Abstract. A large portion of terrestrially-derived carbon outgasses as carbon dioxide (CO2) from streams and rivers to the atmosphere. Particularly, the amount of CO2 outgassing from small headwater streams was indicated as highly uncertain. Conservative estimates suggest that they contribute 36 % (i.e., 0.93 petagrams C yr−1) of total CO2 outgassing from all rivers and streams worldwide. In this study, stream pCO2, dissolved inorganic carbon (DIC) and δ13CDIC data were used to determine CO2 outgassing from an acidic headwater stream in the Uhlirska catchment (Czech Republic). This stream drains a catchment with silicate bedrock. The applied stable isotope model is based on the principle, that the 13C / 12C ratio of its sources and the intensity of CO2 outgassing control the isotope ratio of DIC in stream water. It avoids the use of the gas transfer velocity parameter (k) that is highly variable and mostly difficult to constrain. Model results indicate that CO2 outgassing contributed 80 % to the annual stream inorganic carbon loss in the Uhlirska catchment. This translated to a CO2 outgassing rate from the stream of 5.2 t C yr−1 and to 2.9 g C m−2 yr−1, when normalised to the catchment area. Large temporal variations with maximum values during spring snowmelt and summer emphasise the need for investigations at higher temporal resolution. We improved the model uncertainty by incorporating groundwater data to better constrain the isotope compositions of initial DIC. Due to the large global abundance of acidic, humic-rich headwaters, we underline the importance of this integral approach for global applications.
Groundwater data improve modelling of headwater stream CO<sub>2</sub> outgassing with a stable DIC isotope approach
