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Hydrological response of snowpack under rain-on-snow events:a field study
P. Singh*, G. Spitzbart, H. Hubl, H.W. Weinmeister
Institute for Torrent and Avalanche Control, BOKU A-1190, Peter Jordan Strasse, 82, Vienna, Austria1
Received 27 August 1996; revised 20 November 1996; accepted 9 December 1996
Abstract
The hydrological response of rain-on-snow events has been studied on a plot scale at 2640 m
altitude in the Austrian Alps. Three artificial rain events with different intensities and durations were
simulated over two snow plots on a natural snowpack and the behaviour of emerging outflow was
examined. Measurements of meteorological parameters, soil temperature and snowpack properties
were also made. The investigations show that the impeding characteristics of the ice layers more than
doubled the storage capacity of the snowpack. The speed of water movement was estimated to be
about 6 m h1 when the snow pack was fully saturated. Accelerated metamorphism under saturated
conditions and preferential flow paths created owing to uneven snow surface caused by the impact of
intense rain over the snow surface, are understood to be responsible for the high speed of water flow.
This indicates that heavy rain water moves several times faster than the natural snowmelt under non-
rainy conditions. Moreover, under rainy conditions, natural snowmelt also percolates faster along
with rain water. Observations of the time of arrival of runoff, taand time to equilibrium concentra-
tion of liquid water in snow, te, for different rain events indicate that after conditioning of the
snowpack, a significant reduction of rain intensity (by half in the present study) is not able to change
the distribution of runoff much; fast response of water was also observed under reduced rain-
intensity. Another important aspect of the snowpack worth noting was that most of the input
appeared as runoff. This stage of snowpack with high conductivity is found to be responsible for
the production of high streamflows. Heavy rain either with snowmelt or alone can generate floods
under such conditions. The importance of the role of rain water in conditioning the snowpack to yield
maximum and fast runoff is due much more to input rather than to rain-induced snow melt. 1997
Elsevier Science B.V.
Keywords:Water movement; Snowpack; Rain; Snowmelt; Field study
0022-1694/97/$17.00 1997 Elsevier Science B.V. All rights reserved
PII S 0 0 2 2 - 1 6 9 4( 9 7 ) 0 0 0 0 4 - 8
Journal of Hydrology 202 (1997) 120
* Corresponding author.1 Parent institutes address is National Institute of Hydrology, Roorkee (U.P.), PIN-247 667, India.
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1. Introduction
Rain-on-snow is a common feature in various alpine parts of the world and plays a
significant role in generating high streamflows. Such events have much greater potential
for generating serious floods than do short periods of radiation-induced snowmelt
(Kattelmann, 1985). Rain-on-snow events, in which snowmelt may be limited, are also
hydrologically important (Colbeck, 1975; Berg et al., 1991). Most of the largest floods in
British Columbia, Washington, Oregon and California have been associated with rain-on-
snow (Kattelmann, 1987; Brunengo, 1990). Archer et al. (1994) reported that in Britain,
flooding frequently results from combination of melting snow and rainfall. Recently, in the
middle of January 1996, a combination of thaw with three days of very mild temperatures
and rain increased the level of lakes rapidly in the Lake Ontario basin (Environment
Canada, 1996). Rainfall occurs in the high altitude regions of Himalayas during the active
melting period, however, its significance in terms of causing floods is yet to be investi-
gated (Singh et al., 1995; Singh and Kumar, 1996). Furthermore, runoff from rain-on-snow
triggers landslides and is also considered to be the primary cause of changes in channelmorphology owing to erosion in some areas (Harr, 1981,1986; Christner and Harr, 1982;
Bergman, 1987). For example, at a site in the Oregon Cascades, 85% of all landslides
which could be accurately dated were associated with snowmelt during rainfall (Harr,
1981). Recently Sandersen et al. (1997) reported that triggering of debris flows in Norway
is caused by the combination of rainfall and snow melt. Rain-on-snow events are also
considered a major cause in the release of avalanches. Introduction of liquid water into
snow weakens the bond between grains and alters the snow texture which results in
reduced mechanical strength of the snowpack. Various studies have been carried out on
the role of rain in triggering avalanches in maritime climates (Conway et al., 1988,
Heywood, 1988; Conway and Raymond, 1993).
Rain or snowmelt generated at the snow surface passes through the porous snowpackbefore appearing as streamflow. The major factors affecting the response of outflow
include surface melt, snow metamorphism, water movement through the wet snow, inter-
action of melt water with the underlying soil, and overland flow at the snow cover base.
