Hydrometry - Measurement of snow water equivalent using snow mass registration devices

This Technical Report defines the requirements for the use of snow mass registration devices for
measurement of SWE under natural environmental conditions. It includes weighing and pressure measuring methods.

Hydrometrie - Das Messen des Schnee-/Wasseräquivalentes

Dieser Technische Bericht definiert die Anforderungen an die Verwendung von Geräten zur Erfassung des Wasserwertes der Gesamtschneedecke unter natürlichen Umweltbedingungen. Er enthält ausschließlich Wäge- und Druckmessverfahren.

Hydrométrie - Mesurage de l'équivalent en eau de la neige au moyen de dispositifs d'enregistrement de la masse neigeuse

Le présent rapport technique définit les exigences relatives à l’utilisation de dispositifs d’enregistrement de la masse neigeuse pour le mesurage de l’ÉEN dans les conditions environnementales naturelles. Il inclut des méthodes de mesurage du poids et de la pression.

Hidrometrija - Merjenje količine vode v snegu s pomočjo naprav za registracijo snežne mase

To tehnično poročilo določa zahteve za uporabo naprav za registracijo snežne mase za
merjenje SWE pod naravnimi pogoji okolja. Vključuje metode tehtanja in tlačnega preskušanja.

General Information

Publication Date
Technical Committee
Current Stage
6060 - National Implementation/Publication (Adopted Project)
Start Date
Due Date
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SIST-TP CEN/TR 15996:2010
Hydrometry - Measurement of snow water equivalent using snow mass registration
Hydrometrie - Das Messen des Schnee-/Wasseräquivalentes
Hydrométrie - Mesurage de l'équivalent en eau de la neige au moyen de dispositifs
d'enregistrement de la masse neigeuse
Ta slovenski standard je istoveten z: CEN/TR 15996:2010
07.060 Geologija. Meteorologija. Geology. Meteorology.
Hidrologija Hydrology
SIST-TP CEN/TR 15996:2010 en,fr,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST-TP CEN/TR 15996:2010

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SIST-TP CEN/TR 15996:2010

CEN/TR 15996


February 2010
ICS 07.060
English Version
Hydrometry - Measurement of snow water equivalent using
snow mass registration devices
Hydrométrie - Mesurage de l'équivalent en eau de la neige Hydrometrie - Messung des Schnee-Wasser-Äquivalents
au moyen de dispositifs d'enregistrement de la masse unter Verwendung eines Gerätes zur Erfassung der
neigeuse Schneemenge

This Technical Report was approved by CEN on 11 January 2010. 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, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland,
Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.



Management Centre: Avenue Marnix 17, B-1000 Brussels
© 2010 CEN All rights of exploitation in any form and by any means reserved Ref. No. CEN/TR 15996:2010: E
worldwide for CEN national Members.

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SIST-TP CEN/TR 15996:2010
CEN/TR 15996:2010 (E)
Contents Page
Foreword .3
Introduction .4
1 Scope .5
2 Normative references .5
3 Terms and definitions .5
4 Symbols and abbreviated terms .5
5 Purpose of method .6
6 Principle .6
7 Operational requirements .6
8 Monitoring by hydrostatic pressure measurements .8
9 Monitoring by weight measurements . 10
10 Data collection, transfer and processing . 10
11 Factors affecting accuracy of stationary point measurements of SWE . 10
12 Factors affecting accuracy of snow mass monitoring specifically . 11
13 Evaluation of method . 12
Annex A (informative) List of methods for determination of SWE in total snowpack . 13
Annex B (informative) Snow mass registration stations – networks . 14
Annex C (informative) Manual SWE measurements . 16
Annex D (informative) Snow pillow . 17
Annex E (informative) Snow plate . 19
Bibliography . 21


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SIST-TP CEN/TR 15996:2010
CEN/TR 15996:2010 (E)
This document (CEN/TR 15996:2010) 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.

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Snow water equivalent (SWE) measurements
Snow water equivalent (SWE) is the height of water that would be obtained by melting the snowpack on a
corresponding surface area, and is normally expressed in millimetres (mm).
Knowledge of the SWE is essential for estimation of total runoff and flood forecasting in river basins where
snowfall occurs. Independent of the selected method, the SWE measurements should proportionally represent
the total SWE in the studied area.
The parameter is predominant in avalanche theory and avalanche danger forecasting as well as for risk
assessment of heavy snow loads. Additionally, the development of SWE measurements using satellite
sensors has increased the need for validation and calibration using in-situ measurements.
Annex A is a list of methods for determination of SWE.
Snow mass registration devices
Snow mass registration devices are widely used in North America and Europe. Different problems
experienced in the use of the equipment have resulted in a slow development of the technique, but
improvements in equipment design and data management in recent years have increased interest in the
method. Annex B shows a table of station networks running during publication of this report.

