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*Present address: Department of Geology, Portland State University, Portland, Oregon 97207
In polar regions, slope angle can play a large role in glacial meltwater generation. Because of the low sun angle, steep surfaces, although they face the Sun for only limited spans of time, receive far more intense radiation than corresponding low-angle surfaces. A comparison of terminus and surface ablation stake data from the Canada Glacier, Taylor Valley, indicates that ablation on the terminus cliffs ranges from one to seven times the average surface ablation (Fountain, Lewis, and Dana, Antarctic Journal, in this issue). Consequently, although terminus cliffs represent a much smaller area than the glacier surface ablation zone, the cliffs contribute disproportionately to the meltwater runoff from the glacier. Because terminus cliff melt is far more heavily dependent on time of day than surface melt, we expect to see strong meltwater inputs to the streams at times when the terminus cliffs feeding the streams are facing directly into the Sun.
To assess quantitatively the contribution of meltwater from the ice cliffs on streamflow, we measured the local energy balance of the cliffs at the Canada Glacier terminus. This location was chosen because
In addition, ablation stake, meteorologic, and eddy correlation data are available for the glacier surface.
The terminus meteorologic station was located on the western side of the Canada Glacier roughly 500 meters north of the Lake Hoare shore ( figure 1). The instruments were mounted 1.3 meters from the terminus cliff on an iron bar frozen into the ice at the base of the cliff to measure characteristics associated with the cliff face. The shortwave radiation sensors were oriented horizontally into and away from the cliff face. Both the shortwave sensors and the temperature and relative humidity probe were mounted approximately 1 meter above the ground. Data were collected from 10 December 1995 to 22 January 1996.
The correlation between the air temperatures in excess of 0°C and stream discharge is shown in figure 2. When the terminus cliff air temperature is below freezing, the streamflow declines or shuts down. When temperatures are above freezing, the stream discharge and air temperature curves are positively correlated.
The air temperature on the glacier surface is generally cooler than at the terminus cliffs (Chinn 1987), which allows for a longer melt season on the terminus cliffs than on the glacier surface. In addition, though streamflow slows or shuts down when air temperatures drop below 0°C, melt on the terminus cliffs can occur at air temperatures well below 0°C during periods of large incoming radiation flux. Because the streambed is generally frozen when the air temperature is below 0°C, however, this meltwater refreezes before reaching the stream gauge.
Shortwave radiation receipt on the terminus cliffs differs significantly from that on the glacier surface. Unlike the surface, which receives nearly continual sunlight modified only by surrounding topography (i.e., shading by local mountains), the cliffs receive energy only for a short period of the day when the Sun faces the cliffs. During these periods, the shortwave energy flux can exceed 500 watts per square meter (Wm-2) for a span of a few hours. On average, the cliffs receive roughly 50 Wm-2. In contrast, surface values rarely exceed 300 Wm-2 but average roughly 150 Wm-2.
In general, for any point on the terminus cliff, we would expect to see a daily radiation peak. Instead, we measured two shorter peaks (one at 1500-1700 hours and another at 1900-2400 hours) due to the local topography, which blocks the Sun for a few hours during the middle of its passage across the cliff face. The bi-daily radiation peaks are generally accompanied by corresponding discharge peaks. The timing of the latter shows more variation than the former but in general occurs between 1630-2000 and 2100-0100 hours. Discharge peaks fail to appear during cloudy periods and periods when the air temperatures are well below 0°C. This 2-hour lag between peak radiation and peak melt is probably indicative of the time delay between increased melt and subsequent travel to the streambed.
Unlike on the glacier surface, where up to 70 percent of ice ablation is due to sublimation (Lewis et al. 1995), along most of the terminus cliff wind speeds are low, and thus the sensible and latent heat fluxes are small (Chinn 1987). Net radiation is the dominant source of incoming energy, and most of this energy goes to melt. Therefore, we can estimate the terminus meltwater contribution to Anderson Creek for the summers of 1994-1995 and 1995-1996 by assuming that the ablation recorded on the terminus ablation stakes is due to melt.
The terminus cliff bordering Anderson Creek is roughly 20 meters high by 2,000 meters long. If we assume the melt across this entire face is equal to the average of the ablation stake measurements, 16 centimeters (cm) for the 1995-1996 summer, the resulting flow is 6,400 cubic meters (m3). The recorded seasonal flow for Anderson Creek is 39,200 m3, implying 16 percent of the streamflow is due to terminus melt. Similar calculations for 1994-1995, using an average ablation of 15.5 cm and a total flow of 16,050 m3, indicate 39 percent of the streamflow during that summer came from terminus melt. The difference between terminus meltwater contribution and total flow must be accounted for through glacier surface melt, because no other significant source of available meltwater exists.
Clearly, the terminus cliffs cannot be neglected when calculating glacial water and mass balances. In particular, peak streamflow can be attributed almost entirely to terminus cliff melt.
This research was supported by National Science Foundation grant OPP 92-11773.
References
Chinn, T. 1987. Accelerated ablation at a glacier ice-cliff margin, dry valleys, Antarctica. Arctic and Alpine Research, 19(1), 71-80.
Fountain, A.G., K.J. Lewis, and G.D. Dana. 1996. McMurdo Dry Valleys LTER: Spatial variation of glacier mass balances in Taylor Valley, Antarctica. Antarctic Journal of the U.S., 31(2).
Lewis, K.J., G.D. Dana, A.G. Fountain, and S. Tyler. 1995. The surface-energy balance of the Canada Glacier, Taylor Valley. Antarctic Journal of the U.S., 30(5), 280-282.