The percolation of water into snow can be considered in many ways analogous to the
movement of water into coarse sand (Gerdel, 1954), but there are some differences
between the two media which make application of this analogy difficult. For example,
in the case of water flow in the snowpack, the input (water) and porous medium are the
same substance and phase changes between water and snow can interfere with flow
(Jordan, 1983). Also, snow undergoes continual metamorphism, so the physical properties
of the porous medium are changeable and influence the response of the outflow. Further-
more, through the process of snowmelt, the porous medium ultimately disappears. The
snowpack is built up by individual snowfalls which produce a stratified deposit and eachlayer has its own effect on the water movement.
In order to improve streamflow prediction for reservoir operation, flood control and
design of major structures, models are needed to estimate the timing, amount and rate of
outflow from the snowpack under rain-on-snow events. Such knowledge requires a
thorough understanding of processes associated with liquid water storage, natural and
rain-induced melting, and transmission through the snowpack. Wankiewicz (1978b)
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emphasised the quantitative effect of snow cover on the various runoff mechanisms and
Kattelmann (1985) discussed the necessity of accurate forecasting of snowmelt particu-
larly during rainy conditions. The present study is carried out with a prime objective of
obtaining information on melt water movement and runoff generation from the snowpack
which would be applicable in physically based models. For this purpose, artificial rain was
applied over two snow plots prepared in natural field conditions and outflow from these
plots was observed and studied. Because small-scale studies can provide a better under-
standing of the hydrological processes, studies were conducted on a plot scale.
2. A brief review of water movement through snow and other associated processes
To understand the infiltration of liquid water into natural snowpack, various field studies
were carried out by spreading water-soluble dye at the snow surface and examining the
end conditions by excavating snow pits after rain. These studies have shown that percola-
tion occurs in a highly irregular manner. However, travel time estimated using dye undernormal snowmelt conditions may be different than that under rainy conditions because a
higher water flux into the snowpack will influence the infiltration rate. Real-time studies
monitoring the water movement into the snowpack are hampered by the poor availability
of tools suitable for this purpose. Furthermore, these instruments need to be set in the wall
of a snow pit, and excavations potentially alter the thermal and hydrological properties of
the snowpack. Consequently, only limited studies have been conducted on a real-time
basis (Sturm and Holmgren, 1993; Conway and Benedict, 1994).
Gerdel (1954) described isolated vertical flow channels of coarse grained snow that act
as drains or preferential paths for melt water. The vertical channels occupy only a fraction
of the total snow volume and the local rate of penetration is often much faster than average
rates (Gerdel, 1954; Wakahama, 1975; Colbeck, 1979; Kattelmann, 1985, 1989; Marshand Woo, 1984, 1985). Recently, Conway and Benedict (1994) reported that water pene-
trated through localised channels which often occupied less than 50% of the total volume
of the snowpack and water infiltration into the snowpack was not uniform. Water con-
centrates in these channels and penetrates deeper more rapidly. Kattelmann (1986)
reported that snowpacks in the central Sierra Nevada need not reach a ripe stage (in
which the entire water holding capacity of the snow is satisfied) before outflow begins. Ice
layers are not impermeable but rather are characterised by a variable permeability which
forces the melt water to take numerous sideways steps on its route to ground level (Gerdel,
1948, Langham, 1974). These layers would tend to increase the transient storage of liquid
water at impeding boundaries and reduce the average conductivity of the snowpack.
However, these tend to lose their low-conductivity characteristics as metamorphism pro-
ceeds because of the passage of melt water; ice layers, in particular, can become high-conductivity layers as they break down. The metamorphism of the ice layers and the
development of the concentrations of vertical flow in the snowpack are discussed in detail
by Wankiewicz (1978a). At the time of infiltration of water, development of flow
fingers in the snowpack has been observed by several investigators. These flow
fingers hasten vertical water movement and counter the effect of high density layers
(Kattelmann, 1985, McGurk and Kattelmann, 1988). Conway and Raymond (1993) found
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that water flowed preferentially through a few flow fingers which enlarge at the
expense of others.
