Integrated monitoring and modeling to disentangle the complex spatio-temporal dynamics of urbanized streams under drought stress

We have a poor understanding of how urban drainage and other engineered components interact with more natural hydrological processes in green and blue spaces to generate stream flow. This limits the scientific evidence base for predicting and mitigating the effects of future development of the built environment and climate change on urban water resources and their ecosystem services. Here, we synthesize > 20 years of environmental monitoring data to better understand the hydrological function of the 109-km2 Wuhle catchment, an important tributary of the river Spree in Berlin, Germany. More than half (56%) of the catchment is urbanized, leading to substantial flow path alterations. Young water from storm runoff and rapid subsurface flow provided around 20% of stream flow. However, most of it was generated by older groundwater (several years old), mainly recharged through the rural headwaters and non-urban green spaces. Recent drought years since 2018 showed that this base flow component has reduced in response to decreased recharge, causing deterioration in water quality and sections of the stream network to dry out. Attempts to integrate the understanding of engineered and natural processes in a traditional rainfall-runoff model were only partly successful due to uncertainties over the catchment area, effects of sustainable urban drainage, adjacent groundwater pumping, and limited conceptualization of groundwater storage dynamics. The study highlights the need for more extensive and coordinated monitoring and data collection in complex urban catchments and the use of these data in more advanced models of urban hydrology to enhance management.

Effects of 66 years of water management and hydroclimatic change on the urban hydrology and water quality of the Panke catchment, Berlin, Germany

Long-term records of combined stream flow and water chemistry can be an invaluable source of information on changes in the quantity and quality of water resources. To understand the effect of hydroclimate and water management on the heavily urbanized Panke catchment in Berlin, Germany, an extensive search, collation and digitization of historic data from various sources was undertaken. This integrated a unique 66-year spatially distributed record of stream water quality, a 21-year record of groundwater quality and a 31-year stream flow record. These data were analysed in the context of hydroclimatic variability, as well as the history and technological evolution of water resource management in the catchment. To contextualize the effect of droughts, "average" and wet years the Standard Precipitation Index (SPI) was applied. As upstream sites have been less regulated by human impacts, the flow regime is most sensitive to changes in hydroclimatic conditions, while downstream sites are more influenced by wastewater effluents, urban storm drains and inter-basin transfers for flood alleviation. However, at all sites, a general increase in maximum event discharge was observed until a recent drought, starting in 2018. In general, water quality in the catchment has gradually improved as a result of management change and increasingly effective wastewater treatment, though in some places legacy and/or contemporary urban and rural groundwater contamination may be affecting the stream. Hydroclimatic changes, particularly drought years can affect water quality classes, and alter the chemostatic/dynamic behaviour of catchment export patterns. These insights from the Panke catchment underline the importance of strategic adaptation and improvement of water treatment and water resource management in order to enhance the quality of urban water courses. It also demonstrates the importance of long-term integrated data sets.

Quantifying heterogeneity in ecohydrological partitioning in urban green spaces through the integration of empirical and modelling approaches

Urban green spaces (UGS) can help mitigate hydrological impacts of urbanisation and climate change through precipitation infiltration, evapotranspiration and groundwater recharge. However, there is a need to understand how precipitation is partitioned by contrasting vegetation types in order to target UGS management for specific ecosystem services. We monitored, over one growing season, hydrometeorology, soil moisture, sapflux and isotopic variability of soil water under contrasting vegetation (evergreen shrub, evergreen conifer, grassland, larger and smaller deciduous trees), focussed around a 150-m transect of UGS in northern Scotland. We further used the data to develop a one-dimensional model, calibrated to soil moisture observations (KGE's generally > 0.65), to estimate evapotranspiration and groundwater recharge. Our results evidenced clear inter-site differences, with grassland soils experiencing rapid drying at the start of summer, resulting in more fractionated soil water isotopes. Contrastingly, the larger deciduous site saw gradual drying, whilst deeper sandy upslope soils beneath the evergreen shrub drained rapidly. Soils beneath the denser canopied evergreen conifer were overall least responsive to precipitation. Modelled ecohydrological fluxes showed similar diversity, with median evapotranspiration estimates increasing in the order grassland (193 mm) < evergreen shrub (214 mm) < larger deciduous tree (224 mm) < evergreen conifer tree (265 mm). The evergreen shrub had similar estimated median transpiration totals as the larger deciduous tree (155 mm and 128 mm, respectively), though timing of water uptake was different. Median groundwater recharge was greatest beneath grassland (232 mm) and lowest beneath the evergreen conifer (128 mm). The study showed how integrating observational data and simple modelling can quantify heterogeneities in ecohydrological partitioning and help guide UGS management.

