This study was conducted in the 0.13 km2 forested Västrabäcken subcatchment of the 0.47 km2 Svartberget catchment, which is part of the wider 68 km2 Krycklan catchment (64∘14′ N,19∘46′ E), 50 km north-west of Umeå, Sweden .

The altitude of the catchment ranges from 235 to 310 ma.s.l. . Mean annual temperature (1980–2008) is 1.7 ∘C, with the maximum (14.6 ∘C) and minimum (-9.6 ∘C) in July and January, respectively. Mean annual precipitation (1981–2008) is 612 mm, with approximately 35 % falling as snow . Snow covers the ground on average for 171 days from October to May to a depth of between 43 and 113 cm (1980–2007) . The largest hydrological event during the year is a 4-6 week long snowmelt period in late April/early May which contributes between 40 and 60 % of annual runoff . The Västrabäcken is a first order, low pH (range pH 4.5–5.9) with little variability in the bicarbonate equilibrium system .

Drainage channels were deepened and widened in the 1920s to improve forest drainage . Scots pine (Pinus sylvestris) is the dominant tree species in hillslope areas with some Norway spruce (Picea abies) nearer to the stream. Understorey vegetation is predominately a mix of Vaccinium myrtillus, Vaccinium vitis-idaea and grasses (Deschampsia flexuosa), with mosses (Sphagnum spp., Polytrichum commune) and wood horsetail (Equisetum sylvaticum) in the wetter riparian areas.

The bedrock is gneiss overlain by 10–15 m of locally derived glacial till. Soils consist of well-developed iron podzols comprising a surface 5 cm humus layer overlying a 12 cm thick sandy bleached E-horizon and a 60 cm thick B-horizon . The riparian zone formed through the accumulation of organic matter in low lying areas and extends 4 m on either side of the stream channel. Riparian soils consist of ∼70 cm thick peat transitioning to the underlying till at ∼90 cm depth. Soil organic content is considerably higher in the riparian soil (>80 %) compared to the hillslope podzols (<5 %) . Saturated hydraulic conductivity is about one order of magnitude lower in the riparian zone (6.2×10-6 ms-1) than the hillslope (5.6×10-5 ms-1) decreasing with depth . Porosity and water retention were higher in the riparian soil . The riparian zone studied is representative of headwater till catchments across the larger Krycklan catchment .

Sampling was carried out in both the hillslope podzol soils (15 m perpendicular to the stream) and riparian zone peat soils (1.5 m from the stream) (Fig. ). At each location, two dipwells with a 10 cm perforated sampling window at either 30–40 or 60–70 cm depth were installed, with the dipwells separating the soil water from the surrounding soil allowing the measurement of soil water CO2 concentrations. Dipwells were constructed from 50 mm inner diameter (ID) pipe open at the bottom and sealed at the surface using rubber bungs (Saint Gobain Performance Plastics, France). To prevent damage to the dipwells over winter due to freezing, the short above ground section was covered in insulation foam. Each dipwell contained a Vaisala CARBOCAP GMP221 non-dispersive infra-red (NDIR) CO2 sensor (range 0–5 %). Prior to deployment, sensors were enclosed in a water-tight, gas-permeable membrane . At each sampling point a third dipwell, constructed from 90 mm ID pipe perforated along its entire 1 m length and open at the bottom, was used for the measurement of water table depth (Level Troll 300, In-situ, USA) and soil temperature (CS457A, Campbell Scientific, USA), with sensors suspended at 90 cm depth. All sensors were connected to a data logger (CR1000, Campbell Scientific, USA) which recorded measurements at 30 min intervals. The system ran for three months (from 1 July 2012) to allow it to become stabilised and equilibrated before the main measurement period over one hydrological year (1 October 2012–30 September 2013). Soil moisture content was recorded by time domain reflectometry (TDR) probes .

Measurements were made at one point in the hillslope and riparian zone assuming, as with all transect studies, that these points are representative of the catchment overall. A single transect approach is appropriate for this catchment due to the small area (0.13 km2) and limited variability in soils and vegetation . Furthermore, overland flow or deeper groundwater inputs to the stream have been shown to be limited in this catchment . The sampled riparian zone is consistent with the 13 riparian zones in the wider Krycklan Riparian Observatory .

Stream water dissolved CO2 concentrations (measured using the same Vaisala CO2 sensors), plus discharge were measured continuously in a heated dam house at the catchment outlet , 200 m downstream from and over the same time period as soil measurements.

