Science / project summary
The production, transport, and export of meltwater from the surface of the Greenland Ice Sheet (GrIS) is critically important to our understanding of global sea level rise, yet remains one of the least-studied hydrologic systems on Earth. To date, efforts to measure GrIS contributions to global sea level rise have prioritized solid/dynamical ice losses, gravity anomalies from the Gravity Recovery and Climate Experiment (GRACE) mission, and calculations of surface mass balance from automated weather stations and/or climate modeling. None of these explicitly considers the fast transport of meltwater through supraglacial river networks, despite their ubiquity across much of the ablation zone and being a primary mechanism by which surface meltwater is routed to the englacial and proglacial parts of the ice sheet. This currently poor state of knowledge about supraglacial rivers impairs understanding of GrIS contributions to global sea level rise for at least four reasons. First, there is a recognized positive feedback between the penetration of surface meltwater to the bed and ice sliding velocity, thus impacting ice dynamics. Second, there is a growing appreciation in the Greenland science community that meltwater runoff contributes as much if not more mass loss to the global ocean than does solid/dynamical ice loss. Third, the intensity and areal extent of meltwater production on the GrIS surface are projected to increase. Fourth, virtually nothing is known about the hydraulics of GrIS supraglacial rivers, so numerical modeling of meltwater fluxes flowing over and into the ice sheet cannot be done with confidence. To address this knowledge gap, the researchers seek to answer five science questions: 1) To what extent does ice surface topography dominate the movement of water across the ice sheet? 2) Are fluxes of supraglacial meltwater into the GrIS subsurface uniformly distributed or do some parts of the ice sheet receive greater concentrations of meltwater than others? 3) Do supraglacial river flows attain supercritical velocities? 4) Was a 100% efficient surface water drainage pattern observed following the extreme July 2012 melt event unusual or typical for the ice sheet? 5) How efficiently is GrIS surface meltwater transported off the ice sheet surface and out to the global ocean? To answer these questions, the project will build upon new remote sensing capabilities developed in previous NASA Cryospheric Sciences grant NNX11AQ38G (ending 2014). It will use archived and scheduled high-resolution visible/NIR imagery and stereo digital elevation models from WorldView-1/2, QuickBird, and Geoeye-1 satellites, and cold-season IceBridge Digital Mapping System (DMS) camera imagery, to study supraglacial river drainage pattern and flow efficiency for a ~15,000 km2 area of the ablation zone in western Greenland. Remotely sensed estimates of river flow direction, velocity and flux (discharge) will be calibrated/validated using a dataset of in situ hydraulic measurements from two field campaigns in the ablation zone. The operation and maintenance of four previously established terrestrial river gaging sites near Kangerlussuaq will be continued. Finally, all of these remotely sensed and field datasets will be incorporated into a GIS-based hydrologic modeling framework to enable a first "snow-to-sea" simulation of GrIS meltwater runoff that explicitly includes principles of open-channel flow through supraglacial river drainage networks.