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1 Scope
This Technical Report defines the requirements for the use of snow mass registration devices for
measurement of SWE under natural environmental conditions. It includes weighing and pressure measuring
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
EN ISO 772:2000, Hydrometric determinations ― Vocabulary and symbols (ISO 772:1996)
CEN ISO/TS 25377, Hydrometric uncertainty guidance (HUG) (ISO/TS 25377:2007)
3 Terms and definitions
For the purposes of this document, the terms and definitions given in EN ISO 772:2000 apply.
4 Symbols and abbreviated terms
Table 1 lists the symbols used in this document.
Table 1 — Symbols
Symbol Term Unit
SWE Snow water equivalent mm
M Snow mass kg
ρ Density kg/m
V Volume m
A Area m

Table 2 lists the abbreviated terms used in this document.
Table 2 — Abbreviated terms
Abbreviation Term
SPA Snow Pack Analyzer
GPR Ground-penetrating radar
PVC Polyvinyl chloride

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5 Purpose of method
The use of snow mass registration devices provides stationary single point measurements of the total SWE
and recording of the changes that take place due to further accumulation of snowfall or melting of the
If the equipment is correctly mounted at a location where the snow accumulation is representative of the
surrounding terrain, the data obtained by this method can be used for the prediction of melting water volumes.
It can also be used for calculating the effect of heavy snow loads on structures in the area, avalanche
prediction or just provide information on snow quantities in general.
6 Principle
The mass of the snow on top of a measuring device is equivalent to the mass of the water content in the
overlying snow. This mass can be obtained by measuring the pressure in a pillow filled with anti-freeze
solution, which is the most common method, or by using a flat plate mounted on weight sensors through which
the mass of accumulated snow can be determined.
7 Operational requirements
7.1 General
Snow mass registration devices should be as large as possible to optimise the accuracy of the measurement.
2 2
The most common areas are between 2 m and 15 m .
The measuring device should be installed horizontally on a stable foundation. The top surface should be at the
same level as the surrounding ground to minimize the effects of shear perimeter stress concentrations. To
reduce environmental effects that cause SWE measurement errors the instrument should have low
compressibility and a thermal conductivity similar to that of the surrounding soil (see 12.2).
It is recommended to imitate the properties of the surrounding ground using a permanent cover of, for
example, camouflage netting or synthetic grass. This may also prevent snow blowing off the instrument
following moderate snow falls.
The disturbance of snow accumulation at the measuring site due the measuring equipment should be
The site should be well drained. Water should not be allowed to collect on the device.
The equipment should be protected against interference by animals or unauthorized persons. If necessary,
the installation could be protected by a fence, but it should not interfere with the accumulation and the ablation
of the snow.
To be able to locate the exact position of the measuring device, it can be marked with reference poles of
sufficient height to be seen above the snowpack at the expected maximum snow height.
7.2 Site selection
Detailed site investigations are required to select a representative location. To prevent the snow on the ground
from being considerably influenced by the wind, a location surrounded by bush vegetation or a clearing in an
open forest is preferable. Wind exposed areas should be avoided, as well as pronounced recessions in the

