Wavelet analysis of inter‐annual variability in the runoff regimes of glacial and nival stream catchments,Bow Lake,Alberta

Continuous wavelet analyses of hourly time series of air temperature, stream discharge, and precipitation are used to compare the seasonal and inter‐annual variability in hydrological regimes of the two principal streams feeding Bow Lake, Banff National Park, Alberta: the glacial stream draining the Wapta Icefields, and the snowmelt‐fed Bow River. The goal is to understand how water sources and flow routing differ between the two catchments. Wavelet spectra and cross‐wavelet spectra were determined for air temperature and discharge from the two streams for summers (June–September) 1997–2000, and for rainfall and discharge for the summers of 1999 and 2000. The diurnal signal of the glacial runoff was orders of magnitude higher in 1998 than in other years, indicating that significant ice exposure and the development of channelized glacial drainage occurred as a result of the 1997–98 El Niño conditions. Early retreat of the snowpack in 1997 and 1998 led to a significant summer‐long input of melt runoff from a small area of ice cover in the Bow River catchment; but such inputs were not apparent in 1999 and 2000, when snow cover was more extensive. Rainfall had a stronger influence on runoff and followed quicker flow paths in the Bow River catchment than in the glacial catchment. Snowpack thickness and catchment size were the primary controls on the phase relationship between temperature and discharge at diurnal time scales. Wavelet analysis is a fast and effective means to characterize runoff, temperature, and precipitation regimes and their interrelationships and inter‐annual variability. The technique is effective at identifying inter‐annual and seasonal changes in the relative contributions of different water sources to runoff, and changes in the time required for routing of diurnal meltwater pulses through a catchment. However, it is less effective at identifying changes/differences in the type of the flow routing (e.g. overland flow versus through flow) between or within catchments. Copyright © 2003 John Wiley & Sons, Ltd.     

Suspended sediment and total dissolved solid yield patterns at the headwaters of Urumqi River,northwestern China: a comparison between glacial and non‐glacial catchments

In order to understand the differences in the suspended sediment and total dissolved solid (TDS) yield patterns between the glacial and non‐glacial catchments at the headwaters of Urumqi River, northwestern China, water samples were collected from a glacier catchment and an empty cirque catchment within the region, during three melting seasons from 2006 to 2008. These samples were analyzed to estimate suspended sediment and TDS concentrations, fluxes and erosion rates in the two adjoining catchments. There were remarked differences in suspended sediment and TDS yield patterns between the two catchments. Suspended sediment concentrations were controlled mainly by the sediment source, whereas TDS concentrations were primarily related to the hydrologic interaction with soil minerals. Generally, the glacial catchment had much higher suspended sediment and TDS yields, together with higher denudation rates, than the non‐glacial catchment. Overall, glacial catchment was mainly dominated by physical denudation process, whereas the non‐glacial catchment was jointly influenced by physical and chemical denudation processes. The observed differences in material delivery patterns were mainly controlled by the runoff source and the glacial processes. The melting periods of glacier and snow were typically the most important time for the suspended sediment and TDS yields. Meanwhile, episodic precipitation events could generate disproportionately large yields. Subglacial hydrology dynamics, glaciers pluck and grind processes could affect erodibility, and the large quantities of dust stored on the glacier surface provided additional sources for suspended sediment transport in the glacial catchment. These mechanisms imply that, in response to climate change, the catchment behaviour will be modified significantly in this region, in terms of material flux. Copyright © 2013 John Wiley & Sons, Ltd.     

Hydrological and hydrochemical response of a small canadian shield catchment to late winter rain-on-snow events

