Accuracy of the Copernicus snow water equivalent (SWE) product and the impact of SWE calibration and assimilation on modelled SWE and streamflow was evaluated. Daily snowpack measurements were made at 12 locations from 2016 to 2019 across a 4104 km2 mixed-forest basin in the Great Lakes region of central Ontario, Canada. Sub-basin daily SWE calculated from these sites, observed discharge, and lake levels were used to calibrate a hydrologic model developed using the Raven modelling framework. Copernicus SWE was bias corrected during the melt period using mean bias subtraction and was compared to daily basin average SWE calculated from the measured data. Bias corrected Copernicus SWE was assimilated into the models using a range of parameters and the parameterizations from the model calibration. The bias corrected Copernicus product agreed well with measured data and provided a good estimate of mean basin SWE demonstrating that the product shows promise for hydrology applications within the study region. Calibration to spatially distributed SWE substantially improved the basin scale SWE estimate while only slightly degrading the flow simulation demonstrating the value of including SWE in a multi-objective calibration formulation. The particle filter experiments yielded the best SWE estimation but moderately degraded the flow simulation. The particle filter experiments constrained by the calibrated snow parameters produced similar results to the experiments using the upper and lower bounds indicating that, in this study, model calibration prior to assimilation was not valuable. The calibrated models exhibited varying levels of skill in estimating SWE but demonstrated similar streamflow performance. This indicates that basin outlet streamflow can be accurately estimated using a model with a poor representation of distributed SWE. This may be sufficient for applications where estimating flow is the primary water management objective. However, in applications where understanding the physical processes of snow accumulation, melt and streamflow generation are important, such as assessing the impact of climate change on water resources, accurate representations of SWE are required and can be improved via multi-objective calibration or data assimilation, as demonstrated in this study. Bias corrected Copernicus snow water equivalent (SWE) agreed well with measured data demonstrating the products utility for hydrology applications. Calibration to lake levels, streamflow, and SWE substantially improved model performance compared to a model calibrated to lake levels and streamflow only, thus demonstrating the value of including SWE in multi-objective calibration. image

The mosaic of urban, agricultural and natural covers that typifies most developed landscapes makes it challenging to identify the primary causes of stream flow perturbation in mixed landcover watersheds. This is especially true in southern Ontario, Canada, where approximately 1/3 of the Canadian population lives in urban areas surrounded by agriculture. Whilst previous studies have examined the impacts of urban or agricultural landcover on stream flow separately, they are rarely considered together. Furthermore, major expansions in tile-drained (TD) cropland in Ontario over the past several decades could affect the flow regime; however, this has never been examined. This study assessed the effect of landcover on flow regime at 19 proximal watersheds that varied in agriculture (0%-87%), natural (2%-97%) and urban landcover (2%-96%) using the Richards-Baker index (RBI), the coefficient of variation (CV) and a Baseflow index (BFI). Urbanized watersheds were consistently the most flashy (highest RBI and CV), regardless of season, whereas agricultural watersheds had moderately flashy conditions that varied between the growing (GS) and non-growing seasons (NGS). Natural watersheds were the least flashy and had the highest BFI. Watersheds dominated by TD cropland were flashier during the NGS compared with un-tiled agricultural watersheds. Furthermore, TD-streams were warmer in the NGS and cooler in the GS, such that the former could affect ice breakup. A 50-year analysis at three watersheds showed statistically significant increases in flashiness and decreases in BFI at an urbanizing watershed. In contrast, watersheds that remained agricultural or natural underwent small but significant declines in flashiness and increases in baseflow, potentially due to increases in precipitation and forest maturation. Our results suggest that continued expansions of urban and TD cropland may increase NGS flashiness. In contrast, enhanced soil moisture storage provided by TD could decrease the potential for flooding in the GS in southern Ontario. Southern Ontario watersheds have faced changing landcovers due to urban expansion and agricultural intensification, which affect hydrologic regimes. This research quantified flashiness in streams across a wide range of land covers through both a short-term and long-term analysis. Watersheds with more urban cover or tile drainage showed higher flashiness values in the short-term analysis, and urban expansion increased stream flashiness over a 50 year flow record. image

