Carbon dioxide transport across the hillslope–riparian–stream continuum in a boreal headwater catchment

Abstract. Headwater streams export CO₂ as lateral downstream export and vertical evasion from the stream surface. CO₂ in boreal headwater streams generally originates from adjacent terrestrial areas, so determining the sources and rate of CO₂ transport along the hillslope–riparian–stream continuum could improve estimates of CO₂ export via the aquatic pathway, especially by quantifying evasion at higher temporal resolutions. Continuous measurements of dissolved CO₂ concentrations and water table were made along the hillslope–riparian–stream continuum in the Västrabäcken sub-catchment of the Krycklan catchment, Sweden. Daily water and CO₂ export from the hillslope and riparian zone were estimated over one hydrological year (October 2012–September 2013) using a flow-concentration model and compared with measured lateral downstream CO₂ export.

Total water export over the hydrological year from the hillslope was 230 mm yr⁻¹ compared with 270 mm yr⁻¹ from the riparian zone. This corresponds well (proportional to the relative upslope contributing area) to the annual catchment runoff of 265 mm yr⁻¹. Total CO₂ export from the riparian zone to the stream was 3.0 g CO₂-C m⁻² yr⁻¹. A hotspot for riparian CO₂ export was observed at 30–50 cm depth (accounting for 71 % of total riparian export). Seasonal variability was high with export peaks during the spring flood and autumn storm events. Downstream lateral CO₂ export (determined from stream water dissolved CO₂ concentrations and discharge) was 1.2 g CO₂-C m⁻² yr⁻¹. Subtracting downstream lateral export from riparian export (3.0 g CO₂-C m⁻² yr⁻¹) gives 1.8 g CO₂-C m⁻² yr⁻¹ which can be attributed to evasion losses (accounting for 60 % of export via the aquatic pathway). The results highlight the importance of terrestrial CO₂ export, especially from the riparian zone, for determining catchment aquatic CO₂ losses and the importance of the CO₂ evasion component to carbon export via the aquatic conduit.