Jökull - 31.12.2001, Page 78

Helgi Björnson et al.
Figure 1. Typical location of mass balance stakes on Vatnajökull and Langjökull. Ice divides, separating vari-
ous outlet glaciers, are drawn on Vatnajökull. - Lega mœlistika við afkomumœlingar á Vatnajökli og Langjökli.
Línur á Vatnajökulskorti afmarka skriðjökla.
zero in 1994-1995 but since then it has been negative.
The winter balance was highest during the first years
of the decade, reached a minimum in 1996-97 and
has since then been slowly increasing. The summers
were colder during the first half of the decade, which
is reflected in the summer ablation. The high ablati-
on during the summer of 1997, however, was too a
large extent caused by low average albedo following
the exposure of the tephra layer from the Gjálp erupti-
on in October 1996. Moreover, dust from Skeiðar-
ársandur, originating from the jökulhaup deposits in
November 1996, also spread over large parts of the
ice cap. In contrast the high summer melting in 2000
was primarily attributable to warm and windy wea-
ther. The total mass loss 1991-2001 has been 2.37 m
(water equivalent) or 19.4 km3 (which amounts to 1.5
times the average winter balance of the ice cap), i. e.
the ice cap lost about 0.6% of its total ice mass. In
addition to this surface melting the volcanic eruption
in October 1996 melted 4.0 km3 of ice.
In years of zero net balance, the equilibrium line
on Vatnajökull lies at about 1 lOOm on the southern
outlet glaciers, at 1200-1300 m on the northwestern
and northern outlet glaciers. The accumulation area is
thus typically about 60% of the total glacier area. Dur-
ing 1991-2001 the accumulation area varied from 20-
70% of the total area of the outlet glaciers, the equili-
brium line altitude fluctuated by 200-300 m and the
annual net balance from plus to minus one metre. A
lOOm rise in the equilibrium line would reduce the
net balance by about 0.75 m per year.
The mass balance on Langjökull shows simil-
ar variations as on Vatnajökull. However, Langjök-
ull is smaller and lower than Vatnajökull. Snow
76 JÖKULL No. 51