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