Jökull - 01.01.2013, Blaðsíða 74

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Figure 2. Water-year (1 Sept. – 31 Aug.) streamflow hydrographs for the period 1971–2000 normalized by
the annual mean discharge. Non-glacierized catchments (top) and partly-glacierized catchments (bottom). –
Vatnsrit yfir vatnsár (1. sept. – 31. ágúst) fyrir tímabilið 1971–2000, sem eru normaliseruð að ársmeðaltals
rennsli. Efri mynd: vatnasvið án jöklaáhrifa, neðri mynd: vatnasvið með jöklaáhrifum.
temperature-index melt model that relates air temper-
ature to snow and ice melt rates (e.g. Hock, 2003):
M = { fm(Td − Tm), Td > Tm
0 Td ≤ Tm
(1)
where M is daily melt, fm the melt factor for snow
or ice, Td is daily mean temperature and Tm is the
threshold temperature beyond which melt is assumed
to occur, set to 0 ◦C in this study. At the end of each
hydrological year, on the glaciers only, any remaining
snowpack exceeding 500 mm of accumulated SWE
was arbitrarily set to 500 mm, as in Jóhannesson et al.
(1993), and the excess was assumed to be transformed
into firn and then into ice. The melt factors for snow
and ice were taken from previous glaciological stud-
ies (Aðalgeirsdóttir et al., 2006; Jóhannesson et al.,
2007) in which temperature was extrapolated to the
glaciers from meteorological stations not located on
the glaciers, using different optimized lapse rates for
each glacier. The melt factors were determined from
summer and winter balance observations and assumed
constant throughout the year. For partly-glacierized
catchments, the melt factors from the glacier were
used. For non-glacierized catchments, the average
melt factor for snow from the different glaciers was
74 JÖKULL No. 63, 2013