Previously undocumented subglacial lake beneath Flade Isblink stores most of catchment runoff and delays its release

We report the discovery of a previously undocumented subglacial lake beneath the Flade Isblink Ice Cap in North East Greenland. Using satellite Earth Observation data (ICESat-2 elevation data and Sentinel-1 Double Difference InSAR) in the period October 2018 to December 2024 and outputs from a regional climate model, we quantify the lake's role in regional hydrology. The subglacial lake's volume is characterised by an annual cycle of filling during the melt season and drainage in September-October, with the lake storing up to 63 ± 23% of the yearly runoff from its catchment area. In most years, lake drainage causes a 2-3-month lag between peak surface-meltwater production and downstream discharge into the nearby proglacial lake, Romer Sø. Lake drainage occurred in all years of our observation period except 2022, which had the lowest surface melt rates, suggesting that a minimum water-input threshold is required to initiate lake drainage. Additionally, our dataset does not show any evidence of a hydrological connection between the lake and the nearby, well-known subglacial lake on the high plateau of Flade Isblink. Our findings highlight how subglacial conditions may substantially modify the outflow of subglacial water to the ice margin with potential impacts on downstream hydrology and ecosystems. It further demonstrates the potential of integrating remote sensing with hydrological modelling to understand ice-sheet hydrology.