rebecca harrison (2003)
TRANSCRIPT
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Meltwater Flow in a Snowpack,
Niwot Ridge and Berthoud Pass, Colorado
Rebecca Harrison
This thesis is submitted in part fulfilment of the requirements for the B.Sc degree in
Environmental Science at the University of Lancaster. January 2003
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Abstract
Niwot Ridge (NR) and Berthoud Pass (BP) are both situated in the Eastern Front
Range of the Rocky Mountains, Colorado, USA. Distinguishing the presence of
meltwater pathways is an important research issue in these areas due to local towns andcities relying on snowmelt for a large fraction of their water supply throughout the
summer months. A long established technique in soil, dye tracing was applied to snow in
order to investigate whether or not meltwater flowpaths were present. Experiments were
carried out twice in the two different locations, NR and BP showing how different
snowpack properties affect the meltwater flow. Red food colouring was used as a dye,
added to natural snowmelt water and applied to the snow surface and then left to allow
infiltration to occur. After a short period of time snow was removed in thin one and twocentimetre sections and the resulting snow profile photographed. Stratigraphic and
temperature profiles of the snow were also identified with the use of snowpits. With the
help of the computer program MATLAB the centre of mass of dye in each section,
spread of dye around this point and the roughness of each section in terms of the
presence of dyed and non-dyed areas of snow were established. Comparisons were made
between images, statistical results and stratigraphic snow profiles to help understand the
processes involved. Preferential flowpaths were seen to exist mainly in a snowpack not
isothermal at 0 oC. When the snow became warmer and had a more uniform temperature
structure dye tended to move as one large plume through the snow spreading horizontally
with depth. High amounts of lateral flow were also observed when there was a change in
snowpack properties in the vertical direction. There did not appear to be a lot of
connectivity between flowpaths, vertical movement appeared to occur mainly when a
weak spot was found between layers of different properties, for example ice layers.
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Contents
Abstract 1Contents 2List of Figures 4
List of Tables 5
1. Introduction 6- 1.1 Background 6- 1.2 Flow Processes and Hydrology of a Snowpack 7- 1.3 Dye Tracing 9
- 1.3.1 Dye Tracing in Soil 10- 1.3.2 Dye Tracing on Glaciers 11- 1.3.3 Dye Tracing in Snow 11- 1.3.4 Dyes 12
- 1.4 Research Aims 12- 1.5 Report Structure 13
2. Experimental methods 14 - 2.1 Research Team 14
- 2.2 Field Sites 14- 2.3 Dyes 16- 2.4 The Guillotine 17- 2.5 The Experiment 20- 2.6 Problems 21- 2.7 Statistical Analysis 22
3. Results 25- 3.1 Image Problems 25- 3.2 Fieldwork at Niwot Ridge 25- 3.3 Fieldwork at Berthoud Pass 29
- 3.3.1 BP1 29- 3.3.2 BP2 34
4. Analysis 39- 4.1 Centre of Mass 39
- 4.1.1 Centre of Mass for NR 40- 4.1.2 Centre of Mass for BP 1 42- 4.1.3 Centre of Mass for BP 2 44- 4.1.4 Comparison between Two Snowpacks Centre of Mass 47
- 4.2 Variance around the Centre of Mass 47- 4.2.1 Variance for NR 48- 4.2.2 Variance for BP 1 49- 4.2.3 Variance for BP 2 51- 4.2.4 Comparison between Two Snowpacks Isothermal at 0 oC 52
- 4.3 Roughness 53- 4.3.1 Roughness in all Experiments 53- 4.3.2 Roughness for NR 55- 4.3.3 Roughness for BP 1 56- 4.3.4 Roughness for BP 2 57
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- 4.4 Summary 58
5. Conclusion 60- 5.1 Conclusions 60- 5.2 Suggestions for Further Work 60
References 62Acknowledgements 65Appendices 66
- Appendix A Code used in MATLAB for statistical analysis 66- Appendix B Raw data after statistical analysis 68
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List of Figures
Figure 1.1 Movement of meltwater through snowpack 6Figure 1.2 Representation of channelled and water-film flow in snow 7Figure 1.3 Development of flow fingers 9Figure 1.4 Dye tracing example image 10
Figure 2.1 Location map 15Figure 2.2 Dye sprayed onto snow surface 16Figure 2.3 Mechanism of the guillotine 18Figure 2.4 Image of guillotine 19Figure 2.5 Using the guillotine 20Figure 2.6 Sequential removal of sections in snowpack 20Figure 2.7 Example of image from dye tracing experiments 23Figure 3.1 Image 2 from Niwot Ridge 26Figure 3.2 Image 12 from Niwot Ridge 27Figure 3.3 Image 28 from Niwot Ridge 27Figure 3.4 Image 44 from Niwot Ridge 28Figure 3.5 Image 48 from Niwot Ridge 28Figure 3.6 Image 94 from Niwot Ridge 29Figure 3.7 Image 1 from Berthoud Pass 1 31Figure 3.8 Image 11 from Berthoud Pass 1 31Figure 3.9 Image 20 from Berthoud Pass 1 32Figure 3.10 Image 23 from Berthoud Pass 1 32Figure 3.11 Image 27 from Berthoud Pass 1 33Figure 3.12 Image 40 from Berthoud Pass 1 33Figure 3.13 Image 1 from Berthoud Pass 2 35Figure 3.14 Image 21 from Berthoud Pass 2 35Figure 3.15 Image 25 from Berthoud Pass 2 36Figure 3.16 Image 38 from Berthoud Pass 2 36Figure 3.17 Image 60 from Berthoud Pass 2 37Figure 3.18 Image 78 from Berthoud Pass 2 37Figure 4.1 Centre of mass graph for three experiments 39Figure 4.2 Centre of mass in X an Y co-ordinate, May 9th 40Figure 4.3 Centre of mass as a function of distance through snowpack, May 9th 40Figure 4.4 Centre of mass in X an Y co-ordinate, May 18th 42Figure 4.5 Centre of mass as a function of distance through snowpack, May 18th 43Figure 4.6 Centre of mass in X an Y co-ordinate, June 1st 45Figure 4.7 Centre of mass as a function of distance through snowpack, June 1st 46
Figure 4.8 Variance in centre of mass, May 9th 48Figure 4.9 Variance in centre of mass, May 18th 50Figure 4.10 Variance in centre of mass, June 1st 51Figure 4.11 Schematic diagram of ice layers in the snowpack 52Figure 4.12 Roughness as a function of distance through snowpack 54Figure 4.13 Roughness in X and Y co-ordinates, May 9th 56Figure 4.14 Roughness in X and Y co-ordinates, May 18th 57Figure 4.15 Roughness in X and Y co-ordinates, June 1st 58Figure 4.16 Summary diagram of meltwater flow processes 59
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List Of Tables
Table 1 Snowpit data for Niwot Ridge 26Table 2 Snowpit data for Berthoud Pass 1 30Table 3 Snowpit data for Berthoud Pass 2 34
Table 4 Centre of mass, variance and roughness data for NR 68Table 5 Centre of mass, variance and roughness data for BP1 69Table 6 Centre of mass, variance and roughness data for BP2 70
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1. Introduction
1.1 Background
A reliable water source becomes increasingly important as the population rises inthe western United States. Alpine snowpacks provide a major source of annual
streamflow especially in spring contributing a large fraction of water to the hydrological
system (Michaels, 1985). Knowledge of meltwater routing through snow is important in
the determination of seasonal runoff from snowpacks and glaciers (Pfeffer and
Humphrey, 1996)
Initially, preferential water movement through snow is important for energy
transfer in the liquid phase, solute transport in wet snow and snowpack stability(Schneebeli, 1995). As the snow begins to melt physical movement of water becomes
more important, particularly release of meltwater at the base of a snowpack as in
Colorado the main source of water is from snowmelt (figure 1.1). The rate of release of
meltwater affects the river hydrograph and is especially important for the amount and
timing of the release of water from dams.
Figure 1.1 Movement of meltwater through all stages in the snowpack
The onset of snowmelt can vary by up to two months annually and it is therefore particularly important to understand the processes involved (Kattelmann and Dozier,
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1999). Spatial distribution, solute transport, chemical movement, avalanche forecasting
and hydropower potential are also all important consequences of meltwater flow
(Schneebeli 1995; Williams et al., 1999; Boggild, 2000; Petersen et al., 2001). Despite
research there is still no clear understanding of preferential flow and we are unable to
simulate it (Schneebeli, 1995).
