Mass Balance Determination using Photogrammetric Methods and Numerical Flow Modeling

 

A new indirect method for the determination of mass balance of glaciers has been developed. For this purpose, the kinematic boundary condition at the surface was used. This relation links the mass-balance rate at one point to the velocity at the surface, the surface elevation changes with time, and the surface slopes. Remotesensing methods combined with numerical flow modeling provide the required information with a high spatial resolution without resorting to in-situ field measurements.

In a detailed sensitivity study the new method was applied and tested on Unteraargletscher (Bernese Alps, Switzerland) for two consecutive periods 1996/97 and 1997/98. Digital elevation models (DEM) of the glacier surface and the horizontal velocity field at the surface were obtained by using modern photogrammetric methods. A
three-dimensional numerical flow model calculates the distribution of velocities, stresses, and strain rates.

Direct field measurements of the mass balance and surface velocities were performed for both periods to verify the indirectly evaluated results. A good agreement -- especially for the ablation area of Unteraargletscher -- was found. Some existing deviations are mainly due to inaccurately modeled vertical velocities. This was confirmed by an additional comparison with terrestrially surveyed stake velocities. A sensitivity analysis of the indirect method for the mass balance determination showed clearly that an accurate knowledge of the vertical velocity component at the surface is indispensable for a successful application.