Tracer-aided identification of hydrological and biogeochemical controls on in-stream water quality in a riparian wetland

In-stream water quality reflects the integrated results of hydrological mixing of different water sources and associated biogeochemical transformations. However, quantifying the relative importance of these controls is often challenging, particularly in riparian wetlands due to complex process interactions and marked spatio-temporal heterogeneity in environmental gradients. Here, we established a two-step method to differentiate the dominance of hydrological and biogeochemical controls on water quality in a riparian peatland in northern Germany. First, an isotope-based mixing model was developed for distributed modelling of in-stream water balance over a two-year period. The simulation showed the predominance of groundwater inflows for most of the time period, while lateral inflows and channel leakage became more influential in mid-summer, as stream-groundwater connectivity weakened due to declining groundwater levels. A moderate downstream shift from groundwater to lateral inflow was also observed due to the changing channel network geometries and inflow from field drains. The mixing model was then further applied to predict the in-stream concentrations of nutrients, major ions and trace elements. The predicted concentrations were assumed to be those resulting from hydrological mixing only, while influence of biogeochemical controls were reflected by the prediction deviation from observation. Accordingly, 15 water quality parameters were grouped based on their simulation performances into hydrologically-controlled (Cl^-, Mg, Na, K, and Si), biogeochemically-controlled (DOC, SO₄^2-, Mn, and Zn), or controlled-by-both (SRP, NO₃-N, Ca, Fe, Al, and Cu). The mixing modelling not only reproduced the spatiotemporal in-stream water balance with finer process conceptualisation, but also provided a generic method to quantitatively disentangle the relative strength of hydrological and biogeochemical controls. Such a method can be employed as a robust learning tool before extending a hydrological model for water quality simulation, as when, where and how strong biogeochemical controls are exerted provides a strong indicator on which dominant processes need to be conceptualised.

Assessing land use influences on isotopic variability and stream water ages in urbanising rural catchments

Stable water isotopes are invaluable in helping understand catchment functioning and are widely used in experimental catchments, with higher frequency data becoming increasingly common. Such datasets incur substantial logistical costs, reducing their feasibility for use by decision makers needing to understand multi-catchment, landscape-scale functioning over a relatively short period to assess the impact of proposed land use change. Instead, reconnaissance style surveys (high spatial resolution across the landscape at a lower temporal frequency, over a relatively short period) offer an alternative, complementary approach. To test if such sampling could identify heterogeneities in hydrological functioning, and associated landscape controls, we sampled 27 stream sites fortnightly for one year within a peri-urban landscape undergoing land use change. Visual examination of raw data and application of mean transit time and young water fraction models indicated urbanisation, agriculture and responsive soils caused more rapid cycling of precipitation to stream water, whereas mature forestry provided attenuation. We were also able to identify contiguous catchments which functioned fundamentally differently, meaning their response to land use alteration would also be different. This study demonstrated how stable water isotopes can be a valuable, low-cost addition to tools available for environmental decision makers by providing local, process-based information.

An agent-based model that simulates the spatio-temporal dynamics of sources and transfer mechanisms contributing faecal indicator organisms to streams. Part 1: Background and model description

A new Model for the Agent-based simulation of Faecal Indicator Organisms (MAFIO) is developed that attempts to overcome limitations in existing faecal indicator organism (FIO) models arising from coarse spatial discretisations and poorly-constrained hydrological processes. MAFIO is a spatially-distributed, process-based model presently designed to simulate the fate and transport of agents representing FIOs shed by livestock at the sub-field scale in small (<10 km²) agricultural catchments. Specifically, FIO loading, die-off, detachment, surface routing, seepage and channel routing are modelled on a regular spatial grid. Central to MAFIO is that hydrological transfer mechanisms are simulated based on a hydrological environment generated by an external model for which it is possible to robustly determine the accuracy of simulated catchment hydrological functioning. The spatially-distributed, tracer-aided ecohydrological model EcH₂O-iso is highlighted as a possible hydrological environment generator. The present paper provides a rationale for and description of MAFIO, whilst a companion paper applies the model in a small agricultural catchment in Scotland to provide a proof-of-concept.

An agent-based model that simulates the spatio-temporal dynamics of sources and transfer mechanisms contributing faecal indicator organisms to streams. Part 2: Application to a small agricultural catchment

The new Model for the Agent-based simulation of Faecal Indicator Organisms (MAFIO) is applied to a small (0.42 km²) Scottish agricultural catchment to simulate the dynamics of E. coli arising from sheep and cattle farming, in order to provide a proof-of-concept. The hydrological environment for MAFIO was simulated by the "best" ensemble run of the tracer-aided ecohydrological model EcH₂O-iso, obtained through multi-criteria calibration to stream discharge (MAE: 1.37 L s^-1) and spatially-distributed stable isotope data (MAE: 1.14-3.02‰) for the period April-December 2017. MAFIO was then applied for the period June-August for which twice-weekly E. coli loads were quantified at up to three sites along the stream. Performance in simulating these data suggested the model has skill in capturing the transfer of faecal indicator organisms (FIOs) from livestock to streams via the processes of direct deposition, transport in overland flow and seepage from areas of degraded soil. Furthermore, its agent-based structure allowed source areas, transfer mechanisms and host animals contributing FIOs to the stream to be quantified. Such information is likely to have substantial value in the context of designing and spatially-targeting mitigation measures against impaired microbial water quality. This study also revealed, however, that avenues exist for improving process conceptualisation in MAFIO (e.g. to include FIO contributions from wildlife) and highlighted the need to quantitatively assess how uncertainty in the spatial extent of surface flow paths in the simulated hydrological environment may affect FIO simulations. Despite the consequent status of MAFIO as a research-level model, its encouraging performance in this proof-of-concept study suggests the model has significant potential for eventual incorporation into decision support frameworks.