CO2 sensor output was corrected for temperature and pressure using the method of but using algorithms supplied by the manufacturer specific to the GMP221 sensors. In addition to atmospheric pressure, the correction also accounted for the head of water above the sensor, related to the water table depth at the time of sampling. Corrected concentrations are given in units of ppmv with mg CO2-C L-1 used for export calculations.

The export of water and CO2 from each m2 of hillslope and riparian zone were estimated using a flow-concentration model, a similar approach to the riparian integration model concept used previously in this, and similar, hillslope–riparian systems . Our study used measured water table positions in the hillslope and riparian zone while previous studies have used a correlation between groundwater and runoff dynamics .

The model was constructed by subdividing the 90 cm deep soil profile into 5 cm horizons. The daily lateral water export from each 5 cm soil layer was estimated by combining the measured volumetric water content with lateral saturated hydraulic conductivity estimated by . Total daily water export from the full soil profile was estimated by adding together the lateral flow from all 5 cm horizons below the daily mean water table. As CO2 concentration was only measured at two depths (30–40 and 60–70 cm) these were assumed to represent the concentration above and below 45 cm depth. Daily average CO2 concentrations below the water table, in mg CO2-C L-1, were multiplied by the water export, with CO2 export expressed in units of mg CO2-C m-2 day-1. The model was run twice to estimate the export from (1) podzol hillslope soils and (2) riparian organic soils. Hillslope export was taken to represent the input of water and CO2 into the riparian zone with riparian export representing the total terrestrial water and CO2 export to the stream.

Downstream CO2 export was determined by multiplying daily mean stream water CO2 concentration with discharge at the catchment outlet. Based on the assumption that all stream water carbon is derived from terrestrial inputs, evasion was estimated by subtracting the downstream lateral CO2 export from the export of CO2 from the riparian zone to the stream. Whilst we recognise this assumption may not be completely met and we may be underestimating total inputs and therefore evasion, in stream processing in this catchment is likely to be minimal due to the low temperatures, pH and short water residence time . Our results therefore provide a useful means to consider temporal variability in a flux which is otherwise unmeasurable at high temporal resolution.

Water table was significantly higher in the riparian zone compared to the hillslope, but with similar temporal variability (Fig. ). Mean (±SD) hillslope water table during the hydrological year was -63 ± 16 cm compared with -37 ± 10 cm in the riparian zone (Table ). Volumetric soil moisture content was also higher in the riparian zone (mean ±SD of 0.73 ± 0.008 and 0.46 ± 0.004 m3 m-3 at 30 and 60 cm depths, respectively) compared to the hillslope (0.50 ± 0.01 and 0.45 ± 0.003 m3 m-3 at 30 and 60 cm depths, respectively).

Annual mean (±SD) soil temperature was 4.1 ± 3.0 and 3.9 ± 3.0 ∘C in the hillslope and riparian zone respectively with a strong seasonal trend (Fig. ). Soil temperature (measured at 70 cm depth) did not fall below freezing, with minimum temperatures in the hillslope (0.8 ∘C) and riparian (0.7 ∘C) (Table ) reached at the beginning of May (Fig. ).

Water table was above the 60 cm CO2 measurement depth for 32 and 97 % of the time and above the 30 cm depth 7 and 59 % of the time, in the hillslope and riparian zone, respectively. These values describe the relative proportion of the measurement period over which the CO2 sensors were submerged.

CO2 concentrations were on average higher in the riparian zone than the corresponding depth in the hillslope (Table ). The highest mean (±SD) CO2 concentrations were at 60–70 cm in the riparian zone (23 100 ± 4100 ppmv) with the lowest at 30–40 cm in the hillslope (4410 ± 2570 ppmv). At both depths the hillslope and riparian zone CO2 concentrations were significantly different (Table ).

There was considerable temporal variability in CO2 concentration in the hillslope, which displayed a baseline and peak pattern (Fig. ). Baseline concentrations were generally < 2000 ppmv. In the winter period (January–March), median concentrations at 60–70 cm depth (1600 ppmv) were 24 % less than those at 30–40 cm depth (2110 ppmv). During drier periods in summer (July–September), the baseline was higher with median concentrations at 60–70 cm (1770 ppmv), 61 % less than those at 30–40 cm depth (4530 ppmv). The range was also higher in summer (7640 and 6470 ppmv at 30–40 and 60–70 cm depths) than in winter (680 and 870 ppmv).