A study was undertaken during the winter of 1990–1991 in a small (3.7 ha) Canadian Shield catchment to examine the hydrological and hydrochemical response during rain-on-snow events. The results are presented of two large (37.9 and 34.6 mm) rain-on-snow events occurring in early and late March 1991. Peak and total runoff and the groundwater response from the two events are significantly different. Hydrological data indicate that these differences can be attributed to a combination of meteorological (temperature) and physical conditions (antecedent snowpack ripeness, soil moisture and groundwater levels). An immature snowpack (low temperature and density) combined with low antecedent soil moisture conditions significantly reduced the magnitude of the net hydrological input and runoff from the catchment during the early March event, whereas a more mature snowpack and high antecedent soil moisture conditions led to a large runoff event during late March. During both rain-on-snow events a significant portion of the pre-event snowpack chemical load was lost. Based on the maximum snowpack chemical load measured before the events, the two large rain-on-snow events and a brief mid-March warm period during which there were two much smaller rain-on-snow events removed 78% of the hydrogen ion and 63% of the sulphate and nitrate load from the snowpack, while only reducing snowpack water equivalence by 7%. A two-component (rain and snowmelt) isotopic (δ¹⁸O SMOW %₀) separation of snowmelt lysimeter water during the two events indicated that snowmelt was an important (50 and 65%, respectively) water source available for infiltration and runoff at the snow-soil interface. Considering the high hydrogen ion loadings to the catchment during these two events (3.3 and 3.0 mequiv.m^?2, respectively) streamflow pH was not significantly reduced due to an increase in the discharge of well-buffered groundwater. A two-component isotopic hydrograph separation of peak stream discharge during the 2–3 March event indicated that 75% of the total flow was groundwater. In mid-latitude acid-sensitive catchments, winter rain-on-snow events are an important hydrological occurrence due to their ability to elute much of the chemical load (H⁺, SO₄, NO₃) from the snowpack before the onset of spring melt when the maximum annual hydrological input typically occurs.     

Temporal and spatial variability of cation and silica export in an alpine watershed,Emerald Lake,California

A reaction set of possible mineral weathering reactions is proposed to explain observed cation and silica export for the Emerald Lake watershed, a small Sierra Nevada, California catchment. The reaction set was calculated through a stoichiometric mole‐balance method, using a multiyear record of stream flow and snowpack chemical analyses and site‐specific mineral compositions. Reaction‐set calculations were intended to explore how the processes controlling stream cation and silica export depend on differing bedrock mineralogy across the catchment as snowmelt and runoff patterns change over the year. Different regions within the watershed can be differentiated by lake inflow subdrainages, each exhibiting different stream‐flow chemistry and calculated weathering stoichiometry, indicating that different silica and cation generation processes are dominant in wet steep portions of the catchment. Short‐term differences in stream concentrations were assumed to reflect ion exchange equilibria and rapid biological processes, whereas long‐term persistent stream concentration differences in different areas of the catchment were assumed to reflect spatial variability in mineral weathering stoichiometry. Mineralogical analyses of rock samples from the watershed provided site‐specific chemical compositions of major mineral species for reaction calculations. Reaction sets were evaluated by linear regression of calculated versus observed differences between snowmelt and stream‐flow chemistry and by a combined measure. Initially, single weathering reactions were balanced and evaluated to determine the reactions that best explained observed stream chemical export. Next, reactions were combined, using mineral compositions from different rock types to estimate the dependence of ion fluxes on lithology. The seasonal variability of major solute calculated fluxes is low, approximately one order of magnitude, relative to the observed three orders of magnitude variability in basin discharge. Reaction sets using basin‐averaged lithology and Aplite lithologies gave superior explanations of stream chemical composition. Copyright © 2004 John Wiley & Sons, Ltd.     

An isotopic and geochemical study of snowmelt runoff in a small arctic watershed

‘Old’ water contributions to snowmelt runoff in a stream can be defined as water which was stored in the catchment prior to the start of the runoff event in question. We used mass balance techniques for natural oxygen-18 and several chemical parameters (electrical conductivity, Ca and Mg) to estimate the magnitude and timing of ‘old’ water contributions to snowmelt runoff in the Apex River watershed during the 1983 field season. The Apex River catchment is located in the southern part of Baffin Island, Northwest Territories, Canada. The watershed has an area of 60 km², it is in the zone of continuous permafrost, and the geology is dominantly Precambrian gneiss with sporadic, thin, glacial overburden. The isotopic data indicate that for the snowmelt season of 1983 (third highest peak discharge of 11 years of record), approximately 50 per cent of the peak stream discharge consisted of ‘old’ water. Our data also suggest that about 60 per cent of the entire 1983 hydrograph was ‘old’ water. The chemical parameters give old water contributions which are at least 10 per cent less than the isotopically-derived estimate, but they are consistent with the isotopic estimate during peak flow.     

