Linking travel times and flow pathways to stream chemistry, isotopic composition, and catchment characteristics in a snow-dominatedboreal landscape

Understanding surface waters is critical in many hydrological and biogeochemical investigations. In this study, a particle tracking model approach in Mike SHE was used to investigate the travel time, and pathway of water in 14 partly nested, long-term monitored boreal sub- 30 catchments. These sub-catchments are characterized by long and snow-rich winters with little groundwater recharge and highly dynamic hydrologyrunoff during the following spring snowmelt. The calculated annualgeometric mean of the travel timestime distribution (MTT geo ) for these sub-catchments varied from 0.8-2.7 years. The seasonality caused considerable variation in travel times between The variations were found to be related to the distribution of different seasons and landscape types, with winter mean travel times ranging and their different response to seasonal changes. Winter MTT geo ranged from 1.2-7.7 years and, while 35 spring mean travel times ranging fromMTT geo varied between 0.5-1.9 years. The modelled variation in annual and seasonal travel timesMTT geo and the fraction of young water (less than three(<3 months) was supported by extensive observations of both δ 18 O and base cation concentrations in the stream water. The age ofdifferent streams. The groundwater age was positively correlated to the abundanceareal coverage of low conductive soilssilty sediments (r=0.90, P<0.0001). As a result of lacking synchronicity and contrasting hydrological responses between different soil types (e.g., peatmires and low-conductive soilssilty sediments), mixed 40 catchments typically displayed larger differencesvariability in travel times between winter baseflow and spring flood. Mires wereseasonal MTT geo . The areal coverage of mires was found to affect be especially important for the young water fractions of the stream contribution of young water in spring (r=0.96, P<0.0001) by introducing larger differences in the mean travel times between the seasons compared to forest dominated sub-catchments. ). The main factor for this difference is likely related to thewas attributed to extensive soil frost in mires, causing considerable overland flow in spring. Theduring the snowmelt period. The present study was based on the integration of a large dataset from a previously well-investigated catchment and an advanced distributed 3D hydrological model. The results showed that the groundwater travel times vary considerably on annual and intra-annual scales in the boreal landscape, both as an effect of physical differences between different types of catchments, most notably the hydrological conductivity of the soils, and the response of different landscape units to the changing of the seasons. Yet, The silt fraction effect is especially prominent in winter when the range in MTT geo is between one and almost eight years. The seasonal MTT geo change from winter to spring is also largest for the silt dominated catchments, with, for example, six years difference for C20 compared to two years for the similar-sized till dominated sub-catchment C6. These intra-annual variations can also be linked to another landscape feature, namely the areal coverage of mires. Mires affected the young water fraction but only when new precipitation or snowmelt input into the system occurred in spring and summer. The lack of synchronicity between the response of mire and silt areas caused greater annual MTT geo variation for sub-catchments with both features. For example, the MTT geo for C4, dominated by mires, decreased from 1.5 years to 0.7 years from winter to spring. In contrast, winter MTT geo for the C20 catchment dominated by silt was 7.7 years, which decreased to 1.5 years in spring. The results also show that groundwater recharge is affected by the soil frost in mires. For example, C4 showed more variations in its seasonal MTT geo , although C2 (dominated by forest and till) and C4 (dominated by mires) had almost an equal annual MTT geo (Table 5). In spring, the MTT geo was shorter in C4 than in C2 due to surface runoff on the frozen mire, and in applicability of modelling and the hydrological conditionstravel times by combining a distributed hydrological model with, for example, long dry winters, temporary soil frost, and intensive spring floods that prevail in the boreal region. empirical observations from 14 nested sub-catchments. In the wake of a changing climate and intensified pressure from forestry and other formstypes of land use, this study provides a useful foundationbaseline for assessing the often-intricate connections and feedbacks between hydrological and biogeochemical 875 processes throughout the boreal landscape. Our results showed that water travel times could vary considerably on annual and seasonal scales between different types of catchments. This was mainly related to soil properties, with low conductivity silty sediments leading to the longest travel times on annual and seasonal timescales. In contrast, mires lead to increased fractions of young water, and hence shorter travel times, but mainly in spring when the soil was frozen. As a result of the lower groundwater recharge during the snowmelt, however, the MTTs in mires were, in turn, longer than in forests during the winter. In a warmer with reduced soil frost and decreased snowmelt input, we would expect the effect of mires to be reduced while the impact of till and silty sediment soils likely will remain relatively unaffected.