Periodically, sharp increases in hillslope CO2 concentrations were observed, corresponding to a rise in water table position (Fig. ). CO2 concentrations in the hillslope had a positive correlation with hillslope water table at 30–40 cm ((r2=0.43; P<0.001) and 60–70 cm (r2=0.65; P<0.001) depths. Over the measurement period the two spikes with the highest CO2 concentrations (14 100 and 13 160 ppmv) occurred when the water table was above the level of the deeper (60–70 cm depth) sensor at -41 and -51 cm, respectively.

CO2 concentrations in the riparian zone did not display the same baseline and peak pattern as the hillslope. At 60–70 cm depth, CO2 concentrations peaked at 31 920 ppmv in October before falling to 16 140 ppmv in April (Fig. ). This change in concentration closely followed soil temperature which decreased over a similar period from 9.2 ∘C to the annual minimum of 0.7 ∘C on 26 April 2013 (Fig. ), coinciding with minimum CO2 concentrations. CO2 concentrations had a weak positive correlation with riparian water table at 30–40 cm (r2=0.12; P<0.001) and a weak negative correlation at 60–70 cm (r2=-0.21; P<0.001) depth. At 30–40 cm depth in the riparian) depth. At 30–40 cm depth in the riparian zone there was considerably more temporal variability in CO2 concentrations than at 60–70 cm depth with concentrations at 30–40 cm depth rising during periods of higher water table. Minimum concentrations (4430 ppmv) occurred in April before the spring snowmelt event.

Annual water export from each m2 of hillslope over the hydrological year was estimated at 230 mmyr-1. Across the year, water export was consistently low with flow largely restricted to below ∼ 50 cm depth. Total monthly export was highest in May (109 mmmonth-1) accounting for 47 % of the annual hillslope water export, due to greater flow at 25–35 cm depth (37 mmmonth-1). Hillslope CO2 export over the hydrological year was 1144 mgCO2-C m-2yr-1 and followed a similar pattern to water export. CO2 export was considerably higher in May (482 mgCO2-C m-2month-1), accounting for 42 % of the annual CO2 export, largely due to considerable export (93 mgCO2-C m-2month-1) from 45–50 cm depth (Fig. ).

Annual water export from the riparian zone to the stream channel over the hydrological year was estimated at 270 mmyr-1. Below 65 cm depth, although water export was relatively low (< 3 mmmonth-1) it occurred for > 97 % of the measurement period. Water export was highest in a zone between 25 and 50 cm depth. At shallower depths (0–25 cm) flow was restricted to the wettest months. Total monthly export was highest in May (49 mmmonth-1) due to dominant flow at 25–50 cm depth with maximum monthly flow (9 mmmonth-1) at 35–40 cm depth.

Total annual CO2 export from the riparian zone to the stream channel over the hydrological year was estimated at 3008 mgCO2-C m-2yr-1. Two monthly peaks in riparian CO2 export to the stream occurred over the hydrological year, in December (529 mgCO2-C m-2month-1) and May (522 mgCO2-C m-2month-1). CO2 export was highest in a narrow zone between 25 and 50 cm depth (Fig. ). In December, water flow (5 mmmonth-1) and CO2 export (77 mgCO2-C m-2month-1) were highest from 35–40 cm depth. In May, maximum flow of both water and CO2 was from the same depth (35–40 cm) at 9 mmmonth-1 and 96 mgCO2-C m-2month-1.

Total catchment runoff over the hydrological year was 265 mmyr-1 (Fig. ). Total downstream lateral CO2 export from the catchment was estimated at 1183 mgCO2-C m-2yr-1. There was considerable temporal variability in the downstream lateral export of CO2 from the catchment, related to temporal variability in discharge (Fig. ). Median downstream lateral export was 1.2 mg CO2-C m-2 day-1 with two large spikes (57 and 35 mg CO2-C m-2 day-1) corresponding to sudden increases in discharge after storm events. Over the same period the input of CO2 from the soil to the stream was 3008 mgCO2-C m-2yr-1. Based on the assumption that all stream water CO2 is derived from soil input then by subtraction 1825 mgCO2-C m-2yr-1 is lost between the soil and the stream (i.e. evasion from the stream surface), which accounted for 60 % of export via the aquatic pathway (Fig. ).

