Water ages in the critical zone of long-term experimental sites in northern latitudes

We thank Pernilla Löfvenius (SLU) for providing PET data for Krycklan (via SITES) and Carl Mitchell for snowmelt data in Dorset. We thank Pertti Ala-aho, Paolo Benettin, Sylvain Kuppel, Aaron A. Smith, and Hailong Wang for constructive discussions on the topic. The authors would like to acknowledge the support of the Maxwell computing cluster funded by the University of Aberdeen. The Krycklan component of the study was funded by the KAW Branch-Point project. We thank the European Research Council (ERC, project GA 335910 VeWa) for funding. We acknowledge support by the German Research Foundation (DFG) and the Open Access Publication Fund of Humboldt-Universität zu Berlin. We thank Todd Walter and two anonymous referees for their critical comments to improve the manuscript. Data availability. The underlaying research data are not publicly available in a repository, as they contain 70 GB. However, they can be requested from the authorsPeer reviewedPublisher PD

Ecohydrological separation in wet, low energy northern environments? A preliminary assessment using different soil water extraction techniques

Funded by European Research Council ERC. Grant Number: project GA 335910 VEWA ACKNOWLEDGEMENTS The constructive comments and suggestions from two anonymous reviewers greatly improved an earlier version of this manuscript. Jon Dick, Jason Lesselsand Jane Bang Poulsen are thanked for assistance with data collection; Audrey Innes for sample preparation and assistance with the cryogenic extraction of water samples; Paula Craib for glassware design; Colleagues in Prof. J. Anderson’s lab for day-to-day assistance incryogenic extraction; Todd Dawson and Nathalie Schultz for providing information on extraction techniques and the analysis of vegetation water; Hedda Weitz for help with the centrifugation of soil samples;and Iain Malcolm and colleagues at the Marine Scotland Freshwater Lab for providing meteorological data. We thank Jason Newton and the Scottish Universities Environmental Research Centre (SUERC) Mass Spectrometry Facility Laboratory in East Kilbride for theisotopic analyses of the xylem water samples. The European Research Council ERC (project GA 335910VEWA) is thanked for funding.Peer reviewedPostprin

Tracer-based assessment of flow paths, storage and runoff generation in northern catchments : a review

Peer reviewedPostprin

Measuring and Modeling Stable Isotopes of Mobile and Bulk Soil Water

We thank Audrey Innes for support with the isotope analysis at University of Aberdeen for the Bruntland Burn and Krycklan sites, Johannes Tiwari (SLU) for the isotope sampling in Krycklan, Pernilla Löfvenius (SLU) for providing PET data for Krycklan, Pertti Ala-aho for providing snowmelt simulations for Krycklan, and Kimberely Janzen (University of Saskatoon) for soil water isotope analysis for the Dorset sites. The work at Krycklan was supported by KAW Branch-Points. We thank the European Research Council (ERC, project GA 335910 VeWa) for funding. We thank two anonymous reviewers and the associate editor for their suggestions and comments.Peer reviewedPublisher PD

Comparison of threshold hydrologic response across northern catchments

Funded by Leverhulme Trust. Grant Number: F/00 152/AGPeer reviewedPostprin

Cross-regional prediction of long-term trajectory of stream water DOC response to climate change

There is no scientific consensus about how dissolved organic carbon (DOC) in surface waters is regulated. Here we combine recent literature data from 49 catchments with detailed stream and catchment process information from nine well established research catchments at mid- to high latitudes to examine the question of how climate controls stream water DOC. We show for the first time that mean annual temperature (MAT) in the range from −3° to +10° C has a strong control over the regional stream water DOC concentration in catchments, with highest concentrations in areas ranging between 0° and +3° C MAT. Although relatively large deviations from this model occur for individual streams, catchment topography appears to explain much of this divergence. These findings suggest that the long-term trajectory of stream water DOC response to climate change may be more predictable than previously thought

