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Supraglacial Meltwater Flow (Dye tracing (Fluorometer (i.e. intensity &…
Supraglacial Meltwater Flow
Meltwater routing in snow/firn
= Slow via percolation
Snow acts like porous aquifer / sponge
Delays delivery of water elsewhere
Initially, melt refreezes & warms snowpack
Vertical flux
= f ( permeabiliy & obstruction by ice layers )
Lateral flow at surface
--> occurs in water table within snowpack / firn
Where surface melt goes ?
Surface melt either:
Runs off edge
Enters glacier
Supraglacal meltwater flow
= meltwater routing @ glacier surface
surface conditions
= critical
snow cover vs exposed ice?
debris cover?
thermal regime?
strongly influences nature of glacial runoff
Melt flows through glacier via:
Upper:
Percolates through snowpack
Lower:
Crevasses / moulins
(more dynamic)
Summer surface conditions
Accumulation area
= snow + firn
Ablation area
= exposed ice & debris
Flow efficiency varies
between
different systems (supraglacial/englacial/subglacial)
Ice layer effects
Ice layers in snowpack
Impede water percolation through snowpack
Dye tracing
Fluorometer
i.e. intensity & wavelength
= device used to measure fluorescence
Inject dye at snow surface
Melt w dye percolates through snowpack
Water flows through basal saturated layer
Fluorometer downglacier measures
dye conc with over time
--> derive rate
Before melting can occur
Must warm snowpack to pressure melting point
Flow velocity through snowpack
Very slow percolation rates via Darcian flow
V= 0.13 - 0.49 cm / hr
Snowpack evolution
Through time, snowpack becomes more efficient at routing meltwater
Why ?
Snowpack thins
Snow evolves, becoming more efficient at transmitting water
Measuring meltwater flow
Dye / salt tracing
Flow meters
Meltwater routing over ice
= Fast
Meltwater flows via sheetflow to supraglacial streams
Water travels quickly in streams
∴ little potential for meltwater storage
Surface travel time = f (distance to moulins & crevasses)
Meltwater channels have
low roughness
thus facilitate v fast flow
Supraglacial streams
Fast flowing ~1 - 5 m/s
(faster than rivers)
Various scales
(cm to km)
High elevations
thus
high gradients & crevasses/moulins - infrequent