A lumped conceptual Rainfall–Runoff Model (the NAM model) was applied to quantify simulated intra- and inter-annual discharge from the Mittivakkat glacier catchment (18.4 km2, 78% glacier cover), Ammassalik Island, SE Greenland. Discharge simulations were performed for three periods: 1999–2004 (calibration period), 1993–1995 and 1998/1999 (validation period), and 2071–2100 (scenario period). In periods when observed winter discharges were lacking, visual observations from daily photographic time lapse were used for calibration. The timing and magnitude of simulated discharge were in general in good accordance with observed discharge (R2=0.77). However, discrepancies between simulated and observed discharge occur (maximum daily difference up to 3.4 m3 s−1, up to 11% difference between observed and simulated cumulative discharge, and model predicted river break-up 1–3 d before it actually occurs). For the period 2071–2100 future IPCC A2 and IPCC B2 climate scenarios were used as input for NAM based on HIRHAM RCM and HadCM3 AOGCM model simulations. The IPCC scenarios indicated mean maximum monthly runoff higher than 900 mm w.eq., and mean annual runoff around 3200 mm w.eq. yr−1, approximately one and a half times higher than the runoff in 1993–2004 of approximately 2000 mm w.eq. yr−1. The increasing runoff indicated an approximately three times higher negative glacier net mass balance ranging from about −750 mm w.eq. yr−1 (1961–1990) to approximately −2000 mm w.eq. yr−1 (2071–2100).