Measurement of Snow
Water Equivalent
Kara Przeczek
Outline

Snow water equivalent

Snow pillows

Snow surveys

Snow survey equipment & methods

Examples from 2008 snow surveys
Snow Water Equivalent


“The vertical depth of water which would be
obtained by melting snow”
Snowfall measurement can be problematic
Determining swe from snowpack depth can be
problematic

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Assuming mean density 100 kg/m3
Regional density variations (new snow: 35–101 kg/m3)
Densification over time
Measuring snowpack swe is the standard

by weight of snowpack sample
Snow Pillows

An antifreeze filled bladder of various shapes,
sizes, and materials.

Minimum size based on expected winter swe

Pressure inside the pillow changes in
response to the weight of snow

Fluid pressure changes are measured with a
manometer or pressure transducer

data can be transmitted remotely
http://watershed.montana.edu/hydrology/images/img_1053.jpg
Snow Pillows
Pros:
 A non-destructive sampling technique
 An automatic measure of swe in remote locations
 Identify snowfall and snowmelt events
 Can provide rough estimates of loss of swe
Cons:
 A point swe measurement

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Bridging may occur

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site representativeness important
separation of pillow from overlaying snow
under measurement of swe
Snow pillow is a barrier to heat & moisture fluxes between
snowpack and ground
Snow Pillows

B.C. Ministry of Environment River Forecast Centre

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http://www.env.gov.bc.ca/rfc/
Automatic Snow Pillows (ASPs) and manual snow surveys
view ASP swe graphs (weekly)
download temperature, precipitation, and swe data from
ASP sites (near real-time)
download manual snow survey data
 Historic data 1935 - present
54 active ASP sites and almost 200 snow survey
sites in the province
Sonic Ranger

Automatic snow depth measure

Determines the distance to a target by sending out
ultrasonic pulses and listening for the returning
echoes that are reflected from the target

The time from transmission to return of an echo is
used to obtain the distance measurement

Air temperature correction required for variations in
the speed of sound in air
Snow Surveys


Measure snow depth, density and swe
Snow course is the line of permanently marked
sampling points

Repeat measurements at regular intervals throughout
season

Location and frequency of snow survey depends on
purpose:


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1o as index of swe for spring runoff prediction
2o absolute measure for hydrologic, agricultural, ecologic,
transportation, recreational, and engineering functions
Snow survey data are the ultimate base of comparison
for other methods
Snow Surveys
Considerations

What are you measuring swe for?

Where to set up snow course?

How long should the snow course be?

How many sample points per course?

swe depth sampling ratio?

When & how often to sample?

What equipment to use?
Snow Surveys
Purpose

Index of swe:

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Choose high accumulation area
 Show changes in swe
Represent basin characteristics
 Forest cover; aspect; elevation
Consistent instrumentation and methodology
Absolute estimate of swe:


Account for biases
 Instrument, method, site
Account for variability in snow cover
 Erosional/depositional areas
 Stratify by landscape features
Snow Surveys
Site Selection
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Accessibility
Representativeness of terrain/land cover
Choose slightly sloping terrain
Avoid steep slopes
Avoid areas with land use disturbances

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Avoid microsite irregularities at sample points


i.e. logging, mining, construction
i.e. stumps, logs, ponding areas
Avoid areas with snow removal activity
Snow Surveys
Site Selection

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Snowmelt for testing melt models
North and south aspect

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Open and forest cover

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Radiation differences
Similar location, slope, elevation


Radiation differences
Same weather, vegetation
Access
Snow Surveys
Course Length

Longer in complex terrain

More samples in complex terrain

Snow course can zig-zag

Oversample initially

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Length & sample density adjusted based on selected
precision level
Fewer swe than depth measurements possible

Density shows least variability
 Calculate swe at depth measurement points
Snow Surveys
Course Length cont.

Cumulative coefficient of variation (C.V.)

C.V. =

standard deviation
mean
Measure of variability independent of scale

Plot cumulative C.V. against length of course

How many samples before C.V. levels off ?
Snow Surveys
Sampling Schedule

RFC: sample beginning of month Jan-Jun

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Purpose of snow course?
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Extra mid-month sampling May & June
Snow accumulation
Snow melt
Maximum swe
Snow cover duration
Purpose: melt

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Start: April 1st
Interval: weekly
End: snow melted (June)
Snow Surveys
Equipment

Graduated snow tube with cutter

Various materials, cutter configurations, and sizes

Larger diameter tubes for shallow snow packs

Graduations on outside to measure snow depth
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Slots in tube to view snow core

