Jökull - 01.10.1998, Qupperneq 50

Table 7. Meltwater runoff from drainage basins in Vatnajökull in the summer of 1994 from May 1 to Sept. 30, Ag = glacier area,
ZQS = 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, ZQrs = runoff during summer in the river (June 1 to September 30), Qra = mean river discharge during the year.
-Afrennsli leysingarvatns frá jökli sumarið 1994frá byrjun júní til loka september, 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 byrjunjúní til loka september, Qra er meðalársrennsli jökulár.
River Ag £QS Qs qs Ar ^Qrs £QsÆQrs Summer Qra Qs/Qra Annual
km2 106mV mV 1 s'1 km2 km2 106mV % m3s‘' %
Tungnaá 141 353 11.2 79 1131 1446' 24 96.0' 12
Sylgja 75 84 2.7 36
Jökulsá á Fjöllum 1144 1379 43.7 38 1966 11812 117 72.02 61
Kreppa and Kverká 302 461 14.6 48 862 - - - -
Jökulá á Brú 1406 1959 62.1 44 2090 25733 77 103.F 60
') National Power Company, vhm. 096, Maríufossar.
2) National Energy Authority. Hydrological Service, 1994.
3) National Energy Authority. Hydrological Service, 1998b.
than one metre averaged over the whole glacier area
(Table 4). The equilibrium line altitude on Brúarjökull
was 70 m lower on the eastem than on the westem
part (1030 m and 1100 m, respectively, Fig. 6). No
measurements were done in the ablation area of Síðu-
jökull, but the fim line was close to 1000 m.
Runoff of water from the glaciers
The runoff from the glaciers to the various glacial
rivers, corresponding to the summer balance, is com-
pared with measured runoff at the gauges closest to
the glaciers. In Table 5 rain was a considerable com-
ponent of the runoff in Jökulsá á Brú. The high per-
centage of summer melt in Jökulsá á Fjöllum at Upp-
typpingar (84%) reflects the decrease in the precipita-
tion towards the west on the northem edge of Vatna-
jökull. Further, due to a large groundwater component
in Jökulsá á Fjöllum, fluctuations in ablation and pre-
cipitation from year to year have a smaller effect in
that river than in Jökulsá á Brú. The highest specific
discharge (qs) was found on Tungnaárjökull (Table 5).
III. THE MASS BALANCE 1993-1994
The mass balance was measured at 32 points on
five drainage basins (Table 6, Figs. 9 and 10), the
winter balance on May 2 to 10 and on June 19 to 21
(in Grírnsvötn) and the summer balance on August
30, and on September 9, 15 and 16, and October 10.
No mass balance measurements were carried out on
Síðujökull because of a surge of the glacier in the
spring of 1994. On Brúarjökull and Dyngjujökull, the
measuring program was limited to a single central
flowline on each glacier. The pattem of the mass bal-
ance observed in 1992-93 was used when drawing
maps of the mass balance on these glaciers.
The winter balance in the central part of Vatna-
jökull varied from 2.3 to 2.9 m. The winter balance at
the edge of the westem glaciers was about 1000 mm
higher than on the northern outlets as was also ob-
served in the previous year. The winter balance on
Köldukvíslarjökull increased with elevation by 100
mm/100 m, but four times faster on the northem out-
lets. The variation of the winter balance with eleva-
tion was quite irregular on Tungnaárjökull because of
snowdrift over the rough glacier surface at the begin-
ning of a surge. Winds were frequently from SW to W
and NW. The winter precipitation was above normal
in northern and northeastem Iceland.
The summer balance was negative on the entire
northern outlets but close to zero in a small area in the
uppermost part of Tungnaárjökull and the top of Köl-
48
JOKULL, No. 45, 1998