Influence of Vertical Hydrologic Exchange Flow, Channel Flow, and Biogeochemical Kinetics on CH₄ Emissions From Rivers

CH₄ emissions from inland water are highly uncertain in the current global CH₄ budget, especially for rivers and streams due to sparse measurements and the uncertainty of measurements caused by turbulent water flow. A previous study has revealed that vertical hydrologic exchange flow (VHEF) is the main regulator of CH₄ emissions from riverbed sediments. However, to what extent the understanding obtained from the plot-scale can be extended to the reach scale and basin scale remains unknown. To address this challenge, we developed a process-based model to estimate CH₄ flux at the air-water interface using the attributes available in the national hydrography data set. It calculates the annual mean flux of VHEF, CH₄ production in sediments, and CH₄ transport in the river channel in a sequential manner. Model performance is evaluated by CH₄ efflux observed at the Hanford reach of the Columbia River. We show that reach-wise sediment hydrologic and biogeochemical conditions estimated from the national hydrography data set could serve as a good indicator of CH₄ emissions from rivers. Aerobic methane oxidation and export to the downstream are the dominant ways of total CH₄ loss for the large lowland river. The hotspots of CH₄ emissions are likely to be at the reaches with fine sediments and slow channel velocity. This study demonstrates the possibility of quantifying CH₄ emissions at the reach scale and the modeling framework has the potential to be extended to the basin scale to improve estimates of CH₄ emissions from lotic inland water.