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# The importance of snowmelt spatiotemporal variability for isotope-based hydrograph separation in a high-elevation catchment

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• _boden & grundwasser~ Allgäu GmbH

## Abstract and Figures

Seasonal snow cover is an important temporary water storage in high-elevation regions. Especially in remote areas, the available data are often insufficient to accurately quantify snowmelt contributions to streamflow. The limited knowledge about the spatiotemporal variability of the snowmelt isotopic composition, as well as pronounced spatial variation in snowmelt rates, leads to high uncertainties in applying the isotope-based hydrograph separation method. The stable isotopic signatures of snowmelt water samples collected during two spring 2014 snowmelt events at a north- and a south-facing slope were volume weighted with snowmelt rates derived from a distributed physics-based snow model in order to transfer the measured plot-scale isotopic composition of snowmelt to the catchment scale. The observed δ18O values and modeled snowmelt rates showed distinct inter- and intra-event variations, as well as marked differences between north- and south-facing slopes. Accounting for these differences, two-component isotopic hydrograph separation revealed snowmelt contributions to streamflow of 35 ± 3 and 75 ± 14 % for the early and peak melt season, respectively. These values differed from those determined by formerly used weighting methods (e.g., using observed plot-scale melt rates) or considering either the north- or south-facing slope by up to 5 and 15 %, respectively.
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Hydrol. Earth Syst. Sci., 20, 5015–5033, 2016
www.hydrol-earth-syst-sci.net/20/5015/2016/
doi:10.5194/hess-20-5015-2016
The importance of snowmelt spatiotemporal variability for
isotope-based hydrograph separation in a high-elevation catchment
Jan Schmieder1, Florian Hanzer1, Thomas Marke1, Jakob Garvelmann2, Michael Warscher2, Harald Kunstmann2,
and Ulrich Strasser1
1Institute of Geography, University of Innsbruck, 6020 Innsbruck, Austria
2Institute of Meteorology and Climate Research – Atmospheric Environmental Research,
Karlsruhe Institute of Technology, 82467 Garmisch-Partenkirchen, Germany
Correspondence to: Jan Schmieder (jan.schmieder@uibk.ac.at)
Received: 15 March 2016 – Published in Hydrol. Earth Syst. Sci. Discuss.: 11 May 2016
Revised: 15 November 2016 – Accepted: 26 November 2016 – Published: 19 December 2016
Abstract. Seasonal snow cover is an important temporary
water storage in high-elevation regions. Especially in re-
mote areas, the available data are often insufﬁcient to ac-
curately quantify snowmelt contributions to streamﬂow. The
limited knowledge about the spatiotemporal variability of
the snowmelt isotopic composition, as well as pronounced
spatial variation in snowmelt rates, leads to high uncertain-
ties in applying the isotope-based hydrograph separation
method. The stable isotopic signatures of snowmelt water
samples collected during two spring 2014 snowmelt events
at a north- and a south-facing slope were volume weighted
with snowmelt rates derived from a distributed physics-
based snow model in order to transfer the measured plot-
scale isotopic composition of snowmelt to the catchment
scale. The observed δ18O values and modeled snowmelt rates
showed distinct inter- and intra-event variations, as well as
marked differences between north- and south-facing slopes.
Accounting for these differences, two-component isotopic
hydrograph separation revealed snowmelt contributions to
streamﬂow of 35 ±3 and 75 ±14 % for the early and peak
melt season, respectively. These values differed from those
determined by formerly used weighting methods (e.g., us-
ing observed plot-scale melt rates) or considering either the
north- or south-facing slope by up to 5 and 15 %, respec-
tively.
1 Introduction
In many headwater catchments, seasonal water availability
is strongly dependent on cryospheric processes and under-
standing these processes becomes even more relevant in a
changing climate (APCC, 2014; IPCC, 2013; Weingartner
and Aschwanden, 1992). The seasonal snow cover is an im-
portant temporary water storage in alpine regions. The tim-
ing and amount of water released from this storage is im-
portant to know for water resources management, especially
in downstream regions where the water is needed (drinking
water, snow making, hydropower, irrigation water) or where
it represents a potential risk (ﬂood, drought). Environmental
tracers are a common tool to investigate the hydrological pro-
cesses, but scientiﬁc studies are still rare for high-elevation
regions because of the restricted access and high risk for ﬁeld
measurements in these challenging conditions.
Two-component isotope-based hydrograph separation
(IHS) is a technique to separate streamﬂow into differ-
ent time source components (event water, pre-event water)
(Sklash et al., 1976). The event component depicts water that
enters the catchment during an event (e.g., snowmelt) and is
characterized by a distinct isotopic signature, whereas pre-
event water is stored in the catchment prior to the onset of
the event (i.e., groundwater and soil water, which form base-
ﬂow) and is characterized by a different isotopic signature
(Sklash and Farvolden, 1979; Sklash et al., 1976). The tech-
nique dates back to the late 1960s (Pinder and Jones, 1969)
and was initially used for separating storm hydrographs in
humid catchments. The ﬁrst snowmelt-based studies were
Published by Copernicus Publications on behalf of the European Geosciences Union.
5016 J. Schmieder et al.: The importance of snowmelt spatiotemporal variability
conducted in the 1970s by Dinçer et al. (1970) and Martinec
et al. (1974). These studies showed a large pre-event water
fraction (> 50%) of streamﬂow that changed the understand-
ing of the processes in catchment hydrology fundamentally
(Klaus and McDonnell, 2013; Sklash and Farvolden, 1979)
and forced a paradigm shift, especially for humid temperate
catchments. However, other snowmelt-based studies in per-
mafrost or high-elevation catchments (Huth et al., 2004; Liu
et al., 2004; Williams et al., 2009) revealed a large contribu-
tion of event water (>70 %), depending on the system state
(e.g., frost layer thickness and snow depth), catchment char-
acteristics, and runoff generation mechanisms.
Klaus and McDonnell (2013) highlighted the need to
quantify and account for the spatial variability of the iso-
tope signal of event water, which is still a vast uncertainty
in snowmelt-based IHS. In the literature inconclusive results
prevail with respect to the variation of the isotopic signal
of snowmelt. Spatial variability of snowmelt isotopic com-
position was statistically signiﬁcant in relation to elevation
(Beaulieu et al., 2012) in a catchment in British Columbia,
Canada, with 500 m relief. Moore (1989) and Laudon et
al. (2007) found no statistical signiﬁcant variation in their
snowmelt δ18O data, due to the low gradient and small el-
evation range (approximately 30 and 290m) in their catch-
ments, which favors an isotopically more homogenous snow
cover. The effect of the aspect of the hillslopes on isotopic
variability and IHS results in topographically complex ter-
rain has been rarely investigated. Dahlke and Lyon (2013)
and Dietermann and Weiler (2013) surveyed the snowpack
isotopic composition and showed a notable spatial variabil-
ity in their data, particularly between north- and south-facing
slopes. They conclude that the spatial variability of snowmelt
could be high and that the timing of meltwater varies with the
morphology of the catchment. Dietermann and Weiler (2013)
also concluded that an elevation effect (decrease of snow-
pack isotopic signature with elevation), if observed, is dis-
turbed by fractionation due to melt/refreeze processes during
the ablation period. Aspect and slope are therefore important
factors that affect the isotopic evolution of the snow cover
and its melt (Cooper, 2006). In contrast, there have been var-
ious studies that have investigated the temporal variability
of the snowmelt isotopic signal, e.g., with the use of snow
lysimeters (Hooper and Shoemaker, 1986; Laudon et al.,
2002; Liu et al., 2004; Maulé and Stein, 1990; Moore, 1989;
Williams et al., 2009). During the ablation season the iso-
topic composition of the snowpack changes due to percolat-
ing rain and meltwater, and fractionation caused by melting,
refreezing and sublimation (Dietermann and Weiler, 2013;
Lee et al., 2010; Unnikrishna et al., 2002; Zhou et al., 2008),
which leads to a homogenization of the isotopic proﬁle of the
snowpack (Árnason et al., 1973; Dinçer et al., 1970; Stich-
ler, 1987) and an increase in heavy isotopes of meltwater
throughout the freshet period (Laudon et al., 2007; Taylor
et al., 2001, 2002; Unnikrishna et al., 2002). Therefore, the
characterization and the use of the evolving isotopic signal
of snowmelt water instead of single snow cores is crucial for
applying IHS (Taylor et al., 2001, 2002).
There have been various approaches to cope with the tem-
poral variability of the input signal. If one uses more than
one δ18O snowmelt sample for applying the IHS method,
it is important to weight the values with appropriate melt
rates, e.g., measured from the outﬂow of a snow lysimeter.
Common weighting methods are the volume-weighted av-
erage approach (VWA), as used by Mast et al. (1995), and
the current meltwater approach (CMW), applied by Hooper
and Shoemaker (1986). Laudon et al. (2002) developed the
runoff-corrected event water approach (runCE), which ac-
counts for both, the temporal isotopic evolution and tempo-
rary storage of meltwater in the catchment and overcomes
the shortcoming of the exclusion of residence times by VWA
and CMW. This method was also deployed in several other
snowmelt-based IHS (Beaulieu et al., 2012; Carey and Quin-
ton, 2004; Laudon et al., 2004, 2007).
Tracers have successfully been used in modeling stud-
ies to provide empirical insights into runoff generation pro-
cesses and catchment functioning (Birkel and Soulsby, 2015;
Birkel et al., 2011; Capell et al., 2012; Uhlenbrook and Lei-
bundgut, 2002), but the combined use of distributed mod-
eling and isotope tracers in snow-dominated environments
is rare. Ahluwalia et al. (2013) used an isotope and model-
ing approach to derive snowmelt contributions to streamﬂow
and determined differences between the two techniques of
2 %. Distributed modeling can provide areal melt rates that
can be used for weighting the measured isotopic composi-
tion of meltwater. Pomeroy et al. (2003) described the differ-
ences of insolation between north- and south-facing slopes
in complex terrain that lead to spatial varying melt rates of
the snowpack throughout the freshet period. The use of the
areal snowmelt data from models will likely reduce the un-
certainty that arises from the representativeness of measured
melt rates at the plot-scale.
The overall goal of our study was to quantify the con-
tribution of snowmelt to streamﬂow and hence to improve
the knowledge of hydroclimatological processes in high-
elevation catchments. This study aims to enhance the reli-
ability of isotope-based hydrograph separation by consider-
ing the distinct spatiotemporal variability of snowmelt and
its isotopic signature in a high-elevation study region. This
study has the following three objectives: (1) the estimation
of the spatiotemporal variability of snowmelt and its isotopic
composition, (2) the quantiﬁcation of the impact of the spa-
tial variability in snowmelt rates and its isotopic composi-
tion on IHS, and (3) to assess the combined use of a physi-
cally based snowmelt model and traditional IHS to determine
snowmelt contributions to streamﬂow. Distributed melt rates
provided by a surface energy balance model were used to
weight the measured isotopic composition of snowmelt in or-
der to characterize the event water isotopic composition. Tra-
ditional weighting methods (e.g., using plot-scale observed
melt rates) were compared with the model approach.
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J. Schmieder et al.: The importance of snowmelt spatiotemporal variability 5017
Figure 1. (a) Distribution of slope aspects in the study area; (b) study area (Rofen valley) with underlying orthophoto, sampling, and
measurement locations.
