SIST-TP CEN/TR 16588:2014

SLOVENSKI STANDARD
01-junij-2014
5RþQRPHUMHQMHNROLþLQHYRGHYVQHJX
Manual measurement of snow water equivalent
Manuelle Messung des Schneewasseräquivalents
Mesure manuel de l’équivalent en eau de la neige
Ta slovenski standard je istoveten z: CEN/TR 16588:2014
ICS:
07.060 Geologija. Meteorologija. Geology. Meteorology.
Hidrologija Hydrology
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

TECHNICAL REPORT
CEN/TR 16588
RAPPORT TECHNIQUE
TECHNISCHER BERICHT
March 2014
ICS 07.060
English Version
Manual measurement of snow water equivalent
Mesure manuelle de l'équivalent en eau de la neige Manuelle Messung des Schneewasseräquivalents

This Technical Report was approved by CEN on 3 September 2013. It has been drawn up by the Technical Committee CEN/TC 318.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,
Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and United
Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Avenue Marnix 17, B-1000 Brussels
© 2014 CEN All rights of exploitation in any form and by any means reserved Ref. No. CEN/TR 16588:2014 E
worldwide for CEN national Members.

Contents Page
Foreword .4
Introduction .5
1 Scope .6
2 Terms and definitions .6
3 Symbols . 10
4 Objective . 10
4.1 Spatial estimation of SWE . 10
4.2 Snow load assessment . 11
4.3 Snow profile . 11
4.4 Water content in newly fallen snow . 11
4.5 Reference to automatic SWE measurements . 12
5 Principle of manual SWE measurements . 12
6 Measurement sites. 12
6.1 General . 12
6.2 Manual measurements . 13
7 Measurements . 14
7.1 General . 14
7.2 Snow density . 14
7.3 Snow depth . 15
7.3.1 Manual probing . 15
7.3.2 Manual readings on fixed snow stakes . 15
7.3.3 Automatic recording . 15
7.3.4 Remote sensing . 16
8 Manual SWE sampling methods . 16
8.1 General . 16
8.2 Snow tubes . 16
8.3 Core drills . 17
8.4 Density cutters . 17
9 Spatial estimation . 18
9.1 General . 18
9.2 Interpolation methods . 18
9.3 Snow courses . 19
9.4 Regression modelling . 19
9.5 Hydrologic and land surface modelling . 20
9.6 Remote sensing systems for snow monitoring . 20
10 Maintenance . 21
11 Uncertainties . 21
11.1 Environmental factors . 21
11.2 Technical factors . 21
11.3 Human factors . 22
12 Assessment of quality . 22
13 Measurement uncertainty . 22
14 Recommendations . 22
Annex A (informative) List of methods for determination of SWE in total snowpack . 24
Annex B (informative) Manual SWE measuring bodies in Europe . 25
Annex C (informative) Determination of mass of snow sample . 26
Annex D (informative) Determination of water volume in snow sample . 27
Annex E (informative) Snow stakes . 28
Annex F (informative) List of samplers for detection of SWE . 29
Annex G (informative) Snow tubes . 30
Annex H (informative) Core drills . 31
Annex I (informative) Density cutters . 32
Annex J (informative) On-line glossaries . 33
Bibliography . 34

Foreword
This document (CEN/TR 16588:2014) has been prepared by Technical Committee CEN/TC 318
“Hydrometry”, the secretariat of which is held by BSI.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. CEN [and/or CENELEC] shall not be held responsible for identifying any or all such patent rights.
Introduction
Snow water equivalent (SWE) measurements
Snow water equivalent (SWE), also called “water equivalent of snow”, is the depth of water that would be
obtained by melting the snowpack in a given area, and is normally expressed in millimetres. In other words,
SWE corresponds to the mass of snow over a given area.
Measurements of SWE in snowpack, and new snow, improve the estimation of winter precipitation, especially
in areas with a sparse network of meteorological stations. The measurements are mainly made for the
purpose of estimating the spatial distribution of the total water content in catchment areas, as knowledge of
the SWE in river basins is fundamental for estimating the expected snowmelt runoff.
Information about snow accumulation and daily melt rate is essential in flood forecasting during the snowmelt
season. SWE is also used in avalanche theory and forecasting, as well as for risk assessment of heavy snow
loads. Furthermore, the data is important in glaciological mass balance studies and climate monitoring. The
melt from polar ice sheets is a major factor in sea level rise.
