Toward Modeling Continental‐Scale Inland Water Carbon Dioxide Emissions

Abstract

Inland waters emit significant amounts of carbon dioxide (CO ₂) to the atmosphere; however, the global magnitude and source distribution of inland water CO ₂ emissions remain uncertain. These fluxes have previously been “statistically upscaled” by independently estimating dissolved CO ₂ concentrations and gas exchange velocities to calculate fluxes. This scaling, while robust and defensible, has known limitations in representing carbon source limitations and spatial variability. Here, we develop and calibrate a CO ₂ transport model for the continental United States, simulating carbon transport and transformation in >22 million hydraulically connected rivers, lakes, and reservoirs. We estimate 25% lower CO ₂ fluxes compared to upscaling estimates forced by the same observational calibration data. While precise CO ₂ source distribution estimates are limited by the resolution of model parameterizations, our model suggests that stream corridor CO ₂ production dominates over groundwater inputs at the continental scale. Our results further suggest that the lack of observational networks for groundwater CO ₂ and scalable metabolic models of aquatic CO ₂ production remain the most salient barriers to further coupling of our model with other Earth system components.

Plain Language Summary

Inland water CO ₂ emissions are recognized as an important but highly uncertain component of the global carbon cycle. Estimates rely on methods that statistically upscale point observations that are unable to account for the distribution and limits of CO ₂ sources. Here we present a first step toward distributed process‐based models that link CO ₂ fluxes to water transport in connected rivers, lakes, and reservoirs at the continental scale. We show that using the same data constraints, incorporating water transport results in a 25% reduction relative to previous methods in estimated inland water CO ₂ fluxes over the continental United States. We identify barriers to monitoring and prediction that will enable the incorporation of inland water carbon into earth system models and global budgets.

Key Points

• We develop and calibrate a river network carbon dioxide transport model for the continental United States to estimate emission fluxes

• Compared to previous methods, this model simulates 25% lower carbon dioxide emissions using the same data constraints

• Stream corridor respiration dominates over groundwater sources, but better source constraints are needed for accurate forward predictions