Comparison of threshold hydrologic response across northern catchments

Nine mid-latitude to high-latitude headwater catchments – part of the Northern Watershed Ecosystem Response to Climate Change (North-Watch) programme – were used to analyze threshold response to rainfall and snowmelt-driven events and link the different responses to the catchment characteristics of the nine sites. The North-Watch data include daily time-series of various lengths of multiple variables such as air temperature, precipitation and discharge. Rainfall and meltwater inputs were differentiated using a degree-day snowmelt approach. Distinct hydrological events were identified, and precipitation-runoff response curves were visually assessed. Results showed that eight of nine catchments showed runoff initiation thresholds and effective precipitation input thresholds. For rainfall-triggered events, catchment hydroclimatic and physical characteristics (e.g. mean annual air temperature, median flow path distance to the stream, median sub-catchment area) were strong predictors of threshold strength. For snowmelt-driven events, however, thresholds and the factors controlling precipitation-runoff response were difficult to identify. The variability in catchments responses to snowmelt was not fully explained by runoff initiation thresholds and input magnitude thresholds. The quantification of input intensity thresholds (e.g. snow melting and permafrost thawing rates) is likely required for an adequate characterization of nonlinear spring runoff generation in such northern environments

Analysis of hydrological seasonality across northern catchments using monthly precipitation–runoff polygon metrics

Seasonality is an important hydrological signature for catchment comparison. Here, the relevance of monthly precipitation–runoff polygons (defined as scatter points of 12 monthly average precipitation–runoff value pairs connected in the chronological monthly sequence) for characterizing seasonality patterns was investigated to describe the hydrological behaviour of 10 catchments spanning a climatic gradient across the northern temperate region. Specifically, the research objectives were to: (a) discuss the extent to which monthly precipitation–runoff polygons can be used to infer active hydrological processes in contrasting catchments; (b) test the ability of quantitative metrics describing the shape, orientation and surface area of monthly precipitation–runoff polygons to discriminate between different seasonality patterns; and (c) examine the value of precipitation–runoff polygons as a basis for catchment grouping and comparison. This study showed that some polygon metrics were as effective as monthly average runoff coefficients for illustrating differences between the 10 catchments. The use of precipitation–runoff polygons was especially helpful to look at the dynamics prevailing in specific months and better assess the coupling between precipitation and runoff and their relative degree of seasonality. This polygon methodology, linked with a range of quantitative metrics, could therefore provide a new simple tool for understanding and comparing seasonality among catchments

Testing a spatially distributed tracer‐aided runoff model in a snow‐influenced catchment:effects of multicriteria calibration on streamwater ages

Abstract Integrating stable isotope tracers into rainfall‐runoff models allows investigation of water partitioning and direct estimation of travel times and water ages. Tracer data have valuable information content that can be used to constrain models and, in integration with hydrometric observations, test the conceptualization of catchment processes in model structure and parameterization. There is great potential in using tracer‐aided modelling in snow‐influenced catchments to improve understanding of these catchments’ dynamics and sensitivity to environmental change. We used the spatially distributed tracer‐aided rainfall‐runoff (STARR) model to simulate the interactions between water storage, flux, and isotope dynamics in a snow‐influenced, long‐term monitored catchment in Ontario, Canada. Multiple realizations of the model were achieved using a combination of single and multiple objectives as calibration targets. Although good simulations of hydrometric targets such as discharge and snow water equivalent could be achieved by local calibration alone, adequate capture of the stream isotope dynamics was predicated on the inclusion of isotope data in the calibration. Parameter sensitivity was highest, and most local, for single calibration targets. With multiple calibration targets, key sensitive parameters were still identifiable in snow and runoff generation routines. Water ages derived from flux tracking subroutines in the model indicated a catchment where runoff is dominated by younger waters, particularly during spring snowmelt. However, resulting water ages were most sensitive to the partitioning of runoff sources from soil and groundwater sources, which was most realistically achieved when isotopes were included in the calibration. Given the paucity of studies where hydrological models explicitly incorporate tracers in snow‐influenced regions, this study using STARR is an important contribution to satisfactorily simulating snowpack dynamics and runoff generation processes, while simultaneously capturing stable isotope variability in snow‐influenced catchments