Flow and Turbulence driven Water Surface Roughness and Gas Exchange Velocity in Streams

Abstract

Gas exchange velocity in streams and rivers controls fluxes of atmospheric gases across the air-water interface and is commonly related to the turbulence at the water side. Similarly, river flow hydraulics influences the water surface roughness, which is frequently used (in terms of surface flow types) for eco- and morphological mapping of spatial variations of hydraulic conditions. We investigated the relationships between gas exchange velocity, water surface roughness and flow hydraulics for different surface flow types in a low-mountain stream. We used the flux chamber-method to estimate exchange velocity, a freely floating sphere (equipped with acceleration sensors) to measure water surface roughness, as well as a field-particle image velocimetry system for flow and turbulence measurements. The results demonstrate that the gas exchange velocity in smooth and rippled flows followed the same universal dependence on turbulent dissipation rates (with an empirical scaling coefficient at the upper limit) as observed in wind-driven systems. More rough flows were anisotropic and gas exchange velocity was stronger related to vertical components of turbulence parameters. We further explored the potential of using surface flow type evaluations and water surface roughness measurements for estimating gas exchange velocities at the reach scale and beyond.