Methods and instruments, which have been developed for determination of SWE, are listed in Annex A.
Manual SWE measurements
The first station networks with manual SWE measurements were established in the early 20th century at
meteorological institutes in North America and Europe. Today the measurements are made routinely at
federal and national meteorological and hydrological institutes, within the hydropower industry, and by
universities, in cold climate countries all over the world. Annex B shows a list of manual SWE measuring
bodies in Europe.
Automized methods have been developed to be used in remote areas, as well as to enable continuous
recording, but manual measurements are still more common, as they can provide high quality data for a
relatively low capital cost. The importance of manual measurements is also reflected in their use as reference
to other SWE measuring methods.
1 Scope
This Technical Report defines the requirements for manual measurements of SWE over land, see ice and
glaciers, under natural environmental conditions, and shows methods for calculating the spatial distribution of
the data. It includes measurements with snow tubes, core drills and density cutters.
2 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
Note 1 to entry Primarily ‘The International Classification for Seasonal Snow on the Ground’ (UNESCO), ‘Cryospheric
Glossary’ (NSIDC) and ‘Glossary of Meteorology’ (AMS) has been used as reference.
2.1
ablation
removal of material from the surface of an object by vaporization, chipping, or other erosive processes. In this
case the opposite of snow accumulation
2.2
blowing snow
an ensemble of snow particles raised by the wind to moderate or great heights above the ground; the
horizontal visibility at eye level is generally very poor
Note 1 to entry  See also drifting snow.
2.3
condensation
the change of the physical state of matter from gaseous phase into liquid phase (opposite of evaporation)
2.4
deposition
(1) a process by which water vapour is deposited as ice without first forming liquid water (opposite of
sublimation)
(2) the process by which snow is deposited on the ground either with or without wind action
Note 1 to entry As a result, stationary snow deposits such as snow dunes, snowdrifts, or the snow cover itself may
form.
2.5
drifting snow
snow raised from the snow surface by the wind to a height of less than 2 metres; it does not restrict horizontal
visibility at 2 metres or more above the surface
Note 1 to entry  See also blowing snow.
2.6
evaporation
vaporization of a liquid that only occurs on the surface of a liquid, at temperatures below the boiling point
(opposite of condensation)
2.7
firn
well-bonded and compacted snow that has survived the summer season, but has not been transformed to
glacier ice
–3
Note 1 to entry Typical densities are 400 - 830 kg·m . Thus firn is the intermediate stage between snow and glacial
ice where the pore space is at least partially interconnected. Firn usually results from both melt-freeze cycles and
compaction by overload, or from compaction alone, as in inland Antarctic snow.
2.8
glacier
a mass of land ice formed by the further recrystallization of firn, normally flowing continuously from higher to
lower elevations
2.9
new snow
recently fallen snow in which the original form of the ice crystals can be recognized
Note 1 to entry This is usually the snow which has accumulated on a snow board during the standard observing
period of 24 hours.
2.10
old snow
deposited snow whose transformation into firn is so far advanced that the original form of the ice crystals can
no longer be recognized
2.11
recrystallize
to crystallize again, i.e., to form into new crystals
2.12
redistribution
distribution of previously deposited snow that was eroded and transported by the wind
Note 1 to entry Redistribution features such as snowdrifts are usually formed from densely packed and friable snow.
2.13
perennial snow
snow persisting for an indefinite time longer than one year
Note 1 to entry See also seasonal snow.
2.14
seasonal snow
snow that accumulates during one season and does not last for more than one year
Note 1 to entry See also perennial snow.
2.15
snow accumulation
all processes that add mass to the snow cover, i.e. typically solid and liquid precipitation, ice deposition from
atmospheric water vapour, and snow deposited by wind, avalanches, etc. (opposite of ablation)
2.16
snow avalanche
mass of snow which becomes detached and slides swiftly down a slope
Note 1 to entry Large snow avalanches may contain rocks, soil, vegetation, and/or ice.
