Estimating local surface glacier mass balance from migration of the 1918 Katla eruption tephra layer on Sléttjökull, southern Iceland

We use the apparent horizontal shift of an englacial tephra layer outcrop to calculate local glacier mass balance on Sléttjökull, a lobe of Mýrdalsjökull in Southern Iceland. For this approach, the dipping angle of the englacial tephra layer in the glacier upstream of the outcrop and the flow velocity of the ice need to be known. An earlier investigation was expanded by the application of ground-penetrating radar, detecting the depth of the tephra along tracks with a total length of 10 km. Interpolation between the tracks enables us to derive the dipping angle of the layer along several flow lines. Together with glacier surface velocities, determined from feature tracking, we are able to estimate the local surface mass balance from the horizontal displacement of the tephra outcrop using freely available satellite imagery without additional fieldwork. The earlier local balance series was extended to the period 2014/15 to 2019/20. Although the results for the individual profiles differ slightly from each other, they show the same temporal pattern and clear variations from year to year. The results are compared to traditional mass-balance data from Hofsjökull. The two series show a good agreement in their interannual variability.