1.2 Flow Processes and Hydrology of a Snowpack
Melt is produced at the surface of the snow initially draining vertically and
developing into heterogeneous flow as vertical movement occurs. This is due to variable
hydraulic conductivity caused by different density and grain structures, further modified
by refreezing and changes in liquid water content (Pfeffer and Humphrey, 1996). Liquidwater movement is thought to occur in distinct flow paths through the snow as opposed
to uniform flow through a homogeneous porous medium (op. cit.). The distribution and
size of these preferential flowpaths depends upon the structure of the snowpack and local
weather conditions (Schneebeli, 1995). Percolating meltwater moves in two ways (figure
1.2): (i). 'Channelled water flow', where meltwater flows as a single body of water and
(ii). 'Water-film flow', where meltwater flows slowly surrounding snow grains in a thin
film of liquid water (Wakahama, 1974). Small-scale spatial variation and macroscopiclayering exist in snowpacks due to variations in the bulk density and grain structure
(Tseng et al., 1994).
Figure 1.2 Schematic representation of (i) channelled water flow and (ii) water- film flow in snow. Black dots and areas represent water moving through the snow.
Adapted from Wakahama, 1974.
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Research has also been carried out into the weakening of the mechanical strength
and densification of snow relating to levels of meltwater within it (Wakahama, 1974). It
was found densification rate was double during the day compared to overnight and
suggested this might be due to a decrease in pressure from run-off of meltwater or
mechanical weakening of the snow during the warmer daytime temperatures. Over thecourse of the above research it was found that snow grains would continue to grow even
during the melt season, perhaps having an implication on meltwater flow. Schneebeli
(1995) also noted after an infiltration event snowpack properties may have changed
altering the location of flowpaths in the snowpack.
Tseng et al. (1994) have shown natural deposits of snow rarely remain isothermal
throughout their lifetime due to the wide range of physical processes occurring. Melting
and infiltration cause thinning of the snowpack throughout the melt season, varyinglevels of saturation and refreezing of meltwater both at the surface and within the snow
change the snows hydraulic and thermal properties.
The wetting front is the point at which snow changes from the wet top layer to a
dry bottom layer before the snow becomes isothermal. Macropores (open channels within
the snowpack) and flow fingers, lead to preferential flow of water through the snow
before the entire snowpack has become wet (Marsh and Woo, 1984; Schneebeli, 1995).
After the wetting front has reached the base of the snowpack water can infiltrate
throughout the whole mass of snow.
Kattelmann and Dozier (1999) noted that as the snowmelt season begins liquid
water enters the snowpack and it becomes 'ripe' at a rate and spatial variability influenced
by feedback mechanisms. During this ripening procedure water is retained by capillary
pressure in pore spaces at 0 o C. When pressure becomes equal between the upper and
lower layers of snow water is released and flows with gravity through the snow.
Observations of the snowpack show natural processes to include grain growth and
rounding, ice layer formation, warming of the snow to melting temperature,
densification, capillary retention of liquid water and creation of a water flow network.
After the onset of melt water is either retained by the snowpack or lost as runoff
depending on the temperature and physical structure of the snow (Pfeffer and Humphrey,
1996).
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Figure 1.3 Movement of water through a snowpack showing the development of
flow fingers. Adapted from Marsh and Woo (1984).
Marsh and Woo (1984) showed that ponding occurs at stratigraphic boundaries
within a snowpack due to position of the wetting front and abrupt changes in snow
texture. Ponded water then begins to flow in a downhill direction and spread laterally
forming a wet layer, as water continues to accumulate vertical flow fingers begin to
develop (figure 1.3). Water may also spread sideways forming narrow strips in the snow
according to minor differences in snow properties.
Pfeffer and Humphrey (1998) noted the formation of ice layers at stratigraphic
boundaries in a snowpack when the rate of freezing is greater than the force of water
accumulating and pushing across the boundary. Following the formation of this ice layer,
water drains onto the impermeable boundary and is diverted down slope or laterally
within the snowpack until an area is found where water is able to flow through the
boundary. As a body of ice is formed between the wet and dry layers of snow, latent heat
is released internally, warming the surrounding snow (Marsh and Woo, 1984). In the wet
portion of the snowpack ice decays as this latent heat is released and further melting canoccur. Unusually high rates of water input, infiltration and refreezing causing the
formation of ice layers indicates unusually cold initial conditions or early onset of melt
(Pfeffer and Humphrey, 1998).
1.3 Dye Tracing
Quantitative determination and prediction of preferential flowpaths is difficultdue to spatial and temporal variability involved in different flow processes (Petersen et
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al., 1997). In the past dye tracing experiments have been undertaken to observe primary
flow paths of water and solutes at a high spatial resolution (figure 1.4; Petersen et al.,
2001). Use of this dye tracing technique has enabled researchers to carry out detailed
studies of preferential flow through soil and glaciers. Existence of preferential meltwater
flowpaths in snow are generally recognised, however only limited quantitativeinvestigations have been carried out (Schneebeli, 1995). Understanding flow in different
mediums is important for water velocity and storage within the medium, flow and
transport of water and solutes, for example pesticides, through the medium and release of
water and solutes to the surrounding environment. Within snow, formation of preferential
flowpaths and meltwater movement, including macropore flow and water retention
becomes important for developing models of snowmelt run-off.
Figure 1.4 Example of a dye tracing experiment image taken in soil (Baveye et al., 1998)
1.3.1 Dye tracing in Soil
Dye-tracing work has been undertaken in soil to help improve our understanding
of soil drainage systems for irrigation, chemical leaching, water and nutrient availabilityfor plants and flood potential. The majority of research has concentrated on the use of
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coloured and fluorescent dyes showing preferential flow pathways and transport of water
and solutes through cracks, fissures, fingers, root systems and earthworm burrows in the
soil profile (Ghodrati and Drury, 1990; Petersen et al., 1997). Ghodrati and Drury (1990)
used a dye tracing experiment to characterise water and solute transport in the matrix of a
soil system in three dimensions. The study showed the presence of preferential flowchannels with water moving both laterally and vertically under different soil conditions.
Other soil related studies have investigated water infiltration into different types
of soil for example clay, sand and agricultural soils (Ritsema et al., 1993; Heppell et al.,
2000; Petersen et al., 2001). Soil has also been studied under different environmental
conditions, through dye tracing, including forested regions, frozen ground and hillslopes
(Luxmoore et al., 1990; Stadler et al., 2000).
1.3.2 Dye Tracing on Glaciers
Recently dye-tracing experiments have been used for research into glacial
meltwater. Niewnow et al., (1996; 1998) studied the Haut glacier d'Arolla, Switzerland
by carrying out 415 dye-tracer experiments during 1990 and 1991. The aim of the study
was to determine spatial patterns of meltwater flow and evolution of the drainage system
throughout the summer months. Analysis of the results showed specific characteristics of
the englacial and subglacial drainage systems including mean flow velocity, dispersion
coefficient and cross sectional area of flow. Channelled or distributed flow regime and
the response of seasonal run-off changes through these 2 regimes were also investigated
using dye-tracing experiments.
Subglacial drainage characteristics have also been studied at Dokriani glacier,
India through dye tracing. Dispersion, meltwater velocity, passage geometry and
channelled or distributed flow regimes were all investigated concluding flow is
dependant on the level of meltwater present (Hasnain et al., 2001).
1.3.3 Dye Tracing in Snow
The study of snow is a well-established science, however there appears to have
been few experiments carried out using the dye tracing technique. Comparable with soil
different snowpacks have different characteristics, which can be investigated through dye
tracing. Evidence from experiments so far suggests there is no distinct pattern of flow
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paths and preferential flow depends mainly upon the boundary conditions (Schneebeli,
1995). Schneebeli (1995) carried out dye tracing experiments using four different
coloured dyes to investigate the development and stability of flowpaths. Observations
were made of the existing flowpaths concluding that they do not remain stable in time or
space. Boggild (1999) has also investigated meltwater flow and water retention insnowpacks concentrating research on snow in West Greenland. Ink and water were
mixed to create a dye and sprinkled on the snow surface, ten centimetre sections were
then removed and photographed to create a three dimensional profile of the flow paths.
