| Summary: | 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.
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