Using SWOT to revisit global river gas exchange

Air-water gas exchange influences aquatic ecosystem processes ranging from photosynthesis and respiration to greenhouse gas emissions. The speed of this exchange, termed the gas exchange rate, is a key parameter for carbon transport through the world’s river basins. Because of this, global maps of the gas exchange rate are often constructed from hydrographic and hydraulic datasets as one step towards constraining global river carbon budgets. However, existing maps rely on DEM-derived slopes that are not necessarily indicative of water surface conditions and are subject to other operational uncertainties. And more mechanistically, existing models assume that only bed-shear-induced near-surface turbulence drives river gas exchange, neglecting other potentially influential physical processes like wind-shear-induced turbulence (especially in wide rivers). Decades of theory and experimental work in lakes, estuaries, and the open ocean show that air-water gas exchange is a variable function of turbulence on both sides of the interface, but similar studies in rivers are limited to a few sites. To address these issues, we integrate (1) previous theoretical work on wide-river gas exchange, (2) global SWOT measurements of water surface slope, and (3) a downscaled wind model to calculate near-surface turbulent dissipation rates for global rivers. We then produce a first-order characterization of global river gas exchange rates and explore the impact of both wind-shear-induced turbulence and direct water surface observations from SWOT. We find that wind can play a significant role in unsheltered, wide, and flat rivers that are often (though not always) located near the mouths of the world’s river systems. Our results suggest that river gas exchange maps should leverage direct water surface measurements and account for air-side processes to better constrain global river carbon emissions, and we provide the first steps towards an empirical framework to do this.