1.3.4 Dyes
The dye used in these experiments varies depending on the characteristics of themedium on which the dye tracing is performed. Within soil a variety of dyes have been
used including Acid-Red 1, Dispersed-Orange 3, Rhodamine B and Brilliant Blue FCF
(Ghodrati and Drury, 1990; Luxmoore et al., 1990; Petersen et al., 1997; Stadler et al.,
2000; Petersen et al., 2001;). Specific studies have been carried out in order to test
Rhodamine wt, Lissamine FF, Amino G acid and Brilliant Blue FCF as dye tracers for
their toxicity, mobility, absorption and fluorescence in the soil environment (Trudgill,
1987; Flury and Fluhler, 1995). Other tracers are also used in soil such as KBr (Ritsema
et al., 1993).
During the summers of 1989, 1990 and 1991 Rhodamine -B and Flourescine dyes
were used on Haut glacier d'Arolla, Switzerland and in 2000 Rhodamine - wt was used
on Dokriani glacier, India during a series of glacial dye tracing experiments (Niewnow et
al., 1996; Niewnow et al., 1998; Hasnain et al., 2001). There doesnt appear to be any
specific research published on dye tracers in snow, however, some tracers used in the
past are Brilliant Blue FCF, azofloxine, uranine and Lissamine yellow (Schneebeli,
1995).
1.4 Research Aims
This series of experiments attempts to help understand melt water movement
through snow using the technique of dye tracing. Spatial representation of snowmelt
processes is a research problem yet to be solved and few studies so far have dealt with
the problem of spatially distributed snowmelt models (Horne and Kavvas, 1997). A
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series of dye tracing experiments were designed to try and understand water movement
through snow. The main aim of the study is to better physically understand this water
movement enabling improvement of snowmelt models in the future.
Dye tracing involves mixing a selected dye with natural snowmelt water from the
snowpack and allowing it to infiltrate through the snow over a period of approximatelytwo hours. After this time a cutting device was placed above the dyed area of snow and
used to cut away thin sections of snow to expose the infiltrated dye. A sequential series
of digital images were then taken and used for studying general characteristics of flow in
order to establish relevant patterns. Using the computer program MATLAB the centre of
mass was identified to determine specific flow areas and the variance around the centre
of mass in a particular section compared to other sections within the same snowpack.
Comparisons were also made between different snowpacks.This project has benefited from the involvement in an active research program in
the USA. Members of INSTAAR (Institute of Arctic and Alpine Research) are currently
undertaking several research projects at the University of Colorado into snow and
hydrology. Daily and weekly measurements are taken at the INSTAAR Mountain
Research Station including snowmelt runoff level, snow properties (for example depth,
density and temperature) and meteorological data. Annual data has also been collected
and compiled by this research team since 1994. The specific experiments were carried
out with Tyler Erickson, a PhD student at the University of Colorado and part of the
INSTAAR snow hydrology research group.
1.5 Report Structure
The report initially covers background to the project, why it was carried out and
what previous work has been done in soil, glaciers and snow to initiate this particular line
of research. Chapter two then covers the experimental methods describing field sites,
equipment, the experiments and problems encountered during the experiment and their
solutions. Chapter three shows all the results obtained during the experiment leading to
an analysis and discussion of the results in chapter 4. Finally the work is summarised and
concluded ending with suggestions for further research work.
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2. Experimental M ethods
2.1 Research Team
The overall project aims to gain a better understanding of spatial continuity and
connectivity on varying scales in a snowpack using radar tomography, dye tracing and
lysimeters. Research support came from a grant by NSF (National Science Foundation)
the research being organised and run by Mark Williams, a snow hydrologist at the
University of Colorado. Other members of the research team include Tyler Erickson, a
PhD student with M. Williams leading fieldwork at the research sites and carrying out
geostatistical analyses. Tissa Illangasekare is a groundwater hydrologist from the
Colorado School of Mines working on model development and parameter estimation.Tad Pffefer, a glaciologist at the University of Colorado will work on radar tomography
instrumentation and analysis in the future on this project. Similar work is also being
undertaken at the Mammoth Mountain Ski Area (MMSA) site in California directed and
run by Rick Kattelmann for comparative data collection and analysis. It is also expected
that other graduate and undergraduate students in the future will work in the research
areas mentioned in collaboration with this project.
2.2 Field Sites
Two field sites were chosen for this experiment to enable the study of two
snowpacks with differing characteristics (figure2.1).
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~30km
Figure 2.1 Small area of Colorado map to show research site location
Initially experiments were carried out at the University of Colorado MountainResearch Station, Niwot Ridge (NR), Eastern slope of the Colorado Front Range of the
Rocky Mountains, 5km east of Continental Divide (Williams et al., 1999). The 'Soddie'
site used in this experiment is located at an elevation of 3150 m, below treeline and
surrounded by a forest preventing lateral inflow of meltwater. Snowmelt occurs relatively
fast at NR, by the time the first experiment was carried out the snowpack was isothermal
at 0 o C.
Experiments were also carried out at Berthoud Pass (BP) ski area at an elevation
of 3482m and a slope angle of 18 o, this site is also surrounded by a forested area below
treeline. Snowdrifts at BP were relatively deep at approximately 1.4m depth compared to
those at NR at approximately 90cm depth. During the first experiment (BP1) on 18th
May the snow was not isothermal at 0 o C, by June 1st (BP2) the snowpack had become
isothermal at 0 oC.
These two field areas were chosen due to easy access and undisturbed sites
consisting of several deep snowdrifts within easy travelling distance of Boulder and each
other. Both areas contain snowpacks with different characteristics mainly due to the
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climate and elevation. However, the setting of the specific site within the trees helps to
control other factors such as sunlight, wind and inflow of meltwater. The two field sites
do not retain snow all year round as in other US states and countries, across the Colorado
Rockies complete melting of snow occurs during the spring and summer months.
2.3 Dyes
A dye tracing technique was used during this series of experiments as it was
relatively inexpensive, quick and easy to set up at different field sites without the need
for large amounts of equipment. Food colouring mixed with natural snowmelt water was
used as a tracer during this experiment. The choice of dye is very important for any study
of this kind, as several factors could influence the results. The selected dye must havelow toxicity while maintaining high solubility and visibility. The dye used was red food
colouring as it is non-toxic, inexpensive and easy to obtain with high visibility and
solubility in snow. The ingredients in the dye were water, propylene glycol and red 40.
Other dyes were not tested, however features represented in the snow may have proved to
be different due to different absorption properties.
Figure 2.2 Dye sprayed onto snowpack across a two by two metre area
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30 ml of red food dye were mixed with one (US) gallon (3.78 l) of natural snow
meltwater and spread over an undisturbed two by two metre surface using a weed sprayer
and allowed to infiltrate into the snow (figure 2.2).
2.4 The Guillotine
A cutting device was used during the experiment to produce accurately spaced
and clean sections through the snow pack while studying the flowpaths. The 'guillotine'
(figure 2.3) used was designed and built by Tyler specifically to run the dye tracing
experiments accurately, avoiding the potential hazard of smearing dye on the snow
surface and in order to gain clean cuts through the snow. No one was allowed to disturb
the area to be sprayed with dye before the dye was applied, after application the dyedarea was left completely undisturbed for approximately two hours. After this time, the
guillotine was set up across the dyed area and secured firmly to ensure there was no
movement, which could affect the snow surface, dye infiltration or inaccurate
measurements when cutting the slices.
The guillotine consists of a tubular plastic frame lying on the snow surface
supporting the rest of the structure. Two hollow metal tubes are placed around this plastic
frame and allow forward and backward movement of the guillotine across the snow.
Above these metal tubes is the framework holding a large rectangular metal frame with a
1-cm wide blade at the base in order to cut through the snow. A small snow pit was dug
directly in front of the dyed area and the guillotine used initially to gain a clean section
until the dyed area of snow was reached. The snow was cut away using the guillotine in
one or two cm sections to reveal the flowpaths over an area of approximately 1m 2 (figure
2.4). Initially 2 cm sections of snow were removed however ice layers proved to be a
problem and it was found that 1 cm slices were easier, more accurate and provided a
smoother section to be photographed.
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Figure 2.3 Mechanism of the guillotine from front and side views.
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Figure 2.4 Image showing guillotine device at NR.
Three people are required to operate the guillotine successfully, two people to
pushed the guillotine from the top (figure 2.5) and moved it through the snow as new
sections are required. The third person stood in the snow pit to verify that straight
sections were cut and to operate the digital camera. The camera was fixed relative to the
position of the guillotine frame and the timer used to ensure identical image size and
resolution.
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Figure 2.5 Using the guillotine to cut a section from the snowpack at NR.
