Measurement of Snow Water Equivalent

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Measurement of Snow Water Equivalent

• Kara Przeczek

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Outline

• Snow water equivalent
• Snow pillows
• Snow surveys
• Snow survey equipment methods
• Examples from 2008 snow surveys

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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
• Assuming mean density 100 kg/m3
• Regional density variations (new snow 35101
  kg/m3)
• Densification over time
• Measuring snowpack swe is the standard
• by weight of snowpack sample

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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

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http//watershed.montana.edu/hydrology/images/img_
1053.jpg
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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
• site representativeness important
• Bridging may occur
• separation of pillow from overlaying snow
• under measurement of swe
• Snow pillow is a barrier to heat moisture
  fluxes between snowpack and ground

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Snow Pillows

• B.C. Ministry of Environment River Forecast
  Centre
• 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

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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

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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
• 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

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Snow SurveysConsiderations

• 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?

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Snow SurveysPurpose

• Index of swe
• 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

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Snow SurveysSite Selection

• Accessibility
• Representativeness of terrain/land cover
• Choose slightly sloping terrain
• Avoid steep slopes
• Avoid areas with land use disturbances
• i.e. logging, mining, construction
• Avoid microsite irregularities at sample points
• i.e. stumps, logs, ponding areas
• Avoid areas with snow removal activity

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Snow SurveysSite Selection

• Snowmelt for testing melt models
• North and south aspect
• Radiation differences
• Open and forest cover
• Radiation differences
• Similar location, slope, elevation
• Same weather, vegetation
• Access

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Snow Surveys Course Length

• Longer in complex terrain
• More samples in complex terrain
• Snow course can zig-zag
• Oversample initially
• 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

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Snow SurveysCourse Length cont.

• Cumulative coefficient of variation (C.V.)
• C.V.
• Measure of variability independent of scale
• Plot cumulative C.V. against length of course
• How many samples before C.V. levels off ?

standard deviation mean
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Snow SurveysSampling Schedule

• RFC sample beginning of month Jan-Jun
• Extra mid-month sampling May June
• Purpose of snow course?
• Snow accumulation
• Snow melt
• Maximum swe
• Snow cover duration
• Purpose melt
• Start April 1st
• Interval weekly
• End snow melted (June)

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Snow SurveysEquipment

• 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
• Slots in tube to view snow core
• Spring balance measures swe directly
• Federal (formerly Mt. Rose) snow sampler

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Snow SurveysMethods

• B.C. Snow Survey Sampling Guide
• http//www.env.gov.bc.ca/rfc/river_forecast/snow_s
  urveys_manual.pdf

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Snow SurveysMethods - 1

• 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.

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Snow SurveysMethods - 2

• 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.

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Snow SurveysMethods - 3

• 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.

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Snow SurveysMethods - 4

• 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.

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Snow SurveysMethods rinse repeat

• Give tube a good shake or tap it against toe of
  boot to get out snow core
• Dont 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

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Snow SurveysTips

• Weighing empty tube before each sample is at your
  discretion
• 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

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Site NF GPS 10 U 0588859 5894468 Aspect S Slope 16.1
Date Apr 27, 2008 Start Time 1320 End Time 1415 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 1 500 67 5 88 113
2 1 500 50 7.5 87 100
3 1 500 55 6 86 102
4 1 500 67.5 10 86 94 in nasty tree well

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• Swe increasing at one site while snow depth
  decreasing
• Effect of site on swe

SWE
Depth
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• Density increasing over time
• Effect of site on density
• Interception, shading, wind
• Calculate density from swe and snow depth
• Conversion between mm m density of water

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Snow SurveysIssues

• Different samplers for different conditions
• Federal generally regarded as best all-around
• Generally overestimate swe
• Design of cutting point forces more snow inside
  tube
• Ice layers
• Losing water from the tube during melt
• Gaining extra snow
• Through slots with twisting in deep snowpack
• Dull cutter
• Shrubs, branches, vegetation beneath the snow
• Air pockets
• Check length of core 80 snow depth consistent
  density between samples
• Freezing of snow in the tube
• Particularly when air temp gt 0oC, snow temp lt 0oC

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• April 20th, snow freezing in core resulted in
  anomalously low swe measurement.
• 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

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Site GPS Aspect Slope
Date Start Time End Time Weather
Sampler Recorder N-S Distance (m) E-W Distance (m)
Point Line Direction Depth w/ Soil Soil Plug SWE Empty SWE Full NOTES
1 1
2 1
3 1
4 1