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Data from long-term experimental catchments are the foundation of hydrological sciences and are crucial for benchmarking process understanding, observing trends and natural cycles, and being prerequisites for testing predictive models. Integrated data sets which capture all compartments of our landscapes are particularly important in times of land use and climate change. Here, we present ecohydrological data measured at multiple spatial scales which allow differentiation of "blue" water fluxes (which maintain streamflow generation and groundwater recharge) and "green" water fluxes (which sustain vegetation growth). There are two particular unique aspects to this data set: (a) we measured water stable isotopes in the different landscape compartments (i.e. in precipitation, surface water, soil, groundwater, and plant water), and (b) we conducted this monitoring during the extreme drought of 2018 in central Europe. Stable water isotopes are so useful in hydrology as they provide "fingerprints" of the pathways water took when moving through a catchment. Thus, isotopes allow one to evaluate the dynamic relationships between water storage changes and fluxes, which is fundamental to understanding how catchments respond to hydroclimate perturbations or abrupt land use conversion. Second, as we provide the data until 2020, one can also investigate recovery of water stores and fluxes after extreme droughts. Last but not least, lowland headwaters are often understudied systems despite them providing important ecosystem services such as groundwater and drinking water provision and management for forestry and agriculture. The data are available at 10.18728/igb-fred-826.3 (Dämpfling, 2023). [ABSTRACT FROM AUTHOR]

We introduce EcH2O-iso, a new development of the physically based, fully distributed ecohydrological model EcH2O where the tracking of water isotopic tracers (2H and 18O) and age has been incorporated. EcH2O-iso is evaluated at a montane, low-energy experimental catchment in northern Scotland using 16 independent isotope time series from various landscape positions and compartments, encompassing soil water, groundwater, stream water, and plant xylem. The simulation results show consistent isotopic ranges and temporal variability (seasonal and higher frequency) across the soil profile at most sites (especially on hillslopes), broad model-data agreement in heather xylem, and consistent deuterium dynamics in stream water and in groundwater. Since EcH2O-iso was calibrated only using hydrometric and energy flux datasets, tracking water composition provides a truly independent validation of the physical basis of the model for successfully capturing catchment hydrological functioning, both in terms of the celerity in energy propagation shaping the hydrological response (e.g. runoff generation under prevailing hydraulic gradients) and flow velocities of water molecules (e.g. in consistent tracer concentrations at given locations and times). Additionally, we show that the spatially distributed formulation of EcH2O-iso has the potential to quantitatively link water stores and fluxes with spatiotemporal patterns of isotope ratios and water ages. However, our case study also highlights model-data discrepancies in some compartments, such as an over-dampened variability in groundwater and stream water lc-excess, and over-fractionated riparian topsoils. The adopted minimalistic framework, without site-specific parameterisation of isotopes and age tracking, allows us to learn from these mismatches in further model development and benchmarking needs, while taking into account the idiosyncracies of our study catchment. Notably, we suggest that more advanced conceptualisation of soil water mixing and of plant water use would be needed to reproduce some of the observed patterns. Balancing the need for basic hypothesis testing with that of improved simulations of catchment dynamics for a range of applications (e.g. plant water use under changing environmental conditions, water quality issues, and calibration-derived estimates of landscape characteristics), further work could also benefit from including isotope-based calibration. [ABSTRACT FROM AUTHOR]

Climate and land cover are two major factors affecting the water fluxes and balance across spatiotemporal scales. These two factors and their impacts on hydrology are often interlinked. The quantification and differentiation of such impacts is important for developing sustainable land and water management strategies. Here, we calibrated the well-known Hydrus-1D model in a data-rich boreal headwater catchment in Scotland to assess the role of two dominant vegetation types (shrubs vs. trees) in regulating the soil water partitioning and balance. We also applied previously established climate projections for the area and replaced shrubs with trees to imitate current land use change proposals in the region, so as to quantify the potential impacts of climate and land cover changes on soil hydrology. Under tree cover, evapotranspiration and deep percolation to recharge groundwater was about 44% and 57% of annual precipitation, whilst they were about 10% lower and 9% higher respectively under shrub cover in this humid, low energy environment. Meanwhile, tree canopies intercepted 39% of annual precipitation in comparison to 23% by shrubs. Soils with shrub cover stored more water than tree cover. Land cover change was shown to have stronger impacts than projected climate change. With a complete replacement of shrubs with trees under future climate projections at this site, evapotranspiration is expected to increase by ∼39% while percolation to decrease by 21% relative to the current level, more pronounced than the modest changes in the two components (<8%) with climate change only. The impacts would be particularly marked in warm seasons, which may result in water stress experienced by the vegetation. The findings provide an important evidence base for adaptive management strategies of future changes in low-energy humid environments, where vegetation growth is usually restricted by radiative energy and not water availability while few studies that quantify soil water partitioning exist. [ABSTRACT FROM AUTHOR]