2 Study area
The 98 km2high-elevation catchment of the Rofenache
stream is located in the central eastern Alps (Oetztal Alps,
Austria), close to the main Alpine ridge. The basin ranges in
elevation from approximately 1900 to 3770m.a.s.l. The av-
erage slope is 25and the average elevation is 2930 m.a.s.l.
(calculated from a 50 m digital elevation model). A nar-
row riparian zone (< 100 m width) is located in the valley
ﬂoor. The predominantly south- (southeast) and north-facing
(north-northwest) slopes form the main valley (Fig. 1a),
which trends roughly from southwest to northeast (Fig. 1b).
The study area has a dry inner-alpine climate. Mean annual
precipitation is 800 mm yr1, of which 44 % falls as snow.
The mean annual temperature at the gauging station in Vent
(1890 m.a.s.l., reference period: 1982–2003) is 2 C. Sea-
sonal snow cover typically lasts from October to the end of
June at the highest regions of the valley.
The bedrock consists of mainly paragneiss and mica schist
and is overlain by a mantle of glacial deposits and thin soils
(< 1m). The bedrock outcrops and unconsolidated bare rocks
cover the largest part (42%) of the catchment (CLC, 2006).
Glaciers cover approximately a third of the Rofenache catch-
ment (35 %), while pastures and coniferous forests are lo-
cated in the lowest parts of the catchment and cover less
than 0.5 % (CLC, 2006). Sparsely vegetated areas and nat-
ural grassland cover 15 and 7.5%, respectively (CLC, 2006).
Besides seasonally frozen ground at slopes of various ex-
positions, permafrost is likely to occur at an elevation over
2600 m.a.sl. at the north-facing slopes (Haeberli, 1975). The
annual hydrograph reveals a highly seasonal ﬂow regime.
The mean annual discharge is 4.5 m3s1(reference period:
1971–2009) and is dominated by snow and glacier melt dur-
ing the ablation season, which typically lasts from May to
September. The onset of the early snowmelt season in the
lower part of the basin is typically in April.
3 Methods
3.1 Field sampling, measurements, and laboratory
analysis
The ﬁeld work was conducted during the 2014 snowmelt sea-
son between the beginning of April and the end of June. Two
short-term melt events (3 days) were investigated to illustrate
the difference between early spring season melt and peak
melt. The events were deﬁned as warm and precipitation-free
spells, with clear skies and dry antecedent conditions (i.e., no
precipitation was observed 48 h prior to the event). Low dis-
charge and air temperatures with a small diurnal variation
and low melt rates, as well as a snow-covered area (SCA) of
about 90 % in the basin (Fig. 2a), characterize the conditions
of the early melt event at the end of April (Fig. 3b). In con-
trast, the peak melt period at the end of June is characterized
by high discharge and melt rates, a ﬂashy hydrograph, high
air temperatures with remarkable diurnal variations (Fig. 3c),
and a strongly retreated snow line (SCA: 66 %; Fig. 2c). Dis-
charge data are available at an hourly resolution for the gaug-
ing station in Vent and meteorological data are obtained by
two automatic weather stations (hourly resolution) located in
and around the basin (Fig. 1).
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5018 J. Schmieder et al.: The importance of snowmelt spatiotemporal variability
Figure 2. Comparison of observed and simulated snow distributions for (a, b) 5 May (MODIS scene) and (c, d) 10 June 2014 (Landsat
scene).
The stream water sampling for stable isotope analysis con-
sisted of pre-freshet baseﬂow samples at the beginning of
March, sub-daily samples (temporal resolution ranges be-
tween 1 and 4 h) during the two studied events and a post-
event sample in July as indicated in Fig. 3a (gray-shaded
area). Samples of snowmelt, snowpack, and surface over-
land ﬂow (if observed) were collected at the south- (S1,
S2) and north-facing slope (N1, N2), as well as on a wind-
exposed ridge (Fig. 1b) using a snowmelt collector. At each
test site, a snow pit was dug to install a 0.1m2polyethy-
lene snowmelt collector at the ground–snowpack interface.
The snowmelt collector consists of a pipe that drains the
percolating meltwater into a ﬁxed plastic bag. Tests yield a
preclusion of evaporation for this sampling method. Com-
posite daily snowmelt water samples (bulk sample) were col-
lected in these bags and transferred to polyethylene bottles
in the ﬁeld before the onset of the diurnal melt cycle. Fur-
thermore, sub-daily grab melt samples were collected at S1
(on 23 April) and at N2 (on 7 June) to deﬁne the diurnal
variability of the respective melt event. Unfortunately further
sub-daily snowmelt sampling was not feasible. The pit face
was covered with white styrofoam to protect it from direct
sunlight. Stream, surface overland ﬂow, and grab snowmelt
water samples were collected in 20 mL polyethylene bot-
tles. Snow samples from snow pit layers were ﬁlled in air-
tight plastic bags and melted below room temperature be-
fore being transferred into bottles. Overall, 144 samples were
taken during the study period. Snow water equivalent (SWE),
snow height, snow density, and various snowpack observa-
tions (wetness and hand hardness index) were observed be-
fore the onset of the diurnal melt cycle at the study plots
(Fig. 1). Mean SWE was determined by averaging ﬁve snow-
tube measurements within an area of 20 m2at each site. Daily
melt rates were calculated by subtracting succeeding SWE
values. Sublimation was neglected, as it contributes only a
small percentage (10 %) to the seasonal water balance in
high-altitude catchments in the Alps (Strasser et al., 2008).
All samples were treated by the guidelines proposed by
Clark and Fritz (1997) and were stored in the dark and kept
cold until analysis. The isotopic composition of the sam-
ples (δ18O, δD) was measured with cavity ring-down spec-
troscopy (Picarro L1102-i). Results are expressed in the delta
notation as parts per thousand relative to the Vienna Standard
Mean Ocean Water (VSMOW2). The mean laboratory pre-
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J. Schmieder et al.: The importance of snowmelt spatiotemporal variability 5019
Figure 3. (a) Daily precipitation, air temperature, and discharge at the outlet of the catchment during the complete study period; hourly
hydroclimatologic data of a 7-day period around the (b) early melt and (c) peak melt event. Gray-shaded areas indicate the investigated
events.
cision (replication of eight measurements) for all measured
samples was 0.06 ‰ for δ18O. Due to the covariance of δ2H
(δD) and δ18O (Fig. 5), all analyses were done with oxygen-
18 values.
3.2 Model description
For the simulation of the daily melt rates, the non-calibrated,
distributed, and physically based hydroclimatological model
AMUNDSEN (Strasser, 2008) was applied. Model features
include interpolation of meteorological ﬁelds from point
measurements (Marke, 2008; Strasser, 2008); simulation of
shortwave and longwave radiation, including topographic
and cloud effects (Corripio, 2003; Greuell et al., 1997); pa-
rameterization of snow albedo depending on snow age and
temperature (Rohrer, 1991); modeling of forest snow and
meteorological processes (Liston and Elder, 2006; Strasser et
al., 2011); lateral redistribution of snow due to gravitational-
(Gruber, 2007) and wind-induced (Helfricht, 2014; Warscher
et al., 2013) processes; and determination of snowmelt using
an energy balance approach (Strasser, 2008). Besides hav-
ing been applied for various other Alpine sites in the past
(Hanzer et al., 2014; Marke et al., 2015; Pellicciotti et al.,
2005; Strasser, 2008; Strasser et al., 2004, 2008), AMUND-
SEN has recently been set up and extensively validated for
the Oetztal Alps region (Hanzer et al., 2016). This setup
was also used to run the model in this study for the period
2013–2014 using a temporal resolution of 1 h and a spatial
resolution of 50 m. In order to determine the model perfor-
mance during the study period, catchment-scale snow distri-
bution by satellite-derived binary snow-cover maps and plot-
scale observed SWE data were used for the validation (cf.
Sect. 4.2). Therefore, the spatial snow distribution as simu-
lated by AMUNDSEN was compared with a set of MODIS
(500 m spatial resolution) and Landsat (30m resolution, sub-
sequently resampled to the 50 m model resolution) snow
maps with less than 10 % cloud coverage over the study area
using the methodology described in Hanzer et al. (2016).
Model results were evaluated using the performance mea-
sures BIAS, accuracy (ACC) and critical success index (CSI)
(Zappa, 2008). ACC represents the fraction of correctly clas-
siﬁed pixels (either snow covered or snow free both in the
observation and the simulation). CSI describes the number of
correctly predicted snow-covered pixels divided by the num-
ber of times where snow is predicted in the model and/or
observed, and BIAS corresponds to the number of snow-
covered pixels in the simulation divided by the respective
number in the observation. ACC and CSI values range from
0 to 1 (where 1 is a perfect match), while BIAS values below
1 indicate underestimations of the simulated snow cover, and
values above 1 indicate overestimations. At the plot-scale,
observed SWE values were compared with AMUNDSEN
SWE values represented by the underlying pixels at the lo-
cation of the snow course. Catchment-scale melt rates are
calculated by subtracting two consecutive daily SWE grids,
neglecting sublimation losses, which is also done to achieve
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5020 J. Schmieder et al.: The importance of snowmelt spatiotemporal variability
observed melt rates at the plot scale. Subsequently, the digital
elevation model was used to calculate an aspect grid and fur-
ther to divide the catchment into two parts: grid cells with as-
pects ranging from 270 to 90were classiﬁed as “north
facing”, while the remaining cells were attributed to the class
“south facing”. Finally, these two grids were combined to de-
rive melt rates for the south-facing (melts)and for the north-
facing slope (meltn).
3.3 Isotopic hydrograph separation, weighting
approaches, and uncertainty analysis
IHS is a steady-state tracer mass balance approach, and sev-
eral assumptions underlie this simple principle, which are de-
scribed and reviewed in Buttle (1994) and Klaus and McDon-
nell (2013):
1. The isotopic compositions of event and pre-event water
are signiﬁcantly different.
2. The event water isotopic signature has no spatiotempo-
ral variability, or variations can be accounted for.
3. The pre-event water isotopic signature has no spa-
tiotemporal variability, or variations can be accounted
for.
4. Contributions from the vadose zone must be negligible
or soil water should be isotopically similar to ground-
water.
5. There is no or minimal discharge contribution from sur-
face storage.
The focus of this study is on one of the assumptions: the spa-
tiotemporal variability of the event water isotopic signature
is absent or can be accounted for. The fraction of event water
(fe)contributing to streamﬂow was calculated from Eq. (1).
fe=CpCs
CpCe(1)
The tracer concentration of the pre-event component (Cp)is
the δ18O composition of baseﬂow prior to the onset of the
freshet period, constituted mainly by groundwater and po-
tentially by soil water, which was assumed to have the same
isotopic signal as groundwater. Tracer concentration Csis
the isotopic composition of stream water for each sampling
time. The isotopic compositions of snowmelt samples were
weighted differently to obtain the event water tracer concen-
tration (Ce)using the following ﬁve weighting approaches:
1. volume weighted with observed plot-scale melt rates
(VWO);
2. equally weighted, assuming an equal melt rate on north-
and south-facing slopes (VWE);
3. no weighting, only south-facing slopes considered
(SOUTH);
4. no weighting, only north-facing slopes considered
(NORTH);
5. volume weighted with simulated catchment-scale melt
rates (VWS).
Equation (2) is the VWS approach with simulated melt rates
for north- and south-facing slope as described in Sect. 3.2,
where Mis the simulated melt rate (in mm d1),δ18O is the
isotopic composition of sampled snowmelt, and subscripts
s and n indicate north and south, respectively. For obtain-
ing the value of Cea daily time step (t) is used, considering
daily melt rates and the isotopic composition of the daily bulk
snowmelt samples.
