hydrological response of snowpack under rain-on-snow events a field study (singh et al-1997)

8/10/2019 Hydrological Response of Snowpack Under Rain-On-snow Events a Field Study (Singh Et Al-1997)

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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)

2 P. Singh et al./Journal of Hydrology 202 (1997) 1 20


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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

3P. Singh et al./Journal of Hydrology 202 (1997) 120


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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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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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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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