Conceptual diagram of the effects of catchment legacy nitrogen and drought conditions on stream nitrate-nitrogen concentrations, export pattern (slope b) and export regime (CV c /CV q). As the legacy nitrogen decreases and drought intensifies, stream nitrate-nitrogen concentrations will decrease, and the relationship with streamflow tends toward enrichment pattern and chemodynamic. [Display omitted] • Legacy-N led to stream NO 3 concentrations showing chemostatic behavior. • Export pattern closely linked to export regime of stream NO 3 concentrations. • Future droughts will lead to a decrease in stream NO 3 concentrations. • Droughts will result in stream NO 3 concentrations showing a more positive correlation with streamflow. • Droughts will result in stream NO 3 concentrations showing more chemodynamic behavior. Stream nitrate nitrogen (NO 3) concentrations and concentration-discharge (C-Q) relationships are key indicators of water quality. However, their long-term variability and response to climate change in large-scale catchments are still poorly understood. Here, we report a synoptic survey and long-term observations (2000–2021) in the Spree catchment (10,105 km2), Germany, to identify the spatial–temporal patterns in stream NO 3 concentrations and C-Q relationships, as well as the underlying environmental drivers. We found that in the context of gradual reduction of nitrate inputs, the stream NO 3 concentrations had a decreasing trend, but showed different magnitudes at different spatial scales (-0.01–0.48 mg L-1 yr−1). In the upstream parts of the catchment - with a high proportion of farmland - high levels of stream NO 3 concentration remained with a risk of eutrophication due to the large nitrogen legacy. Especially in winter, stream NO 3 concentrations were much higher due to the groundwater export with high NO 3 concentrations, a decreased dilution effect of rainfall and vegetation uptake became clear. Consequently, the large nitrogen legacy in the upper catchment resulted in NO 3 concentrations not significantly changing with streamflow and showing chemostatic behavior. This was different from NO 3 dynamics in the mid- and lower-catchment areas, which were positively related to streamflow, showing a chemodynamic behavior. Further, we found the export patterns (i.e. enrichment vs. dilution) of stream NO 3 concentrations strongly correlated with the export regimes (i.e. chemostatic vs. chemodynamic), which were also affected by drought conditions. This is probably due to the decrease in stream depth which would enhance benthic removal leading to a decrease in stream NO 3 concentrations. Reduced hydrological connectivity leads to higher spatial heterogeneity of residual NO 3 in soil and drainage water, which will result in chemodynamic behavior of stream NO 3 concentrations. After rewetting, the export of these high NO 3 concentration in soils led to elevated stream NO 3 concentrations, resulting in a positive correlation with streamflow. Our work revealed significant heterogeneity in stream NO 3 concentrations and C-Q relationships at different spatial–temporal scales in large catchments. Critically, current stream NO 3 concentrations and C-Q relationships are likely to respond strongly to future drought, leading to challenges for future land and water management. [ABSTRACT FROM AUTHOR]

Quantifying and partitioning evapotranspiration ( ET) into evaporation and transpiration is challenging but important for interpreting vegetation effects on the water balance. We applied a model based on the theory of maximum entropy production to estimate ET for shrubs for the first time in a low-energy humid headwater catchment in the Scottish Highlands. In total, 53% of rainfall over the growing season was returned to the atmosphere through ET (59 ± 2% as transpiration), with 22% of rainfall ascribed to interception loss and understory ET. The remainder of rainfall percolated below the rooting zone. The maximum entropy production model showed good capability for total ET estimation, in addition to providing a first approximation for distinguishing evaporation and transpiration in such ecosystems. This study shows that this simple and low-cost approach has potential for local to regional ET estimation with availability of high-resolution hydroclimatic data. Limitations of the approach are also discussed. [ABSTRACT FROM AUTHOR]

