Glaciological Measurementdoc Notes
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How would you go about measuring the net mass balance of a 10,000 km2 Arctic ice cap which terminates in the sea?
10,000 km2 is at the very top of the size range of Arctic ice caps. Detailed measurement programs of mass balance are relatively rare for Arctic glaciers, and those that are available mostly relate to smaller glaciers (around 10km 2) terminating on land, and not to larger ice caps above 100km2 (Dowdeswell & Hagen, 2004). Observation of such an ice cap would therefore be quite scientifically valuable. Precedents include the Hans Tausen Ice Cap in Greenland (4000 km2) and the Austfonna Ice Cap (8000 km2) in Svalbard (Dowdeswell & Hagen, 2004).
The equation for calculating total mass balance is V / a = Ma Mm Mc ± Mb (Hagen & Reeh, 2004). There are two main approaches for measuring the mass balance of ice masses, which quantify the left and right sides of this equation respectively: (1) the cartographic method, directly measuring the change in volume by monitoring the changes in surface elevation through remote sensing or (2) the direct glaciological method, seperately measuring each element of accumulation and ablation through local measurements (Reeh, 2006; Hagen & Reeh, 2004).
The direct glaciological method is more resource-intensive than the cartographic method, but is potentially more accurate (Hagen & Reeh, 2004). Since a 10,000 km 2 ice sheet is sufficiently small for direct glaciological methods to be feasible but big enough that remote sensing may be preferable, both methods will be considered here.
Cartographic method The basis of the cartographic method is that the change in volume can be estimated by comparing the topographic changes in an ice mass between different years. Topographic changes can be measured using photography, airborne laser altimetry, or satellite radar altimetry (Hagen & Reeh, 2004), however the accuracy of current satellite radar altimetry is not sufficient for an ice cap of this size, so air-based methods would be required (Dowdeswell & Hagen, 2004). Of the two methods, aerial photography has been more commonly used in the past, but laser altimetry is far more accurate (down to ± 10 cm compared with ± 1-2 m for photography) (Bamber &
Kwok, 2004). Either could potentially be used here.
Hagen & Reeh (2004) describe the process of measuring mass balance using this method. An aerial survey using photography or laser altimetry is made at the same point late in the ablation cycle in different years, and the data is used to create digital elevation models (DEMs) showing the change in topography over the period, which can be converted to the volume change in water equivalent to yield the net mass balance figure.
Cartographic estimation of mass balance based on remote sensing is often used
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