Jökull - 01.10.1998, Blaðsíða 44

Table. 3. Meltwater runoff from drainage basins in Vatnajökull in the summer of 1992, from June 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, qs = specific discharge, corresponding to the summer balance, Ar = area
above the discharge gage, 2Qrs = runoff during summer in the river (June 1 to September 30), Qra = mean river discharge dur-
ing the year.
- Afrennsli leysingarvatns frá jökli sumarið 1992 frá byrjun júní til loka september, Ag er flatarmál jökuls, SQs 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 2QS Qs qs Ar ZQrs Summer Qra Qs/Qra Annual
km2 106 m3 a_l m3 s4 1 s'1 kmJ km2 106m3 a_l % m3 s'1 %
Tungnaá 141 290 9.2 65 1131 14381 20' 87.71 10.5
Kaldakvísl 359 c. 330 c. 10.5 c. 29 834 4682 712
Jökulsá á Fjöllum 1144 c. 1150 c. 36.5 c. 32 1966
‘) National Power Company, vhm 096, Maríufossar. :) National Energy Authority. Hydrological Service, 1998b.
was infrequent. The winter precipitation in Iceland
was above normal except in the northeastern and east-
em part of the country (Veðráttan, 1992).
The summer was cold and cloudy, cyclonic activi-
ty was high and winds from south to west were fre-
quent (Veðráttan, 1992). The summer balance was
positive over a large area in the accumulation area of
Dyngjujökull (by 600 nim at 1900 m elevation and up
to a maximum of 1400 mm at 1500 m) where the
glacier gained mass during the summer due to cold
spells and considerable snowfall in mid-June and late
August. This snowfall increased the albedo and de-
creased ablation. The summer temperature was 0.8 °C
below the average of 1931-60 (Veðráttan, 1992). The
highest positive net balance observed in a single point
was at 1600 m elevation in the uppermost part of
Síðujökull, 5500 mm. This may have been a local
anomaly caused by accumulation by snow drift dur-
ing the summer to the southern slopes of Háabunga.
In the ablation area of Tungnaárjökull, the gradient of
the summer balance with elevation was 800 mm/100
m, but only one quarter of that in the accumulation
area. This reflects the difference between the albedo
of the accumulation and ablation areas, normally pre-
sent, which increased during the summer 1992 due to
new snow falling on the accumulation area. The min-
imum observed value of the summer balance was -
5400 mm at 700 m elevation on Tungnaárjökull.
The specific net balance in the central parts of the
ice cap was from 2000 to 4000 mm. Intergrated over
the drainage area, the net balance of the four outlets
was positive. The equilibrium line was at 1050 m a. s. 1.
on Síðujökull. It rose to 1125 m on Tungnaárjökull
and to 1200 m on Köldukvíslarjökull and Dyngju-
jökull. On the first three glaciers the accumulation
area comprised about 65% of the total area but 75%
on Dyngjujökull. This percentage is about 60% in a
year of zero net mass balance (see discussion later).
Runoff of water from the glaciers
The summer balance of the water drainage basins
gave estimates of meltwater that drained to the glacial
rivers (Table 3). On Tungnaárjökull, the water drainage
basin had a winter balance of 200* 106 m3, a summer
balance of -290* 10f’ m3 and an annual net balance of -
90«106 m3 (Table 3). In contrast, the net balance was
positive by 74» 106 m3 in the ice drainage basin. Equal-
ly distributed over a whole year, the summer balance
discharged 9.2 m3 s4. Adding summer rain and snow-
fall melted during the summer (not measured) to the
290* 106 m3 of negative summer balance, the total annu-
al runoff from Tungnaárjökull may have been 350-
42
JOKULL, No. 45, 1998