The gravity drainage theory of water movement through a ripe snowpack was developed
by Colbeck (1972). Calculated hydrographs at various depths in the unsaturated zone were
tested against field measurements by Colbeck and Davidson (1973) in a very deep snow-
pack in the Cascade Mountains of Washington. Later, Colbeck (1974) treated the move-
ment of water after it has percolated to the bottom of the snowpack and moved downslope
through a thin saturated layer formed at the base of snowpack owing to accumulation of
percolating melt water. Dunne et al. (1976) tested this theory in the homogeneous snow
cover in the Canadian subarctic where the slopes were steep and uniform, and infiltration
to the soil was negligible. In general, measured runoff compared reasonably well with
observed runoff from the various study plots with different sizes, aspects, slopes and
having a variety of vegetative covers. One consistent difference between the calculated
and measured discharges was the overestimation of the lag-time between peak rates of
melt and runoff. It was suggested that this difference may be due to the presence of water
channels underneath the snow cover. Wankiewicz (1976) developed instrumentation forthe measurement of capillary pressure in snow and applied the theory to the study of melt
water movement in deep mountainous snowpacks.
3. Materials and methods
3.1. Study area and setting of the experiment
The study was conducted in the Glatzbach watershed located about 5 km S.W. of
Grossglockner, the highest peak of the Austrian Alps. Normally, a 12 m deep snowpack
is developed during winter in this watershed. The watershed experiences more than 60% ofannual precipitation during spring and summer when active snowmelt takes place.
3.2. Snow plots
For this study, two snow plots were prepared in natural field conditions at an elevation
of 2640 m. Plot I was an isolated plot, whereas plot II was a part of the continuous
snowpack. To prepare plot I, first a larger (3 m 2 m) plot was prepared by digging the
snowpack from all sides. Then carefully, it was further reduced to the dimensions of
2.30 m 1.30 m maintaining the natural structure of the snowpack. It was essential
because such changes can disturb its thermal and hydrological conditions which can affect
the flow processes. Available depth of the snowpack was 1.08 m and it was not changed
during the preparation of the plots. To check the lateral flow from plot I, three sides of thisplot were wrapped by a plastic sheet and the runoff draining side was kept exposed. The
upper surface (airsnow) and lower surface (groundsnow) were not disturbed. To avoid
the possibility of fast percolation of rain water falling near where the edge of the plastic
sheet met the snow, rain was simulated only over the inner 2 m 1 m of this plot, i.e. for
each side, 15 cm space was kept rain-free. The slope of this plot was about 3 .
Plot II was intact with natural snowpack on three sides, i.e. it was part of a continuous
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snowpack. Only the draining side was dug to measure the outflow from this plot. Rain was
simulated over an area of 2 m 2 m. Plot II was located very close to plot I, and the
structure and other conditions of plot II were similar to those of plot I. It took about two
days to make both plots ready for the experiment. A sketch of the snow plot with the rain
simulator and other measuring devices is shown in Fig. 1.
3.3. Rain simulation
Two rainfall simulators, developed at the Institute for Torrent and Avalanche Control,
Vienna, Austria, were installed to simulate rainfall over snow plots. Each rainfall simu-
lator is designed to precipitate over an area of 2 m 2 m, but this area can be reduced
according to the requirements of the investigations. The rainfall simulator is equipped with
a horizontally moving device consisting of two pipes, each 2 m in length and separated by
15 cm, and a net installed about 10 cm below the pipes. Each pipe has 33 equidistant holes
fitted with nozzles. The water pressure produces small jets through nozzles which break
into small water drops before falling on the net. Finally rain drops 34 mm in diameter areproduced through the net. According to the design of the instruments, the net can be
changed to generate rain drops of larger or smaller sizes, but we used the net producing
normal sized raindrops (34 mm) throughout the experiment. The rain drops fell over the
snowpack from a height of about 1 m. This whole rain simulating device moved back and
Fig. 1. A sketch of a snow plot with the rain simulator and other measuring devices.
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forth at a constant speed of 0.20 m s1 over the snow plot providing a uniform distribution
of rain over the study plots.
Water was pumped from a nearby lake through a pipe to the water distributor. The
distributor supplied the water to the regulators. Each rain simulator had a separate
regulator to control the rain intensity between 20 and 200 mm h1. In the present study,
different intensities were used for different events. The rainfall simulator was micro-
processor based and both the rainfall intensity and simulation period were programmed
before starting the rain event. The rain intensity was not changed during the preset time
period of the event. The rainfall intensity was recorded every minute in the data logger.
The whole set was run by the generator in the field.
