to increase by factors of 1.8 to 2.3 over current conditions, from an increase of 30% in annual rainfall. The area of runoff CSAs was predicted to more than triple with climate change, leading to negligible advantage of targeting runoff control BMPs to CSAs under future climate. The areas of sediment, nitrogen, and phosphorus CSAs were predicted to increase by factors of 2 to 3 with climate change, causing BMP-targeting advantage ratios to decrease from approximately 3:1 (baseline) to 2:1 (future). While advantage ratios for suspended and dissolved constituents remain favorable, even under future climate, the much larger area predicted to be covered by CSAs (2 to 3 times current values) suggests that stakeholder involvement and community-oriented participatory approaches will be increasingly important for achieving Chesapeake Bay TMDLs with climate change.