2.17
snow board
in this case a specially constructed board used to identify the surface of snow that has been recently covered
by snowfall
2.18
snow core
a sample of snow, either just the freshly fallen snow or the combined old and new snow on the ground,
obtained by pushing, or drilling, a cylinder down through the snow layer and extracting it
2.19
snow course
an established line, or transect, of measurements of SWE across a snow covered area in a representative
terrain, where appreciable amounts of snow accumulates
2.20
snow cover
in general, the accumulation of snow on the ground surface, and in particular, the areal extent of snow-
covered ground; term to be preferably used in conjunction with the climatologic relevance of snow on the
ground
Note 1 to entry See also snowpack.
2.21
snow creep
a continuous, slow downhill movement of a snow layer
2.22
snow density
the mass per unit volume of snow
Note 1 to entry Sometimes total and dry snow densities are measured separately. Total snow density encompasses
all constituents of snow (ice, liquid water, and air) while dry snow density refers to the ice matrix and air only.
2.23
snow depth
the total height of the snowpack, measured vertically from the base to the snow surface
Note 1 to entry The slope-perpendicular equivalent of snow depth is the snowpack thickness.
2.24
snow distribution
spatial and temporal variability of snow cover affected by snowfall, wind speed, elevation, topography,
vegetation and ablation
2.25
snow erosion
the process by which the surface of the snow cover is worn away, primarily by the action of wind
Note 1 to entry Wind erosion is a very important factor in the redistribution of snow.
2.26
snow height
the vertical distance from a base to a specific level in the snow, or to the snow surface
Note 1 to entry Ground surface is usually taken as the base, but on firn fields and glaciers it refers to the level of
either the firn surface or glacier ice. Height is used to denote the locations of layer boundaries but also of measurements
such as snow temperatures relative to the base. Where only the upper part of the snowpack is of interest, the snow
surface may be taken as the reference. This should be indicated by using negative coordinate values. Snow depth is the
total height of the snowpack.
2.27
snow layer
a layer of ice crystals with similar size and shape
2.28
snow load
the downward force on an object or structure caused by the weight of accumulated snow
2.29
snow metamorphism
the transformation that the snow undergoes in the period from deposition to either melting or passage to
glacial ice
Note 1 to entry Meteorological conditions as well as mechanical or gravitational stresses are the primary external
factors that affect snow metamorphism.
2.30
snow pit
in this case a pit dug vertically into the snowpack where snowpack stratigraphy and characteristics of the
individual snow layers are observed
Note 1 to entry See also snow profile.
2.31
snow profile
a stratigraphic record of the snowpack including characteristics of individual snow layers, usually performed in
snow pits
2.32
snow sample
in this case a sample of snow with a defined volume extracted from the snowpack
2.33
snow sampler
an instrument used for the collection of snow samples in an undisturbed snowpack
2.34
snow season
the time period when the ground usually is covered by snow
2.35
snow surface
the uppermost part of the snow cover, forming the interface to the atmosphere
2.36
snow survey
the process of determining snow parameters, most often depth and density, at representative points, usually
along a snow course
2.37
snow water equivalent (SWE)
the depth of water that would result if a certain amount of snow melted completely
Note 1 to entry It can represent the snow cover over a given region or a confined snow sample over the
corresponding area. The snow water equivalent is the product of the snow height and the snow density divided by the
density of water. It is typically expressed in millimetres of water equivalent, which is equivalent to kilograms per square
metre or litres of water per square metre.
2.38
snowdrift
a mound or bank of snow deposited as sloping surfaces and peaks, often behind obstacles, irregularities, and
on lee slopes, due to eddies in the wind field. (See also deposition)
2.39
snowfall
the quantity of snow falling within a given area in a given time
2.40
snowpack
the accumulation of snow on the ground at a given site and time; term to be preferably used in conjunction
with the physical and mechanical properties of the snow
Note 1 to entry See also snow cover.
2.41
snowpack stratigraphy
the definition and description of the stratified, i.e. layered snowpack
Note 1 to entry See also snow profile.
2.42
snowpack thickness
the total height of the snowpack, measured perpendicularly from base to snow surface, i.e. at right angle to
the slope on inclined snow covers
Note 1 to entry When observers report thickness, they should also include the slope angle with respect to either the
snow surface or a layer within the snowpack, e.g., the bed surface of an avalanche. The slope-vertical equivalent of
snowpack thickness is the snow depth.
2.43
sublimation
the change of state of matter from solid phase to gaseous phase without entering liquid phase (opposite of
deposition)
3 Symbols
The symbols used in this technical report are given in Table 1.