3.4. Runoff observations
Runoff from each snow plot was measured separately by employing a tipping bucket
device. The surface and sub-surface flows were collected in an open pipe channel immedi-
ately after emerging out from the plot and were drained into the tipping bucket. Themeasuring device for each plot was installed very close to the plot at a sufficiently
lower level so that runoff could move immediately to the tipping bucket through a gravity
feed mechanism. Like rainfall, the outflow was also automatically recorded in the data
logger.
3.5. Meteorological data
Continuous measurements of net radiation, air temperature, humidity, and wind velocity
were made at 2 m above the snow surface. All the sensors were mounted on a mast
installed near the experiment plots. The data were observed at intervals of 15 s by the
instruments and averaged out for a period of 10 min before recording in the data logger.All the sensors were calibrated before taking them to the field. This set of instruments was
battery operated. The meteorological records observed during the experiments are shown
in Fig. 2. No precipitation was observed during this time and most of the time, cloudy
conditions prevailed.
3.6. Soil moisture and temperature
The Trase System (Model-6050X1) based on time domain reflectometry (TDR) was
used to measure the soil moisture content under the snowpack. The metal wave guides
were installed at 15 cm below the ground surface. The instrument was programmed to
autolog average moisture content every 10 min. Changes in the soil moisture content
recorded during the experimental period are shown in Fig. 2. The soil temperature wasabout 0C throughout the experiment.
3.7. Stratigraphy
Stratigraphic observations of the snowpack including height, density, ice layers, grain
size and type, liquid water content and temperature profile were made according to the
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international classification for seasonal snow on the ground (Colbeck et al., 1990). The
density of the snow was determined using sampling snow tubes of known volume and the
liquid water content was measured with a dielectric moisture meter with a flat capacitive
sensor. To avoid disturbance of the natural structure of the study plots which could have
influenced the infiltration processes, a few parameters (density, grain size and type and
Fig. 2. Net radiation, air temperature, relative humidity, wind speed and soil moisture content observed on 13th
May 1996 in the the Glatzbach basin located in the Austrian Alps
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liquid water content) were determined at a nearby location before starting the experiment
and assumed to be same for the study plots. However, at the end of the experiments, this
information was collected from the actual study plots.
The rate of settlement and compaction of the snowpack depends upon several processes
causing changes in the form and displacement of crystals within the snowpack. Further-
more, settlement of snow is influenced by the additional weight of the rain. Under the
present investigations, the depth of the snowpack was reduced by 5 cm after all three
simulations as a result of melt and settlement of snow. The density was slightly increased
after the rain simulations.
3.8. Case studies
The study was carried out over two snow plots on 13th May 1996 for three rain-on-snow
events with different rain intensities ranging from 0.80 to 1.66 mm min 1. The average
rain intensity, starting time and duration for different events for each plot are given in
Table 1. As discussed above, the objective of the study was to study the runoff response ofthe snowpack under heavy rain events, therefore, high rain intensities were used. The sky
was overcast during the experiments and air temperature was not too high. These weather
conditions provided an environment very similar to that of natural rainy conditions in the
study area.
3.9. Natural and rain-induced snow-melt
The energy balance equation and other associated parameters used in calculating snow-
melt are given as:
Q =Qn + Qh +Qe + Qr + Qg
where:Qis total energy gained by the snowpack (W m 2);Qnis net radiation (W m2);Qh
is sensible heat flux (W m 2); Q eis latent heat flux (W m2);Q ris rain flux (W m
2); and
Qgis ground heat flux (W m2).
Furthermore sensible, latent and rain heat-fluxes are described as:Qh = Kh P DT V;
Qe =Ke De V; and Qr = 0001 rw Cpw (Tr Ts) R , respectively.
Khand Ke are sensible and latent heat exchange coefficients, respectively and can be
Table 1
Details of the different rain simulations over snow on plot I and plot II
Rain
event
Starting
time
Snow plot I Snow plot II
Average
ain intensity
(mm min1)
Duration
(min)
Average rain
intensity
(mm min1)
Duration
(min)
1 11:30 hours 1.27 120 1.65 120
2 14:30 hours 1.66 120 1.61 120
3 17:30 hours 0.83 60 0.80 60
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written as (Ambach, 1986; Braithwaite, 1995):
Ks =Cpak
2r0
P0ln zz
0w ln zz
0T
Ke =Lk2r00623
P0ln zz0w
ln z
z0e
where,
Tris temperature of rain (C)
Tsis temperature of snow surface (usually 0C)
DTis difference in air and snow surface temperature
Deis difference in air and snow surface vapour pressure
Vis mean wind speed (m s1)
R is rate of rainfall (m s1
)Pis atmospheric pressure at site (Pa)
P0is standard atmospheric pressure (1.013 105 Pa)
Lis latent heat of evaporation (2.514 10 3 kJ kg1)
Cpw is specific heat of water (4.2 kJ kg1
C1)
Cpa is specific heat of air at constant pressure (1.005 kJ kg1
C1)
r0is standard density of air (1.29 kg m3)
rwis density of water (1000 kg m3)
k is Karmans constant (0.41)
zis height of instruments above snow surface (in the present case 2 m)
z0w is roughness parameter for logarithmic wind profile (1.0 104 m)
z0T is roughness parameter for temperature profile (6.0
10
6
m)z0eis roughness parameter for logarithmic vapour pressure profile (6.0 106 m).