2.5 The Experiment
The experiment was carried out on four separate occasions on May 9th and May
15th at NR and May 18th and June 1st at BP (figure 2.1). Sections were removed every 1
or 2 cm by pushing the guillotine back after each cut, an image taken using a digital
camera and the images sequentially analysed (figure 2.6). At BP the slope was
considerably steeper than at NR so the guillotine had to be firmly anchored before it
could be used. Dye was sprayed slightly uphill of the area to be sampled since the dye
was expected to move down hill as well as percolating vertically through the snow this proved to be the case.
Figure 2.6 Inferred snowpack showing how sequential sections are removed
throughout experiment
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2.7 Statistical analysis
All the images taken in the field were transferred to a computer for analysis.
Firstly the blurred and completely out of focus images were discarded and the rest
corrected using the sharpen tool with a graphics package. The images were then all cut
out so the same area appeared on every image, this was done by using the guillotine
frame as a reference point and cutting out the snow area seen on every image. Resizing of
all the images then took place to ensure identical pixel sizes the images were 666 by 654
pixels, a pixel being 0.11 by 0.16cm. An example of an image cut out ready to be
analysed is shown in figure 2.7.
Figure 2.7 Example of an image ready for statistical analysis
Spatial moment analysis was then carried out on each of the images for NR, BP1
and BP2 using the computer program MATLAB (programming details can be seen in
appendix A). Each pixel has a value in the red, green and blue spectrum between 0 and
255, where 0 is white and 255 is pure red for example, these are known as digital
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numbers and were the values used during analysis. Figure 2.7 shows the point of the
origin used for each image during the analysis. Two comparative pixels are also
highlighted to show how dye concentration varies between adjacent pixels. The two
pixels highlighted are at co-ordinates (i,j) 624,320 and 625,320 with digital number
values in the red of 216 and 187, the higher value indicating a higher concentration of dye in that particular pixel.
The first moment involves looking at the centre of mass in each image to
determine where the main mass of flow is in the snowpack and whether this varies
through the snowpack using:
X = (X ij P ij) (1) P ij
Where X is mean centre of mass in x co-ordinate, X ij is the distance from the origin in the
x (i) and y (j) co-ordinates and P ij is the pixel value at the X ij co-ordinate in the pixel
value array. The first moment is calculated in the same way for the y co-ordinate:
Y = (Y ij P ij) (2) P ij
Where Y is the mean centre of mass in y co-ordinate, Y ij is the distance from the origin inthe x (i) and y (j) co-ordinates.
The second moment is based upon variance around the centre of mass using the
equations:
x = (Xij - X) 2 P ij (3) P ij
y = (Yij - Y)2
P ij (4) P ij
The variance attempts to illustrate the spread of dye around the centre of mass, a high
value for the variance indicating a wide spread around the centre of mass.
After analysis using the first and second moments an assessment of the roughness
of each section was carried out in order to investigate the existence of specific flow areas.
The roughness is defined as:
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R x = (P ij - P i+1j)2 (5) n
R y = (P ij - P i+1j)2 (6) n
where R x is the roughness value in x, R y is the roughness value in y, P ij is the pixel value
to the left of pixel P i+1j and n is the number of pixels. All the results were then plotted in
various different ways and the results compared and analysed.
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3. Results
3.1 Image Problems
Certain problems with the images were identified and solutions found. The main
problem found during the fieldwork was that the sun caused a low contrast ratio on someimages reducing the visibility of the flowpaths. A solution to this might have been to
carry out this tracing experiment at night (Schneebeli, 1995) however under the climatic
conditions of Colorado the temperature dropped at this elevation so therefore melt would
not occur as the snowpack refreezes. Cloud also proved to be a problem as scattered
clouds on the research days forced some photographs to be taken during sunny periods
and others during a cloudy spell again altering the contrast ratio within the image.
Computer manipulation of the images helped to remove some of the contrast ratio problems and improved visibility of the flowpaths for statistical analysis.
After the fieldwork was complete some of the images appeared to be blurred as
the automatic focus mechanism on the camera did not appear to work correctly. To
correct for this the camera had to be checked after every photo and the blurred images
sharpened using a computer. Some of the images were not correctable and another
experiment would need to be carried out under the same conditions in order to gain a full
data set for analysis.
3.2 Fieldwork on Niwot Ridge (NR)
The dye tracing experiment was carried out at NR on a cold but sunny day with
an air temperature of 3 oC and snow surface temperature of 0 oC with a few clouds and
slow wind speeds. Unfortunately snowpit data for this experiment is unavailable however
the snow had similar properties on May 15th and therefore the data for this day is shown
in table 1.
48 images were taken and used for statistical analysis at NR, a small sample of
these are seen in figures 3.1 to 3.6. Each image has a number associated with it indicating
the distance from the start of the cutting in centimetres. Each section was cut 2 cm
behind the last so for example image 12 (figure 3.2) was 12 cm behind the first cut and
the 6th image taken in the sequence.
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Table 1 Snowpit data for NR.
Date 15/05/02Location Niwot
Ridge
Temp Net Weight Height above Height above grain(C) (g) ground (cm) ground (cm) shape
183 182- wet snow, slush
180 1770 513 IC discontinuous ice
170 1740 489 ET
160 1680 533 IC distant layer of ice
150 1670 493 ET
140 1510 506 IC ice layers
130 1500 550 ET
120 1180 553 IC very thin ice layer
110 1170 542 ET
100 700 530
900 540
80
Figure 3.1 Image 2 from Niwot Ridge on May 9th 2002. Note the slightly dipping layersin the snowpack forcing the dye to move laterally rather than vertically.
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Figure 3.2 Image 12 from Niwot Ridge on May 9th 2002. The stripe down the centre of the image corresponded to a small piece of ice falling out of the snowpack.
Figure 3.3 Image 28 from Niwot Ridge on May 9th 2002. Image is slightly blurred however this should not affect results as statistics concentrates on contrast between dyed
snow and non-dyed snow.
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Figure 3.4 Image 44 from Niwot Ridge on May 9th 2002.
Figure 3.5 Image 48 from Niwot Ridge on May 9th 2002.
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Figure 3.6 Image 94 (final image) from Niwot Ridge on May 9th 2002. Note theincreased concentration of dye lower in the snowpack comparative to figure 3.1.
3.3 Fieldwork at Berthoud Pass
3.3.1 BP1
Fieldwork was carried out for two days at BP. A snowpit record was also created
and particular properties identified including temperature, net weight, stratifying layers
and grain shape (table 2). Abbreviations used in the snowpit sampling of grains include
ET (equi-temperature), IC (ice), TG (depth hoar) and CR (crust). At an elevation of
3482m, the air temperature was 8 oC with an identical snow surface temperature of 0 oC.
Wind speed was again low and low cloud cover observed although the experiment was
carried out in a tree covered area so changes in sunlight would not greatly affect the
experiment. The guillotine was pushed down to a depth of one metre, 60cm above the
snowpack base creating sections 1cm apart.
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Table 2 Snowpit data for BP1.Date: 18/05/02 12:50
Location: Berthoud Pass1
Local Slope: 18 DegreesAspect: 343 degrees from
north
GPS N: 4405646 MetersGPS E: 433684 Meters
Temp Net Weight Height above Height above Grain(C) (g) ground (cm) Ground (cm) Shape
0 160 1610 454 ET well rounded, wet
150 153-1 484 IC Bonded grains
140 150-1 426 ET well bonded, rounded, wet
130 1450 506 IC
120 143-1 464 ET well bonded
110 136-1 438 ET well rounded, bounded
100 1320 422 ET Slushy
90 1220 391 ET well banded clusters
80 1180 442 ET well banded, rounded
70 1070 455 ET well rounded
60 420 464 ET Rounded facets
50 240 418 CR
40 170 419 ET Rounded angular facets
30 120 420 ET Rounded angular facets
20 00 376
100 376
00
During the BP1 experiment 37 images were taken, however due to focusing
problems with the camera images between 11 and 20cm were not analysed. Figures 3.7
to 3.12 show six of the images used in statistical analysis.
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Figure 3.7 First Image from Berthoud Pass on May 18th 2002. Note the well-defined areas of dye towards the top of the section and the lateral flow across layers within the
snowpack.
Figure 3.8 Image 11 from Berthoud Pass on May 18th 2002. Specific plumes of
meltwater movement are observed on the left of the image and an anomalous patch of dye seen towards the right of the section.
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Figure 3.9 Image 20 from Berthoud Pass on May 18th 2002.
Figure 3.10 Image 23 from Berthoud Pass on May 18th 2002. Higher concentrations of
dye are observed compared to figure 3.8 also more layering is seen towards the top of the section.
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Figure 3.11 Image 27 from Berthoud Pass on May 18th 2002.
