Water Flow Through Snow

1. Water Flow Through Snow

2. 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

3. 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

4. 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

5. 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

6. 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 with Ponding Preferential Flow Paths Water Flow Ice Lens

7. 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

8. 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)

9. Water Flow Through Snow • Liquid Water Transmission Melt and rain water are lagged and attenuated as they move through the snow cover. Function of depth, density, ice layers, grain size, and refreezing. Rain

10. 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

11. 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

12. Wet snow

13. 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

14. 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

15. Dielectric Properties of Snow • Propagation and absorption of microwaves and radar in snow are a function of their dielectric constant • Instrumentation: Denoth Meter, Finnish Snow Fork, TDR

16. 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

17. NWT LYSIMETER ARRAY • 105 snow melt lysimeters • Area = 0.2 m2 • Height = 20 cm • Each drains into dedicated tipping bucket • Semivariograms

18. NWT LYSIMETER ARRAY

19. SNOW GUILLOTINE

20. 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

21. CROSSHOLE RADAR TOMOGRAPHY • Source and receptor antennas • Analogous to CAT scan at a hospital • 250 MHz

24. Fate of Snowmelt

25. Fate of Snowmelt • Depends on slope, snow, and soil conditions Surface Melt 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. Thawed Soil

26. Fate of Snowmelt • Depends on slope, snow, and soil conditions Surface Melt Snowmelt encountering frozen soil at the base of the snow pack, or other impediments to infiltration, may pond at the snow/soil interface. Ponding Frozen Soil

27. 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.

28. 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

29. 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)

30. 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 T0°C • generally work OK in forests, problematic above tree-line

31. Depletion Curves SWE (m) S C A (%) Accum. Degree Days (*C) Accum. Net Rad. (w/m2)

38. 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