Table 1 — Symbols
Most common
Symbol Quantity
units
SWE Snow water equivalent m, mm
D Snow depth m, cm
m Mass kg, g
-3 -3
ρ Density kg·m , g·cm
3 3
V Volume m , cm
2 2
A Area m , cm
4 Objective
4.1 Spatial estimation of SWE
Measurements of SWE are essential for estimation of the total snow water content in catchment areas.
Manual SWE measurements made by meteorological observers are often used as a complement to
precipitation measurements. In areas where water budget calculations are difficult, due to sparse
meteorological networks, additional snow surveys may be required. This is the case especially in mountainous
regions where precipitation at the measuring stations often badly represents the precipitation in the region.
SWE observations are used as input to and verification of models for calculation of river and ground water
flow, water management, flood warnings, snow load assessment, avalanche prediction and glacier mass
balance calculations.
Data from point measurements of SWE can be used to estimate the spatial distribution by means of a number
of methods. These include the following which are described in more detail in clause 9.
Ground measurements can be spatially distributed by use of:
— mathematical regionalization algorithms;
— mean or weighted values from snow courses.
Ground measurements can be used for calibration, validation and updating of:
— meteorological and hydrological models;
— snow distribution models;
— remote sensing systems for snow monitoring.
4.2 Snow load assessment
Collapse of buildings/structures, due to excessive snow loads, is a serious problem both in terms of economic
loss and public safety. The SWE present on roofs often differs a lot from the mean value in the landscape.
Wind and snow creep together with the presence of taller buildings/structures are factors which can decrease
or increase the weight of the snow on a certain building/structure. Measurement of SWE on roofs as well as
on the ground can potentially be of vital importance.
4.3 Snow profile
Periods of melting and freezing, snow falling at different air temperatures, and wind packing the snow, result in
layers of ice, crust, and snow with different densities. By digging a snow pit with a vertical wall, layers in the
snow can be detected and measured separately.
Knowledge of layers with different density is essential in avalanche risk forecasting. Furthermore, it can be
important for the correct assessment of the functioning of automatic measuring instruments.
4.4 Water content in newly fallen snow
Daily data on new snow measurements are very important for, e.g. military services, emergency and civil
protection services, road and airport maintenance services, avalanche forecasting and tourism. Continuous
registration of newly fallen snow can also be a complement to precipitation monitoring, and for verifying
weather forecasts.
Usually the sampling is carried out by use of a snow measurement board, which is made from a thin board
that will not sink into the snow, yet be heavy enough not to be blown away. The board should be pushed into
the snow surface just far enough so that the top of the board is nearly level or slightly below the top of the old
snow. Samples can be taken with a cylinder either at regular intervals or after each snowfall. After each
observation the board should be cleaned and placed in a new location close to the previous sample points.
The snow measurement board may need daily observations to assure that the top remains flush with the old
snow. To reduce the risk of heat absorption the board should be painted white.
The measuring site should be sheltered as much as possible from drifting and blowing snow.
4.5 Reference to automatic SWE measurements
A manual point measurement of the total value of SWE is regarded to give a more accurate value than any
other measuring method, as the assessment of the quality of the measurement is made directly on site.
Therefore the manual measurement is considered to be the reference standard method.
Layers in snowpack can act like bridges thus affecting the distribution of the weight of the snow on the site,
which may lower or raise the pressure on weighing sensors detecting the snow mass. Another problem can be
changes in homogeneity in the snowpack within very short distances, which typically occurs at the very end of
the snow season. Furthermore, measurements can fail due to malfunction of the sensors, or failure of
electronic circuits.
To ensure that readings of automatic sensors are as accurate as possible a quality control programme using
manual measurements should be established. It may be appropriate to undertake frequent manual
measurements following the initial installation of recorders to ensure correct performance of the instrument.
When the reliability of the sensor is proven, the quality program can be less frequent.
5 Principle of manual SWE measurements
A manual point measurement of the total SWE is performed by taking a vertical core from the snow surface to
the bottom of the snowpack, using a tube or core drill.
The water content of the snow in the sample is assumed to be the amount of snow that has fallen on the site,
and is still left after occasional melting and blowing periods. Determination of the SWE in the sample is
performed either by weighing (see Annex C) or melting (see Annex D) the snow.