A modelling framework to simulate field‐scale nitrate response and transport during snowmelt: The WINTRA model

Modelling nutrient transport during snowmelt in cold regions remains a major scientific challenge. A key limitation of existing nutrient models for application in cold regions is the inadequate representation of snowmelt, including hydrological and biogeochemical processes. This brief period can account for more than 80% of the total annual surface runoff in the Canadian Prairies and Northern Canada and processes such as atmospheric deposition, overwinter redistribution of snow, ion exclusion from snow crystals, frozen soils, and snow‐covered area depletion during melt influence the distribution and release of snow and soil nutrients, thus affecting the timing and magnitude of snowmelt runoff nutrient concentrations. Research in cold regions suggests that nitrate (NO₃) runoff at the field‐scale can be divided into 5 phases during snowmelt. In the first phase, water and ions originating from ion‐rich snow layers travel and diffuse through the snowpack. This process causes ion concentrations in runoff to gradually increase. The second phase occurs when this snow ion meltwater front has reached the bottom of the snowpack and forms runoff to the edge‐of‐the‐field. During the third and fourth phases, the main source of NO₃ transitions from the snowpack to the soil. Finally, the fifth and last phase occurs when the snow has completely melted, and the thawing soil becomes the main source of NO₃ to the stream. In this research, a process‐based model was developed to simulate hourly export based on this 5‐phase approach. Results from an application in the Red River Basin of southern Manitoba, Canada, shows that the model can adequately capture the dynamics and rapid changes of NO₃ concentrations during this period at relevant temporal resolutions. This is a significant achievement to advance the current nutrient modelling paradigm in cold climates, which is generally limited to satisfactory results at monthly or annual resolutions. The approach can inform catchment‐scale nutrient models to improve simulation of this critical snowmelt period.     

Hydrochemical and sedimentary responses of paired High Arctic watersheds to unusual climate and permafrost disturbance,Cape Bounty,Melville Island,Canada

High Arctic river responses to changing hydroclimatic and landscape processes are poorly understood. In non‐glacierized basins, snowmelt and rainfall generate river discharge, which provides first order control over fluxes. Further factors include the seasonality of precipitation, seasonal active layer development, and permafrost disturbance. These controls were evaluated in terms of sedimentary and biogeochemical fluxes from paired catchments at Cape Bounty, Melville Island, Nunavut during 2006–2009. Results indicate that the source of runoff can be more important than the amount of runoff for sediment, solutes, and organic yields. Although the snowmelt period is typically the most important time for these yields, heavy late summer precipitation events can create disproportionately large yields. Rainfall increases yields because it hydrologically connects areas otherwise isolated. Inorganic solute yields from late summer rainfall are higher because the thick active layer maximizes hydrologic interactions with mineral soils and generates high solute concentrations. Results also indicate that while the catchments are broadly similar, subtle topographic differences result in important inter‐catchment differences in runoff and suspended and dissolved loads. The East watershed, which had less extensive permafrost disturbance, consistently had higher concentrations of dissolved solids. These higher dissolved fluxes cannot therefore be explained by thermokarst features, but rather by deeper active layer development, due to a greater proportion of south‐facing slopes. Although warm temperatures in 2007 led to extensive active layer disturbance in the West watershed, because the disturbances were largely hydrologically disconnected, the total disturbed area was small, and inter‐annual variability in discharge was high, there was no detectable response in dissolved loads to disturbances. Sediment availability increased after 2007, but yields have largely returned to pre‐disturbance levels. Results indicate that seasonality and frequency‐magnitude characteristics of projected increases in precipitation must be considered along with active layer changes to predict the fluvial sedimentary and biogeochemical response to regional climate change. Copyright © 2011 John Wiley & Sons, Ltd.     

