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Abstract

Iceberg calving is a key process of mass loss at lake-calving glaciers, but there is uncertainty about implementing this process into glacier evolution models. An eight-year timeseries of images from a fixed camera near the terminus of Haupapa/Tasman Glacier is used to derive timeseries of (a) terminus position, (b) ice velocity, (c) height of the terminal ice cliff, and (d) height above flotation. Our results show that the terminus retreated 968 ± 5 m between 2013 and 2021. Ice velocity at the moving terminus averaged 43 ± 2 m/a. Seven large calving events (each greater than 10⁶ m³) accounted for 46% of the retreat of the glacier, while the rest resulted from sub-aerial calving and terminus melt. Buoyancy-driven calving events are associated with terminus uplift of up to 5.2 m over 2-3 days before calving, but the timing of calving is not related to the height of the terminal ice cliff. Repeat bathymetry shows that a sub-aqueous ice-ramp intermittently forms in front of the terminus, increasing the buoyancy of the terminus. We hypothesise that a small height above flotation is a necessary, but not sufficient, condition for calving. Another factor, i.e. an ice ramp or basal crevassing, is needed to initiate calving. The height above flotation stays within a narrow range (0-9%), which is used as a terminus boundary condition for a glacier evolution model to examine how sensitive the future state of the glacier is to details of the terminus boundary condition, and its parameters.