Abstract. It has been argued that the infiltration and retention of meltwater within firn across the percolation zone of the Greenland ice sheet has the potential to buffer up to ~3.6 mm of global sea level rise (Harper et al., 2012). Despite evidence confirming active refreezing processes above the equilibrium line, their impact on runoff and proglacial discharge has yet to be assessed. Here we compare meteorological, melt, firn-stratigraphy and discharge data from the extreme 2010 and 2012 summers to determine the relationship between atmospheric forcing and runoff across the Kangerlussuaq catchment of the Greenland ice sheet, which drains into Watson River. The bulk discharge in 2012 of 6.8 km3 exceeded that of 2010 of 5.3 km3 by 28 %, despite only a 3 % difference in net energy available for melt between the two summers. This large disparity in discharge response can be explained by a 24 % contribution of runoff originating from above the long-term equilibrium line in 2012, triggered by diminished firn retention that culminated in three days of record discharge from 11 July of 3100 m3 s−1 (0.27 km3 d−1) that washed-out the Kangerlussuaq bridge. Throughout the 2010 melt-season, there was a steady increase in the residual difference between integrated melt over the catchment and cumulative proglacial discharge that by mid-September equated to 21 % (~1.1 km3) of the total melt generated being retained within the catchment. In 2012 a similar pattern is observed until 11 July, after which the residual fell by 50 % and further diminished so that less than 0.4 km3 (~5 %) of the total melt was retained by the end of the summer. Cumulative energy receipts versus bulk discharge further indicate a marked contrast between the two melt-seasons, such that in 2012 there was a noteably higher discharge response per unit energy forcing after the 11 July. Density profiles from cores and pits within the accumulation area acquired in April 2012 reveal an extensive, dense, ice-layer between 0.9 to 1.4 m snow depth that extended from the equilibrium line to at least 1840 m elevation. This perched superimposed ice layer can be attributed to melt refreezing during previous summers and we hypothesise that in July 2012, it provided a barrier to further infiltration rendering the underlying pore space inaccessible thereby forcing extensive runoff from the accumulation zone. Discharge was further amplified by catchment hypsometry, leading to a disproportionate increase in the area contributing to runoff as the melt-level rose above the ice sheet plateau in July 2012. Satellite imagery and oblique aerial photographs confirm an active network of supraglacial rivers extending 140 km from the ice margin providing strong support for the hypothesis. Our findings substantiate active infiltration processes across the percolation zone of the Greenland ice sheet though the resulting patterns of refreezing are complex and can lead to spatially extensive, perched superimposed layers within the firn. In 2012, such layers extended to 1840 m providing a low-permeable obstruction to further meltwater storage, thereby promoting runoff into the hydrological system that contributed directly to sea-level rise.;