Riparian wetland rehabilitation and beaver re-colonization impacts on hydrological processes and water quality in a lowland agricultural catchment

Quantifying the catchment water balance and the characterization of its water quality changes are effective tools for establishing the response of catchments to shifting land management practices. Here we assess long-term hydrological partitioning and stream water chemistry over a 30-year period in a rural mixed land use catchment in northern Germany undergoing riparian wetlands and widespread re-colonization by beavers (Castor fiber) along the river network. We used long-term spatially distributed stream discharge, groundwater levels and surface water quality data with a simple monthly water balance model, changes in the variability in discharge measurements, and statistical analysis of spatio-temporal changes in stream water quality to assess long-term changes. Water balance estimates indicated high proportions of evapotranspiration loss (~90% of total precipitation) and relatively low groundwater recharge (<5% of total precipitation) prior to riparian rehabilitation in 2000. Increasing groundwater levels from 2000 to 2017 and the relatively linear nature of the catchment storage - discharge relationship, indicate a gradual increase in groundwater recharge (buts still <10% of total precipitation). Wetland rehabilitation, greatly enhanced by increasing beaver populations, resulted in longer water transit times in the stream network, less linear storage-discharge relationship and a loss of daily stream variability, increased DOC concentrations, isotopic evaporative enrichment downstream, and moderated stream temperatures. There was limited long-term water quality improvements from wetland rehabilitation on either nitrate or total phosphorus concentrations, with unchanged seasonal summer and winter peak concentrations for phosphorus and nitrate, respectively. This likely reflects the long-term legacy of fertilizer use on nutrient reservoirs in the catchment's soils, aquifers, and stream network. These long-term changes in hydrology and stream chemistry resulting from riparian rehabilitation and changes in agricultural management practices provide invaluable insights into catchment functioning and an evidence base for future planning in relation to long-term climatic changes.

Climate-phenology-hydrology interactions in northern high latitudes: Assessing the value of remote sensing data in catchment ecohydrological studies

We assessed the hydrological implications of climate effects on vegetation phenology in northern environments by fusion of data from remote-sensing and local catchment monitoring. Studies using satellite data have shown earlier and later dates for the start (SOS) and end of growing seasons (EOS), respectively, in the Northern Hemisphere over the last 3 decades. However, estimates of the change greatly depend on the satellite data utilized. Validation with experimental data on climate-vegetation-hydrology interactions requires long-term observations of multiple variables which are rare and usually restricted to small catchments. In this study, we used two NDVI (normalized difference vegetation index) products (at ~25 & 0.5 km spatial resolutions) to infer SOS and EOS for six northern catchments, and then investigated the likely climate impacts on phenology change and consequent effects on catchment water yield, using both assimilated data (GLDAS: global land data assimilation system) and direct catchment observations. The major findings are: (1) The assimilated air temperature compared well with catchment observations (regression slopes and R² close to 1), whereas underestimations of summer rainstorms resulted in overall underestimations of precipitation (regression slopes of 0.3-0.7, R² ≥ 0.46). (2) The two NDVI products inferred different vegetation phenology characteristics. (3) Increased mean pre-season temperature significantly influenced the advance of SOS and delay of EOS. The precipitation influence was weaker, but delayed SOS corresponding to increased pre-season precipitation at most sites can be related to later snow melting. (4) Decreased catchment streamflow over the last 15 years could be related to the advance in SOS and extension of growing seasons. Greater streamflow reductions in the cold sites than the warm ones imply stronger climate warming impacts on vegetation and hydrology in colder northerly environments. The methods used in this study have potential for better understanding interactions between vegetation, climate and hydrology in observation-scarce regions.

