Jökull - 01.10.1998, Blaðsíða 56

Fig. 17. Mass balance for the westem and northem outlets of
Vatnajökull, 1991-1995. Specific mass balance in m water
equivalent; bw: winter balance, bs: summer balance, bn: annual
net balance. The mean values are weighted by the glacier area.
- Afkoma einstakra skriðjökla í vestan- og norðanverðum
Vatnajökli, 1991- 1995, vegin með flatarmáli þeirra. bw:
vetrarafkoma, bs: sumarafkoma, bn: ársafkoma.
western outlets but 1200-1300 m for the northwestem
and northem outlets. For a year with zero mass bal-
ance, the accumulation area was estimated to be 55-
65% of the total glacier area. The activity index, de-
fined as the gradient of the net balance with elevation
at the equilibrium line, was lowest for Dyngjujökull
and Köldukvíslarjökull, about 60 mm/100 m, but 75
mm/100 m for Brúarjökull and 90 mm/100 m for
Tungnaárjökull.
The mass balance data provide the first estimates of
the meltwater contribution to the various glacial rivers
systems. For a year of zero net balance, the specific
mnoff corresponding to the summer balance was about
60 1 s'1 km'2 averaged over the entire glacier and the
whole year, dropping down to 30 1 s*1 knr2 in the years
of the most positive glacier mass balance. Rain on the
glaciers during the five summer months may add 10-20
1 s'1 km'2 to the specific discharge from the glacier. This
glacial component, added to precipitation outside the
glacier (which is unknown), could be compared with
measured river mnoff at streamflow gauges closest to
the glacier. Only in the rivers draining Brúarjökull,
however, does the river gauge measure allt the glacial
melt. In two river systems, Jökulsá á Fjöllum (in 1994
and 1995) and Kaldakvísl (in 1995), the glacial sum-
mer balance runoff component (not including precipi-
tation) was greater than the river runoff measured at the
downstream gauges. This demonstrates that glacial
meltwater is discharged in groundwater past the
streamflow gauges. In other years (1992 and 1993) the
meltwater component during the summer was 70% and
85% of the river mnoff during the summer which may
reflect variations in precipitation from one summer to
another. Looking at the whole year of 1993 the glacial
component in both Jökulsá á Fjöllum and Jökulsá á Brú
was only about 30% but about 60% in 1994. This indi-
cates that 1994 was a drier year north of Vatnajökull
-0.5
-1.0
%
C0
n
-1.5
-2.0
1993 •
1992 •
1994 •
1995 •
7.0 8.0
summer temperature (°C)
Fig. 18. Mean specific balance on Vatnajökull compared
with mean values at meteorological stations (Fagurhólsmýri,
Hólar, Teigarhorn and Brú). bw: winter balance compared
with precipitation from September-May and bs: summer bal-
ance compared with temperature from May-September.
- Arsafkoma á Vatnajökli borin saman við veðurþœtti á
fjórum veðurstöðvum (Fagurhólsmýri, Hólum, Teigarhorni
og Brú). Vetrarafkoma (bw) borin saman við úrkomu frá
septemberbyrjun til maíloka. Sumarafkoma (bs) borin saman
við meðalhita frá maíbyrjun til loka september.
54
JÖKULL, No. 45,1998