Reviewed research article Energy balance of Brúarjökull and circumstances leading to the August 2004 floods in the river Jökla, N-Vatnajökull Sverrir Guðmundsson 1 , Helgi Björnsson 1 , Finnur Pálsson 1 , and Hannes H. Haraldsson 2 1 Institute of Earth Sciences, University of Iceland 2 National Power Company of Iceland email: 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 1550 m 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 km 2 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 1