Assessing runoff generation in riparian wetlands: monitoring groundwater-surface water dynamics at the micro-catchment scale

Riparian wetlands (RW) are important variable source areas for runoff generation. They are usually characterised by a combination of groundwater exfiltration-which maintains saturated conditions in low-lying organic-rich soils-and direct precipitation. Both processes interact to generate overland flow as a dominant runoff process. The small-scale details of groundwater-surface water (GW-SW) interactions are usually not well understood in RW. Here, we report the results of a study from an experimental catchment in the Scottish Highlands where spatio-temporal runoff processes in RW were investigated using isotopes, alkalinity and hydrometric measurements. We focused on perennial micro-catchments within the RW and ephemeral zero-order channels draining peatland hollows and hummocks to better understand the heterogeneity in GW-SW interactions. The 12-month study period was dominated by the wettest winter (December/January) period on record. Runoff generation in the RW was strongly controlled by the local groundwater response to direct rainfall, but also the exfiltration of groundwater from upslope. This groundwater drainage is focused in the hollows in ephemeral and perennial drainage channels, but in wet conditions, as exfiltration rates increase, can affect hummocks as well. The hollows provide the dominant areas for mixing groundwater, soil water and direct rainfall to deliver water to the stream network as hollows "fill and spill" to increase connectivity. They also provide wet areas for evaporation which is evident in enriched isotope signatures in summer. Although there is some degree of heterogeneity in the extent to which groundwater influences specific micro-catchments, particularly under low flows, the overall isotopic response is quite similar, especially when the catchment is wet and this responses can explain the isotope signatures observed in the stream. In the future, more longitudinal studies of micro-catchments are needed to better explain the heterogeneity observed.

Using stable isotopes to estimate travel times in a data-sparse Arctic catchment: Challenges and possible solutions

Use of isotopes to quantify the temporal dynamics of the transformation of precipitation into run-off has revealed fundamental new insights into catchment flow paths and mixing processes that influence biogeochemical transport. However, catchments underlain by permafrost have received little attention in isotope-based studies, despite their global importance in terms of rapid environmental change. These high-latitude regions offer limited access for data collection during critical periods (e.g., early phases of snowmelt). Additionally, spatio-temporal variable freeze-thaw cycles, together with the development of an active layer, have a time variant influence on catchment hydrology. All of these characteristics make the application of traditional transit time estimation approaches challenging. We describe an isotope-based study undertaken to provide a preliminary assessment of travel times at Siksik Creek in the western Canadian Arctic. We adopted a model-data fusion approach to estimate the volumes and isotopic characteristics of snowpack and meltwater. Using samples collected in the spring/summer, we characterize the isotopic composition of summer rainfall, melt from snow, soil water, and stream water. In addition, soil moisture dynamics and the temporal evolution of the active layer profile were monitored. First approximations of transit times were estimated for soil and streamwater compositions using lumped convolution integral models and temporally variable inputs including snowmelt, ice thaw, and summer rainfall. Comparing transit time estimates using a variety of inputs revealed that transit time was best estimated using all available inflows (i.e., snowmelt, soil ice thaw, and rainfall). Early spring transit times were short, dominated by snowmelt and soil ice thaw and limited catchment storage when soils are predominantly frozen. However, significant and increasing mixing with water in the active layer during the summer resulted in more damped steam water variation and longer mean travel times (~1.5 years). The study has also highlighted key data needs to better constrain travel time estimates in permafrost catchments.

Using spatial-stream-network models and long-term data to understand and predict dynamics of faecal contamination in a mixed land-use catchment

An 11year dataset of concentrations of E. coli at 10 spatially-distributed sites in a mixed land-use catchment in NE Scotland (52km²) revealed that concentrations were not clearly associated with flow or season. The lack of a clear flow-concentration relationship may have been due to greater water fluxes from less-contaminated headwaters during high flows diluting downstream concentrations, the importance of persistent point sources of E. coli both anthropogenic and agricultural, and possibly the temporal resolution of the dataset. Point sources and year-round grazing of livestock probably obscured clear seasonality in concentrations. Multiple linear regression models identified potential for contamination by anthropogenic point sources as a significant predictor of long-term spatial patterns of low, average and high concentrations of E. coli. Neither arable nor pasture land was significant, even when accounting for hydrological connectivity with a topographic-index method. However, this may have reflected coarse-scale land-cover data inadequately representing "point sources" of agricultural contamination (e.g. direct defecation of livestock into the stream) and temporal changes in availability of E. coli from diffuse sources. Spatial-stream-network models (SSNMs) were applied in a novel context, and had value in making more robust catchment-scale predictions of concentrations of E. coli with estimates of uncertainty, and in enabling identification of potential "hot spots" of faecal contamination. Successfully managing faecal contamination of surface waters is vital for safeguarding public health. Our finding that concentrations of E. coli could not clearly be associated with flow or season may suggest that management strategies should not necessarily target only high flow events or summer when faecal contamination risk is often assumed to be greatest. Furthermore, we identified SSNMs as valuable tools for identifying possible "hot spots" of contamination which could be targeted for management, and for highlighting areas where additional monitoring could help better constrain predictions relating to faecal contamination.

