Integrating daily CO2 concentrations in Topo-SWAT to examine climate change impacts in a karst watershed

Authors: Gunn, Kpoti; Buda, Anthony; GALL, HEATHER - Pennsylvania State University; CIBIN, RAJ - Pennsylvania State University; Kennedy, Casey; Veith, Tameria - Tamie

Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/4/2021
Publication Date: 7/1/2021
Citation: Gunn, K.M., Buda, A.R., Gall, H.E., Cibin, R., Kennedy, C.D., Veith, T.L. 2021. Integrating daily CO2 concentrations in Topo-SWAT to examine climate change impacts in a karst watershed. Transactions of the ASABE. 1-58. https://doi.org/10.13031/trans.13711.
DOI: https://doi.org/10.13031/trans.13711

Interpretive Summary: Plants often respond to elevated carbon dioxide levels by reducing the amount of soil water they transfer to the atmosphere by way of transpiration. As atmospheric carbon dioxide levels continue to rise with human activities, it essential that carbon dioxide be adequately represented in watershed models that are used to study the effects of climate change on hydrology. In this study, we predicted how the water balance of a mixed land use watershed might evolve with climate change using one model that ignored the effects of carbon dioxide and two models that accounted for carbon dioxide and variable plant functions. Results of the modeling experiments suggested that adding carbon dioxide to watershed models significantly reduced plant transpiration and increased runoff relative to models that discounted the effects of carbon dioxide. Findings from the study demonstrate the importance of integrating carbon dioxide effects in future modeling studies of climate change in mixed land use watersheds.

Technical Abstract: Characterizing the effects of CO2-induced climate change on hydrology is important to watershed management. In this study, we used Topo-SWAT to examine the effects of climate change and increasing CO2 on the water balance of Spring Creek, 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 Topo-SWAT’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 unmodified Topo-SWAT against two modified versions of Topo-SWAT. One Topo-SWAT version integrated daily CO2 levels (Topo-SWAT_CO2), while another version added flexible stomatal conductance and LAI routines (Topo-SWAT_CO2+P) to the dynamic CO2 capacity. Under current climate (1985–2015), the three Topo-SWAT 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 unmodified Topo-SWAT and the two modified Topo-SWAT models with CO2. Notably, variable stomatal conductance and leaf area index (LAI) routines produced no detectable effects beyond that of CO2. For the 2020–2065 period, unmodified Topo-SWAT projected ET increases of 0.7 mm yr-1, while Topo-SWAT models with CO2 suggested annual ET could decline by approximately -0.4 mm yr-1 over the same time frame. As a result, the two CO2-based Topo-SWAT models predicted streamflow increases of almost 1.6 mm yr-1 over the 2020–2065 period, which were roughly double the streamflow increases projected by unmodified Topo-SWAT. In general, Topo-SWAT models with CO2 effects produced 22.4% more streamflow in 2045–2065 than the Topo-SWAT model without CO2. Results also showed that adding daily CO2 to Topo-SWAT reduced ET in wetter parts of the Spring Creek watershed, leading to much greater runoff losses from variable source areas compared to unmodified Topo-SWAT. Findings from the study clearly highlight the importance of considering increasing atmospheric CO2 concentrations in water balance simulations with Topo-SWAT in order to gain a fuller appreciation of the hydrologic uncertainties with climate change