It was hypothesised that CO2 concentrations would be higher in the riparian zone as a result of enhanced production (by decomposition of soil organic matter and root respiration) due to the higher organic matter content of riparian peat (> 80 %) compared to hillslope podzols (< 5 %) and greater mobilisation of CO2 due to the generally wetter conditions found in riparian zones . Changes in the soil water content can result in: (1) initial displacement of high CO2 concentration soil atmosphere as soils become saturated and (2) decreased vertical diffusion as soil pore space becomes saturated with water. Weathering of carbonate parent material can also contribute but carbonate bedrock is not found in this catchment . Mean hillslope concentrations (Table ) are within the range (∼ 400 to ∼ 10 000 ppmv) reported by studies conducted in similar forest podzol soils . Mean riparian concentrations at 30–40 cm depth were similar to those in a Finnish riparian zone at the same depth (14 200–16 500 ppmv) but CO2 concentrations at 60–70 cm depth were considerably higher (Table ).

There was considerable temporal variability in CO2 concentrations, especially in the hillslope, corresponding to water table fluctuations (Fig. ). The CO2 concentrations in the hillslope podzol soils over the measurement period, with peaks of 14 100 and 13 160 ppmv, both occurred during high water table conditions, as also observed by . During the winter months (January–April) when the water table was generally low, CO2 concentrations remained relatively stable at ∼ 2000 ppmv (Fig. ) suggesting that the vertical exchange of CO2 is greater than the lateral export during these periods .

Temporal variability in riparian zone CO2 concentrations was more pronounced at 30–40 cm depth which can largely be attributed to water table fluctuations (the sensor was below the water table for 59 % of the measurement period). The 60–70 cm sampling depth was below the water table for 97 % of the measurement period with water table variability alone unlikely to explain the observed pattern in riparian CO2 concentrations. CO2 concentration was linked to soil temperature, which had a strong seasonal cycle (Fig. ) suggesting root respiration and/or organic matter decomposition, driven by changes in soil temperature, provides an additional control on CO2 concentrations. During periods of generally high water tables in the riparian zone, such as during the spring snowmelt event in May, concentrations increased (Fig. ) again highlighting the importance of water table fluctuations on CO2 concentrations in soils.

Overall, the large temporal variability in porewater CO2 concentration, especially in the hillslope, can largely be attributed to water table fluctuations altering the rate of CO2 mobilisation and vertical diffusion of CO2 stored in soil pore space. This confirms the results of others who measured higher CO2 concentrations during periods of water table fluctuation . The importance of the water table for CO2 concentration dynamics suggests that pore water CO2 concentration differences between the hillslope and the riparian zone are strongly influenced by the altered diffusion gradient and surface exchange of CO2 with the atmosphere; the riparian zone had continually higher water tables than the hillslope (Fig. ). However, despite the importance of water table fluctuations, most of the existing studies involving continuous use of CO2 sensors have been focused predominately above the water table, potentially missing this observation . This highlights the importance of considering CO2 concentration changes over the full soil profile depth, especially in horizons which experience frequent fluctuations between wet and dry conditions. In this study, CO2 concentrations were only determined at two sampling depths (30–40 and 60–70 cm). In the riparian zone the shallower measurement depth was below the water table 59 % of the time giving a range of measurements under saturated and unsaturated conditions. The deeper measurement point was below the water table 97 % of the time giving concentrations under saturated conditions. These two sampling depths therefore gave a good overall representation of soil conditions, and their respective CO2 concentrations.

The total export of water per m2 of hillslope was 230 mmyr-1 and from the riparian zone 270 mmyr-1. Both estimates correspond well (proportional to the relative upslope contributing area) to the annual catchment runoff of 265 mmyr-1. The lateral movement of water along the hillslope–riparian–stream continuum is dependent on a number of assumptions, principally that lateral subsurface flow is the dominant hydrological pathway and that these flow paths are perpendicular to the stream. These assumptions have been validated for the study catchment by the observed planar nature of the water table and through stable isotope studies . Alternative flow paths, such as overland flow and groundwater recharge are not important at this site . Downslope lateral water flow occurs in saturated soils, with minimal lateral flow occurring above the water table, in accordance with the transmissivity feedback conceptualisation of subsurface flows in shallow till catchments .

Transmissivity feedback, which has been widely observed in this catchment, is defined as the increase in lateral saturated hydraulic conductivity towards the soil surface, resulting in more lateral flow as water table rises into near surface soil horizons . The drier hillslope podzol soils had a greater potential to store water without increasing water table , limiting the time in which high water table conditions occurred; mean water position in the hillslope was -63 cm. Water table only moved into higher lateral hydraulic conductivity horizons during the spring snowmelt event in May when 42 % of annual hillslope CO2 export occurred, emphasising the importance of the spring snowmelt event in this system. The results from this study show that maximum water export occurs where water table position and soil hydrological properties (lateral hydraulic conductivity and soil water content) combine to produce the set of conditions most conducive to water flow, in agreement with the transmissivity feedback principle.