Ce(t)=Ms(t)δ18Os(t)+Mn(t 18On(t)
Ms(t)+Mn(t) (2)
An uncertainty analysis (Eq. 3) was performed accord-
ing to the Gaussian standard error method proposed by
Genereux (1998):
Wfe=
"CpCs
CpCe2WCe#2
+"CsCe
CpCe2WCp#2
+"1
CpCe2WCs#2
1/2
,(3)
where Wis the uncertainty, Cis the isotopic composition,
fis the fraction, and the subscripts p, s, and e refer to the
pre-event, stream, and event component, respectively. This
assumes negligible errors in the discharge measurements and
the melt rates (modeled and observed). The uncertainty of
streamﬂow (WCs)is assumed to be equal to the laboratory
precision (0.06 ‰). For the uncertainty of the event compo-
nent (WCe), the diurnal temporal variability (standard devi-
ation) of the snowmelt isotopic signal (from one site and 1
day) was multiplied by the appropriate value of the two-tailed
t-table (dependent on sample number) and used for the event,
as proposed by Genereux (1998). This resulted in different
uncertainty values for the early melt event (WCe=0.2 ‰)
and the peak melt event (WCe=0.5‰). An error of 0.04
was assumed for the pre-event component (WCp), which re-
ﬂects the standard deviation of two baseﬂow samples. A 95%
conﬁdence level was used. Spatial variation in snowmelt and
its isotopic composition were not considered in this error cal-
culation method as they represent the hydrologic signal of
interest.
4 Results
4.1 Spatiotemporal variability of streamﬂow and stable
isotopic signature of sampled of water sources
Two major snowmelt pulses (mid-May and beginning of
June) and four less pronounced pulses between mid-March
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J. Schmieder et al.: The importance of snowmelt spatiotemporal variability 5021
Figure 4. Linearly interpolated stream isotopic content of Rofenache for (a) the early melt and (b) the peak melt event. Dots indicate
measurements. Event and pre-event water contributions during (c) the early melt and (d) the peak melt event calculated with the VWS
approach.
Figure 5. Relationship between δ2H and δ18O of water sources
sampled during the snowmelt season of 2014 in the Rofen valley,
Austrian Alps.
to early May occurred during the snowmelt season (Fig. 3a).
Peak melt occurred at the beginning of June with maximum
daily temperatures and runoff of 15 C and 18 mm d1, re-
spectively. The following high ﬂows were affected by rain
(Fig. 3a) and glacier melt due to the strongly retreated snow
line and snow-free ablation area of the glaciers in July. Di-
urnal variations in discharge were strongly correlated with
diurnal variations in air temperature (Fig. 3b and c) with
a time lag of 3–5 h for the early melt event and 2–3 h for
the peak melt event. An inverse relationship between stream-
ﬂow δ18O and discharge was found for the early melt event
(Fig. 4a and c). Small diurnal responses of streamﬂow δ18O
were identiﬁed for both events, but were masked due to miss-
ing data during the recession of the hydrograph.
The quality control of the isotopic data was performed by
the δ2H–δ18O plot (Fig. 5), which indicated no shift in the
linear regression line and thus no secondary fractionation ef-
fects (evaporation) during storage and transport of the sam-
ples. The slope of the linear regression (slope =8.5, n=144,
R2=0.93) of the measurement data slightly deviates from
that of the global meteoric (slope =8) and local meteoric wa-
ter line (slope =8.1) based on monthly data from the Aus-
trian Network of Isotopes in Precipitation sampling site in
Obergurgl, which is located in an adjacent valley (reference
period: 1991–2014). The small deviation (visible in Fig. 5) of
the sampled water (i.e., snowpack and snowmelt) could indi-
cate fractionation effects induced by phase transition (i.e.,
melt/refreeze and sublimation). The signiﬁcant differences
between the isotopic signatures of pre-event streamﬂow and
snowmelt water enabled the IHS.
Overall, the δ18O values ranged from 21.5 to 15.0 ‰,
while snowpack samples were characterized by the most neg-
ative and pre-event baseﬂow samples by the least negative
values. Snowpack samples showed a wide isotopic range,
while streamﬂow samples revealed the narrowest spread, re-
ﬂecting a composite isotopic signal mixing of the water com-
ponents. Figure 6 shows the δ18O data of the water samples
grouped into different categories and split into early and peak
melt data. It shows the different δ18O ranges and medians of
the sampled water sources (Fig. 6a), as well as marked spa-
tiotemporal variations in the isotopic signal (Fig. 6b and c).
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5022 J. Schmieder et al.: The importance of snowmelt spatiotemporal variability
Figure 6. Jittered dot plots for δ18O of collected water samples split into (a) water sources, (b) stage of snowmelt and (c) spatial origin. Gray
circles indicate early melt samples and black circles peak melt samples. The gray and black line represents the median of early and peak melt
data, respectively. neis the number of early melt samples and npis the number of peak melt samples.
It is apparent that the snowpack δ18O values have a larger
variation compared to the snowmelt data due to homoge-
nization effects (Fig. 6a), as was also shown by Árnason et
al. (1973), Dinçer et al. (1970) and Stichler (1987). The me-
dian of the δ18O of snowmelt was higher than that of the
snowpack, which indicates fractionation. The median δ18O
of surface overland ﬂow was higher than that of snowmelt
(Fig. 6a) for the early and peak melt period. Overall, the peak
melt δ18O values (Fig. 6b) were less variable and had a higher
median than the early melt values, because fractionation ef-
fects (due to melt/refreeze and sublimation) most likely al-
tered the isotopic composition of the snowpack over time (cf.
Taylor et al., 2001, 2002). One major ﬁnding was that the
δ18O values on the north-facing slope had a larger range and
a lower median compared to the opposing slope (Fig. 6c).
Samples from the wind-drift-inﬂuenced site (also south ex-
posed) were more depleted in heavy isotopes compared to
the south-facing slope samples (Fig. 6c).
In general, the average snowmelt and snowpack isotopic
composition was more depleted for the early melt period (Ta-
ble 1) and changed over time because fractionation likely al-
tered the snowpack and its melt. It is obvious that the iso-
topic evolution (gradually enrichment) on the south-facing
slope took place earlier in the annual melting cycle of the
snow, and indicates a premature snowpack concerning the
enrichment of isotopes and earlier ripening compared to the
north-facing slope.
Table 1 shows that meltwater sampling throughout the en-
tire snowmelt period is required to account for the tempo-
ral variation in the isotopic composition of the snowpack
(cf. Taylor et al., 2001, 2002). In detail, the snowpack and
snowmelt δ18O data highlighted a marked spatial inhomo-
geneity between north- and south-facing slopes throughout
the study period. The snowpack isotopic composition from
both sampled slopes was statistically different for the early
melt, but not for the peak melt (with Kruskal–Wallis test at
0.05 signiﬁcance level), whereas the snowmelt δ18O showed
a signiﬁcant difference throughout the study period (Fig. 7).
Sub-daily snowmelt samples (n=5) at S1 (23 April 2014)
had a range of 0.1 ‰ in δ18O, and the bulk sample (inte-
grating the entire diurnal melt cycle) was within the scat-
ter of those values (Fig. 8). The intra-daily variability of
snowmelt (n=3) at N2 (7 June 2014) was relatively higher
with values ranging from 17.9 to 18.1 ‰. The bulk sam-
ple (17.9 ‰) was at the upper end of those values (Fig. 8).
Stream water isotopic composition was more enriched in
heavy isotopes during the early melt period and successively
became more depleted throughout the freshet period, result-
ing in more negative values during peak melt (Table 2). The
standard deviation and range of stream water δ18O during
early melt was higher and could be related to an increasing
snowmelt contribution throughout the event and larger diur-
nal amplitudes of snowmelt contribution compared to peak
melt (Table 2).
4.2 Snow model validation and snowmelt variability
Figure 9 shows the values for the selected performance mea-
sures based on the available MODIS and Landsat scenes
during the period March–July 2014. The results indicate a
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J. Schmieder et al.: The importance of snowmelt spatiotemporal variability 5023
Table 1. Average isotopic composition of snowpack and snowmelt with standard deviation for north- and south-facing slopes during the early
and the peak melt event. Values are averages of 3 consecutive days.
North-facing slope South-facing slope
Snowpack δ18O (‰) Snowmelt δ18O (‰) Snowpack δ18O (‰) Snowmelt δ18O (‰)
Early melt event 19.7 ±0.6 (n=12) 18.8 ±0.2 (n=3) 17.3 ±0.3 (n=4) 17.4 ±0.2 (n=8)
Peak melt event 17.6 ±0.4 (n=18) 17.9 ±0.1 (n=3) 17.9 ±0.1 (n=15) 17.1 ±0.0 (n=2)
Table 2. Descriptive statistics of streamﬂow isotopic composition at the outlet of the Rofenache during events of the snowmelt season 2014.
Pre-event Early melt Peak melt Post-event
Date 7 Mar 23–25 Apr 7–9 Jun 11 Jul
Average (δ18O ‰ ) 15.02 15.97 16.87 15.09
Standard deviation (δ18O ‰) 0.04 0.16 0.05
Range (δ18O ‰) 0.05 0.50 0.20
Number of samples 2 17 30 1
Figure 7. Jittered dot plots for δ18O of (a) snowpack
and (b) snowmelt of north- and south-facing slopes. Gray circles
indicate early melt samples and black circles are for peak melt sam-
ples. The gray and black lines indicates the median of the early and
peak melt data, respectively. neis the number of early melt samples
and npis the number of peak melt samples.
reasonable model performance with a tendency to slightly
overestimate the snow cover during the peak melt season
(BIAS > 1). In general the CSI does not drop below 0.7, and
80 % of the pixels are correctly classiﬁed (ACC) through-
out the study period. Figure 2 shows the observed and sim-
ulated spatial snow distribution around the time of the two
events. Despite a higher SCA during the early melt season
(Fig. 2a and b) compared to the peak melt season (Fig. 2c
and d) one can see the overestimation of the simulated SCA
compared to the observed (MODIS/Landsat) SCA. Table 3
Figure 8. Comparison of snowmelt δ18O between the bulk sample
(dashed line) and sub-daily samples (circles) for the two sites (S1,
N2).
shows the observed and simulated SWE values at the plot
scale. The model slightly underestimated SWE during peak
melt, but generally appears to be in quite good agreement,
suggesting well-simulated snowpack processes. Throughout
the study period the model deviates by 13 % from the ob-
served SWE values, but the representativeness (small-scale
effects) of SWE values for the respective 50 m pixels should
be considered.
Snowmelt (observed and simulated daily losses of SWE)
showed a distinct spatial variation between the north-facing
and the south-facing slope for the early melt (23/24 April) pe-
riod, but less marked variations for the peak melt (7/8 June)
period (Fig. 10). Relative day-to-day differences are more
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5024 J. Schmieder et al.: The importance of snowmelt spatiotemporal variability
Table 3. Comparison of observed and simulated (represented by the underlying pixel) SWE values.