Hydraulic modelling of the spatial and temporal variability in Atlantic salmon parr habitat availability in an upland stream

We show how spatial variability in channel bed morphology affects the hydraulic characteristics of river reaches available to Atlantic salmon parr (Salmo salar) under different flow conditions in an upland stream. The study stream, the Girnock Burn, is a long-term monitoring site in the Scottish Highlands. Six site characterised by different bed geometry and morphology were investigated. Detailed site bathymetries were collected and combined with discharge time series in a 2D hydraulic model to obtain spatially distributed depth-averaged velocities under different flow conditions. Available habitat (AH) was estimated for each site. Stream discharge was used according to the critical displacement velocity (CDV) approach. CDV defines a velocity threshold above which salmon parr are not able to hold station and effective feeding opportunities or habitat utilization are reduced, depending on fish size and water temperature. An average value of the relative available habitat (<RAH>) for the most significant period for parr growth - April to May - was used for inter-site comparison and to analyse temporal variations over 40years. Results show that some sites are more able than others to maintain zones where salmon parr can forage unimpeded by high flow velocities under both wet and dry conditions. With lower flow velocities, dry years offer higher values of <RAH> than wet years. Even though <RAH> can change considerably across the sites as stream flow changes, the directions of change are consistent. Relative available habitat (RAH) shows a strong relationship with discharge per unit width, whilst channel slope and bed roughness either do not have relevant impact or compensate each other. The results show that significant parr habitat was available at all sites across all flows during this critical growth period, suggesting that hydrological variability is not a factor limiting growth in the Girnock.

Spatial and temporal patterns of soil water storage and vegetation water use in humid northern catchments

Using stable isotope data from soil and vegetation xylem samples across a range of landscape positions, this study provides preliminary insights into spatial patterns and temporal dynamics of soil-plant water interactions in a humid, low-energy northern environment. Our analysis showed that evaporative fractionation affected the isotopic signatures in soil water at shallow depths but was less marked than previously observed in other environments. By comparing the temporal dynamics of stable isotopes in soil water mainly held at suctions around and below field capacity, we found that these waters are not clearly separated. The study inferred that vegetation water sources at all sites were relatively constant, and most likely to be in the upper profile close to the soil/atmosphere interface. The data analyses also suggested that both vegetation type and landscape position, including soil type, may have a strong influence on local water uptake patterns, although more work is needed to explicitly identify water sources and understand the effect of plant physiological processes on xylem isotopic water signatures.

Modeling the isotopic evolution of snowpack and snowmelt: Testing a spatially distributed parsimonious approach

Use of stable water isotopes has become increasingly popular in quantifying water flow paths and travel times in hydrological systems using tracer-aided modeling. In snow-influenced catchments, snowmelt produces a traceable isotopic signal, which differs from original snowfall isotopic composition because of isotopic fractionation in the snowpack. These fractionation processes in snow are relatively well understood, but representing their spatiotemporal variability in tracer-aided studies remains a challenge. We present a novel, parsimonious modeling method to account for the snowpack isotope fractionation and estimate isotope ratios in snowmelt water in a fully spatially distributed manner. Our model introduces two calibration parameters that alone account for the isotopic fractionation caused by sublimation from interception and ground snow storage, and snowmelt fractionation progressively enriching the snowmelt runoff. The isotope routines are linked to a generic process-based snow interception-accumulation-melt model facilitating simulation of spatially distributed snowmelt runoff. We use a synthetic modeling experiment to demonstrate the functionality of the model algorithms in different landscape locations and under different canopy characteristics. We also provide a proof-of-concept model test and successfully reproduce isotopic ratios in snowmelt runoff sampled with snowmelt lysimeters in two long-term experimental catchment with contrasting winter conditions. To our knowledge, the method is the first such tool to allow estimation of the spatially distributed nature of isotopic fractionation in snowpacks and the resulting isotope ratios in snowmelt runoff. The method can thus provide a useful tool for tracer-aided modeling to better understand the integrated nature of flow, mixing, and transport processes in snow-influenced catchments.

No influence of CO2 on stable isotope analyses of soil waters with off-axis integrated cavity output spectroscopy (OA-ICOS)

RATIONALE

It was recently shown that the presence of CO₂ affects the stable isotope (δ² H and δ¹⁸ O values) analysis of water vapor via Wavelength-Scanned Cavity Ring-Down Spectroscopy. Here, we test how much CO₂ is emitted from soil samples and if the CO₂ in the headspace influences the isotope analysis with the direct equilibration method by Off-Axis Integrated Cavity Output Spectroscopy (OA-ICOS).

METHODS

The headspace above different amounts of sparkling water was sampled, and its stable isotopic composition (δ² H and δ¹⁸ O values) and CO₂ concentration were measured by direct equilibration and by gas chromatography, respectively. In addition, the headspace above soil samples was analyzed in the same way. Furthermore, the gravimetric water content and the loss on ignition were measured for the soil samples.

RESULTS

The experiment with the sparkling water showed that CO₂ does not influence the stable isotope analysis by OA-ICOS. CO₂ was emitted from the soil samples and correlated with the isotopic fractionation signal, but no causal relationship between the two was determined. Instead, the fractionation signal in pore water isotopes can be explained by soil evaporation and the CO₂ can be related to soil moisture and organic matter which both enhance microbial activity.

