Submergence Velocity Measurements at Victoria Lower and Evans Piedmont Glacier

During the 1999/2000 season three submergence velocity devices (Hamilton & Whillans, 2000) for mass balance measurements in the McMurdo Dry Valleys were installed. During the 2004/2005 season two submergence velocity devices have also been installed at EPG. This method is used to determine mass balance by comparing vertical velocity of a marker in firn or ice with long-term, average snow accumulation rates. The movement of the marker is the result of three motions: firn compaction, gravitational glacial flow, and changes in mass balance.

High precision GPS measurements are used to determine absolute position of the tracking point during subsequent years. Trimble 5700 base station and rover unit were used to measure the absolute position of the tracking point of the mass balance devices. At Victoria Lower Glacier, the base station was deployed on a rocky platform at Staeffler Ridge <3km away from all mass balance sites. The proximity of the base station to the rover allowed the tracking points to be measured with a horizontal precision of <1mm and a vertical precision of <5mm. At Evans Piedmont Glacier base station data from the Cape Roberts permanent GPS/GLONASS and tide gauge observatory will be used. All GPS measurements are post-processed using precise orbits, which are published on-line at "http://igscb.jpl.nasa.gov/components/prods_cb.html". These data are corrected using GIPSY-OASIS II software and provide precise point positions by taking into account satellite orbit, Earth orientation, and clock solution from NASA Jet Propulsion Laboratory's independent analysis of globally distributed GPS receivers.

Fig. 13 Cartoon of the 'coffee can' submergence mass balance device (modified after Hamilton and Whillans 2000)and picture of coffee can device deployed at Victoria Lower Glacier

The rate of thickness change H, can then be calculated using (Hamilton et al., 1998):

where: H =rate of thickness change (myr⁻¹) b_m=accumulation rate (Mgm⁻²yr⁻¹) ρ =density at marker depth to account for densification processes (Mgm⁻³) z =vertical component of ice velocity (upward is positive, myr⁻¹) α =surface slope (radians) u =horizontal velocity (myr⁻¹ with azimuth)