ESM 203: Snow in the hydrologic cycle

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ESM 203 Snow in the hydrologic cycle

• Jeff Dozier and Thomas DunneFall 2007

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Worldwide importance of the mountain snowpack
1/6 of Earths population relies on glaciers or
seasonal snow for their water supply
Barnett et al., Nature, 17 Nov 2005
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Topics

• Snow accumulation and runoff
• Energy balance of the snowpack
• Water flow
• Snow metamorphism and melt
• Implications for nutrient cycling in watersheds
• Long-term trends

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Example Merced River, Happy Isle in Yosemite
Valley
Peterson et al., USGS
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SWE (snow water equivalence) and mass balance
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Manual measurement of SWE
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Measurement of SWE and Snowfall
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Automated measurement with snow pillow
Map of pillows in Bales et al. 2006
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Snow-pillow data for Dana Meadows (DAN) and
Tuolumne Meadows (TUM)
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Accumulation and ablation inferred from snow
pillow data, Tuolumne Meadows (TUM) and Dana
Meadows (DAN)
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Example forecast, May 2003

• Kings River below Pine Flat Reservoir, April-July
  unimpaired runoff (units are 1,000 acre-ft)

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Mesoscale/regional distribution

• Synoptic scale storms
• Elevation
• Topographic configuration

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Molecular structures of water and ice Ih
water
Ice Ih
(New York University, The MathMol Hypermedia
Textbookhttp//www.nyu.edu/pages/mathmol/textbook
/info_water.html)
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New snow
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Electron microscope photo of snowflake
From USDA snow microscopy
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Saturation vapor pressure
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Saturation vapor pressure
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Saturation vapor pressure (waterice)
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Rimed new snow
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Rime on chairlift
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Cant measure Sublimation or Melt, so calculate
via energy balance
RN Net radiation H Sensible heat exchange L
Latent heat exchange (energy used in
evaporation) G Heat flow into/out of pack,
direction ?T M Energy used in melting E
Sublimation L/?w(?v?f) Qm Melt M/(?w ?f)

• Dry snow
• M0
• Wet snow
• G0, because T is uniform

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Measurement of snowpack energy exchange (Sierra
Nevada)
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Snow is a collection of scattering grains
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Snow spectral reflectance and absorption
coefficient of ice
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Net solar radiation
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Snowmelt ? Runoff?

• Energy balance methods calculate surface melt
  rate accurately
• Still have to get it to the bottom of the pack
• Melt leads to local convergence of water near
  surface
• Depressions form because of enhanced metamorphism
  and settling
• Additional water flows into these depressions
• Development of flow fingers
• Process re-occurs in snowpack where impediments
  to flow exist (e.g., crusts)

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Rain-on-snow event, Independence Creek near
Truckee
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Rain on snow

• Melt source is condensation on the snow surface,
  not the rain itself
• Compare specific heat of water with latent heats
  of fusion and vaporization
• e.g., 1 kg water at 1C releases 4.2?103 J,
  enough to melt just 0.013 kg ice
• but, 1 kg of condensate releases 2.5 ?106 J,
  enough to melt 7.5 kg ice
• The storm, not the rain, melts the snow
• But the rain helps develop the flow fingers

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Snow metamorphism on the ground
Depth hoar
Typical snow pit
Wet snow
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Saturation vapor pressure
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Growth of grains by vapor diffusion growth of
bonds by grain boundary diffusion
cold
early
warm
late
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Electron microscope photo of a bond between snow
grains
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Molecular structures of water and ice Ih
water
Ice Ih
(New York University, The MathMol Hypermedia
Textbookhttp//www.nyu.edu/pages/mathmol/textbook
/info_water.html)
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Ionic pulse in laboratoryand field lysimeter
Concentration is highest at initial melt, and is
higher in background wetting front than in flow
fingers
(Harrington et al., Hydrol. Process., 1995)
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ESEM photo of impurities in snow
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A changing western snowpack?
Service, R. F., As the West goes dry, Science, 20
Feb 2004
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Dust and algae
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Spectral reflectance of dirty snow and snow with
red algae (Chlamydomonas nivalis)
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Radiative forcing concept
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ESM 236, spring break
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Web pages

• California Cooperative Snow Survey
• UCSB Snow Hydrology Research Group
• Mammoth Mountain research site (near real-time
  data)
• Mammoth Mountain Ski Patrol (data, pits, photos)
• California snowmelt runoff forecasts (Bulletin
  120)
• ESM 236 The Mountain Snowpack
• Snow crystal photos Patty Rasmussen Ken
  Libbrecht, Caltech
• National Snow and Ice Data Center University of
  Colorado