Site Date Stage of SWE [mm] Difference between
snowmelt observed and
season simulated SWE [%]
Observed Simulated
S1 23 Apr 2014 Early melt 141 151 7
N1 23 Apr 2014 Early melt 351 356 1
Wind 24 Apr 2014 Early melt 201 229 14
S1 25 Apr 2014 Early melt 113 78 31
N1 25 Apr 2014 Early melt 270 293 9
N2 7 Jun 2014 Peak melt 594 477 20
N2 8 Jun 2014 Peak melt 568 435 23
N2 9 Jun 2014 Peak melt 537 390 27
Mean deviation between observed and simulated SWE 13
Figure 9. Performance measures of (a) accuracy (ACC), (b) critical success index (CSI), and (c) BIAS as calculated by comparing AMUND-
SEN simulation results with satellite-derived (MODIS/Landsat) snow maps.
pronounced for the early melt season. Both simulated and
observed melt rates are higher for the peak melt event on the
south-facing slope, but not for the north-facing slope. Sim-
ulated melt intensity on the south-facing slope at the end of
April was twice the rate on the north-facing slope, while sim-
ulated melt rates were approximately the same for the op-
posing slopes during peak melt. Simulated (catchment scale)
snowmelt rates were markedly lower during the early melt
(23 and 24 April) on the north-facing slope compared to the
observed (plot scale) melt rates (Fig. 10a), but differences be-
tween them were small during peak melt for both slopes (7
and 8 June; Fig. 10).
4.3 Weighting techniques and isotope-based
hydrograph separation
Differences between the applied snowmelt weighting tech-
niques, induced by the high spatial variability of snowmelt
(Sect. 4.2), led to different event water isotopic compositions
(Ce)for the IHS analyses (Table 4). The event water compo-
nent was depleted in δ18O by roughly 0.3 ‰ for the second
day (24 April) of the early melt event compared to the pre-
ceding day, but inter-daily variation during the peak melt is
Table 4. Isotopic composition of the event water component for the
applied weighting techniques.
Event water isotopic composition
(δ18O ‰)
23 Apr 24 Apr 7 Jun 8 Jun
VWS 17.9 18.2 17.5 17.5
VWO 18.3 18.6 17.4 17.5
VWE 18.1 18.3 17.5 17.5
NORTH 18.6 18.8 17.9 17.9
SOUTH 17.6 17.9 17.1 17.1
almost absent. Especially during early melt (23 to 24 April),
strong deviations between observed plot-scale melt rates and
distributed (areal) melt rates obtained by AMUNDSEN oc-
curred (Fig. 11), and led to more different event water iso-
topic compositions between the VWS and the VWO ap-
proach (Table 4).
The hydrograph and the results of the IHS applied with the
VWS method for the early and peak melt event are presented
in Fig. 4 and highlight the lower ﬂow rates and higher pre-
Hydrol. Earth Syst. Sci., 20, 5015–5033, 2016 www.hydrol-earth-syst-sci.net/20/5015/2016/
J. Schmieder et al.: The importance of snowmelt spatiotemporal variability 5025
Figure 10. Observed (plot scale) and simulated (catchment scale)
daily snowmelt on (a) the north-facing and (b) the south-facing
slope for the early melt (23/24 April) and peak melt (7/8 June).
Figure 11. Relative contribution of the north- and south-facing
slope δ18O values to the catchment average. VWS: volume
weighted with simulated (areal) melt rates. VWO: volume weighted
with observed (plot-scale) melt rates.
event fractions during early melt (Fig. 4c) and vice versa for
the peak melt period (Fig. 4d). The total runoff volume dur-
ing the peak melt period was approximately 6 times higher
than in the early melt period. The fractions of snowmelt (vol-
ume) estimated with the VWS approach were 35 and 75 %
with calculated uncertainties (95 % conﬁdence level) of ±3
and ±14 % for the early and peak melt event, respectively.
The uncertainty calculated from Eq. (3) of the IHS applied
with the VWS method was higher (14 %) for the peak melt
Table 5. Discharge characteristics of the Rofenache for the early
and peak melt event.
Event
Early melt Peak melt
Date 23–25 Apr 7–9 Jun
Mean discharge 1.5 m3s111.5 m3s1
Peak discharge 1.9 m3s117.4 m3s1
Volume runoff 3.3 mm 20.7 mm
Mean-event water fraction 35 ±3 % 75 ±14 %
Peak-event water fraction 44 ±4 % 78 ±15 %
event than for the early melt event because the difference be-
tween isotopic composition of pre-event water and event wa-
ter was smaller than for the early melt event (uncertainty:
3 %) (cf. Tables 2 and 4).
Throughout the early melt event, the snowmelt fraction in-
creased from 25 to 44 % (Fig. 4c; Table 5). This trend mir-
rors the stream isotopic composition, which became more de-
pleted (Fig. 4a). Event water contributions during peak melt
were generally higher but had a smaller range (70 to 78 %;
Fig. 4d). Diurnal isotopic variations of stream water were
small for both events (Fig. 4a and b), and could not clearly
be obtained due to missing data on the falling limb of the
hydrographs.
The use of the different weighting approaches led to
strongly varying estimated snowmelt fractions of streamﬂow
(Fig. 12). Especially the differences between the SOUTH
and the NORTH approach during both investigated events
(up to 24 %), and the differences between the VWS and the
VWO approach (5 %) during early melt (Fig. 12a) are no-
table. Event water contributions estimated by the different
weighting methods ranged from 21–28 % at the beginning
of the early melt event up to 31–55% at the end of the
event (Fig. 12a, Table 6). Minimum event water contributions
during the peak melt were estimated at 60–84 % and max-
ima ranged between 67 and 94 % for the different weighting
methods (Table 6, Fig. 12b). Beside these intra-event varia-
tions in snowmelt contribution, the volumetric variations at
the event-scale were smaller and ranged between 28–40 and
66–90 %, for the early and peak melt event, respectively (Ta-
ble 6).
Considering only spatial variation of snowmelt isotopic
signatures (i.e., comparing the NORTH/SOUTH approach
with the VWE approach) for IHS led to differences in es-
timated event water fractions up to 7 and 14% for the early
and peak melt period, respectively (Table 6). However, con-
sidering only spatial variation in snowmelt rates (i.e., com-
paring the VWS/VWO approach with the VWE approach)
led to differences in event water fraction up to 3 and 2 % for
the early and peak melt period, respectively (Table 6).
Surface overland ﬂow was not considered in the IHS anal-
yses, but if applied, it would most likely increase the cal-
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5026 J. Schmieder et al.: The importance of snowmelt spatiotemporal variability
Table 6. Event water contribution to streamﬂow based on the different weighting techniques. The error indicates the variability (standard
deviation) and the values in parentheses depict the range.
Event water contribution (%)
VWS VWO VWE NORTH SOUTH
Early melt event 35 ±6
(25–44)
30 ±4
(22–35)
33 ±5
(24–39)
28 ±3
(21–31)
40 ±9
(28–55)
Peak melt event 75±2
(70–78)
78 ±3
(71–82)
76 ±2
(70–78)
66 ±2
(60–67)
90 ±3
(84–94)
Figure 12. Comparison of the IHS results for the different weighting techniques used for (a) early melt and (b) peak melt. Scale of yaxis
in (b) differs from that in (a).
culated snowmelt fraction slightly. Furthermore, snowmelt
samples from the wind-exposed site were not used in the IHS
analyses because this site was only sampled on the south-
facing slope during early melt and is not representative for
the catchment due to its limited coverage. However, incorpo-
ration of this data would decrease the calculated snowmelt
fraction by approximately 2 %.
5 Discussion
5.1 Temporal variation in streamﬂow during the
melting season
Snowmelt is a major contributor to streamﬂow during the
spring freshet period in alpine regions and large amounts of
snowmelt water inﬁltrate into the soil and recharge ground-
water (Penna et al., 2014). The hydrological response of the
stream followed the variations of air temperature, as already
observed by Braithwaite and Olesen (1989) (Fig. 3a). The
observed time lags (Fig. 3b and c) between maximum daily
air temperature and daily peak ﬂow are common in mountain
catchments (Engel et al., 2016; Schuler, 2002). During peak
melt, the ﬂashy hydrograph revealed less variation in the tim-
ing of peak discharge of 7-day data (Fig. 3c) compared to the
early melt, as reported by Lundquist and Cayan (2002). The
increase in discharge coincides with decreasing streamﬂow
δ18O during the early melt event (Fig. 4a and c) and con-
ﬁrms the earlier ﬁndings of Engel et al. (2016), who identi-
ﬁed inverse relationships between streamﬂow δ18O and dis-
charge during several 24h events in an adjacent valley on the
southern side of the main Alpine ridge, although their ﬁnd-
ings rely on streamﬂow contributions from snow and glacier
melt. The lower stream water isotopic composition during
peak melt suggests a remarkable contribution of more de-
pleted snowmelt to streamﬂow and therefore conﬁrms the re-
sults of the IHS.
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J. Schmieder et al.: The importance of snowmelt spatiotemporal variability 5027
5.2 Spatiotemporal variability of snowmelt and its
isotopic signature
The rate of snowmelt varies spatially in catchments with
complex topography (Carey and Quinton, 2004; Dahlke and
Lyon, 2013; Pomeroy et al., 2003). This was also demon-
strated for the Rofen valley in this study (Fig. 10, Table 3).
Snowmelt results from a series of processes (e.g., energy ex-
change between snow–atmosphere) that are spatially variable
– especially in complex terrain. This also becomes obvious
when comparing the snowmelt rates on 23 April 2014 in
Fig. 10a. Differences of observed and simulated snowmelt
rates might result from the non-representativeness of point
measurements for catchment averages and refer to the scale
issue of data collection. The peak melt period was charac-
terized by less spatial and day-to-day variation in observed
melt rates (Fig. 10). The modeled daily snowmelt during this
period was similar for north- and south-facing slopes, likely
because of higher melt rates but also a smaller snow-covered
area of the south-facing slope in contrast to the north-facing
slope during peak melt (Fig. 11). The model performance
was good for SWE (Table 3) and snow-cover extent (Figs. 2
and 9). The spatial variation of snowpack isotopic compo-
sition are signiﬁcant, as can be seen in the differences for
north- and south-facing slopes, and also shown by Carey and
Quinton (2004), Dahlke and Lyon (2013), and Dietermann
and Weiler (2013) in their studied high-gradient catchments,
whereas there are unclear differences for the spatial varia-
tion of snowmelt isotopic signals in the literature. It is not
clear to which extent altitude is important, as Dietermann
and Weiler (2013) stated that a potential elevation effect (de-
crease in snowmelt δ18O with elevation) is likely to be dis-
turbed by melting processes (isotopic enrichment) depending
on catchment morphology (aspect, slope) during the ablation
period. Beaulieu et al. (2012) detected elevation as a pre-
dictor, which explained most of the variance they observed
in snowmelt δ18O from four distributed snow lysimeters.
Moore (1989) and Laudon et al. (2007) found no signiﬁcant
difference of δ18O in their lysimeter outﬂows, which was
likely due to the small elevation gradient of their catchments
that favor an isotopically homogenous snowpack, whereas
Unnikrishna et al. (2002) found remarkable small-scale spa-
tial variability. An altitudinal gradient was not considered
in this study, but possible effects on IHS are discussed in
Sect. 5.6. The difference of snowmelt (not snowpack) iso-
topic signature between north- and south-facing slopes was
clearly shown in this study. The dataset is small, but reveals
clear differences induced by varying magnitudes and timing
of melt due to differences in solar radiation on the oppos-
ing slopes (Fig. 7). Temporal variability in snowmelt isotopic
composition is greater for the north-facing slope compared to
the south-facing slope (Fig. 7), which was also pointed out by
Carey and Quinton (2004) in their subarctic catchment. Ear-
lier homogenization in the isotopic proﬁle of the snowpack
and earlier melt out are responsible for this phenomenon (cf.