Table 2
Details of total input from various sources to the snow plots for different events and observed runoff
Rain events Rain input
(mm)
Natural
snow melt
(mm)
Rain-induced
snow melt
(mm)
Total liquid
water input
(mm)
Observed
runoff
(mm)
Snow plot I
1 155.0 1.0 0.9 157.0 3.1
2 200.0 5.2 8.2 213.4 202.1
3 51.0 1.1 1.2 53.3 59.2Total 406.1 7.3 10.3 423.3 264.4
Snow plot II
1 201.0 1.0 1.2 203.2 3.0
2 196.4 5.2 7.9 209.5 203.8
3 49.2 1.1 1.1 51.4 53.3
Total 446.6 7.3 10.2 464.1 260.1
All figures correspond to the unit area of the each plot.
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Both natural and rain-induced snowmelt were computed from the snowpack. In the
present computations, the ground heat flux was ignored because ground surface tempera-
ture was about 0C throughout the experiments and its contribution is considered to be
negligible. To estimate the rain-induced snowmelt, the rain temperature was assumed to be
equal to the air temperature prevailing at that time. The cumulative values for natural
snowmelt and rain-induced snowmelt for each event are shown in Table 2. It can be
noticed that contribution of abundant rainfall to snowmelt was not significant. This was
because of very little energy supplied to the snowpack through rain as compared with other
components of the energy source. During the experiments, the snowpack was at 0C,
therefore, possibility of refreezing the water can be ignored. The total rain remained in
the liquid form in the snowpack.
4. Results and discussion
4.1. Event 1
The first rain simulation was made at 11:30 h and continued for a period of 2 h with an
average intensity of 1.27 and 1.65 mm min 1 over plot I and plot II, respectively. Both
snow plots were isothermal at 0C, therefore, any heat deficiency can be considered to be
negligible. Before the rain started, the average liquid water content of the snowpack for
both plots was about 4% by volume. The snowpack contained five well-distinguished ice
layers ranging from about 2 to 8 mm in thickness. The patterns of input to the snow plots
and response of outflow from each plot for this rain event are shown in Fig. 3. Results
indicate that, in spite of isothermal state of the plots and the heavy rain inputs, negligible
runoff was observed from either plot and this runoff was from the subsurface flow only.
Under this event, total depths of rain water supplied to unit area of plot I and plot II were155 and 201 mm, respectively. No significant change was observed in the soil moisture
content under the snowpack and deep percolation of water can be assumed to be very small
during this short period. This confirms that the total quantity of liquid water was absorbed
and retained in the snowpack.
To understand the storage behaviour of the snowpack, we measured the liquid water
holding capacity of the snow at the end of third rain simulation. These observations were
made after the drainage of excess gravitational water from the snowpack and can be
considered as representative of liquid water holding capacity, defined as the maximum
amount of water absorbed and retained in the snowpack against gravity at a given stage of
metamorphism and density. Liquid water holding capacity was determined at two different
places for each snow compartment between two ice layers. It included one observation in
the centre of snow between each set of two ice layers and the other just above the ice layer.The central observations represented liquid water holding capacity of the homogeneous
snow without impeding the water movement, while the observations just above the ice
layer included the impeding effect owing to the presence of ice layers. The average liquid
water holding capacity of snow without the impeding effect was computed to be 6.6 and
7.0% for plot I and plot II, respectively, but this increased to about 14.4 and 14% near the ice
surfaces. These high values may be because of high saturation of snowpack, impermeability
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of ice layers and lack of sufficient slope which impeded horizontal drainage over the ice
layers resulting in water staying in the snowpack. It indicates that impeding characteristics
of ice layers more than doubled water holding capacity of the snow and enhanced the
water storage capacity substantially. In the absence of ice layers, liquid water deficit in the
Fig. 3. Simulated rainfall, computed snowmelt and observed runoff from plot I and plot II for event 1.