Forests are critical source regions of high-quality drinking water but forest disturbances such as harvesting can alter stream dissolved organic carbon (DOC) concentrations and influence source water treatability. Most stream DOC-centric forest harvesting impact studies report on effects <10 years post-harvest; less is known about the legacy effects of forest harvesting on stream DOC. Here, inter- and intra-catchment variability in stream DOC concentration and export were evaluated in two northern hardwood-dominated headwater catchments (unharvested reference and 24 years post-clearcut). The relationship between stream DOC and the concentration, spatial distribution, and hydrologic connectivity of hillslope solute pool DOC was investigated. Stream DOC concentrations in the legacy clearcut catchment exceeded those in the reference catchment for all flow conditions. Inter-catchment differences in DOC export were inconsistent. Hillslope solute pool DOC concentrations decreased with soil depth but were not significantly different between catchments. Concentration–discharge regression analysis indicated that DOC was primarily transport-limited (flushing) in both catchments. Aqueous potassium silica molar ratio data indicate the influence of groundwater on stream chemistry and streamflow was similar in both catchments. Results suggest that while clearcut harvesting can have detectable decadal-scale effects on stream DOC concentrations in northern hardwood-dominated headwater catchments, the effects are limited and likely do not pose a reasonable threat to downstream drinking water treatment operations.

Much of our understanding on temporary headwater streams is from arid and sub-humid environments. We know less about zero-flow periods in humid headwater catchments that experience seasonal snow cover. Our study characterised the temporal and spatial patterns of zero-flow periods for forested headwater streams in a snow-dominated landscape. We used 36 years of streamflow data from 13 headwater catchments within the Turkey Lakes Watershed located on the Canadian Shield in Ontario, Canada, near the eastern shores of Lake Superior. These headwater catchments differ substantially in their number of May-November zero-flow days (0-166 days per year) despite being clustered in a small geographical area with similar geology, physiography and vegetation cover. The catchments also experience similar continental climatic conditions with relatively even precipitation inputs throughout the year (mean annual precipitation of 1210 mm/year). Inter-annual variability in the number of zero-flow days was primarily associated with May-November precipitation and evapotranspiration. Despite the large seasonal snowpacks that form in this region, the amount of snow did not appear to influence the extent of zero-flow periods. We found that between-catchment variability in zero-flow occurrences was related to differences in catchment area and catchment properties typically associated with greater groundwater influence. Our study suggests that occurrences of zero-flows in headwater streams can be highly variable even over small geographical regions and that flow permanence may be more sensitive to spring to fall weather conditions than the influence of snow due partly to the shallow soils typically found on the Canadian Shield.

The efficacy of field-based, photogrammetric point cloud, orthophoto and light detection and ranging datasets to describe forest structure and resolve forest-snowpack relationships in a mixed forest region was evaluated over two years at the point and transect scales. Hemispheric photo-derived canopy metrics correlated well with remotely sensed metrics, but tree bole metrics were not effectively derived from remotely sensed data. Significant differences in melt rate and snow-free date were found across forest type at the transect scale. Field and remotely sensed estimates of canopy cover were highly correlated with melt rate and snow-free date at the point scale, which aligns with previous literature and understanding of snowmelt processes. However, significant correlations were only present during the 2016 study year, which was attributed to canopy-controlled solar radiation-driven melt in 2016 versus more spatially uniform turbulent flux-driven melt in 2017. Peak snow water equivalent metrics were not correlated well with canopy or tree height metrics, contrary to previous research. This was likely due to mid-winter melt events throughout both study years, where a mix of accumulation and melt processes confounded forest-snowpack relationships. This study demonstrates that widely available remotely sensed data with a broad coverage can be used to: (i) describe forest-snowpack relationships in mixed hardwood, coniferous forests and (ii) elucidate the variability of forest-snowpack relationships under different climate conditions in this environment.