Dinçer et al., 1970; Unnikrishna et al., 2002). Fractionation
processes likely controlled this homogenization of the snow-
pack between the two investigated melt events. The isotopic
homogenization of the snowpack on the south-facing slope
started earlier in the melting period and caused a smaller
spatial and temporal variation compared to the north-facing
snowpack, as was also reported by Unnikrishna et al. (2002)
and Dinçer et al. (1970). The differences between these in-
vestigated snowpacks were larger in the early melt season
than in the peak melt season. This affects the IHS results, es-
pecially because the snowmelt contributions from the south-
and north-facing slope – with marked isotopic differences
– were distinct. Due to melt, fractionation processes pro-
ceeded and the snowpack likely became more homogenous
throughout the snowmelt season. However, inter-daily vari-
ations of snowpack isotopic composition, especially for the
north-facing slope, were still observable during the peak melt
period. The gradual isotopic enrichment of the snowpack was
also observed for snowmelt, as described by many others
(Feng et al., 2002; Shanley et al., 2002; Taylor et al., 2001,
2002; Unnikrishna et al., 2002).
Intra-daily variations of snowmelt δ18O could be quanti-
ﬁed for two sites (Fig. 8). At S1 on the south-facing slope
during the early melt event, the 0.1‰ range in δ18O (n=5)
was smaller than the range at N2 on the north-facing slope
during the peak melt event (n=3, range =0.2 ‰). This sub-
daily variability is markedly smaller than the differences be-
tween the investigated slopes (cf. Table 1), which ranged
from 0.8 ‰ (peak melt) to 1.4 ‰ (early melt). Unnikrishna
et al. (2002) described signiﬁcant temporal variations of
snowmelt δ18O during large snowmelt events (peak melt).
However, these ﬁndings could not be conﬁrmed within in
this study, probably due to the temporally limited data and
should be tested with a larger dataset. The bulk sample at S1
(23 April 2014) was isotopically closer to the sub-daily val-
ues compared to the bulk sample at N2 (7 June 2014) that was
at the upper range of the sub-daily samples (Fig. 8). There-
fore, one could argue that for the south-facing slope there is
a negligible uncertainty if one uses a single snowmelt value
(at one time) for IHS instead of using a bulk sample, but this
is not the case for the north-facing slope (Fig. 8, site N2).
Unfortunately the sample numbers are small, because more
frequent and more distributed sampling (at different sites)
was not feasible due to logistical issues. Hence, these re-
sults should be used with caution and should be investigated
in further studies. If the focus and the scale of the study is
not on the sub-daily variability, the authors recommend the
use of bulk samples, because these integrate (automatically
weighed with snowmelt rate) the diurnal variations.
5.3 Validity of isotopic hydrograph separation
The validity of IHS relies on several assumptions (cf.
Sect. 3.3; Buttle, 1994; Klaus and McDonnell, 2013).
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5028 J. Schmieder et al.: The importance of snowmelt spatiotemporal variability
The assumption that the isotopic composition of event
and pre-event water differ signiﬁcantly (assumption 1) was
successfully proven, because the snowmelt isotopic values
were markedly lower than pre-event baseﬂow values (cf. Ta-
bles 2 and 4, Fig. 5). Spatiotemporal variations of event
water isotopic composition (assumption 2) were accounted
for by collecting daily and sub-daily samples during both
events throughout the freshet period and meltwater sampling
at a north- and south-facing slope, respectively. The spa-
tially variable input of event water was considered by di-
viding the catchment into two parts – a north- and a south-
facing slope. This study supports the ﬁndings of Dahlke and
Lyon (2013) and Carey and Quinton (2004), emphasizing
the highly variable snowpack/snowmelt isotopic composi-
tion in complex topography catchments due to enrichment.
The temporal variability of event water isotopic composition
was considered by using bulk daily samples, which integrate
snowmelt from the entire diurnal melting cycle, but smooth
out a sub-daily signal. Because the focus of this study was
more on the inter-event than the intra-daily scale, this ap-
proach seemed reasonably reliable. The spatiotemporal vari-
ability of the isotopic composition of pre-event water (as-
sumption 3) is a major limitation and could not be clearly
identiﬁed due to a lack of data and was therefore assumed to
be constant. Small differences between the pre-event samples
(15.00 and 15.05 ‰ for δ18O) and post-event stream wa-
ter isotopic composition support this assumption (Table 2).
The assumption of soil water having the same isotopic com-
position as groundwater in time and space (assumption 4) is
critical. Some studies reveal no signiﬁcant differences (e.g.,
Laudon et al., 2007), whereas others do (e.g., Sklash and Far-
volden, 1979). Isotopic differences between groundwater and
soil water were not considered due to a lack of data. Fur-
thermore, it is not known to which degree the vadose zone
contributes to baseﬂow in the study area. Winter baseﬂow
used in the analyses is assumed to integrate mainly ground-
water and partly soil water. Soil water could be hypothesized
to have a negligible contribution to baseﬂow during winter
due to the recession of the soil water ﬂow in autumn and
frozen soils in winter. The assumption that no or minimal
surface storage occurs (assumption 5) is plausible because
water bodies like lakes or wetlands do not exist in the study
catchment and due to the steep topography detention stor-
age is likely limited. The transit time of snowmelt was as-
sumed to be less than 24 h. This short travel time is char-
acteristic for headwater catchments (Lundquist et al., 2005)
with high in-channel ﬂow velocities, steep hillslopes, a high
drainage density with snow-fed tributaries, thin soils, most
snowmelt originating from the edge of the snow line (small
average travel distances), partly frozen soil, and observed
surface overland ﬂow. The state-of-the-art method (runCE)
to include residence times of snowmelt in the event water
reservoir proposed by Laudon et al. (2002) was applied in
several IHS studies (Beaulieu et al., 2012; Carey and Quin-
ton, 2004; Petrone et al., 2007), but was not feasible due to
the short-term character and temporally limited data.
5.4 Hydrograph separation results and inferred runoff
generation processes
Large contributions from snowmelt to streamﬂow are com-
mon in high-elevation catchments. Daily contributions be-
tween 35 and 75 % in the Rofen valley are comparable to the
results of studies conducted in other mountainous regions,
mostly outside the European Alps. Beaulieu et al. (2012)
estimated snowmelt contributions ranging from 7 to 66%
at the seasonal scale for their 2.4 km2catchment and re-
ported contributions of 34 and 62 %, for the early melt and
peak melt, respectively. The hydrograph was dominated by
pre-event water during early melt in April (Fig. 4c), which
is in accordance with the results obtained by other IHS
studies (Beaulieu et al., 2012; Laudon et al., 2004, 2007;
Moore, 1989). The snowmelt contribution increased as the
freshet period progressed and peaked with high contribu-
tions at the beginning of June. Beaulieu et al. (2012) and
Sueker et al. (2000) reported comparable results for their
physically similar catchments during peak melt with 62 and
up to 76 % event water contributions to streamﬂow, respec-
tively. At the event-scale comparable studies are rare. Engel
et al. (2016) report a maximum daily snowmelt contribution
estimated with a three-component hydrograph separation of
33 % for an 11 km2southwest of the Rofen valley with sim-
ilar physiographic characteristics, but on the southern side
of the main Alpine ridge. It should be mentioned that in their
study, runoff was fed by three components (snowmelt, glacier
melt, and groundwater) and lower snowmelt contributions
were prevalent because most of the catchment area (69%)
was snow free.
Initial snowmelt events ﬂush the pre-event water reservoir
as snowmelt inﬁltrates into the soil and causes the pre-event
water to exﬁltrate and contribute to the streamﬂow. As the
soil and groundwater reservoir becomes gradually ﬁlled with
new water (snowmelt), the event water fraction in the stream
increases. The system is also wetter during peak melt. The
dominance of event water in the hydrograph is interpreted as
an outﬂow of pre-event water stored in the subsurface and
the gradual replenishment of the soil and groundwater reser-
voirs by event water. The higher water table – compared to
the early melt period – could cause a transmissivity feed-
back mechanism (Bishop, 1991). This is a common mech-
anism in catchments with glacial till (Bishop et al., 2011)
characterized by higher transmissivities and hence increased
lateral ﬂow velocities towards to the surface. Runoff gen-
eration is spatially very variable in the study area. There
are areas (meadow patches between rock ﬁelds) where sat-
uration excess overland ﬂow is dominant (observed mainly
at plots S1, S2, and Wind) and areas (with larger rocks
and debris) where rapid shallow subsurface ﬂow can be as-
sumed (plot N2). Catchment morphology controls various
Hydrol. Earth Syst. Sci., 20, 5015–5033, 2016 www.hydrol-earth-syst-sci.net/20/5015/2016/
J. Schmieder et al.: The importance of snowmelt spatiotemporal variability 5029
hydrologic processes and hence the shape of the hydrograph.
Upslope residence times of snowmelt are usually smaller
due to thin soils (observed during the ﬁeld work), steeper
slopes (Sueker et al., 2000), and higher contributing areas
of glaciers with impermeable ice (Behrens et al., 1978), and
would be indicators for the more ﬂashy hydrograph during
the peak melt season.
5.5 Impact of spatial varying snowmelt and its isotopic
composition on isotope-based hydrograph
separation and assessment of weighting approaches
Klaus and McDonnell (2013) stress in their review paper
the need to investigate the effects of the spatially varying
snowmelt and its isotopic composition on IHS. This study
quantiﬁed the impact of the spatially varying isotopic com-
position of snowmelt between north- and south-facing slopes
on IHS results for the ﬁrst time. The IHS results were more
sensitive to the spatial variability of snowmelt δ18O than
to spatial variations of snowmelt rates (Table 6). This is
even more pronounced for the peak melt period, because
snowmelt rates were similar for the north- and south-facing
slope, probably due to a ripe snow cover throughout the
catchment. The difference in volumetric snowmelt contribu-
tion to streamﬂow at the event-scale determined using the
ﬁve different weighting methods for IHS is maximum 24%
(NORTH approach vs. SOUTH approach). The data show
that the variations between the weighting approaches (VWS,
VWO, and VWE) are higher throughout the early melt sea-
son (Table 6), because small-scale variability of snowmelt
and its isotopic composition are more pronounced in the
early melt season. Thus, the inﬂuence of spatial variability
of snowmelt and its isotopic composition on the event wa-
ter fraction calculated with IHS is larger during this time.