CONCLUSIONS

We found, despite the high CO₂ emissions from soil samples, no need for a post-correction of the pore water stable isotope analysis results, since there is no relation between CO₂ concentrations and the stable isotope results of vapor samples obtained with OA-ICOS. © 2016 The Authors. Rapid Communications in Mass Spectrometry Published by John Wiley & Sons Ltd.

Biogeochemistry of "pristine" freshwater stream and lake systems in the western Canadian Arctic

Climate change poses a substantial threat to the stability of the Arctic terrestrial carbon (C) pool as warmer air temperatures thaw permafrost and deepen the seasonally-thawed active layer of soils and sediments. Enhanced water flow through this layer may accelerate the transport of C and major cations and anions to streams and lakes. These act as important conduits and reactors for dissolved C within the terrestrial C cycle. It is important for studies to consider these processes in small headwater catchments, which have been identified as hotspots of rapid mineralisation of C sourced from ancient permafrost thaw. In order to better understand the role of inland waters in terrestrial C cycling we characterised the biogeochemistry of the freshwater systems in a c. 14 km² study area in the western Canadian Arctic. Sampling took place during the snow-free seasons of 2013 and 2014 for major inorganic solutes, dissolved organic and inorganic C (DOC and DIC, respectively), carbon dioxide (CO₂) and methane (CH₄) concentrations from three water type groups: lakes, polygonal pools and streams. These groups displayed differing biogeochemical signatures, indicative of contrasting biogeochemical controls. However, none of the groups showed strong signals of enhanced permafrost thaw during the study seasons. The mean annual air temperature in the region has increased by more than 2.5 °C since 1970, and continued warming will likely affect the aquatic biogeochemistry. This study provides important baseline data for comparison with future studies in a warming Arctic.

Continuous Dissolved Oxygen Measurements and Modelling Metabolism in Peatland Streams

Stream water dissolved oxygen was monitored in a 3.2km2 moorland headwater catchment in the Scottish Highlands. The stream consists of three 1st order headwaters and a 2nd order main stem. The stream network is fringed by peat soils with no riparian trees, though dwarf shrubs provide shading in the lower catchment. Dissolved oxygen (DO) is regulated by the balance between atmospheric re-aeration and the metabolic processes of photosynthesis and respiration. DO was continuously measured for >1 year and the data used to calibrate a mass balance model, to estimate primary production, respiration and re-aeration for a 1st order site and in the 2nd order main stem. Results showed that the stream was always heterotrophic at both sites. Sites were most heterotrophic in the summer reflecting higher levels of stream metabolism. The 1st order stream appeared more heterotrophic which was consistent with the evident greater biomass of macrophytes in the 2nd order stream, with resulting higher primary productivity. Comparison between respiration, primary production, re-aeration and potential physical controls revealed only weak relationships. However, the most basic model parameters (e.g. the parameter linking light and photosynthesis) controlling ecosystem processes resulted in significant differences between the sites which seem related to the stream channel geometry.

Water sources and mixing in riparian wetlands revealed by tracers and geospatial analysis

Mixing of waters within riparian zones has been identified as an important influence on runoff generation and water quality. Improved understanding of the controls on the spatial and temporal variability of water sources and how they mix in riparian zones is therefore of both fundamental and applied interest. In this study, we have combined topographic indices derived from a high-resolution Digital Elevation Model (DEM) with repeated spatially high-resolution synoptic sampling of multiple tracers to investigate such dynamics of source water mixing. We use geostatistics to estimate concentrations of three different tracers (deuterium, alkalinity, and dissolved organic carbon) across an extended riparian zone in a headwater catchment in NE Scotland, to identify spatial and temporal influences on mixing of source waters. The various biogeochemical tracers and stable isotopes helped constrain the sources of runoff and their temporal dynamics. Results show that spatial variability in all three tracers was evident in all sampling campaigns, but more pronounced in warmer dryer periods. The extent of mixing areas within the riparian area reflected strong hydroclimatic controls and showed large degrees of expansion and contraction that was not strongly related to topographic indices. The integrated approach of using multiple tracers, geospatial statistics, and topographic analysis allowed us to classify three main riparian source areas and mixing zones. This study underlines the importance of the riparian zones for mixing soil water and groundwater and introduces a novel approach how this mixing can be quantified and the effect on the downstream chemistry be assessed.