Forests are critical water supply regions that are increasingly threatened by natural and anthropogenic disturbance. Evaluation of runoff-generating processes within harvested and undisturbed headwater catchments provides insight into disturbance impacts on water quality and drinking water treatability. In this study, an extensive hydrologic dataset collected at the experimental Turkey Lakes Watershed (TLW) located on the Canadian Shield was used to quantify sources of stormflow in legacy clear-cut (24-years post harvesting) and forested (control) headwater catchments using an end member mixing analysis (EMMA) model. Stream water, groundwater, soil water, and throughfall water quality were evaluated during spring snowmelt, stormflow, and fall wet-up. Groundwater chemistry was similar to stream water chemistry in both catchments, suggesting that groundwater is a major contributor to stream flow. The water chemistry in small wetlands within the study catchments was comparable to stream water chemistry, suggesting that wetlands are also important contributors to stream flow. Differences in wetland position between the legacy clear-cut and control catchments appeared to have a greater influence on source contributions than legacy harvesting. Results from this study provide insight into runoff-generation processes that reflect event/seasonal flow dynamics and the impacts on water quality.

Concentration – discharge ( C – Q ) relationships in streamflow can serve a variety of purposes, including elucidating catchment water sources, quantifying chemical weathering, and estimating solute fluxes. Four main C – Q relationships have been identified: dilution (decreasing C with increasing Q ), flushing (increasing C with increasing Q ), chemostatic (little or no change in C with increasing Q ) and chemostochastic (great variation in C relative to that of Q ) behaviour. We used a 40‐year data record for 10 headwater forested catchments in central Ontario, Canada, to examine C – Q relationships for a range of geogenic, exogenic and bioactive solutes. The thin overburden cover of this landscape and general absence of deep groundwater contributions to streamflow suggest that streamflow solute concentrations and their relationship with discharge would be sensitive to pronounced changes in climate and atmospheric deposition chemistry over the monitoring period. We explored the temporal and spatial stability of C – Q relationships in response to these changes at annual and seasonal timescales for relatively wet and dry conditions and for periods with relatively high and low atmospheric deposition. Chemostasis dominated C – Q relationships at the annual and seasonal scales for geogenic (e.g., SiO 2 ) and exogenic (e.g., Cl − ) solutes, although bioactive solutes (e.g., NO 3 − , NH 4 + ) exhibited non‐chemostatic behaviour for some catchments, time periods and wetness conditions. C – Q relationships derived at the annual timescale often masked pronounced seasonal shifts in bioactive solute concentration response to changes in streamflow and provide only partial insight into the production and delivery of such solutes in this forest landscape. Relatively subtle changes in overburden thickness across this landscape appeared to explain some of the deviations from widespread chemostasis. Nevertheless, this general chemostatic behaviour was largely temporally stable across catchments and solutes, highlighting its resilience in the face of such non‐stationary influences as climate conditions and atmospheric deposition.

Previous studies of harvest impacts on stream chemistry from deciduous forests with seasonal snow cover have shown variable responses and recovery times. We hypothesized that this variability in response and recovery was due to the interactive effects of forest harvesting with regional and global environmental changes. We investigated harvest impacts on stream solute (K+, Na+, Ca2+, Mg2+, Cl-, SO42-, DOC, Si, NO3-, NH4+, total P) concentrations, pH and conductivity under clearcut, selection, and shelterwood harvesting using a before-after control impact experimental approach. The experiment was conducted at the Turkey Lakes Watershed, a shade-tolerant sugar maple forest within the Boreal Shield Ecozone, over a 36 year period (15 year pre-harvest and 21 year postharvest) that has experienced climate change and acidification recovery. Harvest impacts on stream chemistry were greatest in the clearcut, moderate in the selection cut, and least in the shelterwood cut. The largest impacts on stream solutes typically lasted one to six years following harvest, although some solutes were impacted during the entire 21 year post-harvest period. Different patterns in the response of stream solute concentrations included: initial deviations (increase or decrease) followed by a return to pre-harvest levels within two to four years (Na+, Ca2+, Mg2+, conductivity); initial increase followed by a decrease but still not recovered (K+); initial increase followed by a decrease to below pre-harvest levels (NO3-, Na+, Cl-); sustained elevated concentrations (dissolved organic carbon [DOC], Si); and a noisy response (TP, SO42-, pH). The different patterns were consistent with expected changes in nutrient demand, sources, and transport (including changes in hillslope-stream hydrologic connectivity due to harvesting). Comparing harvest impacts to long-term climate change and acidification recovery showed that harvesting had antagonistic (mitigative) effects to environmental changes for some solutes (e.g., cations), while for others (e.g., DOC) harvesting had agonistic (synergistic) effects to environmental changes. These results emphasize the need for long-term monitoring to accurately interpret the consequences of forest harvesting practices on the resilience of forests experiencing environmental change.