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snowpack would have been about 3% and only about 30 mm of the input water would have
been absorbed by the snow plots, but the quantities of water absorbed by plot I and plot II
were 155 and 201 mm, respectively. In other words, this stage of snowpack played an
important role in reducing the risk of high streamflow owing to heavy rainfall.
It is difficult to make accurate calculations of the water storage in the snowpack unless
the internal lateral movement of water from the snowpack is known. However, an approxi-
mate quantitative estimate of the water storage were possible for plot I in which use of the
plastic sheet restricted lateral flow to a known surrounding additional area of about 1 m 2.
Plot II had no provision for restricting the lateral movement of water. Occurrence of lateral
flow in the plot I to the adjoining area was confirmed by observing runoff from the whole
bottom of snowpack (not only from the 2 m2) during the second and third rain simulations.
Because of lateral flow to the known adjoining area, it can be safely assumed for plot I that
water was stored in an area of 3 m2, instead of 2 m2 over which rain was precipitated. This
increase in the water storage area will reduce the depth of stored water from 155 mm to
about 103 mm for unit area, which is about 10% by volume. Bearing in mind the liquid
water capacity of the homogeneous snowpack (about 7%) and retention by the ice layers(about 14%), it seems possible for the snowpack to hold this high quantity of water under
the first event.
The lack of information on the capacity of snow to retain liquid water against gravity, as
a function of some index of the stage of metamorphism, constitutes a major gap in the
knowledge of the storage effect of the snow on runoff. Diurnal fluctuations in the liquid
water content and density of snow indicated some correlation (U.S. Army Corps of
Engineers, 1956). Wankiewicz (1978b) reviewed liquid water holding capacity of snow
reported by various authors for different snow densities and found a considerable variation
for both new and old snow. It was not clear whether liquid water holding capacity is related
to snow density or not. However, broadly speaking, higher water holding capacity was at
higher snow density. Such differences in liquid water holding capacity were suggested tobe due to difficulties in measurement and dependence of the values on factors other than
density. There are very few observations at lower snow densities (100 350 kg m3), but it
is expected to be about 25% (U.S. Army Corps of Engineers, 1956). Recently, Conway
and Benedict (1994) reported the liquid water holding capacity of snow to be 6% during
the midwinter period after drainage of rainwater from the snowpack. Our observed aver-
age liquid water holding capacity values for the snow without impeding effects are com-
parable with these reported values, but the values near the ice layer were remarkably
higher showing the impeding effect of water.
4.2. Event 2
The second rain simulation was conducted at 14:30 h i.e. 1 h after stopping the first rainsimulation. This event was also continued for a period of 2 h with average rain intensities
of 1.66 and 1.61 mm min 1 over plot I and plot II, respectively. The runoffs from the both
plots were measured until the complete recession of outflow and are illustrated in Fig. 4.
Unlike the first event, both snow plots started to produce runoff soon after the rainfall. The
water percolation rate and the response time of the water flowing under the snow to the
stream channel, are principal factors determining the time of arrival, ta and time of
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Fig. 4. Simulated rainfall, computed snowmelt and observed runoff from plot I and plot II for event 2.
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equilibrium concentration, te of liquid water in snow. These two factors are mainly
governed by the structure, transient storage and water flux to the pack. The other factor,
the response time of the channel itself, may be important on the watershed scale, but on the
plot scale this factor is of minimal importance. The time of equilibrium concentration
represents the time of peak streamflow when input is constant and equals the time required
for water to reach the stream from the furthest part of the watershed. The lag-times in
producing runoff were about 10 and 15 min, whereas times of equilibrium concentration
were about 30 and 40 min for plot I and plot II, respectively. It shows that water infiltrated
very fast from both plots. The reasons for this fast speed of water are discussed in Section
4.3. A faster rise and recession of the outflow hydrograph was noticed for plot I which is
possibly largely due to its smaller contributing area to the runoff and limited lateral
movement of water.
