Dynamics and controls of inland water CH4 emissions across the Conterminous United States: 1860-2019

Abstract

Inland waters (rivers, lakes, and reservoirs) have been recognized as hotspots of methane (CH₄) emissions. However, the magnitude and spatiotemporal pattern of CH₄ emissions and their underlying mechanisms remain largely unknown due to a lack of process-based quantification of CH₄ production, consumption, and evasion within the aquatic ecosystem. Here we developed a process-based aquatic CH₄ module within the framework of the Dynamic Land Ecosystem Model (DLEM) to explicitly simulate inland water carbon fluxes and the associated CH₄ processes. We further applied this model to assess the inland-water CH₄ emissions across the conterminous United States (CONUS) as affected by the climate variability, land use, fertilizer nitrogen (N) application, atmospheric N deposition, and rising atmospheric CO₂ concentration during 1860-2019. The inland water CH₄ emissions across the CONUS had doubled from the 1860s (1.65±0.18 Tg CH₄-C∙yr^-1) to the 2010s (3.73±0.36 Tg CH₄-C∙yr^-1). In the 2000s, inland water accounts for 8% of the regional CH₄ budget that offsets 11∼14% of the terrestrial C uptake across the CONUS. Our study showed that the small headwater streams (1^st -3^rd order) account for 49% of the diffusive CH₄, and reservoirs constitute 50% of the ebullitive CH₄ emissions during the 2010s. Climate change and variability played a dominant role in the increased CH₄ emissions from rivers and lakes. This study implies that effective mitigation strategies to reduce CH4 emissions should pay much attention to global climate change and headwater stream management.

Keywords: CH(4) emissions; Climate change; Dynamic Land Ecosystem Model (DLEM); Inland water; The United States.