Estimating surface mass balance patterns from UAV measurements on the ablation area of the Morteratsch-Pers glacier complex (Switzerland)

Abstract. The surface mass balance of a glacier (SMB) provides the link between the glacier and the local climate. For this reason, it is intensively studied and monitored. However, major efforts are required to determine the SMB on a sufficient number of locations to capture the heterogeneity of the SMB pattern. Furthermore, because of the time-consuming and costly nature of these measurements, detailed SMB measurements are carried out on only a limited number of glaciers. In this study, we investigate how to accurately determine the SMB in the ablation zone of Vadret da Morteratsch and Vadret Pers (Engadin, Switzerland) using the continuity-equation method. For this, an elaborate dataset (spanning the 2017-2020 period) of high-resolution data derived from UAV measurements (surface elevation changes and surface velocities) is combined with reconstructed ice thickness fields (based on radar measurements). To determine the performance of the method, we compare modelled SMB with measured SMB values at the position of stakes. Our results indicate that with annual UAV surveys, it is possible to obtain SMB estimates with a mean absolute error of approximately 0.5 metre ice equivalent per year. Yet, our study demonstrates that in order to obtain these accuracies, it is necessary to consider the ice flow over spatial scales of several times the local ice thickness using an exponential decay filter. Furthermore, our study shows the crucial importance of the ice thickness, which must be sufficiently well known in order to apply the method. The latter currently hampers the application of the continuity-equation method to derive detailed SMB patterns on regional to global scales.