Cyanobacteria threaten public and ecosystem health globally through the production of secondary metabolites including potent toxins, and disruption of water treatment processes. Warmer water temperatures and high nutrient availability are key characteristics associated with the occurrence of cyanobacteria. There is typically concern of cyanobacteria blooms (e.g., visible biomass accumulations) occurring in the summer season of eutrophic systems. However, in this study, the proliferation of cyanobacteria in lakes across all seasons and in absence of visual biomass indicators of bloom condition was observed in three oligotrophic lakes of the Turkey Lakes Watershed (TLW) in Ontario, located within a sugar maple dominated forest on the Canadian Shield. Almost 40 years of ice phenology data showed that rising temperatures have led to significantly longer ice-free periods and aquatic growing seasons in TLW. Warming is especially evident in the autumn, with the onset of ice-on periods commencing significantly later in the year. Cyanobacterial communities in three interconnected temperate, oligotrophic lakes were characterized over an 18-month period from July 2018 to January 2020 (across 10 synoptic sampling events) using amplicon sequencing of the V4 region of the 16S rRNA gene. During the winter, there was low abundance or occasional absence of cyanobacteria; however, a non-photosynthetic basal lineage of cyanobacteria (Melainabacteria) was present during periods of ice cover. Notably, photosynthetic populations reappeared in the water column immediately following the loss of ice-cover—they were especially abundant in lakes with surficial geology and lake morphometry that favor greater availability of fine sediment and associated nutrients. Thus, this collective analysis demonstrates that the convergence of key abiotic and biotic factors—climate forcing of hydrological and biogeochemical processes, and intrinsic landscape features—enable increases in the relative abundance of potentially toxic cyanobacteria within the temperate forest biome of Canada over increasingly longer periods of time.

We compared stable isotopes of water in plant stem (xylem) water and soil collected over a complete growing season from five well‐known long‐term study sites in northern/cold regions. These spanned a decreasing temperature gradient from Bruntland Burn (Scotland), Dorset (Canadian Shield), Dry Creek (USA), Krycklan (Sweden), to Wolf Creek (northern Canada). Xylem water was isotopically depleted compared to soil waters, most notably for deuterium. The degree to which potential soil water sources could explain the isotopic composition of xylem water was assessed quantitatively using overlapping polygons to enclose respective data sets when plotted in dual isotope space. At most sites isotopes in xylem water from angiosperms showed a strong overlap with soil water; this was not the case for gymnosperms. In most cases, xylem water composition on a given sampling day could be better explained if soil water composition was considered over longer antecedent periods spanning many months. Xylem water at most sites was usually most dissimilar to soil water in drier summer months, although sites differed in the sequence of change. Open questions remain on why a significant proportion of isotopically depleted water in plant xylem cannot be explained by soil water sources, particularly for gymnosperms. It is recommended that future research focuses on the potential for fractionation to affect water uptake at the soil‐root interface, both through effects of exchange between the vapour and liquid phases of soil water and the effects of mycorrhizal interactions. Additionally, in cold regions, evaporation and diffusion of xylem water in winter may be an important process.

