Modelling mass balance using photogrammetric and geophysical data: a pilotstudy at Griesgletscher, Swiss Alps

The kinematic boundary condition at the glacier surface can be used to give glacier mass balance at a point as a function of changes in the surface el evation, and of the horizontal and vertical velocities. Vertical velocity c an in turn be estimated from basal slope, basal ice velocity and surface st rain. In a pilot study on the tongue of Griesgletscher, Swiss Alps, the app licability of the relation for modelling area-wide ice flow and mass-balanc e distribution is tested. The key input of the calculations, i.e. the area- wide surface velocity field, is obtained using a newly developed photogramm etric technique. Ice thickness is derived from radar-echo soundings. Error estimates and comparisons with stake measurements show an average accuracy of approximately +/-0.3 m a for the calculated vertical ice velocity at the surface and +/-0.7 m a(-1) for the calculated mass balance. Due to photogr ammetric restrictions and model-inherent sensitivities the applied model ap peared to be most suitable for determining area-wide mass balance and ice f low on flat-lying ablation areas, but is so far not very well suited for st eep ablation areas and most parts of accumulation areas. Nevertheless, the study on Griesgletscher opens a new and promising perspective for the monit oring of spatial and temporal glacier mass-balance variations.