Quantifying the impact of climate change and land use change on surface-subsurface nutrient dynamics in a Chesapeake Bay watershed system

Authors: TULADHAR, AVALOKITA - Colorado State University; BAILEY, RYAN - Colorado State University; ABBAS, SALAM - Colorado State University; SHANMUGAM, MOHANA - Asian Institute Of Technology; Arnold, Jeffrey; White, Michael

Submitted to: Journal of Environmental Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/19/2025
Publication Date: 4/1/2025
Citation: Tuladhar, A., Bailey, R.T., Abbas, S.A., Shanmugam, M.S., Arnold, J.G., White, M.J. 2025. Quantifying the impact of climate change and land use change on surface-subsurface nutrient dynamics in a Chesapeake Bay watershed system. Journal of Environmental Management. https://doi.org/10.1016/j.jenvman.2025.125101.
DOI: https://doi.org/10.1016/j.jenvman.2025.125101

Interpretive Summary: In this study, a new combination of hydrologic/water quality models was applied to the Nanticoke River Watershed, which eventually drains into the Chesapeake Bay. The primary concern is excessive nitrate, which can lead to harmful environmental effects like eutrophication. The team used the Soil and Watter Assessment Tool (SWAT) and a new groundwater model (gflow) module to simulate and predict the impact of climate change and land use on nitrate levels. Under future climate change the models predict an 18-34% decrease in the river's annual average streamflow and a 4-22% decrease in annual average nitrate loading. These decreases are linked to higher temperatures causing more evaporation and less plant growth. Reduced plant growth means less nitrate uptake, potentially leading to increased nitrate concentration in the soil and groundwater. With urbanization or agriculture expansion, the models predicted these factors to be less significant compared to climate change. The research helps in formulating better strategies for nutrient management in watersheds and suggests that SWAT+ with the gflow module is an effective combination for this purpose.

Technical Abstract: Nutrients such as nitrogen can be harmful to aquatic organisms in excessive amounts. Climate change, through possible increases in temperature and variable rainfall, may cause changes in nutrient loading patterns from watersheds. This study assesses the potential impact of climate and land use change on nitrate (NO3) loading in the Nanticoke River Watershed (NRW), Chesapeake Bay region, USA, using an updated version of SWAT+ watershed model that simulates groundwater nitrate fate and transport in a process based spatially distributed manner. The nutrient loadings from the NRW eventually drain into the Chesapeake Bay, exacerbating eutrophication. The model was simulated for the 2000-2015 timeframe, and tested against measured streamflow, in-stream nitrate loadings, and groundwater head at various stream gages and monitoring wells. Once tested, the model was used to simulate changes in hydrological and nitrate fluxes under two future climates, according to Representative Concentration Pathways (RCP) 4.5 and 8.5, and land use changes as projected by USGS’s FORE-SCE model. The model predicts that in the future climate change could be responsible for an 18-34% and 22-33% decrease in annual average streamflow and a 4-22% and 4-11% decrease in annual average nitrate loading as projected under RCP 4.5 and RCP 8.5 scenarios for future timelines (near 2024-2048, mid 2049-2073 and far future 2074-2099), respectively. The overall decrease in future streamflow is due to higher temperatures resulting in higher evapotranspiration during summer months, offsetting the additional precipitation. The decrease in nitrate loading in the channel is influenced by reduced runoff, and elevated nitrate concentration in the soil, leading to increased leaching to groundwater. This surge in soil nitrate concentration results from reduced plant uptake of nitrate due to decreased plant growth/lower crop yields. The stunted plant growth is due to reduced mineralization of nitrogen in the soil which, in turn, is linked to decreasing soil water content and water stress from higher surface temperatures. As compared to the influence of climate, land use change resulted in a minor decrease in future nitrate loading. These results and insights can be used in future nutrient management for similar landscapes. In addition, we show that the updated SWAT+ model can be a useful tool in quantifying and investigating nitrate fate and transport dynamics in coupled surface-soil-aquifer-channel systems, particularly for systems with a strong hydraulic connection between the unconfined aquifer and channel network.