Spring balance measures swe directly

Federal (formerly Mt. Rose) snow sampler
Snow Surveys
Methods

B.C. “Snow Survey Sampling
Guide”

http://www.env.gov.bc.ca/rfc/river_fo
recast/snow_surveys_manual.pdf
Snow Surveys
Methods - 1
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1. Weigh and record empty tube swe.
2. Push tube straight into snow pack to ground.
3. Push and twist into ground to obtain soil plug.
Snow Surveys
Methods - 2
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1. Weigh and record empty tube swe.
2. Push tube straight into snow pack to ground.
3. Push and twist into ground to obtain soil plug.
4. Record snow depth from outside of tube.
5. Carefully pull tube straight out of snow pack.
6. Check for soil plug.
Snow Surveys
Methods - 3
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1. Weigh and record empty tube swe.
2. Push tube straight into snow pack to ground.
3. Push and twist into ground to obtain soil plug.
4. Record snow depth from outside of tube.
5. Carefully pull tube straight out of snow pack.
6. Check for soil plug.
7. Remove soil plug and estimate depth of soil in
tube.
Snow Surveys
Methods - 4
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1. Weigh and record empty tube swe.
2. Push tube straight into snow pack to ground.
3. Push and twist into ground to obtain soil plug.
4. Record snow depth from outside of tube.
5. Carefully pull tube straight out of snow pack.
6. Check for soil plug.
7. Remove soil plug and estimate depth of soil in
tube.
8. Weigh and record filled tube swe.
*If hit obstruction during sampling or if no soil plug
obtained re-take snow sample.
Snow Surveys
Methods – rinse & repeat

Give tube a good shake or tap it against toe
of boot to get out snow core

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Don’t hit tube or cutter end against anything hard
because it is easy to damage
Check that no snow remains in tube between
samples

No ‘significant’ snow in tube
Snow Surveys
Tips
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Weighing empty tube before each sample is at your
discretion
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Dependent on snow conditions; tube-clearing skill; level of
accuracy desired
Wax inside of tube before use (Turtle Wax works
well)
Bring small, slotted screwdriver and old knife to
remove soil plug, or tricky snow core, from tube
Bring many pairs of gloves, grippy palms are a good
idea
Waterproof paper and pencil for data recording
Site
NF
GPS
10 U 0588859
5894468
Aspect
S
Slope
16.1%
Date
Apr 27,
2008
Start Time
13:20
End Time
14:15
Weather
Overcast,
light drizzle
Sampler
Kara
Recorder
Audrey
N-S
Distance (m)
7
E-W
Distance (m)
20
Point
Line
Direction
Depth w/ Soil
Soil Plug
SWE Empty
SWE Full
NOTES
1
1
5:00
67
5
88
113
2
1
5:00
50
7.5
87
100
3
1
5:00
55
6
86
102
4
1
5:00
67.5
10
86
94
in ‘nasty’
tree well
…
…
…
…
…
…
…
…
SWE
Depth
45
60
20
35
40
75
90 105
125
Snow Depth (cm)
30
SWE (cm)
25
145
45
165
50
55
60
185
Effect of site on swe
5-Jun
30
15

30-May
15
10
Swe increasing at one site while snow depth
decreasing
23-May
17-May
9-May
2-May
27-Apr
20-Apr
12-Apr
0
5
SO
SF
NO
NF
1-Apr
5-Jun
30-May
23-May
17-May
9-May
2-May
27-Apr
20-Apr
12-Apr
1-Apr
0

SO
SF
NO
NF
500

350
380

Density
increasing over
time
Effect of site on
density
290
320

230
260

p snow 
5-Jun
30-May
23-May
17-May
9-May
2-May
27-Apr
20-Apr
12-Apr
1-Apr
swem 
 kg 
 kg 
p snow  3  
 p water  3 
 m  snowdepthm 
m 
swemm
snowdepthm 
Interception,
shading, wind
Calculate density
from swe and
snow depth

200
Density (kg/m3)
410
440
470
SO
SF
NO
NF
Conversion
between mm &
m = density of
water
Snow Surveys
Issues

Different samplers for different conditions


Generally overestimate swe

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Through slots with twisting in deep snowpack
Dull cutter
Shrubs, branches, vegetation beneath the snow


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Design of cutting point forces more snow inside tube
Ice layers
Losing water from the tube during melt
Gaining extra snow


Federal generally regarded as best all-around
Air pockets
Check length of core ≥ 80% snow depth; consistent density between samples
Freezing of snow in the tube

Particularly when air temp > 0oC, snow temp < 0oC
35

30

60
40
April 20th, snow
freezing in core
resulted in
anomalously low
swe measurement.

25
15
20

5
10


5-Jun
30-May
23-May
17-May
9-May
2-May
27-Apr
20-Apr
12-Apr
1-Apr
0
SWE (cm)
45
50
55
SO
SF
NO
NF
Sunny, cold day
Strongest effect at
open sites
Wear gloves
Wax the inside of
the snow tube
Keep the tube filled
with snow between
samples
Site
GPS
Aspect
Slope
Date
Start Time
End Time
Weather
Sampler
Recorder
N-S
Distance (m)
E-W
Distance (m)
Point
Line
1
1
2
1
3
1
4
1
Direction
Depth w/ Soil
Soil Plug
SWE Empty
SWE Full
NOTES