The duration of rainfall, tr was greater than te, therefore, outflow reached a roughly
constant value, slightly higher than rain input for both plots. A runoff higher than the
rainfall input can be expected because of snowmelt contribution. The atmospheric condi-
tions allowed some natural snowmelt during this rain simulation period and some rain-induced melt also occurred at the same time. This snowmelt added to the rainfall runoff
and emerged as combined outflow. Variations in rainfall intensity and snowmelt contrib-
uted to the minor fluctuations in the runoff. The recession of the hydrograph started soon
after stopping the rain and reached a very low outflow from both plots. The major part of
the recession occurred within first 15 20 min after stopping the rain, but a slow recession
continued for about 1 h. This was because of a major contribution of rainfall to the out-
flow. Measurement of soil moisture content did not show any significant change during
this event. This indicates that the soil was already saturated and allowed the maximum
amount of water reaching the bottom of the snowpack to appear as runoff.
4.3. Event 3
A third rainfall simulation was started at 17:30 h after complete recession of the outflow
from the previous simulation. Under this event, the rain was simulated with an average
intensity of 0.83 and 0.80 mm min1 for a period of 1 h for plot I and plot II, respectively.
This rain intensity was about half that of the second rain simulation. Runoff started
draining from both plots after about 10 min of starting the rainfall (Fig. 5). This can be
considered as a minimum time-lag for the rain water to emerge as outflow because the
snowpack was already fully conditioned during the first and second rainfall simulations. It
shows that water infiltrated at a high speed of 6 m h under this event. The lengths of time to
equilibrium concentration were observed to be about 35 and 45 min for plot I and plot II,
respectively. For this event also,trwas greater thanteand weather conditions did not allow
much natural snowmelt during this period. Moreover, there was not much rain-inducedmelt. Therefore, magnitude of observed runoff reached a constant value approximately
equal to the rain input afterte. This was the stage of snowpack when runoff was produced
virtually without any loss. Such conditions are possible during an active melt period
allowing maximum contribution from rain and snowmelt to the emerging outflow with
some delayed effect.
Jordan (1983) reported an average speed of the melt water wave to be about 0.22 m h1.
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Fig. 5. Simulated rainfall, computed snowmelt and observed runoff from plot I and plot II for event 3.
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Other investigators have also reported normal snowmelt movement approximately within
this range. In our case, water moved at a speed of about 6 m h 1 when the snowpack was in
its fully saturated condition and this speed was higher than the reported normal snowmelt
infiltration speed. It can be explained on the basis of heavy rain, changes in metamorphism
in the saturated snow and existence of preferential flow paths in the snowpack. The rate of
metamorphism is greatly accelerated under water-saturated conditions and grains are
coarsened (Wakahama, 1975). Raymond and Tusima (1979) observed grain coarsening
in saturated snow and reported significant change in the size of snow particles within a
short time. These larger grains are responsible for the increase in hydraulic conductivity
(Glass et al., 1989). Furthermore, because of larger grain size in the saturated condition,
the intrinsic permeability of the porous medium is higher than in the unsaturated condition
(Colbeck, 1974). Moreover, differential snow settlement during the rainy conditions
caused more undulation over the snow surface which helped in creating preferential
flow paths in the snowpack. These vertical preferential flow paths tend to reduce the
transient storage and travel time of water in the snowpack in comparison to that which
would occur if uniform melting was taking place on the surface. Under heavy rain con-ditions, such changes in the physical characteristics of the snowpack are responsible for
the fast movement of water through the snowpack. It is to be noted that in such situations
the normal snow melt will combine with rain water and pass through in a time faster than
its normal travel time enhancing the probability of flood.
A comparison of runoff distribution during the second and third rain simulations
demonstrates the effect of reduction in rain intensity on ta and te. For the third event,
rain intensity was half that of the second event, and we expected larger taand teowing to
this lower rate of water influx. However, we observed that tewas extended by about 5 min
for both plots and no change was noticed in taas compared with that for the second rainfall
simulation. It shows that for the subsequent event, even with lower rain-intensity, the
water infiltrated at high speed and the distribution of runoff was not changed significantly.This may also be due to more and/or wider preferential paths owing to coarser grains for
the third event. The snowpack was under highly saturated conditions for longer time
(event 1 plus event 2) before the third event. It provided accelerated metamorphism for
a longer period resulting in coarser grains than for the first and second events. For the third
event, more intense undulation was created over the snow surface as compared with
previous events because rain had fallen for a longer time prior to this event. Furthermore,
coarser grains provide more and/or wider preferential paths in the snowpack and water
moves at a higher speed.
