Water Flow Through Snow
LEAST UNDERSTOOD ASPECT OF SNOW HYDROLOGY
• Timing and magnitude of snow melt runoff
• Biogeochemical processes
• Geomorphological processes
• Wider application of operational snow melt models to more sites
• Account for year-to-year variability
• Glacial hydrology
Water Flow through Snow• Wide Range of Flow Velocities
– 2 - 60 cm/min– Depends on several factors
• internal snow pack structure
• condition of the snow pack prior to introduction of water
• amount of water available at the snow surface
Water Flow Through Snow• Flow through Homogeneous
Snow– At melting temperature, a thin
film of water surrounds each snow grain
• Much of the water can flow through this film
– Once pores are filled, laminar flow can occur
• Very efficient mechanism for draining the snow pack
Water Flow through Snow• Four Liquid Water Regimes
• Capillary: < 1% free water– water doesn’t drain due to capillary tension
• Unsaturated: 1-14% free water– water drains by gravity, but air spaces are continuous
– Pendular Regime
• Saturated: > 14% free water– water drains by gravity, but air spaces are discontinuous
– Funicular Regime
• Melt/Freeze– water melts and refreezes, possible several times, before it
drains from the snow pack
Water Flow Through Snow• Flow through
Heterogeneous Snow– Preferential Flow Paths
• Dye studies reveal vertical channels or macropores in most natural snowpacks
– Ice Layers• Develop from surface melt or
refreezing
• Relatively impermeable
• Forces ponding of water and lateral flow
Ice Lens
Water Flow
Ice Lenswith Ponding
Preferential Flow Paths
MELTWATER MOVES IN PREFERENTIAL CHANNELS
• Once melt occurs at the snow surface, we lose track of it as liquid water infiltrates into the snowpack
• Preferential flow common, but don’t understand from first principles
Water flow through snow
• 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)
Water Flow Through Snow• Liquid Water Transmission
Melt and rain water arelagged and attenuated as they move through the snow cover.
Function of depth, density, ice layers, grain size, and refreezing.
122 123 124 125 126 127 128 129 1300
2
4
6
130 131 132 133 134 135 136 137 1380
2
4
6
138 139 140 141 142 143 144 145 1460
2
4
6
146 147 148 149 150 151 152 153 1540
2
4
6
Snow Melt at SurfaceOutflow from Base
Niwot Ridge, ColoradoMay 2-30, 1995
Day of Year
Rain
Snow Metamorphism• Wet Snow Metamorphism:
• Liquid water in the snow pack
• Acts like supercharged Dry ET metamorphism– rates are accelerated
– small grains are destroyed preferentially
– large grains become rounded (equilibrium forms)
• Melting and refreezing results in large, bonded grain clusters
Wet snow metamorphism
• Low water content – Water is held by capillary tension in the
crevices in grain clusters– Melt-freeze cycles strengthen the clusters
• High water content (slush)– Continuous connection between water
inclusions, i.e., the water surrounds the grains– Rapid grain growth– Lack of intergranular bonding
Wet snow
FROZEN ICE COLUMNS
• When liquid water infiltrates into a cold snow pack, we get frozen ice columns
• Ice columns the residual networks of preferential flowpaths
Tad Pfeffer photo, Greenland
Tad Pfeffer photo, Greenland
SUFFER FROM LACK OF APPROPRIATE TOOLS
• Sparse and invasive sampling
• Difficult to study time and space-dependent processes such as evolution of preferential flowpaths
R Kattelmann photo
R Kattelmann photo
Dielectric Properties of Snow
Material Dielectric Constant
Air 1.0
Ice 3.2
Quartz 4.3
Water 80
• Propagation and absorption of microwaves and radar in snow are a function of their dielectric constant
• Instrumentation: Denoth Meter, Finnish Snow Fork, TDR
SPATIAL PATTERNS
• An outstanding problem is whether there is any spatial pattern to meltwater flow through snow
• Frozen ice columns exhumed through sublimation
R Kattelmann photo
R Kattelmann photo
NWT LYSIMETER ARRAY
• 105 snow melt lysimeters
• Area = 0.2 m2
• Height = 20 cm• Each drains into
dedicated tipping bucket
• Semivariograms
NWT LYSIMETER ARRAY
0
100000
200000
300000
400000
500000
600000
700000
0 2 4 6 8 10
separation distance [m]
sem
ivar
iog
ram
[ti
ps^
2]
Day 150 (5/30/98)
Day 175 (6/24/98)
SNOW GUILLOTINE
SNOW GUILLOTINE
• Dye tracer experiment• Precise cutting
instrument (100 cm x 100 cm x 2 cm)
• Digital Camera• 3-D data cube• Indicator variograms
and connectivity stats
CROSSHOLE RADAR TOMOGRAPHY
• Source and receptor antennas
• Analogous to CAT scan at a hospital
• 250 MHz
Fate of Snowmelt
Fate of Snowmelt
• Depends on slope, snow, and soil conditions
Snowmelt encountering thawed, permeable soil at the base of the snow pack, at a rate less than the infiltration rate, will enter the soil.
Snowmelt in this case behaves much like rainfall would.
Surface Melt
Thawed Soil
Fate of Snowmelt
• Depends on slope, snow, and soil conditions
Snowmelt encountering frozensoil at the base of the snow pack, or other impediments to infiltration, may pond at the snow/soil interface.
Surface Melt
Frozen Soil
Ponding
Fate of Snowmelt
• Basal Ice Development
On shallow slopes, ponded meltwater may refreeze at the base of the pack, forming ice layers that may impede further meltwater infiltration.
Fate of Snowmelt
• Subnivean Flow on a Slope
Lateral flow of basal ponded water may develop, depending on slope. If snow is still present, lateral flow is still through a porous medium. Presence of liquid water in base of snow pack causes rapid destruction of small snow grains, leaving larger grains, and allowing more rapid flow.
Surface Melt
Thickening of Basal Flow Layer
Snow Modeling
• Point Models– Degree Day Methods– Semi-Physical Methods (e.g. SNOW-17)
• Distributed Models– Physically Based– Gridded or Polygon Discretization– Assimilation Systems (e.g. SNODAS)
Snowmelt Runoff?
• Energy balance methods calculate surface melt rate accurately
• In absence of data, many forecasting schemes use degree-day methods– accumulated melt = T “factor” when
T0°C– generally work OK in forests, problematic
above tree-line
00.20.40.60.8
11.2
April May June
S
C
A
(%)
Accum. Net Rad. (w/m2)Accum. Degree Days (*C)
SW
E (
m)
Depletion Curves
Degree Day
0
20
40
60
80
100
0 100 200 300 400 500
2839 - 2947m
3055 - 3163m
3272 - 3380m
Net Radiation
0 2000 4000 6000 8000 10000
2839 - 2947m3055 - 3163m3272 - 3380m
POINT-SNOW MELT MODELS
• Precipitation submodel for storage and distribution of SWE
• Energy balance model at the snow surface
• Snowpack model: liquid water retention, refreezing, diurnal temperature changes
• Snow depletion model; areal depletion curve
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