The SWE profile of the snowpack is normally measured from the wall of a snow pit by use of a density cutter,
with samples taken horizontally or vertically, but the principle of determination of density and SWE is the
same.
See clause 8 for further explanation of the methods.
6 Measurement sites
6.1 General
The criteria for selection of SWE measuring sites are independent of the measuring method, and similar to
siting precipitation gauges for measurement of snowfall.
Sites for both single point measurements and snow courses should be chosen to be representative of the area
of interest.
Where the snow is distributed homogeneously over the area, a few single point measurements could be
sufficient. In locations where snow depth and density changes are caused by wind drift, and interception play
an important role, snow courses are recommended.
A totally open area where the distribution of snow is more affected by wind should if possible be avoided, as
well as pronounced recesses and summits of the terrain.
If the total accumulation of snow mass is of interest, the site should be chosen at elevations and exposures
where there is as little melting as possible prior to the peak accumulation;
Recommended locations of SWE measurement sites are:
— at places where the terrain is horizontal in order to minimize the affect from snow creep;
— at clearings in bush land and open forests sufficiently large so that snow can fall to the ground without
being intercepted by the branches. Trees in the distance may be helpful in making a wind break,
preventing snow drift, and thus providing for a more even distribution of the snow accumulation;
— at places sufficiently distant from larger trees, rock outcrops and buildings which could disturb natural
accumulation and melting of the snow. The closest recommended distance between the sampling point
and the nearest obstacle is roughly equal to the height of the obstacle;
— at places where the environment is rather constant over a long period of time. This will ensure that
conditions on sampling sites remain consistent.
If the measurements are made for estimating the SWE in whole regions, sites where the temperature can be
strongly unrepresentative should be avoided. This means that measuring sites should not be located in urban
areas, or at locals where direct solar radiation or shadow could have considerable thermal influences.
Bogs should be avoided because of possible influence from water underneath and difficulty in measuring the
accurate snow depth.
Hollows and gullies, ridges and tops should be avoided because of possible negative wind effects.
In order to avoid any systematic error because of drifting snow it may be necessary to perform extensive
survey measurements prior to finally determining the location of measuring sites, and length and sampling
distance for snow courses (9.3).
There is an advantage in installing SWE measuring stations at or close to meteorological stations since
meteorological parameters are important for evaluation and validation of SWE, and vice versa.
NOTE Further recommendations can be found in the WMO Guide to Hydrological Practices No. 168.
6.2 Manual measurements
Manual measurements provide the possibility of controlling sources of error and assessment of the result
directly in the field, which enables assurance of the data quality. A disadvantage is the low sampling
frequency.
The ground can be irregular under the snowpack, and the snow depth, and thus the SWE, can vary within a
short distance. If it is important for the measuring program that measurements always are carried out on the
same spot, the measuring sites should be marked, or positioned by GPS.
To ensure continuity of the measurements the sites should be easily accessible even when the possibility of
transportation in the terrain, or the weather, is bad. The personnel who perform the measurements should be
well trained to carry out the measurements correctly, even under bad weather conditions.
Melting and freezing periods during the winter can result in layers of ice inside the snowpack. Problems might
occur to penetrate thick ice layers with the sampler, and digging a pit to the ground could be necessary to
identify the layers and measure them separately.
7 Measurements
7.1 General
Data on snow depth is often more requested then data on SWE, and is thus a more common parameter in
national weather service and road service station networks. For calculation of SWE, however, the snow
density is required. Equation 1 shows how SWE is calculated in a sampling point.
ρ
snow
SWE= ×D
ρ
water
Equation 1
where:
SWE is the snow water equivalent (m)
-3
ρ is the snow density (kg·m )
snow
-3
ρ is the water density (1000 kg·m )
water
D is the snow depth (m)
If a SWE measuring site is situated close to a meteorological station, a model for calculation of SWE at the
site can be set up, where the only input data required are the meteorological parameters and snow depth. It is
recommended that at least 5 years of measurements of snow density and depth are used for model
calibration.
7.2 Snow density
The density of a snow sample is the snow mass per volume unit of the undisturbed snow sample (Equation 2).
The volume of the undisturbed snow sample is calculated by multiplying the snow depth at the sampling point
with the inner area of the sampler. By the weighing method (Equation 3) the mass of the sample is determined
by a balance or scale, and by the volumetric method (Equation 4) the mass of water is calculated from the
volume of melted water of the sample, and the density of water.
