Discharge Fluctuations

The discharge of meltwater from glaciers varies dramatically both on a diurnal (daily) and seasonal basis. Diurnal discharge variations reflect atmospheric air temperatures and therefore the pattern of daily ablation on a glacier. Discharge is usually low in the early morning and rises in the late afternoon or evening (Figure 4.10A). This diurnal fluctuation is suppressed in winter, but increases towards the late summer when the rate of daily ablation reaches its maximum. Seasonal fluctuations are equally dramatic (Figure 4.10B). They reflect two factors: (i) the seasonal nature of ablation; and (ii) the seasonal development of the internal drainage network within warm-based glaciers. We can develop a simple model for the drainage pattern for glaciers in a strongly seasonal climate.

1. Spring melt. On the glacier ablation of winter snow begins in the spring and as a consequence water pressures within the glacier begin to increase. In front of the glacier, ice in proglacial rivers breaks -up and melting of winter snow proceeds rapidly.

2. Late spring melt. Ablation of winter snowfall is well advanced on the glacier surface. Discharge in all channels, conduits and tunnels increases. The conduits grow in size and the internal drainage network within the glacier develops. The amount of water available within the glacier exceeds that which can be discharged by the internal drainage network. Discharge from the glacier into the proglacial channel system steadily increases.

3. Early summer. With the development of a well connected drainage network within the glacier much of the stored water is released. The daily discharge exceeds the amount of daily melt on the glacier surface and in the course of a few weeks the drainage system discharges the vast majority of its total annual discharge (nival flood). This increase in discharge may be associated with a sudden rise in glacier velocity, the so-called 'spring event' (Box 4.5).

4. Late summer. The drainage network within the glacier has reached its optimum efficiency. All the stored water within the glacier has been discharged. Daily discharge matches the amount of melt achieved each day. Water pressures within the glacier are usually at their minimum.

98 Glacier Hydrology A

Discharge (m3 s 1)

Late June

Early June

2 3 4 Days

Late June

Early June

2 3 4 Days

Break up

Nival flood Late summer

Actual runoff

Daily runoff (x10-3 m3) 500

Break up

Nival flood Late summer

Actual runoff

Daily runoff (x10-3 m3) 500

August

Sept

Figure 4.10 Schematic discharge fluctuations for alpine glacial meltwater streams. (A) Diurnal (daily) fluctuations, these become more pronounced as the melt season proceeds. (B) Seasonal fluctuations. The difference between estimated potential runoff and actual runoff reflects the efficiency of the subglacial drainage network. At the start of the melt season the channel and tunnel network is poorly developed and therefore actual runoff is less than potential (i.e. water is stored in the glacier). As the melt season proceeds, the drainage network grows in efficiency so that actual and potential runoffs become similar. [Modified from: Elliston, G.R (1973), Symposium on the hydrology of glaciers, International Association of Scientific Hydrology, 45, figure 1, p. 80]

August

Sept

Figure 4.10 Schematic discharge fluctuations for alpine glacial meltwater streams. (A) Diurnal (daily) fluctuations, these become more pronounced as the melt season proceeds. (B) Seasonal fluctuations. The difference between estimated potential runoff and actual runoff reflects the efficiency of the subglacial drainage network. At the start of the melt season the channel and tunnel network is poorly developed and therefore actual runoff is less than potential (i.e. water is stored in the glacier). As the melt season proceeds, the drainage network grows in efficiency so that actual and potential runoffs become similar. [Modified from: Elliston, G.R (1973), Symposium on the hydrology of glaciers, International Association of Scientific Hydrology, 45, figure 1, p. 80]

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