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The site selected should be a considerable distance from larger trees, rock outcrops and buildings which
could disturb natural accumulation and melting of the snow. An ideal distance between the instrument and the
nearest obstacle is between 2/3 and three times the height of the obstacle.
The slope of the surrounding terrain should be such as to minimize the affect of snow creep on the measuring
Locations should be selected in order to avoid the risk of rising water levels affecting the installation. The soil
should allow good natural drainage.
Snow mass registration devices are preferably located at, or close to, a climate station, since meteorological
parameters are important for evaluation and validation of SWE measurements.
If the equipment on the site has to be supplemented with external power an accumulator can be used,
recharged by a solar panel. In this case, sun conditions have to be considered.
7.3 Validation
Rapid and unexpected changes in the monitored SWE might be a result of snow metamorphoses, formation
or break up of snow bridges, or shear stress concentrations along the perimeter of the measuring device (see
Clauses 11 and 12). It can also be a result of leakage in the pressure system in the snow pillow, or a defective
sensor. To be able to detect measurement errors it is necessary to establish a control programme.
Regular checks utilizing manual measurements of the SWE should be performed with samples being taken
within a few metres from the instrument. It may be appropriate to undertake frequent manual measurements
following the initial installation to ensure the correct performance of the instrument. Annex C summarizes the
techniques for manual measurements.
Registration of meteorological parameters such as precipitation, air temperature, wind speed and the
temperature of the snow at the surface and slightly above the ground at the site is valuable for checking the
accuracy of the monitoring.
By use of simple numerical modelling of SWE based on observations of for example precipitation and air
temperature, the registration can be evaluated continuously.
Additional measurements of snow depth in connection to the SWE recording system can also assess the
performance of the SWE measurements. The combination of SWE and snow depth measurements can be
used to derive an estimate of the snow density (Equation (1)):
SWE[kgm ]
ρ []kgm =                                                 (1)
d []m
A calculated snow density outside typical values can be used as indication of problems with the measurement
-3 -3
system. The maximum range for snow densities is between 50 kg·m to 450 kg·m , but typical values of a
-3 -3
late winter snow pack is often around 250 kg·m to 350 kg·m .
7.4 Maintenance
In summertime the equipment including all complementary and protective devices on site, should be cleaned
and checked and the zero point checked.
In addition, the site itself should be inspected for changes. New vegetation and obstacles that could have an
affect of the measurements should be removed and drainage of the site may have to be improved.

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8 Monitoring by hydrostatic pressure measurements
8.1 Description
Snow pillows for determination of SWE were developed in the early 1960s. The snow pillow consists of a flat
bag completely filled with an anti-freeze fluid. The pillow should have a valve for filling the pillow and removing
air bubbles. It can be in various shapes, sizes and materials. The most common snow pillows are circular,
rectangular or hexagonal shaped and made of UV-resistant butyl, neoprene rubber or stainless steel.
The surface area of the snow pillow should be sufficiently large to minimize the affects of shear stress along
the edges of the pillow or bridging in the snowpack (see Clause 12). Recommendations from United States
Department of Agriculture (USDA) and Norwegian Water Resources and Energy Directorate (NVE) on the
minimum area in relation to the maximum expected SWE is shown in Figure 1.

Y Area (m) X SWE (mm)
— USDA steel pillow – – USDA butyl pillow --- NVE butyl pillow
Figure 1 — Recommendations on the minimum area of snow pillows in relation to the maximum
expected SWE
A snow pillow with a depth of about 10 cm is recommended. This means that a pillow with an area of 3 m
requires approximately 300 l of fluid.
With a good installation, periodical control, and normal use and care, snow pillows should have a working life
in excess of ten years.
8.2 Fluids
The snow pillow should be filled with an anti-freeze solution suitable for the minimum temperature expected. It
is recommended to use environmental friendly solutions.
NOTE Where anti-freeze is used then the compliance to existing national environmental regulations should be
adhered to.
Table 3 shows the most common anti-freeze mixtures, their freezing point, and their rate of toxicity.


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Table 3 — Anti-freeze solutions used in snow pillows

Solution (50:50 by mass) Freezing point Toxicity
ethanol : water
- 32 °C low

propylene glycol : water
- 34 °C low

propylene glycol : ethanol

methanol : water
- 54 °C high

ethylene glycol : methanol
- 40 °C high

ethylene glycol : water
- 34 °C high

The pressure in the fluid corresponds to the weight of snow lying on the pillow and is measured by a pressure
sensor inside the pillow, or in a riser pipe connected to the pillow (see Annex D). The pressure sensor as well
as any other equipment in contact with the anti-freeze fluid should be resistant to its corrosive effects.
A tracer in the fluid can be used to detect leaks and also to facilitate manual readings on a riser pipe scale.
8.3 Installation
In order to allow the surface of the pillow to level with to the surrounding area the ground should be
excavated, or the site should be aggraded with filling material. Care must be taken to ensure that there is
nothing that would damage the underside of the pillow. A padding of sand or crushed rock with a maximum
particle size of 3 mm is ideal to protect the pillow. Local material may be used provided that the material has
full contact with the bottom surface of the pillow and supports the load of the filled pillow system. If necessary
a geotextile membrane can be used between the soil and the pillow.
It is recommended that two separate sensors are used to provide redundancy and also to be able to detect
sensor errors.
If it is necessary to protect the unit from lateral pressure caused by the weight of the surrounding soil,
protective me

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