A preliminary assessment of water partitioning and ecohydrological coupling in northern headwaters using stable isotopes and conceptual runoff models

We combined a conceptual rainfall-runoff model and input-output relationships of stable isotopes to understand ecohydrological influences on hydrological partitioning in snow-influenced northern catchments. Six sites in Sweden (Krycklan), Canada (Wolf Creek; Baker Creek; Dorset), Scotland (Girnock) and the USA (Dry Creek) span moisture and energy gradients found at high latitudes. A meta-analysis was carried out using the Hydrologiska Byråns Vattenbalansavdelning (HBV) model to estimate the main storage changes characterizing annual water balances. Annual snowpack storage importance was ranked as Wolf Creek > Krycklan > Dorset > Baker Creek > Dry Creek > Girnock. The subsequent rate and longevity of melt were reflected in calibrated parameters that determine partitioning of waters between more rapid and slower flowpaths and associated variations in soil and groundwater storage. Variability of stream water isotopic composition depends on the following: (i) rate and duration of spring snowmelt; (ii) significance of summer/autumn rainfall; and (iii) relative importance of near-surface and deeper flowpaths in routing water to the stream. Flowpath partitioning also regulates influences of summer evaporation on drainage waters. Deviations of isotope data from the Global Meteoric Water Line showed subtle effects of internal catchment processes on isotopic fractionation most likely through evaporation. Such effects are highly variable among sites and with seasonal differences at some sites. After accounting for climate, evaporative fractionation is strongest at sites where lakes and near-surface runoff processes in wet riparian soils can mobilize isotopically enriched water during summer and autumn. Given close soil-vegetation coupling, this may result in spatial variability in soil water isotope pools available for plant uptake. We argue that stable isotope studies are crucial in addressing the many open questions on hydrological functioning of northern environments. © 2015 The Authors. Hydrological Processes published by John Wiley & Sons Ltd.

Stream water age distributions controlled by storage dynamics and nonlinear hydrologic connectivity: Modeling with high-resolution isotope data

To assess the influence of storage dynamics and nonlinearities in hydrological connectivity on time-variant stream water ages, we used a new long-term record of daily isotope measurements in precipitation and streamflow to calibrate and test a parsimonious tracer-aided runoff model. This can track tracers and the ages of water fluxes through and between conceptual stores in steeper hillslopes, dynamically saturated riparian peatlands, and deeper groundwater; these represent the main landscape units involved in runoff generation. Storage volumes are largest in groundwater and on the hillslopes, though most dynamic mixing occurs in the smaller stores in riparian peat. Both streamflow and isotope variations are generally well captured by the model, and the simulated storage and tracer dynamics in the main landscape units are consistent with independent measurements. The model predicts that the average age of stream water is ∼1.8 years. On a daily basis, this varies between ∼1 month in storm events, when younger waters draining the hillslope and riparian peatland dominates, to around 4 years in dry periods when groundwater sustains flow. This variability reflects the integration of differently aged water fluxes from the main landscape units and their mixing in riparian wetlands. The connectivity between these spatial units varies in a nonlinear way with storage that depends upon precipitation characteristics and antecedent conditions. This, in turn, determines the spatial distribution of flow paths and the integration of their contrasting nonstationary ages. This approach is well suited for constraining process-based modeling in a range of northern temperate and boreal environments.

Application of a linear regression model to assess the influence of urbanised areas and grazing pastures on the microbiological quality of rural streams

Faecal coliform (FC) bacteria were used as a proxy of faecal indicator organisms (FIOs) to assess the microbiological pollution risk for eight mesoscale catchments with increasing lowland influence across north-east Scotland. This study sought to assess the impact of urban areas on microbial contaminant fluxes. Fluxes were lowest in upland catchments where populations are relatively low. By contrast, lowland catchments with larger settlements and a greater number of grazing populations have more elevated FC concentrations throughout the year. Peak FC counts occurred during the summer months (April-September) when biological activity is at its highest. Lowland catchments experience high FC concentrations throughout the year whereas upland catchments exhibit more seasonal variations with elevated summer conditions and reduced winter concentrations. A simple linear regression model based on catchment characteristics provided scope to predict FC fluxes. Percentage of improved grazing pasture and human population explained 90 and 62 % of the variation in mean annual FC concentrations. This approach provides scope for an initial screening tool to predict the impact of urban space and agricultural practice on FC concentrations at the catchment scale and can aid in pragmatic planning and water quality improvement decisions. However, greater understanding of the short-term dynamics is still required which would benefit from higher resolution sampling than the approach undertaken here.

Storage dynamics in hydropedological units control hillslope connectivity, runoff generation, and the evolution of catchment transit time distributions

We examined the storage dynamics and isotopic composition of soil water over 12 months in three hydropedological units in order to understand runoff generation in a montane catchment. The units form classic catena sequences from freely draining podzols on steep upper hillslopes through peaty gleys in shallower lower slopes to deeper peats in the riparian zone. The peaty gleys and peats remained saturated throughout the year, while the podzols showed distinct wetting and drying cycles. In this region, most precipitation events are <10 mm in magnitude, and storm runoff is mainly generated from the peats and peaty gleys, with runoff coefficients (RCs) typically <10%. In larger events the podzolic soils become strongly connected to the saturated areas, and RCs can exceed 40%. Isotopic variations in precipitation are significantly damped in the organic-rich soil surface horizons due to mixing with larger volumes of stored water. This damping is accentuated in the deeper soil profile and groundwater. Consequently, the isotopic composition of stream water is also damped, but the dynamics strongly reflect those of the near-surface waters in the riparian peats. "pre-event" water typically accounts for >80% of flow, even in large events, reflecting the displacement of water from the riparian soils that has been stored in the catchment for >2 years. These riparian areas are the key zone where different source waters mix. Our study is novel in showing that they act as "isostats," not only regulating the isotopic composition of stream water, but also integrating the transit time distribution for the catchment.

