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INTRODUCTION
Mass-balance studies address the spatial and temporal changes in the ice mass of a glacier. Winter snowfall and summer temperature alter the amount of snow collected on the glacier and the amount of snow and ice lost by melt, thus changes in glacier mass are linked to changes in climate. Changes in the mass balance influence the dynamics of the glacier, which results in a change in the position of the glacier terminus (Paterson, 1994). Because of the relation between glacier geometry and climate, are glaciers seen as important indicators of climate change.
There are five commonly used methods for measuring the mass changes of the glaciers (Vaughan and others, 2013). Traditionally, the mass balance is observed using the glaciological method, i.e. the mass balance is derived from repeated stake measurements of snow and ice surface. The surface elevation changes are summed to give the total surface mass balance (SMB). In the glaciological method pointwise measurements are generalized to represent larger areas of the glacier, and estimates of the net mass balance (B n ) are deduced. However, local climate and glacial processes can create significant spatial differences in the SMB. Drifting snow, avalanching and shading by surrounding mountains can affect the accumulation and ablation patterns of a glacier, and thus localized observations can lead to biases in the estimate of mass balance (Barrand and others, 2010). The second method, often referred to as the geodetic method (Barrand and others, 2010), converts observed volume changes of the glacier to mass change. Surface elevation changes are obtained from repeated elevation profiles or DEMs. The geodetic mass balance derived from full coverage DEMs provides a better means to assess the spatial variations of the SMB. Nevertheless, the method is unable to reveal whether the local surface elevation changes are caused by accumulation, melt, or glacier flow redistributing the ice. The conversion from volume to mass is also prone to errors due to the limited information on snow and firn density on glaciers (Gardner and others, 2013). Since the successful launch of the Gravity Recovery and Climate Experiment (GRACE)-mission in 2003, mass changes of land ice can be estimated from satellite measurements of gravity...