Figure 3.12 Image 40 (final image) from Berthoud Pass on May 18th 2002.
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3.3.2 BP2
BP2 experimental images consisted of 78 sections split 1cm apart in the
snowpack. The weather was cooler during this experiment relative to BP1 although the
temperature was the same as at NR at 3o
C although under cloudy skies on this occasion.During the time between the two experiments snow properties changed from a varied
temperature stratification to a snowpack isothermal at 0 oC as seen from snowpit data for
BP2 in table 3. Snow depth at BP2 was 14cm lower than at BP1, the guillotine was used
to 1m depth in the snow, 46cm above the snow base.
Table 3 Snowpit data from BP2. Note the isothermal at 0 oC temperature structure of the snowpack
Date: 01/06/02Location: Berthoud Pass
2Local Slope: 18 Degrees
Aspect: 343 degrees from northGPS N: 4405646 MetersGPS E: 433684 Meters
Temp Net Weight snowpack height Layer height grain shape(C) (cm) above ground (cm) Above ground (cm)
146 146ET
140 141
0 429 IC130 140
0 479 ET120 138
0 441 IC110 135
0 434 ET100 132
0 411 ET90 117
0 403 IC80 114
0 471 ET70 0
0 40960
0 46250
0 41340
0 46230
0 42420
0 445 100
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A selection of the images taken at BP2 are observed in figures 3.13 to 3.18. Note
the increase in concentration and spread of dye in the snowpack in later images.
Figure 3.13 First Image from Berthoud Pass (i.e. 0cm from start) on June 1st 2002. Notetwo layers dominating in the centre of the sections and the high dye concentration
between them.
Figure 3.14 Image 21 from Berthoud Pass on June 1st 2002.
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Figure 3.15 Image 25 from Berthoud Pass on June 1st 2002.
Figure 3.16 Image 38 from Berthoud Pass on June 1st 2002.
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Figure 3.17 Image 60 from Berthoud Pass on June 1st 2002.
Figure 3.18 Image 78 (final image) from Berthoud Pass on June 1st 2002. Much wider spread of dye was observed in this image compared to figures 3.13 to 3.17.
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After all the data was collected the images were cut out and adjusted to the same
resolution as seen in figures 3.1 to 3.18. All the images then underwent statistical
analysis as described in chapter two. Results of the statistical analysis are seen and
analysed in chapter 4 and plotted to enable comparisons through individual snowpacks,
between the two different snowpacks and between the same snowpack at different times.
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4. Analysis
4.1 Centre of Mass
Figure 4.1 shows the first moment, centre of mass in each section through the
snowpack during the three experiments: NR, BP1 and BP2. Each data point on the graphindicates one sections centre of mass (Raw data for these analyses is seen in appendix B).
0.000
10.000
20.000
30.000
40.000
50.000
60.000
70.000
80.000
90.000
100.000
0.000 10.000 20.000 30.000 40.000 50.000 60.000 70.000 80.000 90.000
Centre of Mass in X coordinate (cm)
C e n
t r e o
f M a s s
i n Y c o o r d
i n a
t e ( c m
)
BP2
BP1
NR
Figure 4.1 Centre of mass in x and y co-ordinates for all three experiments on May 9th at Niwot Ridge (NR), May 18th (BP1) and June 1st (BP2) at Berthoud Pass. Axis indicate
size of each section seen in cm.
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4.1.1 Centre of Mass for Niwot Ridge
30.0035.00
40.00
45.00
50.00
55.00
60.00
65.00
70.00
30.00 32.00 34.00 36.00 38.00 40.00 42.00
Centre of Mass in X (cm)
C e n
t r e o
f M a s s
i n Y ( c m
)
Figure 4.2 Co-ordinates for the centre of mass in x and y according to the dyeconcentration, variation in centre of mass was shown by concentrating on specific area
of graph (figure 4.1). Niwot Ridge, May 9th 2002
0.00
10.00
20.00
30.00
40.00
50.00
60.00
70.00
0 20 40 60 80 100
Sections through snowpack (cm)
C e n
t r e o
f M a s s
( c m
XY
Figure 4.3 Centre of mass in x and y according to the dye concentration. Axis showcentre of mass varying as a function of distance from the start i.e. number of sections cut
(from 0 cm). Niwot Ridge, May 9th 2002
Figures 4.2 and 4.3 were compared with the images taken at NR in order to
establish how the centre of mass varies through the snowpack statistically and visually.The images initially showed a concentration of dye towards the top of the section with
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two main layers slightly tilted towards the right (figure 3.1). A definite vertical
movement of the dye was seen in the snowpack, in which there appeared to be a fairly
uniform spread in the horizontal axis. As the centre of mass decreased in x there was a
slight increase in y indicating a vertical rise associated with the shift in the centre of mass
to the left. In sections about 12cm from the start of the sample area the movement of dyeappeared to trend slightly towards the left of the section seen in figure 4.3 as centre of
mass increased in the horizontal direction. 28 centimetres from the beginning of the cut
area a slight drop was seen in the centre of mass in the x and y directions. This was
difficult to distinguish on the images however the concentration of dye appeared to be
slightly higher in image 28 and therefore may slightly affect the results (figure 3.3).
Another rise in both x and y co-ordinates was seen at roughly 48cm, therefore shifting
the centre of mass vertically upwards and horizontally to the right (figure 3.5). Nosignificant movement was seen on the images however dark images causing a low
contrast ratio might have proved to be the problem (e.g. figure 3.4).
Figure 4.3 shows a general upwards trend in the y co-ordinate indicating the
centre of mass moving vertically from roughly 40 cm above the ground to 60 cm above
the ground. This was not obvious in the images as it was thought there was a gradual
drop in the main area of dye throughout the snowpack (figures 3.1 to 3.6). This drop may
be due to the extra time required to cut earlier sections of snow away and therefore
allowing further infiltration of the dye through the snow. However higher concentration
of dye retained higher in the snowpack may have caused this rise in the centre of mass
not easily observed. As reflected in figure 4.3 there was a fairly uniform distribution of
dye in the x co-ordinate.
Later on in the sample area there was a plume of dye concentrated about halfway
down the section in the centre of the horizontal axis reflecting the average point for the
centre of mass seen in figures 4.1 and 4.2. Plumes of dye were apparent on the images
but seen towards the left and right of the sections horizontally resulting in a centre of
mass towards the middle although observed to be slightly to the left of the centre. There
appeared to be a strong negative correlation between the centre of mass in the x and y co-
ordinates as seen in figure 4.2, as the concentration of dye in x moves horizontally to the
right, vertically the centre of mass falls. Observations of the images show this was the
case, as dye concentration appeared to move vertically downwards with time and shift
towards the right of each section.
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4.1.2 Centre of Mass for Berthoud Pass 1
15.0020.00
25.00
30.00
35.00
40.00
45.00
50.00
36.00 37.00 38.00 39.00 40.00
Centre of Mass in X coordinate (cm)
C e n
t r e o
f M a s s
i n Y c o o r d
i n a
t
( c m
)
Figure 4.4 Co-ordinates for the centre of mass in x and y according to the dyeconcentration, variation in centre of mass was shown by concentrating on specific area
of graph (figure 4.1). Berthoud Pass, May 18th 2002
Figure 4.1 appeared to show a wide spread in the centre of mass for BP1 that was
also seen in the experiments carried out on NR (figure 4.1). When compared to the
images early in the snowpack a concentration of dye was seen towards the top of the
sections where there were three main layers observed and highlighted by dye (figure 3.7).
Towards the top of the snowpack movement appeared to be concentrated towards the
right of the section, as you move vertically downwards the centre of mass remained
concentrated to the right. This is shown in figure 4.4 where there was a slight negative
correlation between the x and y co-ordinates as the centre of mass in y moves upwards
the centre of mass in x shifts to the left. The pattern is confirmed for the horizontal in
figure 4.5 as in the x co-ordinate there was a constant level for the centre of mass. This
was not reflected in the y co-ordinate however and there does not appear to be any
pattern in the centre of mass through the snowpack.
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0.00
5.00
10.00
15.00
20.00
25.0030.00
35.00
40.00
45.00
50.00
0 5 10 15 20 25 30 35 40
Sections through snowpack (cm)
C e n
t r e o
f M a s s
( c m
XY
Figure 4.5 Centre of mass in x and y according to the dye concentration. Axis showcentre of mass varying as a function of distance from the start i.e. number of sections cut
(from 0 cm). Berthoud Pass, May 18th 2002
Unfortunately fewer sections were made during BP1 experiment compared to the
other two experimental days reflected in figures 4.4 and 4.5 due to problems with the
camera. Useful results were still obtained, only across a shorter sample area of snow.