The concept of a riparian chemosphere has been conceptualised by and shown to exist in the catchment studied here , with the chemistry of water flowing laterally through the catchment determined by soil interactions. The final soil encountered by subsurface waters (in this study the riparian peat soils) will determine the composition of water transported across the terrestrial–aquatic interface . Over the hydrological year, total CO2 export per m2 of hillslope was 1144 mgCO2-C m-2yr-1 with export from the riparian zone estimated at 3008 mgCO2-C m-2yr-1. The organic rich peat soils of the riparian zone therefore represent a significantly greater CO2 export source than the podzol hillslope soils, despite accounting for < 10 % of the catchment area. This can be related to the higher measured CO2 concentrations (Fig. ). The importance of riparian–stream CO2 export is in agreement with the results for other catchments and for total organic carbon in this catchment .

Few studies have estimated carbon export across the terrestrial–aquatic interface, especially for CO2. The total annual export of CO2 in this study (3.0 gCO2-C m-2yr-1) was similar, given the large inter-annual variability, to the estimate for DIC (3.2 g DIC m-2 yr-1) produced from spot measurements and the 2.3–6.9 g DIC m-2 yr-1 estimated over a longer period (1997–2009) in the same catchment . As expected, CO2 export was lower than in peat dominated catchments in Sweden (3.1–6.0 gCO2-C m-2yr-1) and Scotland (11.2–15.5 gCO2-C m-2yr-1) .

The results of this study suggest that the riparian zone, and not the wider hillslope, is the dominant source of CO2 entering the stream but the contribution of the riparian zone to water and CO2 transport has been shown to be episodic, resulting in hotspots and hot moments when export is greatest . CO2 export was highest in a narrow band between 30 and 50 cm depth, which accounted for 71 % of CO2 export. The results from this study suggest that the riparian zone contains two distinct sources of CO2 export; high export rates at 30–50 cm depth as water table moves into more superficial horizons and a deeper (> 65 cm depth) continuous but smaller export (Fig. ). The presence of these two water sources in this catchment has also been shown isotopically and indicated by the observed bimodal frequency distributions in stream water CO2 concentrations during storm events . Therefore, the riparian export rates of water and CO2 measured here can be used to explain the observed changes in stream water CO2 concentrations. This approach can be used as the catchment chosen for this study is relatively simple in terms of the water flow paths (with water transported laterally through the soil at < 1 m depth with groundwater and overland flow not significant) and the consistency of the riparian lateral extent down the stream reach. In catchments with more complex hydrology or where the riparian lateral extent is variable, riparian CO2 export alone may not account for all variability in stream water CO2 dynamics and additional sources would need to be considered.

Peaks in water and CO2 export occurred in late autumn (October–December) and May. The export of water from the riparian zone was highest during May at 49 mmmonth-1 when the spring snowmelt event occurred, accounting for 18 % of the annual water export from the riparian zone. In May, at the onset of spring snowmelt, CO2 concentrations were close to minimum values of 3840 and 14 400 ppmv at 30–40 and 60–70 cm depths in the riparian zone, resulting in relatively low CO2 export despite large water export at this time. In October–December, despite only moderate water export, CO2 export was high (275–529 mgCO2-C m-2month-1) (Fig. ). During these months, CO2 concentrations in the riparian soil were at or close to their maximum values (Fig. ), coinciding with maximum soil water DOC concentrations in late summer/autumn observed in the catchment . This observed pattern of peaks in CO2 export suggests that riparian export is a function of both season and runoff. This highlights the importance of capturing hydrological extremes when quantifying annual estimates of downstream export and evasion of CO2 across catchments and scales.

The lateral export of riparian dissolved CO2 is therefore the main source of stream water CO2, with the amount of CO2 exported related to the depth of water flow through the riparian zone. The riparian export of CO2 can therefore be used as an estimate of the catchment CO2 export to partition the export of CO2 via the aquatic conduit.