M
ρ =
snow
V
Equation 2
M
ρ =
snow
A×D
Equation 3
V ×ρ
water water
ρ =
snow
A×D
Equation 4
where:
-3
ρ is the snow density (kg·m )
snow
M is the mass of the snow sample (kg)
V is the volume of the undisturbed sample (m )
A is the inner cross-sectional area of the snow sampler (m )
D is the snow depth at the sampling point (m)
is the melted water volume of the snow sample (m )
V
water
-3
is the water density (1000 kg·m )
ρ
water
The snow density at the site should be taken as the average of at least three density samples, taken close to
each other. The deviation of each sample should not differ more than 10 % from their mean value; otherwise
more samples should be taken until at least three samples are within the range. Samples outside the range
may not be used in the calculation.
Calculation of the snow density gives an indication of the quality of the measurement since the value should
-3
be within an expected range. Typical values of a late winter snowpack are often between 250 kg·m to 450
-3 -3 -3
kg·m . The maximum range in nature is between 30 kg·m in very dry new snow to 600 kg·m in very wet
or/and compact snow. However, in very thick snow layers, for example on glaciers or high alpine areas,
-3
densities above 600 kg·m are possible.
7.3 Snow depth
7.3.1 Manual probing
For manual measurement of snow depths up to 1,5 m a graduated stick can be used. In deeper snow it is
preferred to use a so-called avalanche probe, made up by several metal rods which connected together
normally has a total length between 2 m to 4 m.
Additional snow depth measurements during snow surveys will give a better spatial estimation of the SWE, if
estimation of snow density at the depth measurement points is possible. Manual measurement of the snow
depth is much easier, less time consuming, and cheaper than measurement of density. Therefore a frequent
sounding during the survey can be cost effective.
Manual probing can also be necessary for verification of snow depth measurements performed with other
methods, and for checking the snow depth prior to SWE measurements (see clause 8).
7.3.2 Manual readings on fixed snow stakes
A common method for the determination of snow depth is by the use of fixed stakes (see Annex E). The entire
length of a snow stake should have a graduated scale with the zero point exactly at the ground level. This
enables readings to be taken from a distance without disturbing the snow surface close to the stake. Whilst
taking measurements from the snow stake it is important to survey against the surrounding snow surface from
a horizontal position.
A snow stake should be painted white to minimize undue melting of snow around the stakes caused by
absorption of solar radiation. If doubt exists about the reading, due to snow drift or ablation around the stake,
the true snow depth should be checked using an independent measuring device.
It is important that the snow close to the stake is left untouched. Therefore, it is recommended to approach the
site always from the same direction. Any snow pits should be filled after measurement.
7.3.3 Automatic recording
Automatic measurements of snow depth make it is possible to follow the snow accumulation and ablation in
detail. Today acoustic and optoelectronic sensors mounted above the snowpack can be purchased.
Acoustic (ultrasonic) instruments measure the time interval from when the transmitted ultrasonic pulse is sent
until its reflection against the snow surface is received.
Optoelectronic (laser) sensors use optoelectronic principles. Eye-safe visible laser is emitted against the snow
surface. Reflected light is received, and from the phase shift the distance to the snow is calculated.
7.3.4 Remote sensing
Laser scanning provides spatial snow depth measurements covering large areas in inaccessible terrain. The
measurements can be airborne (ALS, Airborne Laser Scanning) and ground-based (TSL, Terrestrial Scanning
Laser).
With ground penetrating radar (GPR) large lateral distances can be measured in a short period of time.
Normally, the two-way travel time for radar waves is measured and converted into snow depth or SWE
estimates. The measurements can be either air-born (e.g. from a helicopter) or ground based (e.g. from a
snowmobile).
8 Manual SWE sampling methods
8.1 General
Generally, weighing of the snow sample is accomplished by means of a spring scale or by a special balance.
The spring scale is the most practical approach as it is relatively useful even in strong wind. However, spring
scales used for snow measurements in field are accurate only to about 5 g -10 g, and the error in weighing by
this method may be appreciable for small diameter samplers and shallow depths of snow. Scale balances,
potentially more accurate, are very difficult to use under windy conditions.