Seasonal snowpack dynamics and runoff in a cool temperate forest: lysimeter experiment in Niigata,Japan

Seasonal snowpack dynamics are described through field measurements under contrasting canopy conditions for a mountainous catchment in the Japan Sea region. Microclimatic data, snow accumulation, albedo and lysimeter runoff are given through the complete winter season 2002–03 in (1) a mature cedar stand, (2) a larch stand, and (3) a regenerating cedar stand or opening. The accumulation and melt of seasonal snowpack strongly influences streamflow runoff during December to May, including winter baseflow, mid‐winter melt, rain on snow, and diurnal peaks driven by radiation melt in spring. Lysimeter runoff at all sites is characterized by constant ground melt of 0·8–1·0 mm day⁻¹. Rapid response to mid‐winter melt or rainfall shows that the snowpack remains in a ripe or near‐ripe condition throughout the snow‐cover season. Hourly and daily lysimeter discharge was greatest during rain on snow (e.g. 7 mm h⁻¹ and 53 mm day⁻¹ on 17 December) with the majority of runoff due to rainfall passing through the snowpack as opposed to snowmelt. For both rain‐on‐snow and radiation melt events lysimeter discharge was generally greatest at the open site, although there were exceptions such as during interception melt events. During radiation melt instantaneous discharge was up to 4·0 times greater in the opening compared with the mature cedar, and 48 h discharge was up to 2·5 times greater. Perhaps characteristic of maritime climates, forest interception melt is shown to be important in addition to sublimation in reducing snow accumulation beneath dense canopies. While sublimation represents a loss from the catchment water balance, interception melt percolates through the snowpack and contributes to soil moisture during the winter season. Strong differences in microclimate and snowpack albedo persisted between cedar, larch and open sites, and it is suggested further work is needed to account for this in hydrological simulation models. Copyright © 2005 John Wiley & Sons, Ltd.     

Solute provenance,transport and denudation in a high arctic glacierized catchment

Recent understanding of chemical weathering in glacierized catchments has been focused on mid-latitude, Alpine catchments; comparable studies from the high latitudes are currently lacking. This paper attempts to address this deficiency by examining solute provenance, transport and denudation in a glacierized catchment at 78°N in the Svalbard High Arctic archipelago. Representative samples of snow, glacier ice, winter proglacial icing and glacier meltwater were obtained from the catchment during spring and summer 1993 and analysed for major ion chemistry. Seasonal variations in the composition of glacier meltwater occur and are influenced by proglacial solute acquisition from the icing at the very start of the melt season, and subsequently by a period of discharge of concentrated snowmelt caused by snowpack elution; weathering within the ice-marginal channels that drain the glacier, particularly carbonation reactions, continues to furnish solute to meltwater when suspended sediment concentrations increase later in the melt season. Partitioning the solute flux into its various components (sea-salt, crustal, aerosol and atmospheric sources) shows that c. 25% of the total flux is sea salt derived, consistent with the maritime location of the glacier, and c. 71% is crustally derived. Estimated chemical denudation, 160 meq m⁻² a⁻¹ sea salt-corrected cation equivalent weathering rate, is somewhat low compared with other studied glacierized catchments (estimates in the range 450–1000 meq m⁻² a⁻¹), which is probably attributable to the relatively short melt season and low specific runoff in the High Arctic. A positive relationship was identified between discharge and CO₂ drawdown owing to carbonation reactions in turbid meltwater. © 1997 John Wiley & Sons, Ltd.     

The hydrochemistry of meltwaters draining a polythermal-based,high Arctic glacier,south Svalbard: I. The ablation season

Solute and runoff time-series at Finsterwalderbreen, Svalbard, provide evidence for considerable basal routing of water and the existence of at least two contrasting subglacial chemical weathering environments. The hydrochemistry of a subglacial upwelling provides evidence for a snowmelt-fed subglacial reservoir that dominates bulk runoff during recession flow. High concentrations of Cl⁻ and crustal ions, high pCO₂ and ratios of [*SO²⁻₄/(*SO²⁻₄+HCO⁻₃)] close to 0·5 indicate the passage of snowmelt through a subglacial weathering environment characterized by high rock:water ratios, prolonged residence times and restricted access to the atmosphere. At higher discharges, bulk runoff becomes dominated by icemelt from the lower part of the glacier that is conveyed through a chemical weathering environment characterized by low rock:water ratios, short residence times and free contact with atmospheric gases. These observations suggest that icemelt is routed via a hydrological system composed of basal/ice-marginal, englacial and supraglacial components and is directed to the glacier margins by the ice surface slope. Upwelling water flows relatively independently of icemelt to the terminus via a subglacial drainage system, possibly constituting flow through a sediment layer. Cold basal ice at the terminus forces it to take a subterranean routing in its latter stages. The existence of spatially discrete flow paths conveying icemelt and subglacial snowmelt to the terminus may be the norm for polythermal-based glaciers on Svalbard. Proglacial mixing of these components to form the bulk meltwaters gives rise to hydrochemical trends that resemble those of warm-based glaciers. These hydrochemical characteristics of bulk runoff have not been documented on any other glacier on Svalbard to date and have significance for understanding interactions between thermal regime and glacier hydrology. © 1998 John Wiley & Sons, Ltd.     