Data sharing is not applicable to this paper as no datasets were generated or analysed during the current study.

Plant–soil water isotopic dynamics in northern forests have been understudied relative to other forest types; nevertheless, such information can provide insight into how such forests may respond to hydroclimatic change. This study examines the co-evolution of xylem water and soil water stable isotopic compositions in a northern mixed forest in Ontario, Canada. Gross precipitation, bulk soil water and xylem water were sampled from pre-leaf out to post-senescence in 2016 for eastern white cedar, eastern hemlock, red oak and eastern white pine. Near-bole soil water contents and mobile soil water isotopic compositions were measured for the last three species. Mobile soil water did not deviate significantly from the local meteoric water line (LMWL). In contrast, near-surface bulk soil water showed significant evaporative enrichment relative to the LMWL from pre-leaf out to peak leaf out under all tree canopies, while xylem water was significantly depleted in 18O and particularly 2H relative to bulk soil water throughout the growing season. Inter-species differences in deviation of xylem water from the LMWL and their temporal changes emerged during the growing season, with coniferous species xylem water becoming isotopically enriched, while that of red oak became more depleted in 2H and 18O. These divergences occurred despite thin soil cover (generally <0.5 m depth to bedrock) which would constrain inter-species differences in tree rooting depths in this landscape. Isotopic fractionation at the tree root and fractionation of xylem water via evaporation through the tree bark are among the most plausible potential explanations for deviations between xylem and soil water isotopic compositions. Differences in the timing and intensity of water use between deciduous and coniferous trees may account for inter-specific variations in xylem water isotopic composition and its temporal evolution during the growing season in this northern forest landscape.

•We reviewed 82 studies of wildfire impacts to surface hydrology in North-America.•The number of studies has increased since 1980, with main focus on water chemistry.•Recent increase in studies linking fire to community water supplies.•Lack of standardized method to report post-fire environment in burned watersheds.•Need more water supply focus using spatial analysis, remote-sensing, and modelling.

Estimating streamflow is time and labour intensive due to the necessity of developing a rating curve. The development of a rating curve involves acquiring in‐field measurements of streamflow across a wide range of flow levels, which can be costly and impractical in remote regions with limited seasonal access. Here we showcase an automated system which estimates streamflow multiple times each day, greatly facilitating the development of rating curves for remote or seasonally inaccessible sites. The system uses an emerging technique referred to as particle image velocimetry (PIV) to track the movement of objects and flow structure features on the mobile water surface to generate velocity vector grids. Velocity grids were used to calculate streamflow and facilitate the development of a rating curve. This work contributes to our field‐based knowledge for using automated PIV systems to estimate streamflow in small streams (<5 m wide) using automatically distributed particles.

The time it takes water to travel through a catchment, from when it enters as rain and snow to when it leaves as streamflow, may influence stream water quality and catchment sensitivity to environmental change. Most studies that estimate travel times do so for only a few, often rain-dominated, catchments in a region and use relatively short data records (<10 years). A better understanding of how catchment travel times vary across a landscape may help diagnose inter-catchment differences in water quality and response to environmental change. We used comprehensive and long-term observations from the Turkey Lakes Watershed Study in central Ontario to estimate water travel times for 12 snowmelt-dominated headwater catchments, three of which were impacted by forest harvesting. Chloride, a commonly used water tracer, was measured in streams, rain, snowfall and as dry atmospheric deposition over a 31 year period. These data were used with a lumped convolution integral approach to estimate mean water travel times. We explored relationships between travel times and catchment characteristics such as catchment area, slope angle, flowpath length, runoff ratio and wetland coverage, as well as the impact of harvesting. Travel time estimates were then used to compare differences in stream water quality between catchments. Our results show that mean travel times can be variable for small geographic areas and are related to catchment characteristics, in particular flowpath length and wetland cover. In addition, forest harvesting appeared to decrease mean travel times. Estimated mean travel times had complex relationships with water quality patterns. Results suggest that biogeochemical processes, particularly those present in wetlands, may have a greater influence on water quality than catchment travel times.

