From precipitation to stream

Understanding the journey water makes from precipitation entering a catchment, traveling through soils, and the time it takes before it exits as stream water are questions of great relevance for both scientists and environmental managers. Natural stable isotopes such as δ¹⁸O and δ²H have been extensively used over the last decades to trace water through diverse catchments across the world. In this thesis I analyzed over 2500 isotope samples to create long-term time series of precipitation and stream water data, as well as studying spatial and temporal variability of flow pathways in the Krycklan catchment in Northern Sweden. Based on these isotope samples, I observed that streams draining forested catchments were fed by soil water from different horizons throughout the year. In contrast, stream water from mire dominated catchments was linked primarily to one hydrological active layer with the exception of the winter season when both catchment types showed influence of old/deep groundwater. ²³⁴U/²³⁸U isotope ratios further enhanced the mechanistic understanding of old groundwater where δ¹⁸O signature could not be used to disentangle sources. During a winter baseflow survey I found a the contribution of old groundwater to stream water among 78 sub-catchments, which increased with area ranging from ~20 % contribution in the smaller headwater sub-catchments up to 70-80 % for catchments with areas 10.6 km² or larger. Additionally, I found that the spatial variability of old groundwater contribution to catchments below ~10.6 km² was influenced by differences in structural properties across sub-catchments. Furthermore, dissolved organic carbon (DOC) was negatively correlated with old groundwater contribution, while base cations and pH were positively correlated. Finally, annual water transit time in the snow-dominated boreal catchment with the most complete isotopic record ranged between 300 and 1400 days and was negatively related to annual rain input. This relationship may have implications for our understanding of future hydrological and biogeochemical processes in boreal regions, given that warmer winters are forecasted, which would translate to larger proportions of precipitation falling as rain. Overall, this thesis has taken us one step further in the search for mechanistic understanding of hydrological flow paths and transit times in small to meso-scale boreal catchments