Inter-decadal variability of degree-day factors on Vestari Hagafellsjökull (Langjökull, Iceland) and the importance of threshold air temperatures

The skill of degree-day glacier melt models is highly dependent on the choice of degree-dayfactor (������), which is often assumed to remain constant in time. Here we explore the validity of this assumption in a changing climate for two locations on Vestari Hagafellsjökull (1979-2012) using a Surface Energy Balance (SEB) approach that isolates the effect of changes in theprevailing weather on the ������. At lower-elevation, we observe stable ������ during the period 10 of study; however, at higher elevation, ������ is noted to be more variable and a statistically- significant downward trend is observed. This is found to result from an inappropriate threshold air temperature (���������� 12 ) from which to initiate the positive-degree-day sum, and is removed by setting ���������� to -1.83°C, rather than the usual value of 0°C used in degree-day melt models. The stationarity of ������ once ���������� is adjusted contradicts previous research and lends support to the use of constant ������ for projecting future glacier melt. Optimizing ���������� also improves the skill of melt simulations at our study sites. This research thus highlights the importance of ���������� for both melt model performance and the evaluation of ������ stationarity in a changing climate.