Melt rates strongly differ between the south- and the north-
facing slope (Fig. 11), which were deceptively gathered by
manually measured SWE, likely due to micro-topographic
effects. As the contributions from both slopes are used in
Eq. (3), they strongly inﬂuence the average isotopic com-
position of event water. The weighting method SOUTH (or
NORTH) represents the hypothetical and most extreme sce-
nario in which only one sampling site is used for the IHS
analysis. Because snowmelt is more enriched in δ18O and
closer to pre-event water isotopic composition on the south-
facing slope during peak melt, this scenario has the greatest
effect on IHS and leads to the strongest deviation in estimated
snowmelt fractions (up to 15 % overestimation compared to
the VWS approach). These scenarios (NORTH/SOUTH) are
theoretical and it is obvious that it is not recommended to
conduct a IHS analysis by using only samples from either
north- or south-facing slopes in catchments with complex
terrain. Similar to the VWE method, snowmelt isotopic data
were not volume weighted in other studies (e.g., Engel et al.,
2016) where snowmelt data were not available. This has a
more distinct effect on IHS during the early melt season be-
cause of the higher spatiotemporal variability in snowmelt
(and its isotopic composition) compared to the peak melt
season and led to a deviation in the snowmelt fraction in
streamﬂow of 2 and 3 % compared to the VWS and VWO ap-
proaches, respectively. These differences are small, because
the differing snowmelt and isotopic values offset each other
in this particular case (Table 6). Nevertheless, the results of
VWS are more correct for the right reason, because single
observed plot-scale melt rates do not represent distributed
snowmelt contribution at the catchment scale. Therefore, one
can hypothesize that distributed simulated melt rates enhance
the reliability of IHS, whereas plot-scale weighting intro-
duces a large error caused by the difﬁculty in ﬁnding loca-
tions that represent the average melt rate in complex terrain.
5.6 Limitations of the study
Collecting water samples in high-elevation terrain is chal-
lenging due to limited access and high risk (e.g., avalanches),
limiting high-frequency sampling. Hence, some limitations
are inherent in this study. Potential elevation effects on
snowmelt isotopic composition were not tested. The oppos-
ing sampling sites (S1–N1 and S2–N2) were at the same ele-
vation (Fig. 1). It was assumed that the differences in north-
and south-facing slopes were much greater than a possible
altitudinal gradient in snowmelt isotopic composition. This
hypothesis was not tested, but based on the results of other
studies (Dietermann and Weiler, 2013). However, account-
ing for a potential altitudinal gradient (decrease in snowmelt
δ18O with elevation) would lead to more depleted isotopic
signatures of event water and hence to lower event water frac-
tions.
Another disadvantage is that no snow survey was con-
ducted prior to the onset of snowmelt (peak accumulation)
to estimate spatial variability in bulk snow δ18O. Because
snowmelt is used for applying IHS, it is not clear to which de-
gree the spatial variability of the snowpack isotopic composi-
tion is important. Two-component isotope-based hydrograph
separation was successfully applied using the snowmelt and
baseﬂow endmembers, but potential contributions of glacier
melt were neglected (here deﬁned as ice/ﬁrn melt). Because
glaciers in the catchment were still covered by snow during
the peak melt season, a signiﬁcant contribution from ice/ﬁrn
melt was assumed to be unlikely. Nevertheless, negligible
amounts of basal (ice) meltwater could originate from tem-
perate glaciers. No samples could be collected during the re-
cession of the hydrograph (at night). Even though the spa-
tial variability of the event water signal was the focus of
the study, only temporal variability was considered in the
Genereux-based uncertainty analyses. Although the tempo-
ral variability of winter baseﬂow isotopic composition seems
to be insigniﬁcant, the sample number (n=2) could be too
small to characterize the pre-event component and should
be clearly investigated in future work. Penna et al. (2016)
used two approaches to determine the isotopic composition
www.hydrol-earth-syst-sci.net/20/5015/2016/ Hydrol. Earth Syst. Sci., 20, 5015–5033, 2016
5030 J. Schmieder et al.: The importance of snowmelt spatiotemporal variability
of pre-event water and described differences in the estimated
event water contributions during snowmelt events. They ad-
vise to take pre-event samples prior to the onset of the melt
season because pre-event samples taken prior to the onset of
the diurnal melt cycle could be affected by snowmelt wa-
ter from the previous melt pulses and therefore could lead
to underestimated snowmelt fractions and high uncertainties.
Furthermore, model results and observed discharges were as-
sumed to be free of error in the analyses. As pointed out, in-
strumentation and accessibility are major problems for high-
elevation studies. For this study it turned out that compos-
ite snowmelt samples were easier to collect, representing the
day-integrated melt signal. A denser network of melt collec-
tors would be desirable, as well as a snow lysimeter to gain
high-frequency data automatically. Representative samples
of the elevation zones and different vegetation belts could be
important too, especially in partly forested catchments with
a distinct relief (cf. Unnikrishna et al., 2002).
6 Conclusions
This study provides new insights into the variability of the
isotopic composition in snowmelt and highlights its impact
on IHS results in a high-elevation environment. The spa-
tial variability in snowmelt isotopic signature was considered
by experimental investigations on south- and north-facing
slopes to deﬁne the isotopic composition of the snowmelt
endmember with greater accuracy. This study clearly shows
that distributed snowmelt rates obtained from a model based
on meteorological data from local automatic weather sta-
tions affect the weighting of the event water isotopic sig-
nal, and hence the estimation of the snowmelt fraction in
the stream by IHS. The study provides a variety of relevant
ﬁndings that are important for hydrologic research in high-
alpine environments. There was a distinct spatial variability
in snowmelt between north- and south-facing slopes, espe-
cially during the early melt season. The isotopic composi-
tion of snowmelt water was signiﬁcantly different between
north-facing and south-facing slopes, which resulted in a pro-
nounced effect on the estimated snowmelt contributions to
streamﬂow with IHS. The IHS results were more sensitive
to the spatial variability of snowmelt δ18O than to spatial
variation of snowmelt rates. The differences in the estimated
snowmelt fraction due to the weighting methods used for IHS
were as large as 24 %. This study also shows that it is hardly
possible to characterize the event water signature of larger
slopes based on plot-scale snowmelt measurements. Apply-
ing a distributed model reduced the uncertainty of the spa-
tial snowmelt variability inherent to point-scale observations.
Hence, applying the VWS method provided more reasonable
results than the VWO method. This study highlighted that the
selection of sampling sites has a major effect on IHS results.
Sampling at least north-facing and south-facing slopes in
complex terrain and using distributed melt rates to weight the
snowmelt isotopic composition of the differing exposures is
therefore highly recommended for applying snowmelt-based
IHS.
7 Data availability
Isotope and ﬁeld data (i.e. snow observations) can be ob-
tained from the corresponding author upon request.
Acknowledgements. The authors wish to thank the Institute of
Atmospheric and Cryospheric Sciences of the University of Inns-
bruck, the Zentralanstalt für Meteorologie and Geodynamik, the
Hydrographic Service of Tyrol and the TIWAG-Tiroler Wasserkraft
AG for providing hydrological and meteorological data, the Amt
der Tiroler Landesregierung for providing the digital elevation
model, the Center of Stable Isotopes (CSI) for laboratory support,
as well as many other individuals, who have helped to collect
data in the ﬁeld. We also thank the reviewers for their valuable
suggestions that have much improved the manuscript, and the
editor for the careful handling of the manuscript.
Edited by: I. van Meerveld
Reviewed by: S. Pohl and one anonymous referee
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... With global warming, the environmental systems in mountain areas has been disproportionately suffering from decreasing snow cover areas and glacial recession in past decades (Hill et al., 2020, Déry andBrown, 2007;Farinotti et al., 2015, Chen et al., 2016Tang et al., 2017;Scherrer et al., 2013). Scientists have devoted enormous effort to estimating the contribution ratios of snowmelt water to runoff through stable isotope approach (Lutz et al., 2014;Bravo et al., 2017;Chen et al., 2019;Schmieder et al., 2016;Yang et al., 2021). For instance, Ohlanders et al. (2013) found the cryosphere meltwater accounted for 50% to 90% of runoff in the Central Chile. ...
... Water stable isotopes have a long-standing tradition as tracers in hydrology researches with advantages of tracking, integration and indication (Bowen and Good, 2015;Klaus and McDonnell, 2013). At present, isotopic methods have been widely used to reveal hydrological process, separate different sources of streamflow and estimate the residence time of water at various catchment (Bravo et al., 2017;Chen et al., 2019;Schmieder et al., 2016). Winter precipitation falling as snow generally has isotope composition differing from that of summer precipitation. ...
... This phenomenon mainly resulted from that the relative higher altitudes with lower temperature and weaker evaporation than lower altitudes. In addition, the snow maybe reacted with the soil elements during snowmelt period, which resulted to snowmelt water with different value of δ 18 O and d-excess in different altitudes (Schmieder et al., 2016). The δ 18 O values of rainfall decreased with elevation, it showed significantly altitude effect and rainfall amount effect with altitude gradient about 0.24‰/100 m (p < 0.05). ...
Article
Snow is an important form of water storage which profoundly affects the hydrological processes in alpine watersheds, especially the runoff during snowmelt period. As the strong impact of climate change, the concern about snowmelt water to runoff in alpine watersheds is dramatically growing. However, the studies are mainly focused on arid and semi-arid areas, the research on semi-humid regions sensitive to the climate change need to be strengthened. In this study, for evaluating the contribution of snowmelt water to runoff in snowmelt period, we calculated the stable isotope values (δD, δ¹⁸O and d-excess) of different water bodies including river, snow, rain, groundwater, and snowmelt water in the Erdaobaihe River watershed of Changbai Mountains area. The results indicated that the δ¹⁸O values of snowmelt water showed an increasing trend with increasing temperature, while the d-excess values of snowmelt water showed a decreasing trend. During the snowmelt period, the snow cover ratio varied from 20.31% to 0.02%. The contribution ratio of snowmelt water to runoff was from 14.4% to 59.8% with average of 42.60% and the total contribution discharge was about 36.56×10⁶ m³, which was higher than inland mountainous areas as different elevation and climate. As the stable water source of river, the groundwater contributed 19.2% to 61.6% with average of 43.88%. The snowmelt water plays an important role in regional water balance. Owing to climate change, the snowfall period in Changbai Mountains was gradually shortening and extreme snowfall events were increasing. The results will provide evidence for further hydrological studies and help to plan future water management strategies in the alpine watershed.
... Flow within the aquifer is further controlled by gravity and its hydraulic properties (Welch and Allen, 2014). Various methods can be used to characterize groundwater flow dynamics, storage, and recharge areas including (i) hydrochemistry (Cervi et al., 2012), (ii) hydromechanical modelling (Cappa et al., 2004), (iii) reactive and conservative tracer tests Vallet et al., 2015;Hilberg, 2016;Ronchetti et al., 2020), and (iv) noninvasive geophysical methods Siemon et al., 2009;Chalikakis et al., 2011;Zieher et al., 2017;Lajaunie et al., 2019). The use of tracers to track water through the slope's subsurface entails an auspicious way to assess and characterize the hydrogeological forcing of a landslide. ...
... On a regional scale, Blasch and Bryson (2007) identified seasonality and the dominating areas of recharge within several subbasins by using stable isotope values of precipitation and groundwater. On a catchment scale, Schmieder et al. (2016) exploited stable isotopic signatures combined with a physically based snow model for hydrograph separation and quantification of snowmelt contributions to streamflow. Subsequently, for better understanding and quantification of landslide hydrology, a stable-isotope-supported characterization of recharge, constraining groundwater flow systems, and estimating mean recharge elevations are proven methodologies (Scanlon et al., 2002;Bouchaou et al., 2008;Guglielmi et al., 2002;Mikoš et al., 2004). ...
