The role of stream heterogeneity in gas emissions from headwater streams

Headwater streams as hotspots of carbon dioxide evasion from surface water, and therefore represent a key component of the global carbon cycle. The gas transfer velocity at the water-air interface, k, modulates gas emissions from rivers and streams and is physically related to the energy dissipated by the flow field, ε. Here, we developed mathematical tools for quantifying the fraction of carbon emissions that can be related to localized height drops in the riverbed (e.g. in steps or step-pool formations, which constitute localized energy losses). Direct measures of stream CO₂ outgassing in an Italian headwater catchment and numerical simulations are also part of the study. Our results show that high energy heterogeneous streams are characterized by significantly higher gas transfer velocities than that of an homogeneous stream. The empirical data also suggests the presence of a pronounced heterogeneity of outgassing along a river network. In particular, in many settings the total gas evasion may be dominated by localized gas emissions in correspondence of hydraulic discontinuities. These results offer a clue for the interpretation of empirical data about stream outgassing in heterogeneous reaches, and provides insight into the development of more advanced models for the large-scale estimation of CO₂ outgassing from mountain rivers.