Nutrient concentration patterns in streams draining alpine and subalpine catchments, Fraser Experimental Forest, Colorado

Streamwater samples were collected during 1987–1988 from two adjacent gauged watersheds, the subalpine-alpine East St. Louis and the Fool Creek Alpine, in the Fraser Experimental Forest, Colorado. The study objective was to compare the relationships between streamwater discharge and ion concentration in alpine and alpine-subalpine watersheds at a site receiving low inputs of atmospheric contaminants. Streamwater discharge accounts for much of the variation in ion concentration. Trajectories of time, discharge, and ion concentration suggest that patterns of nutrient flux are controlled primarily by the magnitude of streamwater discharge, and seasonal differences in the relative contributions of snowmelt and soil water. In the subalpine catchment, increased streamwater discharge accounted for most of the decline in concentration of ions, with high concentrations in soil water relative to precipitation. This relationship was not seen in the alpine catchment, probably because of the influence of large diurnal variation in the ratio of snowmelt to soil water. In both catchments, ions with comparatively high concentrations in precipitation and the snowpack relative to soil water showed less concentration decline with increased streamwater discharge. The recurring nature of the trajectories, especially in the subalpine catchment, suggests that the time, discharge, and ion concentration patterns may represent a general characteristic in moderate-sized, undisturbed Rocky Mountain catchments which do not receive high inputs of airborne contaminants.     

One‐year‐long runoff forecast by a single snowpack evaluation

A degree‐day‐based model is presented for a 1 year ahead runoff forecast, with 1 day time steps. The input information is a single snowpack evaluation collected at the beginning of the snowmelt season. The snow‐cover dynamics, the key information for long‐term snowmelt forecast, are described by the snow‐line dynamics, i.e. by the movements of the downhill snowpack limit. The snowmelt volume, estimated by the snow‐line dynamics, is the exogenous input of an autoregressive transformation model. The model is calibrated by a least‐squares procedure on the basis of observed daily runoff data and the corresponding measurements of the snowpack volume (one measurement per year). A real‐world case study on the Alto Tunuyan River basin (2380 km², Argentinean Andes) is presented. The 1 year ahead Alto Tunuyan River runoff patterns, computed for both calibration and validation periods, reveal high agreement with observed streamflows. Copyright © 2004 John Wiley & Sons, Ltd.     

On the identification of ENSO-induced rainfall and runoff variability: a comparison of methods and indices

Abstract

It is known that the El Niño Southern Oscillation (ENSO) phenomenon induces marked climate variability across many parts of the world. However, in seeking useful relationships between ENSO and climate, several indices are available. In addition to the choice of index, previous studies assessing ENSO effects have employed a range of different methods to classify periods as El Niño, La Niña or Neutral. It is therefore clear that significant subjectivity exists in the adoption of ENSO classification schemes. In this study, several ENSO classification methods are applied to a range of ENSO indices. Each method-index combination is investigated to determine which provides the strongest relationship with rainfall and runoff in the Williams River catchment, New South Wales, Australia. The results demonstrate substantial differences between the methods and indices. The Multivariate ENSO Index (or MEI) is found to provide the best classification irrespective of method. The potential for forecasting ENSO-related effects on rainfall, runoff and river abstractions is then investigated. A “rise rule” to account for dynamic ENSO trends is also assessed. Strong relationships were found to exist with runoff (rainfall) up to nine (eight) months in advance of the Summer/autumn period. Implications for improved forecasting of potential river abstractions are apparent.     

