Jökull - 01.10.1998, Síða 53

Table 9. Meltwater runoff from drainage basins in Vatnajökull in the summer of 1995 from May 1 to Sept. 30, Ag = glacier area,
2QS = runoff during summer from the glacier, corresponding to the summer balance , Qs = mean summer discharge from the
glacier, corresponding to the summer balance, 4 = specific discharge, corresponding to the summer balance, Ar = area above the
discharge gage, XQrs = runoff during summer in the river (June 1 to September 30), Qra = mean river discharge during the year.
-Afrennsli leysingarvatns frá jökli sumarið 1995frá byrjun júní til loka september, Ag erflatarmál jökuls, IQS er heildarrennsli
vegna neikvœðrar sumarafkomu jökulsins, Qs er meðalrennsli sumars frá jöklinum vegna neikvœðrar sumarafkomu, qs er
afrennsli áflatar- og tímaeiningufrá jöklinum vegna neikvœðrar sumarafkomu, Ar erflatarmál vatnasviðs ofan við rennslismœli
á jökulá, IQrs er sumarrennsli í jökulá frá byrjun júní til loka september, Qra er meðalársrennsli jökulár.
River Ag £Qs Qs qs Ar ^Qrs SQsÆQrs Summer Qra Qs/Qra Annual
km2 106 m3 a 1 m3 s’1 1 s'1 km2 km2 106m3a'' % m3 s'1 %
Kaldakvísl 359 680 21.6 60 834 516‘ 142 27.1' 80
Jökulsá á Fjöllum 1144 1678 53.2 47 1966 15592 108
Kreppa and Kverká 302 592 18.8 62 862
Jökulá á Brú 1406 2548 80.8 57 2090 31952 80
') National Energy Authority. Hydrological Service, 1998b.
2) National Energy Authority. Hydrological Service, 1998a.
winter balance April 29 to May 5 and in the Gríms-
vötn area June 13. The summer balance was measured
September 26 to 28.
During this winter cyclonic activity frequently
caused northerly winds and the highest amounts of
snowfall during the winter were found in northem Ice-
land. This is reflected in higher values of winter bal-
ance at 900 m elevation on the terminus of Brúarjökull
(1000 mm) than on the westem termini (750 mm). The
winter balance gradient was about 140 mm/100 m on
the westem outlets, but three to four times that on the
northem outlets. The highest values of the winter bal-
ance (2.1 to 2.5 m) were observed on the central ice
cap, from the Grímsvötn area eastwards along the cen-
tral ice cap. On Örœfajökull (1800 m a. s. 1.) the win-
ter balance was 5.8 m water equivalent.
The summer balance was negative (-0.2 to -1.2 m)
on the highest parts of the accumulation areas of all
the northem outlets. The summer was relatively cold
in June and July but warmer in August compared with
the average of 1931-60 (Veðráttan, 1995). Anticy-
clonic circulation was more frequent than in the pre-
ceding three summers with frequent winds from south
to west, and relatively warm but most importantly
sunny weather. The specific summer balance was
similar at a given elevation in all the accumulation
areas but different in the ablation areas; lowest on
Köldukvíslarjökull, second on Dyngjujökull and
highest on Brúarjökull (Figs. 11 and 12). The most
negative summer balance was estimated on Dyngju-
jökull -6300 mm at 750 m elevation, -6100 mm at 600
m on Brúarjökull and -5400 mm at 900 m on Köldu-
kvíslarjökull.
The specific net balance was less than 2 m (1 to
1.8 m) in the central areas of Vatnajökull, for the first
time since the monitoring progam started in 1991/
1992. This was mainly due to high melting by solar
radiation during the summer. The net balance inter-
grated over the glacier area was negative on Köldu-
kvíslarjökull and on Brúarjökull, but almost zero or
slightly positive on Dyngjujökull due to its unusual
area distribution. However, 65% of the glacier area
was situated above the equilibrium line of 1310 m.
Runoff of water from the glaciers
The glacial summer balance runoff component to
both Kaldakvísl and Jökulsá á Fjöllum was greater
than the river runoff measured at the downstream
gauges, 142% and 108%, respectively. Both rivers run
through lava-fields wich contain groundwater storage
and glacial meltwater is discharged from springs
downstreams of the streamflow gauge at Upptypp-
ingar and Þveralda.
JOKULL, No. 45, 1998
51