Climate change projections indicate reduced summer precipitation and increased air temperature for the northern high latitudes. These climate changes are likely to alter forest water budgets of which plant transpiration ( T ) forms a significant component. Plant transpiration is regulated by stomata behavior of particular species, which is constrained by ambient air and soil conditions. Here, we measured sap flow in a Scots pine ( Pinus Sylvestris ) plantation in a low energy Scottish headwater catchment during the main summer growth period. Effects of rainfall on forest transpiration, as well as the response of T to four environmental variables were investigated at a daily scale. In this boreal environment, transpiration was mainly restricted by radiation and vapor pressure deficit. Air temperature was the least important controlling factor. Soil water became an important factor when rainfall was limited. Frequent but small rain events dictated that precipitation met short-term transpiration demand most of the time. The trees needed supplementary water from antecedent soil water stores when weekly rainfall was below ∼8 mm, but such periods were rare. Water exchange mainly occurred in the canopy or upper 10 cm of the soil, with 47% of rainfall transpired, 45% intercepted and <8% evaporated from the soil surface. Understanding interactions between forests and their hydroclimate, as well as the role of forests in water partitioning is crucial to assist a sustainable land and water management in a changing climate. Whilst such studies are common in semi-arid regions, they are limited in boreal zones, therefore, our findings are a valuable contribution to understanding plant-water relations in a changing environment. [ABSTRACT FROM AUTHOR]

To explore the link between catchment characteristics and sensitivity to environmental change, longer-term data from 21 contrasting mesoscale (67-690 km²) catchments in northern Scotland were analysed to examine the influence of hydroclimatic drivers on hydrological response. Cluster analysis was used to classify the catchments into four distinct groups, with topography and annual precipitation being the two most significant differentiating factors. Using 12 years of data from the 21 sites, inter-catchment coherence of intra- and inter-annual variation of precipitation, air temperatures and stream discharge were assessed both within and between clusters. Whilst catchments in clusters characterised by lower elevations exhibited coherence in hydroclimatic drivers and streamflow response, the higher altitude catchments did not. Annual trends were evident for widespread increases in temperature and also increasing values of precipitation in the higher elevation catchments. Seasonal trends indicate a lack of consistent change during winter, but most other seasons exhibit an increase in temperatures, with isolated increases in measures of precipitation and discharge particularly during the transitional seasons of spring and autumn. These trends ultimately could not be related with catchment typology where increasing upland area results in a breakdown of hydroclimatic coherence between sites. [ABSTRACT FROM AUTHOR]

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. EditorZ.W. Kundzewicz;Associate editorK. Heal CitationAli, G., Tetzlaff, D., Kruitbos, L., Soulsby, C., Carey, S., McDonnell, J., Buttle, J., Laudon, H., Seibert, J., McGuire, K., and Shanley, J., 2013. Analysis of hydrological seasonality across northern catchments using monthly precipitation–runoff polygon metrics.Hydrological Sciences Journal, 59 (1), 56–72. [ABSTRACT FROM AUTHOR]

Snowmelt-dominated basins in northern latitudes provide critical habitat for salmonids. As such, these systems may be especially vulnerable to climate change because of potential shifts in the frequency, magnitude, and timing of flows that can scour incubating embryos. A general framework is presented to examine this issue, using a series of physical models that link climate change, streamflow, and channel morphology to predict the magnitude and spatial distribution of streambed scour and consequent risk to salmonid embryos at basin scales. The approach is demonstrated for a mountain catchment in the Northern Rocky Mountains, USA. Results show that risk of critical scour varies as a function of species and life history and is modulated by local variations in lithology and channel confinement. Embryos of smaller-bodied fall spawners may be at greater risk because of shallow egg burial depths and increased rain-on-snow events during their incubation period. Scour risk for all species is reduced when changes in channel morphology (width, depth, and grain size) keep pace with climate-driven changes in streamflow. Although climate change is predicted to increase scour magnitude, the frequency of scouring events relative to typical salmonid life cycles is relatively low, indicating that individual year classes may be impacted by critical scour, but extirpation of entire populations is not expected. Furthermore, refugia are predicted to occur in unconfined portions of the stream network, where scouring shear stresses are limited to bankfull stage because overbank flows spread across alluvial floodplains; conversely, confined valleys will likely exacerbate climate-driven changes in flow and scour. Our approach can be used to prioritize management strategies according to relative risk to different species or spatial distributions of risk and can be used to predict temporal shifts in the spatial distribution of suitable spawning habitats. A critical unknown issue is whether biological adaptation can keep pace with rates of climate change and channel response. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

