A key measure of the mass balance of a glacier is the equilibrium line altitude (ELA). Mass balance for non-calving glaciers is the difference between snow accumulation on a glacier and snow and ice loss from the glacier. The ELA is the point at which accumulation equals melting. On temperate alpine glaciers this is the snowline where snow transitions to bare glacier ice. Its elevation at the end of the summer marks the annual ELA. For a glacier to be in equilibrium at least 50-70% of the glacier must be in the accumulation zone still at the end of the summer. The range of percentages is determined by the specific regional climate and glacier geographic characteristics. A key observation for mass balance calculation is the shift of the ELA-snowline during the course of the melt season. The magnitude of the shift over a given time period is a measure of the melt rate. The shift can be observed in satellite imagery and when combined with field measurements quantifies melting. As the melt season begins the snow cover extent is large on a glacier. The key is how rapidly it rises during the melt season. On Taku Glacier for example in 2004 and 2006 a series of satellite images indicates the rise in the ELA. Below are images from May 26, 2006, then July 29, 2006 and then Sept. 15 2006. Indicating the rise of the snowline. These are followed by annotated images showing the rise of the ELA during the summer melt season in 2004 and 2006 on Taku Glacier.
On the ground the Juneau Icefield Research Program measures the snow depths and snow melt during July and August. The above images from May 26, July 29 and Sept. 15 2006 indicate the rise of the ELA during the course of the melt season, from 1200 to 2600 to 3200 feet. Snow depths at the the Sept. 15 ELA, where snowpack=0, was 2 m on July 22. Thus, we had 2 meters of snow melt at 3200 feet between July 22 and Sept. 15. In 2004 the snowline was at 2800 feet on July 15, snowpack was 1.6 meters at the eventual Sept. 1 snow line of 3400 feet. In 45 days 1.6 m of snow was lost at 3400 feet. In 2004 the melt rate was 0.036 meters per day and in 2006 0.038 meters per day. In 2009 July began with a low ELA of 500 m after a cool snowy winter and spring. By the end of the month the ELA was over 800 meters. This 300 meter rise and the associated melt was a record for July as were temperatures in Southeast Alaska in July 2009. As the 2010 melt season begins the Taku Glacier remains snowcovered, though the blue colors near the terminus indicate melt water is saturating the snowpack on 4/10/2010. What will the 2010 melt season bring? Check back here to see.