The water balance analyses of all rain events are given in Table 2. No loss of water in
the form of infiltration to the soil is considered because of the saturated conditions. It can
be seen that roughly the total input to the plots emerged as runoff from both snow plots for
the second and third rain events. The snowpack became fully conditioned during theseevents because of sufficient rain input and therefore, most of the rainwater infiltrated
through the snowpack very quickly to appear as runoff. Any heavy rain with snowmelt
can result in high streamflow causing floods under such conditions. Furthermore, it can
also be seen that water retained by the unit area of plot II was higher than that of plot I
(Table 2). This is possible because of more lateral flow from plot II to the adjoining area of
the snowpack.
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5. Conclusions
This paper describes the storage and transmission characteristics of water through snow
for rain-on-snow events and provides quantitative experimental data obtained from field
investigation in the Austrian alpine region at an altitude of 2640 m. These investigations
were carried out on a plot scale by simulating artificial rain with different intensities and
durations, and studying the behaviour of emerging outflow. Snowmelt was computed
using an energy balance approach and the required data were collected simultaneously
at the study site. Soil moisture content and temperature were also observed. Some snow-
pack properties such as depth, liquid water holding capacity and density were also
observed before and after the experiment. Both snow plots were isothermal at 0C
before starting the rain simulations. The following conclusions are drawn from these
investigations.
The average liquid water holding capacity of the snowpack without impeding effect of
water was about 6.8%, but it increased to about 14.2% near the ice layers owing to
additional water impounded on their relatively impermeable surfaces. This increasedthe storage capacity of the pack substantially and snow plots absorbed and retained a
very high quantity of rain during the first rain event. It shows that heterogeneities in the
snow roughly doubled the water storage and retention capacity of the snowpack and at this
stage, the snowpack played an important role in reducing the risk of floods owing to heavy
rainfall over the snowpack.
The speed of water movement under heavy rain conditions was observed to be about
6 m h1. The snowpack was fully saturated and conditioned at this stage. The changes in
metamorphism in the saturated snow and the existence of preferential paths owing to
differential snow settlement under the rainy conditions, are understood to be the main
factors producing water percolation at high speed. These vertical preferential flow paths
tend to reduce the transient storage and travel time of water in the snowpack in comparisonto that which would occur if uniform melting was taking place on the surface. This
indicates that heavy rain water moves several times faster than the natural snowmelt
under non-rainy conditions. Moreover, under rainy conditions natural snowmelt also
percolates faster along with rain water enhancing the probability of flood. Furthermore,
most of the total water input appeared as runoff because negligible loss occurred from the
input water flux. This stage of the snowpack is responsible for generating floods either
from heavy rain alone or in combination with snowmelt.
For an average intensity of about 1.60 mm min 1, the times to arrival of runoff,ta, were
recorded as 10 and 15 min, and times of equilibrium concentration of liquid water in snow,
te, were 30 and 40 min for plot I and plot II, respectively. For a subsequent event (third
event) with 80 mm min1,tawas 10 min for both plots and tewas 35 and 45 min for plot I
and plot II, respectively. It shows that after conditioning of the snowpack, a halving of rainintensity, could not change the distribution of runoff significantly and fast percolation of
rain water was observed even under reduced rain intensity. The snowpack was saturated for a
longer time before the third event which resulted in coarser grains. The rain amount during
the third event on the plot added to more and/or wider preferential flow paths as compared
to previous events. Such changes enhanced the conductivity of the snowpack for the third
event and it resulted in fast percolation of rain water even under reduced rain intensity.
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The results indicate that rainwater plays a much more important role in the conditioning
of the snowpack to yield maximum runoff than in contributing to additional melting of
snow. Even heavy rain simulations could not produce significant rain-induced melt, but
they saturated and conditioned the snowpack quickly to produce maximum and fast runoff.
Further investigations studying the effect of varying snowpack conditions on runoff
from rainfall are required. For this purpose, actual snowpack conditions should be evalu-
ated simultaneously to properly assess the snowpacks immediate storage potential with-
out disturbing its natural structure. In order to examine the effect of liquid water storage on
runoff, attempts should be made to develop an under standing of the relationship between
the liquid water holding capacity of snow and a possible index of the stage of meta-
morphism. Moreover, studies are required in order to understand the influence of ice
layers on runoff on the watershed scale.
Acknowledgements
We are very thankful to Josef Eitzinger from the Institute of Meteorology, BOKU,
Vienna, for his help in calibrating the instruments. Assistance rendered by Peter Peringer,
Fritz Zott and Alfred Elmar in conducting the experiments is duly acknowledged. P. Singh
is grateful to the Austrian Academic Exchange Program for providing financial support for
his stay in Austria to carry out this work.
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