Observed on the images small patches of red dye appear lower than the main area of flow, which may shift the centre of mass vertically but not horizontally due to the
occurrence of these patches on both the left and right of the section. The main patch
appeared to be on the right side of the images and became more concentrated as more
sections were removed (e.g. figures 3.8 and 3.11). There was no evidence of a flow path
for this movement seen during any part of the experiment. Explanations for this anomaly
may be an area of flow around the side of the section seen due to an ice layer for example
or due to fast vertical movement in which there was no adhesion to the snow above the
point at which the dye was seen. Concentration of dye at this anomalous point may be
due to a change in snow properties reducing or stopping the flow of water through the
snow.
As more sections of snow were removed flow fingers were noted forming in the
snow. This may have been due to some areas of snow where water was retained while in
other areas flow fingers may have developed due to differences in snowpack properties.
As seen in figure 4.5 there were a few results missing between 12 and 20 cm, this
was due to out of focus images being taken. Between the missing data points a change in
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snow properties was noted with more layering seen in the snowpack after 20cm and the
dye concentrated towards the top of these sections relative to the lower concentration
observed in the first set of sections (figure 3.8 compared to figure 3.9). Two low centres
of mass were noted after 20cm at 22 and 23cm from the start as seen in 4.5. However this
was not easily seen when comparing images and was evidently due to a difference in theway the dye was concentrated with respect to the snow (figure 3.10).
Towards the end of the sampled snow area the dye appeared to be concentrated at
the top of the snow sections with a few lower patches of dye seen but no obvious
pathways. In this area of snow the centre of mass appeared to rise in the vertical direction
unusually fast, when compared to the images dye was seen in high concentrations
towards the top of each section reflecting the retention of meltwater held in layers
towards the top of the snowpack (figure 3.11). Snowpit data showed that towards the topof the snowpack at BP1 the snow was at a temperature of -1 oC at a depth of 7 to 15cm
and 17 to 28cm (table 2) explaining the retained meltwater.
4.1.3 Centre of Mass for Berthoud Pass 2
The centre of mass at BP2 (figure 4.6) was seen trending towards the right of the
section between 30 and 40cm in the horizontal scale compared to BP1 where the centre
of mass was trending towards the left of this area (figure 4.4). Variation in the centre of
mass appeared to be fairly small and there was a definite concentration towards one
specific area of the snow. The pattern seen in the images was two main layers roughly
halfway down each section concentrating the dye and evidently retaining meltwater at
this point. Small plumes of dye were observed below these layers moving meltwater
vertically through the snowpack (figure 3.13). Moving through the snow with the
removal of more sections meltwater concentration appeared to be between the two layers
mentioned above however, there was development of plumes below these layers not
altering the centre of mass as they were spread fairly evenly across the section.
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48.000
49.000
50.00051.000
52.000
53.000
54.000
55.000
56.000
35.000 36.000 37.000 38.000 39.000 40.000
Centre of Mass in X coordinate (cm)
C e n
t r e o
f M a s s
i n Y c o o r d
i n a
t e ( c m
Figure 4.6 Co-ordinates for the centre of mass in x and y according to the dyeconcentration, variation in centre of mass was shown by concentrating on specific area
of graph (figure 4.1). Berthoud Pass, June 1st 2002
Experiments carried out at BP2 showed the smoothest pattern in terms of
movement of the centre of mass through different sections in the snowpack (figure 4.7).
In both the x and y co-ordinate there was nearly a straight line with a slight rise towards
the end of the sample area showing the increased height of the centre of mass. Vertical
differences in the concentration of the dye was seen in the images with dye highlighting
the presence of layers in the upper parts of the section between 21 and 29cm (figures
3.14 and 3.15). This was not reflected in figure 4.7 as there were vertical plumes also
seen towards both the left and right of the section towards the snowpack base therefore
changes in the horizontal centre of mass were small with a slight change in the vertical
centre of mass.
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0.000
10.000
20.000
30.000
40.000
50.000
60.000
0 20 40 60 80
Sections through snowpack (cm)
C e n
t r e o
f m a s s ( c m
Max mass in XMax mass in Y
Figure 4.7 Centre of mass in x and y according to the dye concentration. Axis showcentre of mass varying as a function of distance from the start i.e. number of sections cut
(from 0 cm). Berthoud Pass, June 1st 2002
Further on in the snowpack a more uniform spread of dye was seen across each
section increasing towards the surface. This was surprising as it was expected dye would
continue to infiltrate through the snow and a downward trend seen in the centre of mass
as more vertical infiltration occurs. Figure 4.1 showed BP2 having the smallest change inthe centre of mass, over the course of the experiment this centre of mass appeared to
remain at the same point skewed slightly left horizontally but remaining in the centre
vertically. In comparison to the images this vertical distribution was due to the two layers
in the centre of the snowpack retaining the meltwater for a relatively long period of time
(figure 3.13). Early in the experiment from observations of the images it seemed the
centre of mass would be towards the right of the sections, however moving through the
snowpack leads to a bias towards the left of the sections especially with the formation of
the meltwater plumes towards the bottom left of the section.
Towards the end of the sample area, centre of mass in the vertical moved upwards
and in the horizontal moved to the right of the section, however more snow was seen in
the sections compared to earlier relative to the concentration of dye. The movement of
the centre of mass noted in figure 4.7 from 60cm through the section to the end at 78cm
was also seen in the images (figures 3.17 and 3.18). There was continued development of
plumes in the right and centre of the section although movement was also noted towards
the left of the sections.
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4.1.4 Comparison between the Centre of Mass for Berthoud Pass on May
18th and June 1st
At BP1 the snowpack had two distinct layers at -1o
C with the rest at 0o
C. Thecolder layers measured at BP1 were towards the top of the snowpack, 7 to 10 cm and 15
to 17cm below the top of the snowpack. Figure 4.5 shows the centre of mass in the
horizontal to have been at a depth between 60 and 65cm from the top of the snowpack
more than 40cm below the point at which there were colder layers present. There are also
a couple of layers highlighted by a concentration of dye towards the top of the images,
which may correspond to the lower temperature layers mentioned above (figure 3.7).
When studying the snow grains in each layer it was discovered that one particular layer in the snow was described as 'slushy' at a depth of 60cm the same depth as the main
centre of mass. Well-banded clusters of snow grains underlay this layer of slushy snow,
evidently a meltwater retaining layer in the snowpack.
By the time BP2 experiment was carried out the snow had become isothermal at
0oC and 14 cm of snow had melted and removed from the snow depth during the short
period between the experiments. The experiment was still carried out to 1m depth
however fewer distinct layers were observed when the snowpack was examined and
below 32 cm from the surface no changes in snowpack properties were noted (table 3).
Layers observed in the snowpack were however seen lower than this and the centre of
mass was clustered around 50 to 55cm depth. The snowpit was examined after the
experiment had taken place and by the end of the experiment the main meltwater layers
had migrated towards the snowpack surface and therefore were not seen during the
snowpack analysis. To correct for this the snow should have been observed at both ends
of the pit left and compared to remove these sources of error.
4.2 Variance Around the Centre of Mass
After the centre of mass for each section was complete, the variance of this centre
of mass was also calculated to show the spread in centre of mass through the snowpack
in the x and y co-ordinates. Variance has been calculated for every section and plotted for
each experiment in figures 4.8, 4.9 and 4.10.
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in variance around the centre of mass between 0 and approximately 36cm was noted and
in the images there appeared to be a wider spread of dye as you move through the
snowpack as opposed to clustering around one central point in the snow, for example
figure 3.1 compared to figure 3.3. This area may have been at a slightly different
temperature or the snow had slightly different properties to the surrounding snowpack for example grain shape, size or density where the experiment was also carried out due to
this higher spread of meltwater through the snow. Later in this sample area there was a
plume of dye towards the bottom right of the sections, however slightly less dye was
observed to the left of the sections which may explain the slightly lower variance seen in
the x and y directions towards the end of the experiment (figure 3.5). After 78cm there
appeared to be relatively high infiltration compared to the surrounding sections and
slightly less dye was observed in the upper part of the image, this was seen where therewas a slightly lower centre of mass in figure 4.3. This lower centre of mass and reduction
in surrounding dye concentration has reduced the variance seen in figure 4.8 and the last
section appeared to be an anomaly as through the rest of the sample area dye was spread
across the whole of each section.