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

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

Hydroclimatic and channel snowpack controls over suspended sediment and grain size transport in a High Arctic catchment

Temporal variability in suspended sediment delivery processes was studied during three seasons in a 7·9 km² catchment at Cape Bounty, Melville Island, Nunavut in the Canadian High Arctic. Discharge was controlled primarily by the magnitude of snowmelt, with limited inputs from ground ice melt and precipitation. Years with greater snowpack non‐linearly increased sediment yield and resulted in seasonal counter‐clockwise hysteresis, while a year with low snowpack resulted in reduced sediment yield and clockwise hysteresis, and indicates that sediment was increasingly available after the onset of streamflow. In addition to the event‐scale hysteresis observed during the nival discharge peak, diurnal clockwise hysteresis was observed during all three seasons and suggests daily exhaustion of sediment supplies. These results indicate that the channel snowpack plays a primary role over sediment accessibility during the nival discharge peak. Similarly, grain size analysis of suspended material in the river showed that the coarsest mean grain size was transported during the early phase of peak nival discharge and indicates that isolated sources of coarse material were being accessed by high velocity flow. Snowpack is present through the peak nival period and conditions sediment availability by isolating channel sediments from high‐energy flow. These results indicate sediment delivery characteristics in small High Arctic catchments reflect complex interactions with channel snowpack and disproportionate responses to flow conditions that differ from glacial and temperate settings. Copyright © 2008 John Wiley & Sons, Ltd.     

Basal sediment evacuation by subglacial meltwater: suspended sediment transport from Haut Glacier d'Arolla,Switzerland

Proglacial suspended sediment transport was monitored at Haut Glacier d'Arolla, Switzerland, during the 1998 melt season to investigate the mechanisms of basal sediment evacuation by subglacial meltwater. Sub‐seasonal changes in relationships between suspended sediment transport and discharge demonstrate that the structure and hydraulics of the subglacial drainage system critically influenced how basal sediment was accessed and entrained. Under hydraulically inefficient subglacial drainage at the start of the melt season, sediment availability was generally high but sediment transport increased relatively slowly with discharge. Later in the melt season, sediment transport increased more rapidly with discharge as subglacial meltwater became confined to a spatially limited network of channels following removal of the seasonal snowpack from the ablation area. Flow capacity is inferred to have increased more rapidly with discharge within subglacial channels because rapid changes in discharge during highly peaked diurnal runoff cycles are likely to have been accommodated largely by changes in flow velocity. Basal sediment availability declined during channelization but increased throughout the remainder of the monitored period, resulting in very efficient basal sediment evacuation over the peak of the melt season. Increased basal sediment availability during the summer appears to have been linked to high diurnal water pressure variation within subglacial channels inferred from the strong increase in flow velocity with discharge. Basal sediment availability therefore appears likely to have been increased by (1) enhanced local ice‐bed separation leading to extra‐channel flow excursions and[sol ]or (2) the deformation of basal sediment towards low‐pressure channels due to a strong diurnally reversing hydraulic gradient between channels and areas of hydraulically less‐efficient drainage. Copyright © 2005 John Wiley & Sons, Ltd.     

Proglacial sediment dynamics from daily to seasonal scales in a glaciated Alpine catchment (Bossons glacier,Mont Blanc massif,France)

The sediment yields of Alpine catchments are commonly determined from streamload measurements made some distance downstream from glaciers. However, this approach indiscriminately integrates erosion processes occurring in both the glacial and proglacial areas. A specific method is required to ascertain the respective inputs from (i) subglacial and supraglacial sediments, (ii) proglacial hillslopes and (iii) proglacial alluvial areas or sandurs. This issue is addressed here by combining high‐resolution monitoring (2 min) of suspended sediment concentrations at different locations within a catchment with discharge gauging and precipitation data. This methodological framework is applied to two proglacial streams draining the Bossons glacier (Mont Blanc massif, France): the Bossons and Crosette streams. For the Bossons stream, discharge and suspended load data were acquired from June to October 2013 at 1.15 and 1.5 km from the glacial terminus, respectively upstream and downstream from a small valley sandur. These hydro‐sedimentary data are compared with the Crosette stream dataset acquired at the outlet of the Bossons glacier subglacial drainage system. A fourfold analysis focusing on seasonal changes in streamload and discharge, multilinear regression modelling, evaluation of the sandur flux balance and probabilistic uncertainty assessment is used to determine the catchment sediment budget and to explain the proglacial sediment dynamics. The seasonal fluctuation of the sediment signal observed is related to the gradual closing of the subglacial drainage network and to the role of the proglacial area in the sediment cascade: the proglacial hillslopes appear to be disconnected from the main channel and the valley sandur acts as a hydrodynamic sediment buffer both daily and seasonally. Our findings show that an understanding of proglacial sediment dynamics can help in evaluating paraglacial adjustment and subglacial erosion processes. Copyright © 2017 John Wiley & Sons, Ltd.     

