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Bulk Discharge Variability (S. Cascade Glacier Hydrograph Evo (Diurnal Q…
Bulk Discharge Variability
S. Cascade Glacier Hydrograph Evo
Diurnal Q evoluton through melt season
A.
Discharge increased
B.
Diurnal variation increased
(amplitude)
C.
Lag time decreased
(Hodge, 1975)
Form & Evolution
Seasonal pattern driven by ablation
Winter
- very
low
Q
Small amount percolation from basal melt
Summer
- briefly
high
Q
During short melt season
Driver Bulk Q Pattern
Diurnal (daily) pattern driven by:
Surface energy inputs
notably insolation - clearest cycle on sunny days
Surface conditions
(snow/ice?)
more snow --> longer lag time
Glacial drainage system evolution
more efficient system --> shorter lag time
Diurnal cycle
reflects:
Slowly varying baseflow
[snowmelt]
snowmelt
- slow delivery by percolation
distributed drainage
- delivers melt away from channels
Rapidly routed diurnal peaks
[ice melt]
Surface ice melt
Rapid transport
over surface / via channels
Diurnal variation Vernagtfermer 1992
Snowline retreat as melt season progresses
∴ Reduced retention capacity
Result:
Reduced delay (lag time)
Faster runoff generation
Hydrograph expression
Higher amplitude
Higher Q values
Timing Peak Q
Peak Q occurs earlier in day
later in melt season
Decreasing lag btwn peak melt & Q as season progresses
Bulk Q exhibits
Diurnal cycle
Seasonal cycle
(broader & superimpsed atop)
Leverett Glacier
Late winter
Tiny bit basal melt - friction
Little discharge
rivers frozen
surface T ~ -15°C
Mid summer
Remove snowpack,
darker ∴ lower albedo
Snowpack melts faster
Delivery faster
Late summer
Falling discharge
(1/5 or 1/10 peak summer Q)
Response glacial Q to warming
Bulk Q ~ mass balance proxy
High melt Q
--> lotsa ice/snow loss
-> Negative mass balance
Less melt
--> Low mass loss
--> Possible positive MB
Vernagtferner, Alps
Increasingly negative m.b.1980s onward
Reflected by
increasing summer runoff
Discharge differs btwn +ve & -ve mb years
Whilst global warming causing
lotsa melt discharge
Eventually
will deplete glacier store of ice
∴ Melt discharge will decrease
back to similar level as positive mass balance years