Article
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Continuous and slow-moving deep-seated landslides entail challenges for the effective planning of mitigation strategies aiming at the reduction of landslide movements. Given that the activity of most of these landslides is governed by pore pressure variations within the shear zone, profound knowledge about their hydrogeological control is required. In this context, the present study presents a new approach for the spatial assessment of probable recharge areas to better understand a slope's hydrogeological system. The highly automated geo-statistical approach derives recharge probability maps of groundwater based on stable isotope monitoring and a digital elevation model (DEM). By monitoring stable isotopes in both groundwater and precipitation, mean elevations of recharge areas can be determined and further constrained in space with the help of the DEM. The approach was applied to the Vögelsberg landslide, an active slab of a deep-seated gravitational slope deformation (DSGSD) in the Watten valley (Tyrol, Austria). Resulting recharge probability maps indicate that shallow groundwater emerging at springs on the landslide recharges between 1000 and 1650 m a.s.l. In contrast, groundwater encountered in wells up to 49 m below the landslide's surface indicates a mean recharge elevation of up to 2200 m a.s.l. matching the highest parts of the catchment. Further inferred proxies, including flow path length, estimated recharge area sizes, and mean transit times of groundwater, resulted in a profound understanding of the hydrogeological driver of the landslide. It is shown that the new approach can provide valuable insights into the spatial pattern of probable recharge areas where mitigation measures aiming at reducing groundwater recharge could be most effective.
... In 1969, Hubert et al. [1] published their pioneering paper on hydrograph separation using stable isotope tracers. Since then, isotope hydrograph separation (IHS) has gradually become the principal method for determining the relative contributions of different sources of runoff or streamflow (i.e., event or pre-event water) [2][3][4][5]. Buttle [6] summarized the hydrological processes, including groundwater ridging, lateral throughflow, preferential flow, saturation overland flow, kinematic waves, and output from surface storage, which significantly improved the knowledge of the mechanisms of runoff generation. In 2013, Klaus and McDonnell [7] published a qualitative and comprehensive review of IHS and concluded that, despite certain limitations, water stable isotopes were still the most effective technology for understanding runoff generation processes and mechanisms. ...
... In the 'variation'/'chemistry'/'distribution' relative to subperiod II, there was increasing interest in 'variation', with 68 publications, suggesting that more research focused on the spatiotemporal heterogeneity of runoff sources [5,16,17,93,94]. Moreover, many studies evaluated the 'uncertainty' of end-member contributions to runoff [87,95]. ...
Article
Full-text available
Scientific research into isotope hydrograph separation (IHS) has rapidly increased in recent years. However, there is a lack of systematic and quantitative research to explore how this field has evolved over time. In this study, the methods of text mining and bibliometric analysis were combined to address this shortcoming. The results showed that there were clear periodical characteristics in IHS studies between 1986 and 2019. High-frequency words, e.g., catchment, stable isotope, runoff, groundwater, precipitation, runoff generation, and soil, were the basic topics in IHS studies. Forest and glacier/snow were the main landscapes in this research field. ‘Variation’, ‘spatial’, and ‘uncertainty’ are hot issues for future research. Today, studies involving the geographical source, flow path, and transit/residence time of streamflow components have enhanced our understanding of the hydrological processes by using hydrometeorological measurements, water chemistry, and stable isotope approaches. In the future, new methods, such as path analysis and ensemble hydrograph separation, should be verified and used in more regions, especially in remote and mountainous areas. Additionally, the understanding of the role of surface water in streamflow components remains limited and should be deeply studied in the future.
... The uncertainty for snow and snowpacks was highest than those of other components. This is attributed to the altitude, seasonal variability of air temperature and post-depositional processes that enhance the spatial and temporal variability of isotopic composition of snowpacks ( Dahlke and Lyon, 2013 ;Dietermann and Weiler, 2013 ;Schmieder et al., 2016 ;Zuecco et al., 2019 ). ...
... Globally, various studies about the high spatial variability in the isotopic composition of snowpacks have been related to the topography ( Carey and Quinton, 2004 ;Dahlke and Lyon, 2013 ;Dietermann and Weiler, 2013 ;Schmieder et al., 2016 ), elevation ( Beaulieu et al., 2012 andOhlanders et al., 2013 ), canopy ( Allen et al., 2016 ), snow sublimation ( Earman et al., 2006 ;N'da et al., 2016 ), snow metamorphism and snow redistribution ( Friedman et al., 1991 ;Taylor et al., 2001 ;Mott et al., 2010 ), etc. In alpine catchments like the Sindh and Rambiara with high relief, the meltwater released from the snowpack cover a wide isotopic range, which depend not only on the altitude but also the melting stage of the snowpack ( Jeelani et al., 2018 ). ...
Article
Snow and glaciers are important freshwater resources in the Himalayan catchments for all biological activities. However, under recent climate change, these resources are decreasing, thereby posing a serious threat to the future freshwater resources among the Himalayan catchments. Therefore, understanding the hydrological behavior and sources of streamflow is necessary for better assessment, planning and management of these waters in the mountainous catchments. This study is based on the use of δ¹⁸O and d-excess to identify and estimate the different sources of streamflow in the western Himalayas. The isotopic composition of different source water varied over space and time. These variabilities are attributed to the seasonal meteorological changes, precipitation form, differential melting of snowpacks and moistures sources. The vertical isotope gradients were observed between the sampling sites and the elevation of the basin. The results suggest that snow and snowpack dominantly (up to 55%) contribute to the stream flow during high flow period, while as groundwater maintains the flow in the streams during lean period and contributes up to 78%. The study provides the important insights about the understanding of spatio-temporal variability of different source waters contributing to runoff. In addition, the changing climate could have serious impact on the water resources of the region and thereby affecting the economic and social values of the downstream populations.
... The intensity of snow hydrological processes varies in space. Spatial factors affecting snow isotopic composition have included altitude [21][22][23], aspect [15,24], snow depth [21], and canopy interception [25][26][27]. Most of these factors affect snow sublimation fluxes, which leads to enrichment in heavier isotopes of the remaining snow cover [16,22,28,29]. ...
Article
Full-text available
The stable water isotopes in snow (primarily 18O and 2H) are widely used for tracing hydrological and ecological processes. However, isotopic signatures of snow can be significantly modified by topography and land cover. This study assesses spatial and temporal variability of the bulk snowpack isotopic composition (δ18O, δ2H, d-excess) between forested (pine and birch) and open areas in the West Siberian forest steppes. Isotopic samples were collected over the peak snow accumulation in 2017–2019. The snow isotopic composition within forested areas differed from open steppes, mainly in reducing d-excess (1.6‰ on average). We did not find a significant effect of canopy interception on snow enrichment in heavier isotopes. Snowpack in the pine forests was even lighter by 3.6‰ for δ2H compared to open areas, probably, due to low energy inputs and interception capacity. Additionally, snow depth significantly influenced the isotopic composition spatial variability. As snow depth increased, δ18O and δ2H values decreased due to conservation within the snowpack and less influence of sublimation and moisture exchange with the soil. However, this pattern was only evident in winters with below-average snow depth. Therefore, taking into account snow depth spatial and seasonal variability is advisable when applying the isotopic methods.
... Limited knowledge is available on comparisons between snowpack and meltwater isotopes from field studies. Isotopic fractionation of early meltwater can be different from peak and post-peak meltwater isotopic composition (Schmieder et al., 2016) due to the low sublimation caused by the relatively low ambient temperature at the onset of melting (Evans et al., 2016;Gustafson et al., 2010) and different rates of liquid-ice isotopic fractionation (Feng et al., 2002;Lee et al., 2010;Taylor, Feng, Renshaw, et al., 2002). Thus, the process understanding of earlier meltwater isotopic composition as compared to peak and post-peak meltwater isotopic composition is also limited. ...
Article
Full-text available
This study provides a detailed characterization of spatiotemporal variations of stable water ¹⁸O and ²H isotopes in both snowpack and meltwater in a subarctic catchment. We performed extensive sampling and analysis of snowpack and meltwater isotopic compositions at 11 locations in 2019 and 2020 across three different landscape features: (a) forest hillslope, (b) mixed forest, and (c) open mires. The vertical isotope profiles in the snowpack's layered stratigraphy presented a consistent pattern in all locations before snowmelt, and isotope profiles homogenized during the peak melt period; represented by a 1–2‰ higher δ $\delta$¹⁸O value than prior to melting. Our data indicated that the liquid‐ice fractionation was the prime reason that caused the depletion of heavy isotopes in initial meltwater samples prior to the peak melt period. The liquid‐ice fractionation was influenced by snowmelt rate, with higher fractionation during slow melt. The kinetic liquid‐ice fractionation was evident only in close examination of meltwater lc‐excess values, not δ $\delta$¹⁸O values alone. Meltwater was isotopically heavier and more variable than the depth‐integrated snowpack; the weighted mean of meltwater isotope values was higher by 0.62–1.33‰ δ $\delta$¹⁸O than the weighted mean of snowpack isotope values in forest hillslope and mixed forest areas, and 1.51–6.37‰ δ $\delta$¹⁸O in open mires. Our results reveal close to 3.1‰ δ $\delta$¹⁸O disparity between the meltwater and depth‐integrated snowpack isotope values prior to the peak melt period, suggesting that proper characterization of meltwater δ $\delta$¹⁸O and δ $\delta$²H values is vital for tracer‐based ecohydrological studies and models.
... Snowmelt isotope signals will vary with snow inputs and aging of the snowpack from accumulation through final melt and produce seasonally variable runoff isotope signals [16,[19][20][21]27,41,44,[46][47][48][49][50][51][52][53][54]. Additionally, these internal changes in snowpack isotope signals and subsequent snowmelt and runoff isotope signals can vary across a landscape because of temperature and insolation differences from elevation or orientation [45,55,56]. The evolution of the snowpack at different elevations in the study area likely contributes different snowmelt/runoff isotope signals to the surface-water and groundwater systems fed by this mountain snowpack reservoir. ...
Article
Full-text available
A snowpack’s δ2H and δ18O values evolve with snowfall, sublimation, evaporation, and melt, which produces temporally variable snowpack, snowmelt, and runoff isotope signals. As a snowpack ages, the relatively depleted δ2H and δ18O values of snow will become less depleted with sublimation and evaporation, and the internal distribution of isotope signals is altered with melt moving through and out of the snowpack. An examination of δ2H and δ18O values for snowpack, snowmelt, and ephemeral creek water in the Palouse Range of northern Idaho indicated an evolution from variably depleted snowpack to enriched snowmelt and relatively consistent isotope signals in springtime ephemeral creeks. Within the primary snow band of the mountain range and during the winter–spring period of 2019–2020, the snowpack had an isotope range of −130 to −75‰ for δ2H and −18 to −10.5‰ for δ18O with resulting snowmelt values of −120 to −90‰ for δ2H and −16.5 to −12.5‰ for δ18O. With runoff of snowmelt to ephemeral creeks, the isotope values compressed to −107 to −104‰ for δ2H and −15.5 to −14.5‰ for δ18O. Aging of the snowpack produced increasing densities in the base, middle, and upper layers along with a corresponding enrichment of isotope values. The highest elevation site indicated the least enrichment of δ2H and δ18O in the snowpack base layer, and the lowest elevation site indicated the strongest enrichment of δ2H and δ18O in the snowpack base layer. Deuterium excess decreased with snowpack aging processes of accumulation and melt release, along with the migration of water vapor and snowmelt within the snowpack. It is likely that winter melt (early depleted signal) is a primary contributor to creeks and groundwater along the Palouse Range, but the strong variability of snowpack isotope signals provides a wide range of possible isotope signals to surface-water and groundwater systems at the mountain front.
... Sampling frequency and timing also substantially affect MTT estimates (i.e. Birkel et al., 2012;Schmieder et al., 2016). An alternate method using annual amplitudes of precipitation and streamflow water isotopes to estimate the fraction of water less than ~10 weeks old,"F yw " (Kirchner 2016a(Kirchner , 2016b, can seem to underestimate young water fractions when a watershed hosts overwinter snowpack (Jasechko et al., 2016;Campbell et al., 2020). ...
