Distributed mass-balance and climate sensitivity modelling of Engabreen, Norway Thomas V. SCHULER, 1,4 Regine HOCK, 1 Miriam JACKSON, 2 Hallgeir ELVEHØY, 2 Matthias BRAUN, 3 Ian BROWN, 1 Jon-Ove HAGEN 4 1 Department of Physical Geography and Quaternary Geology, Stockholm University, SE-106 91 Stockholm, Sweden E-mail: t.v.schuler@geo.uio.no 2 Norwegian Water Resources and Energy Directorate (NVE), PO Box 5091, Majorstua, NO-0131 Oslo, Norway 3 Zentrum fu ¨r Fernerkundung der Landoberfla ¨che, University Bonn, Walter-Flex-Strasse 3, D-53113 Bonn, Germany 4 Department of Geosciences, University of Oslo, PO Box 1047, Blindern, NO-0316 Oslo, Norway ABSTRACT. Assessing the impact of possible climate change on the water resources of glacierized areas requires a reliable model of the climate–glacier-mass-balance relationship. In this study, we simulate the mass-balance evolution of Engabreen, Norway, using a simple mass-balance model based on daily temperature and precipitation data from a nearby climate station. Ablation is calculated using a distributed temperature-index method including potential direct solar radiation, while accumulation is distributed linearly with elevation. The model was run for the period 1974/75–2001/02, for which annual mass-balance measurements and meteorological data are available. Parameter values were determined by a multi-criteria validation including point measurements of mass balance, mass-balance gradients and specific mass balance. The modelled results fit the observed mass balance well. Simple sensitivity experiments indicate a high sensitivity of the mass balance to temperature changes, as expected for maritime glaciers. The results suggest, further, that the mass balance of Engabreen is more sensitive to warming during summer than during winter, while precipitation changes affect almost exclusively the winter balance. INTRODUCTION Glacier mass-balance changes are important for global sea- level changes (e.g. Arendt and others, 2002), as well as being significant on a local and regional scale for many aspects of water resource management including flood protection, water supply and the operation of hydroelectric facilities (Hock and others, 2004). To more thoroughly understand the consequences of global climate change, it is necessary to predict the response of glaciers to such changes. A common procedure is to calibrate a mass-balance model using historic climate and mass-balance data, and then to run the model with a perturbated climate using either hypothetical (e.g. Braithwaite and Zhang, 1999) or model-predicted climate changes (e.g. Schneeberger and others, 2003). The purpose of our study is to use the long-term mass- balance measurements available at Engabreen, a maritime glacier in northern Norway that is exploited for hydropower, to calibrate a distributed temperature-index-based mass- balance model and to use the model to assess the static mass-balance sensitivity to hypothetical temperature and precipitation changes. STUDY SITE Engabreen (66840’ N, 13845’ E) is an outlet glacier from the western Svartisen ice cap in a maritime climate in northern Norway (Fig. 1). Its elevation ranges from 1590 m to just a few m a.s.l. and it covers an area of 38 km 2 , of which 86% is located on a plateau above 1100 m a.s.l. From there, the ice flow is channelled into a narrow valley. On this steep descent, the ice ruptures and the glacier tongue is heavily crevassed. Front position changes have been monitored annually by the Norwegian Water Resources and Energy Directorate (NVE) since 1970, but measurements exist back to 1903. Before 1930, the glacier terminus covered the present proglacial lake, and after a rapid retreat of almost 2 km during the 1940s the tongue position became approxi- mately stable. During the 1990s the terminus of Engabreen advanced by roughly 200 m, but it has recently started retreating again (e.g. Kjøllmoen, 2003). INPUT DATA Meteorological data The nearest long-term meteorological data series is recorded at Glomfjord (39 m a.s.l.) roughly 20 km north of Engabreen. The station has been operated by the Norwegian Meteoro- logical Institute since 1912, and a continuous time series of diurnal air temperature and precipitation data is available from 1974. The mean annual air temperature (1974–2002) at this site is 5.38C, with mean monthly winter temperatures just below 08C (Fig. 2). The daily temperature record shows peri- ods of positive air temperature throughout the winter, indi- cating the occurrence of melting events on the lower part of Engabreen. Mean annual precipitation amounts to 2045 mm, with higher precipitation during the autumn/winter. An additional automatic weather station has operated at Skjæret (1364 m a.s.l.), a nunatak in the accumulation area, since 1995, although with some data gaps. Comparison of the air-temperature record (1999–2002) from this station with the corresponding data from Glomfjord yields an average temperature lapse rate of –0.0074 K m –1 . Mass-balance data In association with hydropower production from the Svart- isen area, NVE started a glacier monitoring programme at Annals of Glaciology 42 2005 395