Total downstream lateral CO2 export, calculated from the CO2 concentration and discharge at the catchment outlet, over the hydrological year was 1.2 gCO2-C m-2yr-1. This is within the range 0.9–1.3 gCO2-C m-2yr-1 of estimates for the same catchment and a peatland catchment in southern Scotland . Due to the greater range in discharge (0.02–15.3 L s-1) than dissolved CO2 concentrations (daily mean CO2 concentration ranged from 1.8–7.2 mg CO2-C L-1); discharge had a greater influence on temporal variability in downstream lateral CO2 export (Fig. ).

By subtracting downstream lateral CO2 export (1183 mgCO2-C m-2yr-1) from the estimate of soil export over the same period (3008 mgCO2-C m-2yr-1), an estimated 1825 mgCO2-C m-2yr-1 was released to the stream but not accounted for by downstream lateral export. Photosynthetic uptake by aquatic plants or in stream respiration is unlikely to be important in this catchment due to the short water residence times (typically ∼ 4.5 h) and the low temperatures . Hence the remaining CO2 is likely to be evaded from the stream surface. Our evasion estimate from the stream surface (1825 mgCO2-C m-2yr-1) was at the lower end of evasion rates determined from direct point measurements in the same catchment . However, evasion accounted for 60 % of export via the aquatic pathway supporting previous findings of the rapid CO2 loss from the stream .

Estimating evasion rate based on this approach is dependent on two assumptions: (1) all stream water CO2 is derived from lateral soil inputs from the riparian zone and (2) a short water transport time between the riparian zone and the stream to allow rapid exchange of water and CO2 during events. In headwater systems, in-stream productivity (through biological respiration and photo-degradation of DOC) is limited by the cold temperatures, low pH and short water residence times . Additionally, CO2 evasion from headwater streams has been widely shown to be composed predominately of recently fixed, plant derived CO2 transported from the surrounding soil . The transport time for water between the riparian zone and the stream varied with depth from <1 h at <15 cm depth up to ∼25 h at >70 cm depth. The lateral exchange of water and CO2 between the riparian zone and the stream was rapid enough for the approach to be valid at the daily scale that was used in the model. Additionally, the total volume of water exported from the riparian zone (270 mmyr-1) corresponded well with the annual runoff from the catchment (265 mmyr-1) suggesting that no other inputs of water are contributing. Riparian CO2 export to the stream is therefore sustaining the lateral downstream export and vertical evasion of CO2.

There was considerable temporal variability in the evasion estimate (Fig. ) related to variability in CO2 dynamics in both the terrestrial (Fig. ) and aquatic systems (Fig. ). During the two largest storm events, in which discharge increased suddenly over a very short time period (< 1 day), downstream lateral export spiked but without a corresponding increase in soil export. Evasion was therefore not estimated during these periods. This also occurred during the onset of the spring flood. This suggests that during storms there may be a rapid input of overland flow or direct channel water input contributing to discharge without interacting with the soil. The model used to estimate soil CO2 export did not include overland flow so may be underestimating the total flow of water and CO2 into the stream during storm events. Overland flow has been shown to be a relatively minor flow path within the Västrabäcken catchment . However, during the spring flood, up to 20 % of stream runoff was found to be derived from snowmelt transported via overland flow . Thus during the spring snowmelt period, water and CO2 stored within the snowpack may be an additional export source, bypassing the soil profile. In two Finnish catchments, CO2 concentrations in the snow pack of 500–1900 ppmv were recorded with snowmelt estimated to contribute 35–46 % of downstream lateral CO2 export during the spring snowmelt period . Over the course of the spring snowmelt event in the same Finnish catchments the isotopic signature of evaded CO2 showed a decreasing contribution from recently fixed, plant derived CO2 from near surface soil horizons with a corresponding increase in the atmospheric CO2 component, likely derived from the melting snow pack . In some headwater catchments, the effects of melt water and overland flow would need to be accounted for in annual estimates of riparian CO2 export.

Although the annual estimate of riparian export and evasion in the study compare well to estimates from discrete measurements , the results of this study highlight the importance of high frequency, direct measurements of CO2 concentrations given the high temporal variability in CO2 dynamics in the terrestrial and aquatic systems. Terrestrial processes have been shown to have an important role in determining CO2 export via the aquatic pathway in a wide range of catchments . The results from this catchment indicate that terrestrial–aquatic export of CO2 was controlled by riparian water table dynamics, highlighting the potential importance of riparian zones in headwater catchments. Changes in climate, especially greater variability in precipitation patterns , have the potential to alter riparian water table dynamics and since carbon export via the aquatic pathway has been shown to be positively correlated with precipitation , impact on the export of CO2 to streams and the NECB of boreal headwater catchments.