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.

Automated summary

Uncertainty in CH4 emissions from inland water, especially rivers and streams, is a challenge due to sparse measurements and measurement uncertainty caused by turbulent water flow. This study developed a process-based model using national hydrography dataset to estimate CH4 flux at the air-water interface. The model showed that sediment hydrologic and biogeochemical conditions estimated from the national hydrography dataset can serve as indicators of CH4 emissions from rivers.