How catchment characteristics influence hydrological pathways and travel times in a boreal landscape

Abstract

Abstract. Understanding travel times and hydrological pathways of rain and snowmelt water transported through the landscape to recipient 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 pathway and its associated travel time of water in 14 partly nested, long-term monitored boreal sub-catchments of the Krycklan catchment (0.12–68 km2). This region is characterized by long and snow-rich winters with little groundwater recharge and highly dynamic runoff during spring snowmelt. The geometric mean of the annual travel time distribution (MTTgeo) for the studied sub-catchments varied from 0.8 to 2.7 years. The variations were related to the different landscape types and their varying hydrological responses during different seasons. Winter MTTgeo ranged from 1.2 to 7.7 years, while spring MTTgeo varied from 0.5 to 1.9 years. The modelled variation in annual and seasonal MTTgeo and the fraction of young water (<3 months) was supported by extensive observations of both δ18O and base cation concentrations in the different streams. The travel time of water to streams was positively correlated with the area coverage of low-conductive silty sediments (r=0.90, P<0.0001). Catchments with mixed soil–landscape settings typically displayed larger variability in seasonal MTTgeo, as contrasting hydrological responses between different soil types (e.g. peat in mires, till and silty sediments) are integrated. The areal coverage of mires was especially important for the young water contribution in spring (r=0.96, P<0.0001). The main factor for this was attributed to extensive soil frost in mires, causing considerable overland flow during the snowmelt period. However, this lower groundwater recharge during snowmelt caused mire-dominated catchments to have longer stream runoff MTTgeo than comparable forest catchments in winter. Boreal landscapes are sensitive to climate change, and our results suggest that changes in seasonality are likely to cause contrasting responses in different catchments depending on the dominating landscape type.