Mass balance is measured by determining the amount of snow accumulated during winter, and that is remaining at the end of the melt season, and measuring the amount of snow and ice removed by melting in the summer. The difference between these two parameters is the mass balance. If the amount of snow accumulated during the winter is larger than the amount of melted snow and ice during the summer, the mass balance is positive and the glacier has increased in volume. On the other hand, if the melting of snow and ice during the summer is larger than the supply of snow in the winter, the mass balance is negative and the glacier volume decreases. Mass balance is reported in meters of water equivalent. This represents the average thickness gained (positive balance) or lost (negative balance) from the glacier during that particular year. A typical glacier that is not calving must have 60-70% of its area snowcovered at the end of the summer to be in balance.
Ablation is measured by emplacing stakes in the glacier at the end of the previous melt season or the beginning of the melt season. As the glacier surface melts the amount of the stake emerging from the glacier is measured. The total melt at each stake by the end of the melt season is the net ablation. Most of the stakes must be reemplaced during the summer.
Accumulation is measured by either probing or crevasse stratigraphy to determine the annual snowpack thickness at many locations. Crevasse layering is evident in the picture at right. It is similar to reading tree ring width for climate analysis.
These measurements are completed both in August and again in late September, the end of the hydrologic year, each summer on 10 glaciers in the North Cascades by NCGCP. Measurements of ablation are made at 3-6 locations and accumulation at 60-200 locations. Mean annual balance has been -0.54 m/a on North Cascade glaciers. The impact of the number of measurements is examined in The Impact of Sampling Density on Mass Balance. The changes in accumulation and ablation with location and during recent years indicates the importance of monitoring multiple glaciers as they are unique. Crevasse stratigraphy is used to determine annual snow layer thickness. In the slide at right you can see four annual layers. A thin surface layer, and than two approximately one meter thick layers, and then a partially shown lowest layer that is much dirtier. The continuity of the layer thickness provides a better measure than a point measure in a snowpit or with probing. We utilize probing where crevasses do not exist, above right. Snowpits such as seen at lower left are not used in the North Cascades since the density of the snowpack by August has been found to be constant. Snowpits provide a point measurement and are time consuming.
A map of the glacier is then prepared contoured for the observed gain or loss of snow or ice in water equivalent units.
Probing snowpack on Easton Glacier August 2005
Crevasse Stratigraphy on Easton Glacier illustrating a horizontal annual layer.
Measuring snow depth in crevasse on Rainbow Glacier.
Snowpit on Taku Glacier