Nitrate loading projection is sensitive to freeze-thaw cycle representation

Qianfeng Wang; Junyu Qi; Jia Li; Jefferson Cole; Stephanie T Waldhoff; Xuesong Zhang

doi:10.1016/j.watres.2020.116355

Nitrate loading projection is sensitive to freeze-thaw cycle representation

Water Res. 2020 Nov 1:186:116355. doi: 10.1016/j.watres.2020.116355. Epub 2020 Aug 29.

Authors

Qianfeng Wang¹, Junyu Qi², Jia Li³, Jefferson Cole³, Stephanie T Waldhoff¹, Xuesong Zhang⁴

Affiliations

¹ Joint Global Change Research Institute, Pacific Northwest National Laboratory and University of Maryland, College Park, MD 20740, USA.
² Earth System Science Interdisciplinary Center, University of Maryland, College Park, 5825 University Research Ct, College Park, MD, 20740, USA. Electronic address: junyuqi@umd.edu.
³ U.S. Environmental Protection Agency, 1200 Pennsylvania Avenue, NW (6207 A), Washington, DC 20460, USA.
⁴ Earth System Science Interdisciplinary Center, University of Maryland, College Park, 5825 University Research Ct, College Park, MD, 20740, USA. Electronic address: xzhang14@umd.edu.

Abstract

Climate change can have substantial impacts on nitrogen runoff, which is a major cause of eutrophication, harmful algal blooms, and hypoxia in freshwaters and coastal regions. We examined responses of nitrate loading to climate change in the Upper Mississippi River Basin (UMRB) with an enhanced Soil and Water Assessment Tool with physically based Freeze-Thaw cycle representation (SWAT-FT), as compared with the original SWAT model that employs an empirical equation. Driven by future climate projections from five General Circulation Models (GCMs) from 1960 to 2099 under the Representative Concentrations Pathways (RCP) 8.5 scenario, we analyzed changes in riverine nitrate loadings, as well as terrestrial surface and subsurface contributions of the UMRB in the 21st century relative to the baseline period of 1960-1999. By the end of the 21st century, the original SWAT model predicted about a 50% increase in riverine nitrate loadings which is nearly twice as much as that estimated by SWAT-FT (ca. 25%). Such a large difference in projected nitrate changes can potentially mislead mitigation strategies that aim to reduce nitrogen runoff from the UMRB. Further analysis shows that the difference between the original SWAT model and SWAT-FT led to substantial discrepancies in the spatial distribution of surface and subsurface nitrate loadings in the UMRB. In general, SWAT-FT predicted more nitrate leaching for northwestern parts of the UMRB which are more sensitive to freeze-thaw cycle, mainly because SWAT-FT simulated less frequent frozen soils. This study highlights the importance of using physically based freeze-thaw cycle representation in water quality modeling. Design of future nitrogen runoff reduction strategies should include careful assessment of effects that land management has on the freeze-thaw cycles to provide reliable projection of water quality under climate change.

Keywords: Climate change; Freeze-thaw cycle; Nitrate leaching; Nitrate runoff; Water quality.

MeSH terms

Mississippi
Models, Theoretical*
Nitrates / analysis
Rivers*
Water Quality

Substances

Nitrates

Grants and funding

EPA999999/ImEPA/Intramural EPA/United States