KEY POINTS

Hillslope connectivity is controlled by small storage changes in soil unitsDifferent catchment source waters mix in large riparian wetland storageIsotopes show riparian wetlands set the catchment transit time distribution.

Land use and hydroclimatic influences on Faecal Indicator Organisms in two large Scottish catchments: towards land use-based models as screening tools

Faecal Coliform (FC) bacteria were used as Faecal Indicator Organisms (FIOs) for assessment of microbiological pollution risk in two large, mixed land use catchments in Scotland. FC counts varied spatially in relation to land use and human population and resulting trophic status. These were highest in catchments with a high cover of improved pasture (which was assumed to be a proxy for cattle and sheep grazing densities) and significant human populations. FC counts were lowest in oligotrophic upland areas, where domesticated animal populations were low. In both lowland and upland catchments, peak FC counts occurred under periods of elevated flows during summer. However, in lowland agricultural catchments of higher trophic status, contamination appears to be chronic and occurs all year round. In contrast, upland headwater catchments exhibit more episodic peaks in relation to high flow events. Larger scale catchments integrate the inputs from contrasting head water streams. Spatial variations in stream FC concentrations can be predicted to a first approximation using multiple regression based on catchment characteristics. Land cover was the most important factor, with percentage improved pasture being the primary control and human population being of secondary importance. These two factors could explain 78% of the variation in mean annual FC concentrations and 65% of the 95th percentile. This simple linear model provides a screening tool for rapid assessment of pollution risk in unmonitored catchments. However, improved prediction of short-term dynamics and peak values requires higher resolution sampling and process-based models of FC production, survival and transport. A particularly important need is an improved characterisation of the hydrological connectivity which controls the flux from pollutant reservoirs on the landscape into river channel networks.

Characterizing Pb mobilization from upland soils to streams using (206)Pb/(207)Pb isotopic ratios

Anthropogenically deposited lead (Pb) binds efficiently to soil organic matter, which can be mobilized through hydrologically mediated mechanisms, with implications for ecological and potable quality of receiving waters. Lead isotopic ((206)Pb/(207)Pb) ratios change down peat profiles as a consequence of long-term temporal variation in depositional sources, each with distinctive isotopic signatures. This study characterizes differential Pb transport mechanisms from deposition to streams at two small catchments with contrasting soil types in upland Wales, U.K., by determining Pb concentrations and (206)Pb/(207)Pb ratios from soil core profiles, interstitial pore waters, and stream water. Hydrological characteristics of soils are instrumental in determining the location in soil profiles of exported Pb and hence concentration and (206)Pb/(207)Pb ratios in surface waters. The highest Pb concentrations from near-surface soils are mobilized, concomitant with high dissolved organic carbon (DOC) exports, from hydrologically responsive peat soils with preferential shallow subsurface flows, leading to increased Pb concentrations in stream water and isotopic signatures more closely resembling recently deposited Pb. In more minerogenic soils, percolation of water allows Pb, bound to DOC, to be retained in mineral horizons and combined with other groundwater sources, resulting in Pb being transported from throughout the profile with a more geogenic isotopic signature. This study shows that (206)Pb/(207)Pb ratios can enhance our understanding of the provenances and transport mechanisms of Pb and potentially organic matter within upland soils.

Is the composition of dissolved organic carbon changing in upland acidic streams?

The quantity and composition of dissolved organic carbon (DOC) exported from upland soils to surface waters is a key link in the global carbon cycle and economically important for treating potable waters. The relationship between ultraviolet (UV) absorbance and DOC concentrations can be used to infer changes in the proportion of hydrophobic (aromatic, recalcitrant) carbon and hence biodegradability of DOC. This study describes a significant change in the relationship between UV absorbance and DOC over 22 years at two upland moorland catchments in Scotland, UK. Despite increases in long-term DOC concentrations, analysis suggests that the proportion of hydrophobic material has declined. A statistical mixed-effect modeling approach was used to examine the likely mechanisms that could explain these observations. Annual nonmarine sulfate load was the only significant forcing factor that could explain the observed long-term trend in the UV absorbance-DOC relationship at both sites. It is hypothesized that enhanced heterotrophic decomposition of organic matter and increased solubility of carbon compounds in soils where sulfate driven acidification is being reversed are the dominant mechanisms behind this change in DOC composition. These trends will impact on carbon substrate dynamics by potentially increasing biodegradability of exported organic matter, influencing carbon cycling in terrestrial and aquatic ecosystems.