Assessing the impacts of future climate conditions on the effectiveness of winter cover crops in reducing nitrate loads into the Chesapeake Bay Watersheds using SWAT model

Authors: LEE, SANGCHUL - University Of Maryland; Sadeghi, Ali; YEO, IN-YOUNG - School Of National Engineering Techniques Of Agricultural And Food Industries (ENITIAA); McCarty, Gregory; Hively, Wells - Dean

Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/27/2017
Publication Date: 10/27/2017
Citation: Lee, S., Sadeghi, A.M., Yeo, I., McCarty, G.W., Hively, W.D. 2017. Assessing the impacts of future climate conditions on the effectiveness of winter cover crops in reducing nitrate loads into the Chesapeake Bay Watersheds using SWAT model. Transactions of the ASABE. 60(6):1939-1955. https://doi.org/10.1016/j.rse.2017.12.012.
DOI: https://doi.org/10.1016/j.rse.2017.12.012

Interpretive Summary: Winter cover crops (WCCs) are used as a best management practice (BMP) for their effectiveness in reducing nitrate loads from agricultural lands into streams in the Chesapeake Bay watershed (CBW). There is, however, a concern that WCCs may not be as effective in the future, considering the harsher and uncertain future climate conditions (FCCs) that could potentially exacerbate water quality degradation in the CBW region. The goal of this study was to assess the impacts of FCCs on WCCs’ nitrate reduction efficiency in the Choptank Watershed, one of the larger watersheds in the CBW, using the Soil and Water Assessment Tool (or SWAT) model. For this assessment, three FCC scenarios that considered three levels of potential increase in CO2 emissions along with six WCC scenarios, based on their species and planting dates, were selected. Overall, we found from our model simulations that, as CO2 concentrations become higher and with warmer winter temperatures, the WCCs produce greater biomass that also result in higher nitrate reduction efficiency. Specifically, results showed a WCC biomass increase of, on average, nearly 58 % under FCC scenarios that can be attributed to the climate conditions conducive to the WCC growth and substantial nitrate loads reduction of nearly 22 %. Findings from this study suggest that: i) WCCs implementation are an effective BMP that can reasonably respond to the expected increased in nitrate loads, caused potentially by the hypotactic FCC scenarios; and 2) greater emphasis is needed on the role of WCCs, not only to mitigate nitrate loads, but also to diversify agricultural practices in the face of uncertain FCCs.

Technical Abstract: Winter cover crops (WCCs) have been widely implemented in the Coastal Plain of the Chesapeake Bay watershed (CBW) due to their high effectiveness at reducing nitrate loads. However, future climate conditions (FCCs) are expected to exacerbate water quality degradation in the CBW by increasing nitrate loads from agriculture. Thus, the question remains whether WCCs are sufficient to mitigate increased nutrient loads caused by FCCs. In this study, we assessed the impacts of FCCs on WCC nitrate reduction efficiency on the Coastal Plain of the CBW using Soil and Water Assessment Tool (SWAT) model. Three FCC scenarios (2085 – 2098) were prepared using General Circulation Models (GCMs), considering three Intergovernmnental Panel on Climate Change (IPCC) Special Report on Emissions Scenarios (SRES) greenhouse emission scenarios. We also developed six representative WCC implementation scenarios based on the most commonly used planting dates and species of WCCs in this region. Simulation results showed that WCC biomass increased by ~ 58 % under FCC scenarios, due to climate conditions conducive to the WCC growth. Prior to implementing WCCs, annual nitrate loads increased by ~ 43 % under FCC scenarios compared to the baseline scenario (2001 – 2014). When WCCs were planted, nitrate loads were substantially reduced and WCCs indicated ~ 5 % higher nitrate reduction efficiency under FCC scenarios, due to increased WCC biomass. The increase rate of WCC nitrate reduction efficiency varied by FCC scenarios and WCC planting methods. As CO2 concentration was higher and winters were warmer under FCC scenarios, WCCs had greater biomass and therefore showed higher nitrate reduction efficiency. In response to FCC scenarios, the performance of less effective WCC practices (e.g., barley, wheat, and late planting) under the baseline indicated ~ 14 % higher increase rate of nitrate reduction efficiency compared to ones with better effectiveness under the baseline (e.g., rye and early planting), due to warmer temperatures. According to simulation results, WCCs were effective to mitigate nitrate loads accelerated by FCCs and therefore the role of WCCs in mitigating nitrate loads is even more important in the given FCCs.