We investigate the storage-discharge relationships of two nested (3·6 and 30·4 km2) upland catchments using rainfall-runoff models: (1) a nonlinear discharge sensitivity function and (2) a tracer-constrained process-based conceptual model. Both approaches explicitly acknowledge that water storage is neither time nor space invariant and this provided insight into the respective scaling relationships. Both modelling approaches consistently demonstrated small seasonal storage fluctuations consistent with the wet, cool Scottish climate: the smaller catchment exhibited a greater average dynamic storage ( ca 55 mm) compared to the larger scale ( ca 40 mm). However, there are differences in simulated storage quantities and ranges inferred from both models, which can largely be explained by model uncertainties and model assumptions. In contrast to the dynamic (active) storage indicated by the rainfall-runoff models, input-output relationships of δ18O in both catchments were used to estimate the passive storage available for mixing and tracer damping in streams. This showed that catchment storage is an order of magnitude greater (ranging from 500 to 900 mm) than the dynamic storage estimated by both models, though again, storage estimates were greater for the smaller catchment. The passive storage inferred for mixing indicates that discussion of dynamic storage revealed by water balance considerations masks a much larger catchment storage which may in turn determine sensitivity to environmental change. Copyright © 2011 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The article presents a study on the higher mid-latitudes of the Northern Hemisphere that are sensitive to climate change with the aid of international inter-catchment program North-Watch. It states that the catchments are placed in various locations including Sweden, the U.S., and Canada. It details the initial stage of the North-Watch program, how the catchments were identified, the seasonal pattern and synchronicity of fluxes in the monthly scale.

The article presents a study on the snowmelt water mean transit times (MTTs) of the boreal catchments in Sweden. Methods used include parameter convolution approach to estimate the MTTs and digital elevations model to study the topography. Results show an average snowmelt water MTT of 54 days in all catchments and lower MTTs on catchments with mire wetlands. Results show that it is possible that climatic changes present competing effects on the northern boreal catchment's hydrologic response.

Abstract The extended National Waters Inventory of Scotland (NWIS) monitoring network in combination with an extensive, supplementary low flow sampling campaign was used to create isoscapes of surface water for management purposes at high spatial resolution (100 m grid) across Scotland. The δ2H isoscape shows a strong isotopic separation along a north-south and east-west topographic (mountainous to the north and west and lowlands to the east) and climatic (wetter west, drier east) gradients. Isotopes were enriched in the western domain and depleted in the east and central Highland domains. The surface water d -excess isoscape show more complex spatial variability mainly related to contrasting moisture sources (sub-tropical North Atlantic Ocean, the North Sea, Polar Continental, and the Arctic) as well as secondary evaporation processes. The two-year NWIS isotope record exhibited a significant seasonal evaporative effect on surface water isotopes that progresses from winter through to a maximum in autumn as indicated by Local Evaporation Lines (LELs). The surface water isoscapes can be efficiently reproduced with geographically weighted regression (GWR) models using gridded annual precipitation, remotely sensed actual evapotranspiration, land cover, soil wetness, catchment area, and mean elevation. The GWR models showed potential to assess isotopic changes under future climate and land use change. Highlights • Strong isotopic separation along a north-south and east-west topographic and climatic gradient across Scotland. • Contrasting moisture sources and secondary evaporation processes affect surface water d -excess spatial variability. • Isoscapes can be predicted using precipitation, evapotranspiration, land cover, soil wetness, catchment area and elevation. • Insights help to estimate groundwater recharge, the streamflow contribution, water partitioning, sources of precipitation. [ABSTRACT FROM AUTHOR]

Streams in humid tropical countries provide a wide range of ecosystem services, yet a good understanding of their hydrological functioning is severely limited by lack of data. The transit time of water is a fundamental characteristic of catchment functioning and can often be related to water quality dynamics, thus providing potentially important information for water managers in these environments. In this study, we applied the widely used lumped convolution integral model in a moving window approach to acknowledge the time-variance of transit time distributions (TTD) and resulting moments such as the mean transit time (MTT). We show that for a two-year (2012–2014) rainfall-runoff stable isotope record from almost daily sampling in a humid tropical 30 km 2 catchment in southern Costa Rica, the MTTs are generally short (3 m/year) and events occurring throughout the year, the seasonality of rainfall and distinct moisture origin (Pacific, Atlantic and land surface), the likely dominance of quick near-surface flow paths and relatively low subsurface storage of the underlying volcanic and sedimentary rocks. The moisture origin of rainfall was found to be the most dominant driver of time-variable TTDs as indicated by the changing average wind directions following the transition from the dry into the wet season. This isotope study revealed a highly dynamic system that is likely to be sensitive to environmental change. [ABSTRACT FROM AUTHOR]