High-latitude forests of North America are characterized by their natural dependence on large and severe wildfires. However, these wildfires also pose a range of social, economic, and environmental risks, with growing concern regarding persistent effects on stream flow volume, seasonal timing of flow, water quality, aquatic ecosystem health, and downstream community drinking water treatment. Here, we present the outcomes of a comprehensive scoping review of post-fire hydrologic studies in high-latitude forests of North America (Canada and Alaska). Our objectives were to (1) create an inventory of studies on post-fire hydrologic effects on surface water; (2) analyze those studies in terms of watershed characteristics and the type and duration of hydrologic effects; (3) identify and evaluate the link between upstream hydrologic effects with hydrologic ecosystem services; and (4) propose a research agenda addressing the link between wildfire science and hydrologic ecosystem services. We screened 2935 peer-reviewed articles and selected 82 studies to include based on their relevance according to a systematic, multi-step selection process. Next, we classified the papers into five themes: (a) runoff volume and flow regimes, (b) erosion and sediment transport, (c) water chemistry, (d) hydromorphology, and (e) aquatic food webs. For each study, we documented location, fire regime, watershed characteristics, and ecosystem services. The annual number of published studies on post-fire hydrology in high-latitude forests and, in particular, those addressing hydrologic ecosystem services, has increased steadily in recent years. Descriptions of wildfire characteristics, watershed characteristics, and effects on hydrologic ecosystem services were highly variable across studies, hindering cross-study comparisons. Moreover, there were limited efforts to extend study results to implications for forest or water management decisions regarding ecosystem services from source watersheds. Most studies focused on fire impacts on aquatic habitats and water chemistry while services of direct concern to communities, such as drinking water, were rarely addressed. We contend that study standardization, further use of geospatial technologies, and more studies directly addressing ecosystem services will help mitigate the increasing risks to water resources in northern forests.

Hydrological ProcessesVolume 34, Issue 26 p. 5686-5689 INVITED COMMENTARY Clarifying misconceptions regarding the relationship between Hewlett and Hibbert's translatory flow process and ecohydrological separation James Buttle, Corresponding Author James Buttle jbuttle@trentu.ca orcid.org/0000-0001-7231-8972 Geography, Trent University, Canada Correspondence James Buttle, Geography, Trent University, Canada. Email: jbuttle@trentu.caSearch for more papers by this author James Buttle, Corresponding Author James Buttle jbuttle@trentu.ca orcid.org/0000-0001-7231-8972 Geography, Trent University, Canada Correspondence James Buttle, Geography, Trent University, Canada. Email: jbuttle@trentu.caSearch for more papers by this author First published: 12 October 2020 https://doi.org/10.1002/hyp.13940Citations: 1Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Citing Literature Volume34, Issue2630 December 2020Pages 5686-5689 This article also appears in:HPToday: Invited Commentaries RelatedInformation

Water storage dynamics modulate fluxes within catchments, control the rainfall-runoff response and regulate the velocity of water particles through mixing associated processes. Tracer-aided models are useful tools for tracking the interactions between catchment storage and fluxes, as they can capture both the celerity of the runoff response and the velocity of water particles revealed by tracer dynamics. The phase-space reconstruction of modelled systems can help in this regard; it traces the evolution of a dynamic system from a known initial state as phase trajectories in response to inputs. In this study, we compared the modelled storage-flux dynamics obtained from the application of a spatially distributed tracer-aided hydrological model (STARR) in five contrasting long-term research catchments with varying degrees of snow influence. The models were calibrated using a consistent multivariate methodology based on discharge, isotope composition and snowpack water equivalent. Analysis of extracted modelled storage dynamics gave insights into the system functioning. Large volumes of total stored water needed to be invoked at most sites to reconcile celerity and travel times to match observe discharge and isotope responses. This is because changes in dynamic storage from water balance considerations are small when compared to volume of storage necessary for observed tracer dampening. In the phase-space diagrams, the rates of storage change gave insights into the relative storage volume and seasonal catchment functioning. The storage increase was dominated by hydroclimatic inputs; thus, it presented a stochastic response. Furthermore, depending on the dominance of snow or rainfall inputs, catchments had different seasonal responses in storage dynamics. Decreases in storage were more predictable and reflected the efficiency of catchment drainage, yet at lower storages the influence of ET was also evident. Activation of flow paths due to overland and near-surface flows resulted in non-linearity of catchment functioning largely at high storage states. The storage-discharge relationships generally showed a non-linear distribution, with more scattered states during wettest condition. In turn, all the catchments exhibited an inverse storage effect, with modelled water ages decreasing with increasing storage as lateral flow paths were activated. Insights from this inter-comparison of storage-flux-age dynamics show the benefits of tracer-aided hydrological models in exploring their interactions at well-instrumented sites to better understand hydrological functioning of contrasting catchments.

