Integrating daily CO₂ concentrations in SWAT-VSA to examine climate change impacts on hydrology in a karst watershed

Characterizing the effects of climate change on hydrology is important to watershed management. In this study, we used SWAT-VSA to examine the effects of climate change and increasing atmospheric CO₂ (CO₂) on the water balance of Spring Creek watershed, a mixed land-use karst basin in the Upper Chesapeake Bay watershed. First, we modified the stomatal conductance and leaf area index (LAI) routines of SWAT-VSA’s Penman-Monteith evapotranspiration (ET) procedure and enabled the model to accept daily CO₂ data. Using downscaled climate projections from nine global climate models (GCMs), we then compared water balance estimations from baseline SWAT-VSA against two modified versions of SWAT-VSA. One SWAT-VSA version integrated daily CO₂ levels (SWAT-VSA_CO2), while another version added flexible stomatal conductance and LAI routines (SWAT-VSA_CO2+Plant) to the dynamic CO₂ capacity. Under current climate (1985-2015), the three SWAT-VSA models produced generally similar water balance estimations, with 51% of precipitation lost to ET and the remainder converted to runoff (10%), lateral flow (9%), and percolate (30%). For future climate (2020-2065), water balance simulations diverged between baseline SWAT-VSA and the two modified SWAT-VSA models with CO₂. Notably, variable stomatal conductance and LAI routines produced no detectable effects beyond that of CO₂. For the 2020-2065 period, baseline SWAT-VSA projected ET increases of 0.7 mm year^-1, while SWAT-VSA models with CO₂ suggested that annual ET could decline by approximately -0.4 mm year^-1 over the same period. As a result, the two CO₂-based SWAT-VSA models predicted streamflow increases of almost 1.6 mm year^-1 over the 2020-2065 period, which were roughly double the streamflow increases projected by baseline SWAT-VSA. In general, SWAT-VSA models with CO₂ effects produced 22.4% more streamflow in 2045-2065 than the SWAT-VSA model without CO₂. Results also showed that adding daily CO₂ to SWAT-VSA reduced ET in wetter parts of Spring Creek watershed, leading to greater runoff losses from variable source areas compared to baseline SWAT-VSA. Findings from the study highlight the importance of considering increasing atmospheric CO₂ concentrations in water balance simulations with SWAT-VSA in order to gain a fuller appreciation of the hydrologic uncertainties with climate change.