4.2.2 Variance for Berthoud Pass 1
Variance in the x direction at BP1 was observed to be steady and fairly uniform
throughout the sample area indicating while the centre of mass and the variance around
this point varies in the vertical there was very little or no change in the horizontal (figure
4.9). Due to the properties of the snow during this experiment the dye did not move in
one large mass through the snowpack but instead through specific channels and flow
fingers for example. Variance in the centre of mass was therefore harder to distinguish
than on NR where obvious areas of dye existed or not depending on the section although
there was a uniform horizontal layering system seen in the images which was evidently
areas where there was a change in snowpack properties.
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15.00
17.00
19.00
21.00
23.00
25.00
27.00
29.00
31.00
33.00
35.00
0 5 10 15 20 25 30 35 40 45
Distance from start (cm)
V a r
i a n c e
( c m
2 )
Variance in XVariance in Y
Figure 4.9 Variance in centre of mass as a function of distance from the start i.e. number of sections cut (from 0 cm). Berthoud Pass, May 18th 2002
In the y co-ordinate there was quite a lot of difference between the variances for
different sections reflected in the centres of mass seen in figure 4.5. Further through the
sample area, dye appeared to become more concentrated within the snowpack varying in
the centre of mass and therefore altering the variance around this central point. The pattern observed after 20cm in the vertical axis was unusual although there appeared to
be a reasonably constant variance with the dye concentrated towards the top of each
section. Figure 4.9 shows a very low variance in the vertical at 23cm, surprising as
sections cut both before and after show very similar dye distributions (figures 3.9, 3.10
and 3.11).
After sequential analysis of all the images associated with this experiment there
was an obvious difference in the dye concentration spread through the snowpack betweenwidely spaced sections, for example 11cm and 27cm (figures 3.8 and 3.11). There
appeared to be a lower centre of mass vertically and very little variation at 11cm and a
higher dye concentration in the vertical direction at 27cm. Interestingly these two
sections had a very similar centre of mass (figure 4.5) but completely different variances
in the vertical axis indicating a much higher spread around the centre of mass at 27cm
(figure 4.9). Images from sections directly adjacent to one another appeared to have a
fairly uniform variance compared to the sections split apart with different variances even
though they were only a few centimetres apart in the snowpack.
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4.2.3 Variance for Berthoud Pass 2
Of the three experiments there was least change in variance through the snowpack
at BP2 compared to the two other experimental days as seen in figure 4.10. An almostconstant spread was seen in each section in the horizontal direction possibly due to layers
observed in the snowpack retaining meltwater at one specific snow depth. Through the
horizontal section area it was not obvious from the images how the variance around the
centre of mass remains reasonably constant as the concentration of dye increases through
the snowpack. As seen in figure 4.7 centre of mass also stayed fairly constant moving
slightly to the right and up in the vertical direction with distance through the snowpack,
consistent with the slight increase in variance around this centre of mass.In the vertical axis there were also fairly uniform variances however the variance
slightly increased further through the sample area. The increase in variance around the
centre of mass corresponded with the slight vertical rise in the centre of mass seen in
figure 4.7. This was observed also in the images as the dye became more dispersed
throughout the sample area, for example figure 3.13 compared to figure 3.18. Causes of
this might be due to the snowpack being isothermal at 0 oC and plumes of meltwater
moving as one mass through the snow becoming deeper towards the end of the sample
area as more time was allowed for infiltration to occur.
1 5 . 0 0 0
1 7 . 0 0 0
1 9 . 0 0 0
2 1 . 0 0 0
2 3 . 0 0 0
2 5 . 0 0 0
2 7 . 0 0 0
2 9 . 0 0 0
3 1 . 0 0 0
3 3 . 0 0 0
3 5 . 0 0 0
0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0
D i s t a n c e f r o m s t a r t ( c m )
V a r
i a n c e
( c m
2 )
Va r i a n c e i n X
Va r i a n c e i n Y
Figure 4.10 Variance in centre of mass as a function of distance from the start i.e.number of sections cut (from 0 cm). Berthoud Pass, June 1st 2002
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The spread of meltwater through the snow highlighted the large area of the
snowpack being responsible for meltwater flow. Variance seen at BP2 was relatively
high indicating an even spread of meltwater throughout the whole snowpack.
4.2.4 Comparison between Two Different Snowpacks Isothermal at 0 oC
On NR and at BP2 the snowpacks had achieved equilibrium with the whole
snowpack remaining at a temperature of 0 oC. As observed in the images the dye appeared
to move as one plume through the snow as it had uniform temperature and properties.
Snow grains were equi-temperature throughout the snowpack on both days with a couple
of ice layers also observed. At BP2 these ice layers were fairly shallow at depths of 5cmand 8cm beneath the snow surface however these were not highlighted by the dye. Lower
in the vertical direction two layers were observed, one of these might have corresponded
to the ice layer observed in the snow pit survey at 30 cm depth. Meltwater movement
through the snow was seen to be similar in both snowpacks as layers existed in both,
although slightly higher in the vertical direction at NR compared to BP2, as seen in
figures 3.6 and 3.18. Below these layers very similar development of meltwater plumes
was also observed most likely due to weak areas in these layers allowing the downwards
movement of meltwater (figure 4.11).
Figure 4.11 Schematic diagram of ice layers in the snowpack showing weak areas and meltwater flow. Adapted from Kattelmann and Dozier, 1999.
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The two snowpacks evidently had differing properties despite having a common
temperature, location was also important with a different elevation affecting particular
melt processes. Dye concentration in the snow was higher on BP2 compared to NR
however this may be due to photographic exposure, clouds or infiltration time for the dye
and is unlikely to affect the overall demonstration of meltwater movement.
4.3 Roughness
4.3.1 Roughness in all Experiments
Contrast between dyed areas and non-dyed areas within the snow were shown
with an estimate of the roughness. Firstly roughness is seen in a direct comparison across
all experiments moving through the snowpack. In later sections roughness is plotted for
each individual experiment.
A comparison between roughness in the different snowpacks in the x and y co-
ordinates are shown in figure 4.12. Low roughness values indicate a smooth, uniform
flow of dye through the snowpack and high roughness indicates a high contrast ratio
between the snow and the dye indicating the existence of specific meltwater channels.
The highest roughness was expected to be in a snowpack non-isothermal at 0 oC due tothe presence of meltwater flow channels i.e. During BP1 experiment according to the
snowpit data collected and observed in the sections for example figure 3.8 shows the dye
following distinct vertical pathways.
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Roughness Comparison
0.000
0.500
1.000
1.500
2.000
2.500
3.000
0 10 20 30 40 50 60 70 80 90 100Sections through snowpack (cm)
R o u g
h n e s s
BP2 - X BP2 - Y NR - X
NR - Y BP1 - X BP1 - Y
Figure 4.12 Roughness in dye passing through the snowpack. Comparison of roughness
between the three experiments in x and y co-ordinates as a function of distance from the
start.
Figure 4.12 shows on NR there was firstly a negative trend in the roughness
values, leading to a slight positive correlation where roughness increased. Indication of an increasing value of roughness was seen in both the horizontal and vertical axis
although the sections appeared to be rougher in the horizontal direction as opposed to the
vertical. Changes in roughness were difficult to observe when the images were seen as a
sequential set, however the roughness value differences were fairly small in comparison
with each other and will account for changes in intensity of the dye not seen by the
human eye.
Roughness at BP1 was highly scattered between adjacent centres, a similar
pattern to that observed in the centre of mass and the variance in the y co-ordinate
(figures 4.5 and 4.9). As opposed to NR, roughness in the horizontal was lower than the
roughness in the vertical direction indicating a slightly more defined pattern in the
vertical between clean and dyed snow areas. Later in the snowpack as you go past 20cm
there was a definite vertical shift upwards of dye in the snow, which appeared to be a
more concentrated area of dye relative to the earlier sections observed. This vertical shift
was seen when comparing figures 3.7 and 3.9. Roughness values decreased towards the
end of this experiment indicating a smoother spread of dye with a vertical rise in the
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centre of mass. This was confirmed by the higher variance observed in the vertical
direction in figure 4.9.
As observed in the centre of mass and variance data from BP2 showed a smooth
pattern throughout the snowpack in terms of roughness. Roughness in the vertical
direction was seen to rise initially until around 25cm then become more uniform asopposed to vertical roughness, which rose until around 38cm before becoming steady.
