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

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.