Storage is a fundamental but elusive component of drainage basin function, influencing synchronization between precipitation input and streamflow output and mediating basin sensitivity to climate and land use/land cover (LULC) change. We compare hydrometric and isotopic approaches to estimate indices of dynamic and total basin storage, respectively, and assess inter-basin differences in these indices across the Oak Ridges Moraine (ORM) region of southern Ontario, Canada. Dynamic storage indices for the 20 study basins included the ratio of baseflow to total streamflow (baseflow index BFI), Q (99) flow and flow duration curve (FDC) slope. Ratios of the standard deviation of the streamflow stable isotope signal relative to that of precipitation were determined for each basin from a 1 year bi-weekly sampling program and used as indicators of total storage. Smaller ratios imply longer water travel times, smaller young water fractions (F-yw, < similar to 2-3 months in age) in streamflow and greater basin storage. Ratios were inversely related to BFI and Q (99), and positively related to FDC slope, suggesting longer travel times and smaller F (yw) for basins with stable baseflow-dominated streamflow regimes. Inter-basin differences in all indices reflected topographic, hydrogeologic and LULC controls on storage, which was greatest in steep, forest-covered headwaters underlain by permeable deposits with thick and relatively uniform unsaturated zones. Nevertheless, differential sensitivity of indices to controls on storage indicates the value of using several indices to capture more completely how basin characteristics influence storage. Regression relationships between storage indices and basin characteristics provided reasonable predictions of aspects of the streamflow regime of test basins in the ORM region. Such relationships and the underlying knowledge of controls on basin storage in this landscape provide the foundation for initial predictions of relative differences in streamflow response to regional changes in climate and LULC.

The forest canopy can play a significant role in modifying the amount and isotopic composition of water during its passage throughout the near-surface critical zone. Here, partitioning of gross rainfall into interception, throughfall, and stemflow and its implications for the amount and isotopic composition of soil water was studied for red oak, eastern white pine, and eastern hemlock trees in a northern hardwood-conifer forest in south central Ontario, Canada. Stemflow production was greatest for red oak as a result of its upward-projecting branches and least for eastern white pine due to its horizontal branches and rougher bark. These stemflow contributions to the near-bole soil surface failed to produce consistently wetter soils relative to distal locations from the bole for all tree species. There was also no consistent evidence of isotopic enrichment of throughfall and stemflow relative to gross rainfall or of stemflow relative to throughfall for red oak or eastern hemlock. However, there was isotopic enrichment of both throughfall and stemflow for eastern white pine with increasing maximum atmospheric vapour pressure deficit, which may reflect the potential for evaporative fractionation as a result of retention and detention of water moving through the canopy by the rougher bark of this species. Dry soil conditions limited sampling of mobile soil water during the study, and there was no consistent evidence that either throughfall or stemflow fluxes controlled temporal changes in the isotopic signature of soil water beneath the tree. Thus, the potential for throughfall and stemflow fluxes in northern hardwood-conifer forests to modify the isotopic composition of water taken up by the tree via transpiration remains an open question.