Improving the understanding of how stream flow dynamics are influenced by landscape characteristics, such as soils, vegetation and terrain, is a central endeavor of catchment hydrology. Here we investigate how spatial variability in stream flow is related to landscape characteristics using specific discharge time series from 14 partly nested subcatchments in the Krycklan basin (0.12 – 68 km2). Multivariate principal component analyses combined with univariate analyses showed that while variability in landscape characteristics and specific discharge were strongly related, the spatial patterns varied with season and wetness conditions. During spring snowmelt and at the annual scale, specific discharge was positively related to the sum of wetland and lake area. During summer, when flows are lowest, specific discharge was negatively related to catchment tree volume, but positively related to deeper sediment deposits and catchment area. The results indicate how more densely forested areas on till soils become relatively drier during summer months, while wet areas and deeper sediment soils maintain a higher summer base flow. Annual and seasonal differences in specific discharge can therefore be explained to a large extent by expected variability in evapotranspiration fluxes and snow accumulation. These analyses provide an organizing principle for how specific discharge varies spatially across the boreal landscape, and how this variation is manifested for different wetness conditions, seasons and time scales.

In this paper we combined a multimodel ensemble based on 15 regional climate models with a multicatchment approach to explore the hydrologic sensitivity of 14 neighboring and rather similar catchments to changing climate conditions. Current and future (2062-2090) streamflow was simulated with the HBV model. A diagnostic approach was used, which considered major behavioral catchment functions by using hydrologically relevant signatures related to overall water balance, flow duration curves and hydrograph attributes. Projected increases in temperature and precipitation resulted in increased total available streamflow, with lower spring and summer flows, but substantially higher winter streamflow. Furthermore, significant changes in flow durations with lower chances of both high and low flows can be expected in boreal Sweden in the future. This overall trend in projected streamflow pattern changes was comparable among the analyzed catchments but the magnitude of change differed considerably. This suggests that catchments belonging to the same region can show distinctly different degrees of hydrological responses to the same external climate change signal. We reason that differences in spatially distributed physical catchment properties within catchments are not only of great importance for current streamflow behavior, but also play a major role in the sensitivity of catchments to changing climate conditions.

Streams and rivers emit large quantities of carbon dioxide (CO2) to the atmosphere. The sources of this CO2 are in‐stream mineralization of organic carbon (OC) and CO2 input via groundwater inflow, but their relative importance is largely unknown. In this study, we quantified the role of in‐stream OC mineralization as a source of CO2 in a number of nested boreal headwater streams. The results showed that mineralization of stream OC contributed 3% of CO2 supersaturation at time scales comparable to the estimated water travel times in the streams (<24 h). Mass balances showed that downstream losses of OC were ≤3% in low‐order streams, whereas up to 16% of the OC was lost in the largest (fourth order) streams. In contrast, 85% of the CO2 was lost along the stream network (longest total stream length = 17 km). Under the assumption that in‐stream OC mineralization was the main source of stream CO2, higher rates of OC mineralization (6% of OC) than those reported across the literature (≤0.7% of OC) would be required to sustain observed CO2 supersaturation. Further, model results indicated that groundwater inflows were sufficient to sustain observed stream CO2 concentrations. We hence conclude that in‐stream OC mineralization was a minor source of CO2 in these boreal headwater systems and that the main source of stream CO2 was inflowing groundwater transporting CO2 originating from soil respiration.

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Nearby catchments in the same landscape are often assumed to have similar specific discharge (runoff per unit catchment area). Five years of streamflow from 14 nested catchments in a 68 km 2 landscape was used to test this assumption, with the hypothesis that the spatial variability in specific discharge is smaller than the uncertainties in the measurement. The median spatial variability of specific discharge, defined as subcatchment deviation from the catchment outlet, was 33% at the daily scale. This declined to 24% at a monthly scale and 19% at an annual scale. These specific discharge differences are on the same order of magnitude as predicted for major land-use conversions or a century of climate change. Spatial variability remained when considering uncertainties in specific discharge, and systematic seasonal patterns in specific discharge variation further provide confidence that these differences are more than just errors in the analysis of catchment area, rainfall variability or gauging. Assuming similar specific discharge in nearby catchments can thus lead to spurious conclusions about the effects of disturbance on hydrological and biogeochemical processes.