Article
A clear concept of the recharge pathways for surface flow in mountain rivers is important for understanding the effects of climate change on streamflow in mountain block hydrology. In a mountain river in Alberta, Canada, three subsurface end-member sources were identified using silica, sulfate, and the isotopic composition of sulfate: interflow (water that has not undergone significant rock-water interaction), and groundwater derived from two types of hydrogeologic units (carbonate and siliciclastic fractured rock aquifers). A three end-member mixing model was used to determine their relative contributions to river discharge and infer relative groundwater ages using silica dissolution kinetics and tritium. In the three years of sampling, proportional contributions from subsurface end-members varied up to 25% within a given year while mean contributions between years varied as much as 19%. Interflow is the dominant source for the majority of the study period, contributing ∼50% of river discharge during spring melt and 40-60% over the rest of the season. Siliciclastic and carbonate aquifer contributions varied interannually and seasonally. Although carbonate rocks comprise the majority of the watershed area, siliciclastic rocks contribute nearly equally to annual streamflow (∼30% each), suggesting that siliciclastic aquifers transmit more water. Silica- and tritium- based ages suggest residence times on the order of <5 to 10 years. The combination of short residence times and significant variability in streamflow provenance suggest streamflow in this river will be rapidly affected by climatic change.
Article
In the Yellow River source area (SAYR), a large-scale isotope hydrological monitoring effort of landscape heterogeneity was implemented. To assess and determine whether permafrost has a controlling influence on hydrological processes in the watersheds, the IsoSource and MixSIAR models were used to partition runoff during permafrost thaw season in five nested basins (from 20,930 km² to 121,972 km²). The analysis results showed that there were significant differences in the factors controlling runoff generation in the five basins, as well as significant differences in runoff sources between upper watersheds with predominantly permafrost cover and lower watersheds with predominantly seasonal frozen soil cover. The influence of permafrost on runoff generation was significantly reduced with increasing basin size. Soil permeability was influenced by permafrost thawing which impacted runoff generation in the permafrost zone. However, this only became effective under a specific combination of landscape characteristics and topography. In basins with less permafrost cover, the topographic characteristics and conditions of the catchment itself controlled the runoff generation mechanism. In general, the presence of permafrost influences the runoff generation, but the extent of the influence depends on the characteristics of the basin itself and, in particular, the control of topography increases with the scale of the basin.
Article
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Snow-dominated and glacierized catchments are important sources of fresh water for biological communities and for population living in mountain valleys. Gaining a better understanding of the runoff origin and of the hydrological interactions between meltwater and streamflow is critical for natural risk assessment and mitigation as well as for effective water resources management in mountain regions. This study is based on the use of stable isotopes of water and electrical conductivity as tracers to identify the water sources for runoff and their seasonal variability in a glacierized catchment in the Italian Alps. Samples were collected from rainfall, snow, snowmelt, ice melt and stream water (from the main stream at different locations and from selected tributaries) in 2011, 2012 and 2013. The tracer-based mixing analysis revealed that, overall, snowmelt and glacier melt were the most important end-members for stream runoff during late spring, summer and early fall. The temporal variability of the tracer concentration suggested that stream water was dominated by snowmelt at the beginning of the melting season (May–June), by a mixture of snowmelt and glacier melt during mid-summer (July–early August), and by glacier melt during the end of the summer (end of August–September). The same seasonal pattern observed in streamflow was also evident for groundwater, with the highest electrical conductivity and least negative isotopic values found during periods of limited melting. Particularly, the application of a two-component mixing model to data from different springs showed that the overall snowmelt contribution to groundwater recharge during the three study years ranged between 58% (±15%) and 72% (±15%). These results provided new insights on the isotopic characterization of the study catchment and the presented approach could offer further understanding of the spatio-temporal variability of the main water sources contributing to runoff in other snow-dominated and glacierized Alpine catchments.
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The aim of this study was to analyse and predict the mean stable water isotopic composition of the snow cover at specific geographic locations and altitudes. In addition, the dependence of the isotopic composition of the entire snow cover on altitude was analysed. Snow in four Swiss catchments was sampled at the end of the accumulation period in April 2010 and a second time during snowmelt in May 2010 and analysed for stable isotope composition of 2H and 18O. The sampling was conducted at both south-facing and north-facing slopes at elevation differences of 100 m, for a total altitude difference of approximately 1000 m. The observed variability of isotopic composition of the snow cover was analysed with stepwise multiple linear regression models. The analysis indicated that there is only a limited altitude effect on the isotopic composition when considering all samples. This is due to the high variability of the isotopic composition of the precipitation during the winter months and, in particular in the case of south-facing slopes, an enrichment of heavy isotopes due to intermittent melting processes. This enrichment effect could clearly be observed in the samples which were taken later in the year. A small altitudinal gradient of the isotopic composition could only be observed at some north-facing slopes. However, the dependence of snow depth and the day of the year were significant predictor variables in all models. This study indicates the necessity to further study the variability of water isotopes in the snow cover to increase prediction for isotopic composition of snowmelt and hence increase model performance of residence time models for alpine areas in order to better understand the accumulation processes and the sources of water in the snow cover of high mountains.
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In this study, the fully distributed, physically based hydroclimatological model AMUNDSEN is set up for catchments in the highly glacierized Ötztal Alps (Austria, 558 km² in total). The model is applied for the period 1997–2013, using a spatial resolution of 50 m and a temporal resolution of 1 h. A novel parameterization for lateral snow redistribution based on topographic openness is presented to account for the highly heterogeneous snow accumulation patterns in the complex topography of the study region. Multilevel spatiotemporal validation is introduced as a systematic, independent, complete, and redundant validation procedure based on the observation scale of temporal and spatial support, spacing, and extent. This new approach is demonstrated using a comprehensive set of eight independent validation sources: (i) mean areal precipitation over the period 1997–2006 derived by conserving mass in the closure of the water balance, (ii) time series of snow depth recordings at the plot scale, (iii–iv) multitemporal snow extent maps derived from Landsat and MODIS satellite data products, (v) the snow accumulation distribution for the winter season 2010/2011 derived from airborne laser scanning data, (vi) specific surface mass balances for three glaciers in the study area, (vii) spatially distributed glacier surface elevation changes for the entire area over the period 1997–2006, and (viii) runoff recordings for several subcatchments. The results indicate a high overall model skill and especially demonstrate the benefit of the new validation approach. The method can serve as guideline for systematically validating the coupled components in integrated snow-hydrological and glacio-hydrological models.
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During the summer of 1994 a glaciometeorological experiment was carried out on the Pasterze (a glacier in Austria). This paper reports on the data from six energy-balance stations ranging in altitude from 2075 to 3225 m above sea level (asl). The wind regime was dominated by the glacier wind. On the tongue, directional constancies ranged between 0.94 and 0.97. Mean 2 m wind speed and specific humidity were almost constant along the glacier. The variation in the 2 m temperature along the glacier cannot be described by the usually assumed constant decrease with elevation. On the tongue the 2 m temperature even increased with elevation. A much better description of the temperature distribution is given by a linear relation between the potential temperature and the distance along the flow line. This can be understood from a simple thermodynamic analysis of the glacier-wind layer. It is further shown that changes in clear-sky global radiation with elevation are due mainly to changes in local albedo and relief and hardly at all to changes in absolute optical path length and atmospheric water vapor and aerosol content. On the tongue the laterally averaged ice albedo is almost constant with elevation. The flux of incoming long-wave radiation during clear-sky conditions at U2 (2310 m asl) was 48W/m2 higher than the flux at U5 (3225 m asl), on average. More than half of the difference was due to systematic differences in the shape of the temperature profile (probably the inversion depth is larger at U2). The rest can be ascribed to higher 2 m temperatures and larger amounts of upper hemisphere slopes at U2. The distributions of the meteorological variables and the parameterizations described in this paper might be incorporated in surface energy-balance models designed to simulate the surface mass balance.
Chapter
Environmental isotope studies can be used as a tool to get better insight into snow hydrology. The isotope labelling of the snowcover of its single layers, of the precipitations, and of the discharge concerning its origin gives possibilities to study the mass exchange between meltwater and snowcover and between the individual snow layers. Measurements of isotope contents have also be used to estimate the net evaporation rate of a snowcover during the ablation period, the fractions of groundwater and surface water discharge from catchment areas with a temporary snowcover, the fraction of snow meltwater and ice meltwater in a glacier discharge. In arctic areas the environmental isotopes enable to date firn and glacier ice cores and the long-term conservation of seasonal variations of isotope concentrations in the glacier ice permits a much better identification of annual ice layers than is possible by conventional strati-graphic methods.
Article
During the summer of 1994 a glaciometeorological experiment was carried out on the Pasterze (a glacier in Austria). This paper reports on the data from six energybalance stations ranging in altitude from 2075 to 3225 m above sea level (asl). The wind regime was dominated by the glacier wind. On the tongue, directional constancies ranged between 0.94 and 0.97. Mean 2 m wind speed and specific humidity were almost constant along the glacier. The variation in the 2 m temperature along the glacier cannot be described by the usually assumed constant decrease with elevation. On the tongue the 2 m temperature even increased with elevation. A much better description of the temperature distribution is given by a linear relation between the potential temperature and the distance along the flow line. This can be understood from a simple thermodynamic analysis of the glacier-wind layer. It is further shown that changes in clear-sky global radiation with elevation are due mainly to changes in local albedo and relief and hardly at all to changes in absolute optical path length and atmospheric water vapor and aerosol content. On the tongue the laterally averaged ice albedo is almost constant with elevation. The flux of incoming long-wave radiation during clear-sky conditions at U2 (2310 m asl) was 48 W/m2 higher than the flux at U5 (3225 m asl), on average. More than half of the difference was due to systematic differences in the shape of the temperature profile (probably the inversion depth is larger at U2). The rest can be ascribed to higher 2 m temperatures and larger amounts of upper hemisphere slopes at U2. The distributions of the meteorological variables and the parameterizations described in this paper might be incorporated in surface energy-balance models designed to simulate the surface mass balance.
Article
We analysed contributions to runoff using hourly stream water samples from seven individual melt-induced runoff events (plus one rainfall event) during 2011, 2012 and 2013 in two nested glacierized catchments in the Eastern Italian Alps. Electrical conductivity and stable isotopes of water were used for mixing analysis and two- and three-component hydrograph separation.High-elevation snowmelt, glacier melt and autumn groundwater were identified as major end-members. Discharge and tracers in the stream followed the diurnal variations of air temperature but markedly reacted to rainfall inputs. Hysteresis patterns between discharge and electrical conductivity during the melt-induced runoff events revealed contrasting loop directions at the two monitored stream sections. Snowmelt contribution to runoff was highest in June and July (up to 33%) whereas the maximum contribution of glacier melt was reached in August (up to 65%). The maximum groundwater and rainfall contributions were 62% and 11%, respectively. Runoff events were generally characterised by decreasing snowmelt and increasing glacier melt fractions from the beginning to the end of the summer 2012, while runoff events in 2013 showed less variable snowmelt and lower glacier melt contributions than in 2012.The results provided essential insights into the complex dynamics of melt-induced runoff events and may be of further use in the context of water resource management in Alpine catchments. This article is protected by copyright. All rights reserved.