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 (𝐷𝐷𝐹), 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 𝐷𝐷𝐹. At lower-elevation, we observe stable 𝐷𝐷𝐹 during the period 10 of study; however, at higher elevation, 𝐷𝐷𝐹 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 (𝑇𝑐𝑟𝑖𝑡 12 ) from which to initiate the positive-degree-day sum, and is removed by setting 𝑇𝑐𝑟𝑖𝑡 to -1.83°C, rather than the usual value of 0°C used in degree-day melt models. The stationarity of 𝐷𝐷𝐹 once 𝑇𝑐𝑟𝑖𝑡 is adjusted contradicts previous research and lends support to the use of constant 𝐷𝐷𝐹 for projecting future glacier melt. Optimizing 𝑇𝑐𝑟𝑖𝑡 also improves the skill of melt simulations at our study sites. This research thus highlights the importance of 𝑇𝑐𝑟𝑖𝑡 for both melt model performance and the evaluation of 𝐷𝐷𝐹 stationarity in a changing climate.