Local processes with a global impact: unraveling the dynamics of gas evasion in a step-and-pool configuration

Headwater streams are important sources of greenhouse gases to the atmosphere. The magnitude of gas emissions originating from such streams, however, is modulated by the characteristic microtopography of the riverbed, which might promote the spatial heterogeneity of turbulence and air entrainment. In particular, recent studies have revealed that step-and-pool configurations, usually found in close sequences along mountain streams, are important hotspots of gas evasion. Yet, the mechanisms that drive gas transfer at the water-air interface in a step-and-pool configuration are not fully understood. Here, we numerically simulated the hydrodynamics of an artificial step-and-pool configuration to evaluate the contribution of turbulence and air entrainment to the total gas evasion induced by the falling jet. The simulation was validated using observed hydraulic features (stage, velocity) and was then utilized to determine the patterns of energy dissipation, turbulence-induced gas exchange and bubble-mediated transport. The results show that gas evasion is led by bubble entrainment and is mostly concentrated in a small and irregular region of a few square decimeters near the cascade, where the local gas transfer velocity (k) peaks at 500 md-1. The enhanced spatial heterogeneity of k in the pool does not allow one to define a priori the region of the domain where the outgassing takes place and makes the value of the spatial mean of k inevitably scale-dependent. Accordingly, we propose that the average mass transfer velocity should be used with caution to describe the outgassing in spatially heterogeneous flow fields, such as those encountered in step-and-pool rivers.