Patterns seen in the roughness were reflected in observations of the images as moving
further through the snowpack specific patches of dye and plumes of meltwater movement
were seen. Figures 3.13, 3.14 and 3.17 show this gradual progression of dye developing
meltwater plumes within the snowpack. Experiments carried out on NR showed higher
roughness in the horizontal direction than the vertical as was the case at BP2, this may be
an indication of higher horizontal spread through the snowpack due to the isothermaltemperature (0 oC) properties of the snow.
4.3.2 Roughness for Niwot Ridge
When comparing horizontal and vertical roughness values a similar pattern was
seen. Indications from figure 3.4 compared to figure 3.5 appeared to show that with an
increase in dye in the vertical direction an increase was also observed in the horizontal.
Towards the end of the sample area plumes of dye were observed low in the sections,
these appeared to move as one mass through the snow shown in figure 3.6 and
corresponding well with figure 4.13 where there was an increase in the x direction there
is also an increase in the y direction. A strong positive correlation was noted between
roughness in the x and y directions on all three days but particularly on NR and BP2
where an increase was observed in the roughness in the horizontal, an increase was also
seen in the vertical direction. The direct relationship seen between the vertical and
horizontal direction indicates an isotropic structure to the snowpack with equal
distributions in both the horizontal and the vertical direction.
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0.5
0.7
0.9
1.1
1.3
1.5
0.5 0.7 0.9 1.1 1.3 1.5
Roughness in X
R o u g
h n e s s
i n Y
Figure 4.13 Roughness at Niwot Ridge, May 9th 2002 with roughness in x plotted against roughness in y.
4.3.3 Roughness for Berthoud Pass 1
At BP1 the snow was not isothermal at 0 oC and therefore it was expected this
experiment would show development of flow paths in the snowpack. This appeared to be
the case when studying the images as dye was concentrated in particular areas throughout
the upper part of the snowpack an example of which is seen in figure 3.8. Concentration
of the dye was mainly in the horizontal plane however specific channels appeared to be
present vertically also.
According to the calculations for roughness the existence of flow paths was not
obvious, it was expected that the roughness values would be highest in this data set due
to the high contrast between dyed and non-dyed snow. The reasons for this may have
been due to the fact that a lot of snow was exposed towards the bottom of the section
with no dye present indicating a smooth surface at this level. The top of the snowpack is
likely to have been rough due to the high contrast between snow and dye. A short test
was performed for this by cutting away the lower part of a few images and doing
roughness calculations once again. The results showed the roughness to be on the order
of 1.5 to 2.7 as opposed to the roughness values seen in figure 4.14 on the order of 0.5 to
1.6.
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Roughness at BP1 was slightly anisotropic opposed to NR as figure 4.14 shows
there is a higher roughness in the vertical direction compared to the horizontal indicating
the dominance of a horizontal layering structure from the roughness values observed.
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
1.0 1.2 1.4 1.6 1.8 2.0
Roughness in X
R o u g
h n e s s
i n
Figure 4.14 Roughness at Berthoud Pass, May 18th 2002 with roughness in x plotted against roughness in y.
4.3.4 Roughness for Berthoud Pass 2
In contrast with NR and BP1, during experiments at BP2 (figure 4.15) there
appeared to be higher roughness values in the horizontal than the vertical and a much
more constant vertical roughness. Roughness values were also higher for BP2 on the
order of 2 to 2.5 compared to 0.5 to 2 seen during the previous experiments. Higher
roughness values seen here may be due to the wider spread of dye observed in the
snowpack with variable patches of dye and snow as seen in figure 3.16 for example. Theother two experiments may have had lower roughness values due to the more uniform
spread of dye on NR and very small areas of dye at BP1 causing a high roughness in
these areas but relatively smooth elsewhere in the section.
Images taken of these sections showed an increase in dye concentration through
the snowpack with a higher spread of dye observed towards the end of the sample area as
seen in figure 3.18 compared to figure 3.13. Also seen was the fact that initial vertical
movement of dye was not obvious but later in the snowpack more specific dye plumes
became apparent. Increases in vertical movements were accompanied by some spread in
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the horizontal direction. As reflected in figure 4.15 horizontal spread did not increase
greatly as initially observed on the images as there was a high concentration of dye
roughly halfway down snowpack spread completely across each section between two
layers of differing snow properties (figure 3.13). As in the other isothermal snowpack at
NR a direct relationship is seen between roughness in the horizontal and the verticaldirections indicating a steady structure of flow through the snow.
.
1.5
1.6
1.7
1.8
1.9
2.0
2.1
2.2
1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8
Roughness in X
R o u g
h n e s s
i n Y
Figure 4.15 Roughness at Berthoud Pass, June 1st 2002 with roughness in x plotted against roughness in y.
4.4 Summary
Overall experiments carried out NR, BP1 and BP2 showed some similar and some
different characteristics of snow under varying conditions. Figure 4.16 attempts to
summarise the general characteristics observed in these snowpacks.
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Figure 4.16 Summary diagram to show meltwater flow processes in a snowpack.
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5. Conclusion
5.1 Conclusions
Dye tracing in snow was carried out in this experiment in order to establish the
presence of preferential flow and better physically understand spatial movement of water.
Non-toxic food colouring mixed with natural snowmelt water was applied to a two by
two metre area of the snowpack surface and allowed to infiltrate. After this time one or
two centimetre sections of snow were cut and a sequential set of images taken as more
sections were removed. The experiment was repeated twice at two different locations in
order to compare different snowpacks. After completion of the fieldwork images were
transferred to a computer for analysis using the computer program MATLAB.It appears flowpaths depend on nature of the snow and boundary conditions.
Statistical analysis showed meltwater moves through a snowpack isothermal at 0 oC as
one general plume unless stratigraphic layers are encountered where lateral spread of
meltwater occurred. In comparison to this snowpacks with layers of differing temperature
appeared to show specific flow channels. Small-scale changes were common and
different rates of flow observed within very small areas of the snowpack. Spatial scale
observations need to be carried out studying the snowpack in detail as opposed to over large areas of melt in order to understand both microscale and mesoscale melt processes.
Overall flowpaths exist in a snowpack moving water through preferential
channels towards the snowpack base. Flow rates increase as the snowpack becomes
warmer and then meltwater movement is continuous through the entire snowpack
moving in large plumes spreading horizontally as movement progresses. The level of
connectivity of flowpaths appears to be higher when the snow has uniform properties,
connectivity between flowpaths in a non-isothermal snowpack is through lateral spread
of meltwater across layers of different snow properties.
5.2 Suggestions for Further Work
As a result of the information found here it is suggested that further dye tracing
experiments are carried out at the same locations under similar snow conditions to see if results are related between different years of snowfall. It would also be useful to carry
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out further experiments earlier in the melt season especially at NR to gain further
understanding of melt processes prior to the snowpack becoming isothermal at 0 oC.
For a more thorough investigation a technique known as radar tomography should
be used in the future as it is less destructive to the environment and therefore repeatable
in the exact location, however this technique is quite expensive. The radar techniqueinvolves high frequency radar sensing melt water features in the snowpack at a distance
up to one metre as tested by Albert et al. (1999). A snow trench is dug along the line of
investigation and portable tracks installed along which the radar can be pulled at a
constant rate. Old refrozen melt fingers as well as new wet fingers are observed in the
study of meltwater flowpaths. Investigation at Niwot Ridge and Berthoud Pass would be
possible using this technique along with the dye tracing experiments in order to compare
different results using the two different techniques.After the data has been collected more geostatistics should be applied in order to
attempt to more fully understand the snow. An example of further statistical analysis
might be using variograms to characterise heterogeneity of each section. This process
was used successfully by Rea and Knight (1998) in correlating data from radar and
hydrogeology of an area using dielectric and hydraulic properties in the subsurface of a
sedimentary bed. This could be applied in the same manner to snowpack data in order to
study spatial variation in meltwater flow in the snow. Due to the low roughness values
seen during experiment BP1 due to the low level of infiltration further statistical work
should be carried out in the higher levels of the snowpack where the dye is present. This
would ensure that the areas of infiltrated dye are properly represented. The other
alternative would be to wait longer for dye to infiltrate during periods when the snow is
at a colder temperature.
A lot of research work has been carried out into the need for specific dyes in soil
dye tracing experiments according to the specific study. Research into the use of different
dyes in snow would be useful to establish if any particular dyes absorb more fully to the
snowpack and therefore indicate flowpaths better than a different dye for example.
In conclusion future research work into snow is vital for water supply, flood
management, recreation and tourism and modelling problems in relation to water supply
and long and short-term climate change and stability.
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