Hydraulics are a first‐order control on CO 2 efflux from fluvial systems

Evasion of carbon dioxide (CO 2 ) from fluvial systems is now recognized as a significant component of the global carbon cycle. However, the magnitude of, and controls on, this flux remains uncertain, and improved understanding of both is required to refine global estimates of fluvial CO 2 efflux. CO 2 efflux data show no pattern with latitude suggesting that catchment biological productivity is not a primary control and that an alternative explanation for intersite variability is required. It has been suggested that increased flow velocity and turbulence enhance CO 2 efflux, but this is not confirmed. Here using contemporaneous measurements of efflux (range: 0.07–107 µmol CO 2  m −2  s −1 ), flow hydraulics (mean velocity range: 0.03–1.39 m s −1 ), and p CO 2 (range: 174–10712 µatm) at six sites, we find that flow intensity is a primary control on efflux across two climatically different locations (where pH is not a limiting factor) and that the relationship is refined by incorporating the partial pressure of CO 2 ( p CO 2 ) of the water. A remaining challenge is how to upscale from point to reach or river basin level. Remote imaging or river surface may be worth exploring if subjectivity in interpreting surface state can be overcome.