Another approach is to store the samples in sealed plastic containers or bags and return them to a base
station where they may be accurately weighed or melted and measured with a graduated cylinder. In practice,
this procedure is difficult to carry out as the samples should be bagged without loss, carefully labelled, and
carried back to the base. The advantage of measurement in the field is that any gross errors due to plugging
the sampler, or losses due to part of the sample falling out, may be readily recognized, and repeat readings
can be taken at once.
The results may be recorded on site with other pertinent observations and, if a good notebook is used, there
can be little chance of confusion as to the location or the sampling conditions. In all measurements of this
type, the extremely difficult physical conditions under which observations should frequently be made should
always be kept in mind, and practical consideration should prevail in sampler designs.
Snow can easily freeze inside the sampler. This is often a problem when the equipment has been stored
warm before the measurement. Therefore the measurement should not start until the sampler has the same
temperature as the air. Preparation of the inside of the sampler with silica spray prior to the sampling may
reduce the build-up of ice. Using the silica on the outside may make penetration of the snowpack easier.
A list of samplers is shown in Annex F. The table includes the operator, dimensions, material and the
approximate cost of the equipment.
8.2 Snow tubes
Snow tubes (see Annex G) are used for measurement of the SWE of total snowpack from the snow surface to
the ground. The sampling can be performed without digging and is hence carried out quickly. Normally, about
30 - 60 samples can be taken during an eight hour working day under normal snow conditions, if the snow
depth is less than 2 m and the sampling sites are easily accessed.
Most snow tubes are made of aluminium, stainless steel, PVC or fibreglass. The tube is graduated on the
outside for reading of the snow depth. A sharp edged cutter at the lower end facilitates insertion into the snow,
especially if there are harder crust layers inside the snowpack. If the edge of the cutter consists of sharpened
saw teeth even relatively thick ice layers can be penetrated. Small diameter cutters retain the sample much
better than large cutters, but larger samples increase the accuracy in weighing.
Prior to the measurement the snow depth should be checked by sounding. This enables to choose the correct
length of sampler to be used, and makes it possible to identify hard crust and ice layers in the snow pack
which otherwise could be mistaken for the ground.
In order to cut the core, the sampler is forced vertically downward through the snow cover until it reaches the
ground. If snow conditions permit, a steady downward thrust, causing an uninterrupted flow of the core into
the tube, is best. A minimum amount of turning the tube is possible without interrupting the downward thrust.
This brings the cutter into play, which is desirable for quick penetration of thin ice layers.
After reaching the ground the sampler can be turned a last time to force some soil into the cutter. The snow
depth is obtained by reading on the ruler on the snow sampler, after which the sampler should be pulled up
gently. A soil plug can prevent loss of core through the cutter while the sampler is withdrawn, and indicates
that the whole snowpack has been penetrated. Any soil, though, should be removed from the sample and its
depth excluded from the total measured depth. On wetland sites it may be difficult to feel when the sampler
has reached the ground, if it is not frozen, and the tube may go relatively deep into the soil.
In order to prevent loss of core through the cutter while the sampler is withdrawn from the snow, sufficient soil
can be gathered in the cutter to serve as a plug. The extent to which this will have to be done depends on the
condition of the snow. The inside of the cutter can be conic-shaped, and the inner diameter of the tube a little
larger than the inner diameter of the cutter. It can also help to compress the snow from the upper end of the
tube by use of a rod.
If the snowpack is thicker than the length of the tube, an adjacent pit should be dug to the level where the
mouth of the sample is situated. The next sample is taken from this level to the ground. If the ground is not
reached this procedure is repeated.
To enable measurements in very deep snowpack snow tubes can be made in sections which can be
connected together.
Normally, the SWE is calculated by weighing the snow samples, either by weighing it together with the
sampler or by pouring the snow into for example a bag or bucket and weighing it separately (see Annex C).
A cylindrical brush mounted on a thin shaft can be used for cleaning the inside of the tube between the
measurements.
8.3 Core drills
If the snow is very deep and the density is high, measurements with snow tubes may be very difficult and time
consuming. Use of core drills (see Annex H) may be preferred when the snow depth exceeds 3 m - 4 m. This
method is mainly used for taking samples from glaciers.
When measuring on glaciers it is recommended that a snow pit is dug to identify the bottom of the snow layer
of the last season.
To be able to use a core drill, the snow should be hard enough to prevent the s
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