Jökull - 01.01.2005, Síða 121

Reviewed research article
Energy balance of Brúarjökull and circumstances leading to
the August 2004 floods in the river Jökla, N-Vatnajökull
Sverrir Guðmundsson1, Helgi Björnsson1, Finnur Pálsson1, and Hannes H. Haraldsson2
1Institute of Earth Sciences, University of Iceland, Sturlugata 7, 101 Reykjavík
2 National Power Company of Iceland
sg@raunvis.hi.is
Abstract — We describe the energy balance and melting of the Brúarjökull outlet glacier of the Vatnajökull
ice cap and the exceptional circumstances leading to two extreme floods in August 2004 in the main river Jökla
draining the glacier. The energy balance was estimated using observations from three automatic weather sta-
tions and energy balance maps produced for the entire outlet glacier. Runoff calculated from the energy balance
data satisfactorily agreed with measured river discharge. The results show that the first flood was forced by
intensive rain and the second by exceptionally warm and sunny weather accompanied by unusually low glacier
surface albedo. The energy balance data were used to optimize and evaluate three different empirical models
that correlate glacial melting with air temperature measured 2 m above a non-glaciated surface, 20 km away
from the glacier front. The glacier’s peak-runoff was satisfactorily predicted with empirical models incorpo-
rating theoretically calculated clear-sky irradiance, but overestimated with a model that uses only estimated
degree-days on the glacier, scaled with two constants that differ for snow and firn/ice.
INTRODUCTION
Vatnajökull ice cap (Figure 1) is located close to the
maritime southeastern coast of Iceland. The north-
facing, gently sloping Brúarjökull (1550 km 2) is the
largest outlet of the ice cap, ranging in elevation from
600 to 1550m a.s.l with a mean equilibrium line close
to 1200 m a.s.l. (Björnsson et al., 1998). Jökla is
the main river draining 1250 km2 of Brúarjökull (Fig-
ure 1). Two exceptionally large flood events were ob-
served in the river during the periods August 3–6 and
9–14, 2004. By applying the river discharge model
AQUARIVER that uses air temperature and precip-
itation away from the glacier, Hólm and Sigurjóns-
son (2004) concluded that the first flood was caused
by intensive rain whereas the second was related to
high air temperature. In the present paper we discuss
the generation of these floods based on meteorologi-
cal measurements on the glacier. We present energy
budget calculations for the glacier during the entire
summer of 2004 and compare the result to the energy
budget since 1996. We also compare the correspond-
ing glacial melt in 2004 with the measured river dis-
charge. This provides a detailed description of the en-
ergy fluxes during production of the floodwater and
explains how the flood events are related to weather
parameters, the glacier winter balance and the surface
albedo. Finally, the energy budget calculations are
used to evaluate three distinct empirical ablation mod-
els based on a regression to air temperature measured
2 m above a non-glaciated surface at Eyjabakkar 655
m a.s.l., located ∼20 km away from the glacier front
(Figure 1).
OBSERVATIONS
River discharge and the August 2004 floods
Hourly values for the discharge of the river Jökla were
measured at Brú á Jökuldal, ∼40 km from the glacier
margin (Figure 1). The water draining towards the
discharge gauge is accumulated from 1250 km 2 of
JÖKULL No. 55 121