Spatio‐temporal controls of snowmelt and runoff generation during rain‐on‐snow events in a mid‐latitude mountain catchment

A network of 30 standalone snow monitoring stations was used to investigate the snow cover distribution, snowmelt dynamics, and runoff generation during two rain‐on‐snow (ROS) events in a 40 km² montane catchment in the Black Forest region of southwestern Germany. A multiple linear regression analysis using elevation, aspect, and land cover as predictors for the snow water equivalent (SWE) distribution within the catchment was applied on an hourly basis for two significant ROS flood events that occurred in December 2012. The available snowmelt water, liquid precipitation, as well as the total retention storage of the snow cover were considered in order to estimate the amount of water potentially available for the runoff generation. The study provides a spatially and temporally distributed picture of how the two observed ROS floods developed in the catchment. It became evident that the retention capacity of the snow cover is a crucial mechanism during ROS. It took several hours before water was released from the snowpack during the first ROS event, while retention storage was exceeded within 1 h from the start of the second event. Elevation was the most important terrain feature. South‐facing terrain contributed more water for runoff than north‐facing slopes, and only slightly more runoff was generated at open compared to forested areas. The results highlight the importance of snowmelt together with liquid precipitation for the generation of flood runoff during ROS and the large temporal and spatial variability of the relevant processes. Copyright © 2015 John Wiley & Sons, Ltd.     

A laboratory investigation of the leaching of solute from snowpack by rainfall

Under controlled laboratory conditions, artificial rain leaches solute from snow columns, and gives rise to leachate with a composition similar to snowmelt, in addition to the solute initially present in the artificial rain. the initial concentration of ions in the leachate, normalized to the concentration of ions found in the original snow and corrected for the solute present in the artificial rain, is similar to those reported in other laboratory and field studies of snowmelt composition, but there is some evidence that the concentration of leached ions declines more rapidly than during snowmelt. Similarly, as in snowmelt studies, not all ions are leached with the same efficiency. Bearing in mind the confounding influences of snow crystal morphology and snow column hydrology, it seems likely that rain will leach solute from snowpack during rain-on-snow events, in a manner similar to leaching by snowmelt, and that the precise composition of the leachate will depend on the hydrological routing of rain-meltwater mixtures through the snowpack.     

Ecohydrologic separation alters interpreted hydrologic stores and fluxes in a headwater mountain catchment

Recent studies have demonstrated that compartmentalized pools of water preferentially supply either plant transpiration (poorly mobile water) or streamflow and groundwater (highly mobile water) in some catchments, a phenomenon referred to as ecohydrologic separation. The omission of processes accounting for ecohydrologic separation in standard applications of hydrological models is expected to influence estimates of water residence times and plant water availability. However, few studies have tested this expectation or investigated how ecohydrologic separation alters interpretations of stores and fluxes of water within a catchment. In this study, we compare two rainfall‐runoff models that integrate catchment‐scale representations of transport, one that incorporates ecohydrologic separation and one that does not. The models were developed for a second‐order watershed at the H.J. Andrews Experimental Forest (Oregon, USA), the site where ecohydrologic separation was first observed, and calibrated against multiple years of stream discharge and chloride concentration. Model structural variations caused mixed results for differences in calibrated parameters and differences in storage between reservoirs. However, large differences in catchment storage volumes and fluxes arise when considering only mobile water. These changes influence interpreted residence times for streamflow‐generating water, demonstrating the importance of ecohydrologic separation in catchment‐scale water and solute transport.