Internally drained catchments dominate supraglacial hydrology of the southwest Greenland Ice Sheet

Internally drained catchments (IDCs) are hydrologic units on the Greenland ice sheet (GrIS) surface that collect and drain meltwater through supraglacial stream/river networks to terminal moulins or lakes. Their areas and shapes constrain the volumes and locations of supraglacial meltwater penetration into the ice. We map IDCs of the southwest GrIS using Landsat-8 OLI panchromatic imagery and a moderate-resolution digital elevation model (DEM). In total 919 IDCs are mapped between 400 m and 2000 m a.s.l., together with their associated supraglacial river networks (total length 21,129 km), supraglacial lakes (436), and terminal moulins (872). A complex yet broadly predictable surface drainage pattern is revealed, with both IDC areas and the optimal DEM depression-filling threshold generally increasing with ice surface elevation H. Historical air photos suggest possible transferability of the first relationship over space and time. Intersection of IDC boundaries with MAR (Modèle Atmosphérique Régional) regional climate model runoff simulations shows >50 % of runoff modeled for elevations >1600 m is not transported downstream to lower elevations, but instead drains to a small number (51 out of 872) of terminal moulins, indicating modest but non-trivial penetration of surface meltwater at high elevations. In sum, IDCs provide a new, fine-scale hydrologic unit for study of GrIS surface hydrologic processes.