Sentinel‐1 Detects Firn Aquifers in the Greenland Ice Sheet. Issue 3 (10th February 2020)

• Record Type: Journal Article
• Authors: Brangers, I.; Lievens, H.; Miège, C.; Demuzere, M.; Brucker, L.; De Lannoy, G. J. M.
• Is Part Of: Geophysical research letters. Volume 47:Issue 3(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-02-10
• Subjects: Greenland -- firn aquifers -- Sentinel‐1 -- Operation IceBridge; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2019GL085192
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: Firn aquifers in Greenland store liquid water within the upper ice sheet and impact the hydrological system. Their location and area have been estimated with airborne radar sounder surveys (Operation IceBridge, OIB). However, the OIB coverage is limited to narrow flight lines, offering an incomplete view. Here, we show the ability of satellite radar measurements from Sentinel‐1 to map firn aquifers across all of Greenland at 1 km 2 resolution. The detection of aquifers relies on a delay in the freezing of meltwater within the firn above the water table, causing a distinctive pattern in the radar backscatter. The Sentinel‐1 aquifer locations are in very good agreement with those detected along the OIB flight lines (Cohen's κ = 0 . 84 ). The total aquifer area is estimated at 54, 800 km 2 . With continuity of Sentinel‐1 ensured until 2030, our study lays a foundation for monitoring the future response of firn aquifers to climate change. Plain Language Summary: The Greenland ice sheet encloses vast amounts of liquid water beneath its surface, within a layer of old compacted snow referred to as firn. These water reservoirs, named firn aquifers, are a peculiar feature with potentially widespread impacts on ice sheet temperature, hydrology, and contribution to sea level. Monitoring aquifer areas is therefore critical in assessing the vulnerability of the Greenland ice sheet to climate change. Current knowledge on aquifer locations relies on aircraft flights of NASA Operation IceBridge, surveying only narrow stretches of the ice sheet along its flight lines. Satellite observations are necessary to provide full coverage. Here, we present the first satellite‐based map of Greenland's firn aquifers, using radar observations from the Sentinel‐1 constellation. The detection with Sentinel‐1 relies on a strong and prolonged absorption of the emitted radar signal by the liquid water within the firn, yielding low values in the measured return signal. Very good agreement is found when comparing Sentinel‐1 aquifer locations with those of Operation IceBridge. The total aquifer area estimated by Sentinel‐1 is 54, 800 km 2 . The long‐term continuity provided by the satellite observations offers great potential for the monitoring of changes in aquifer location and area related to climate change. Key Points: We present a method to map Greenland firn aquifers using Sentinel‐1 for the first time The spaceborne aquifer detection agrees well with that of airborne Operation IceBridge The firn aquifer area is estimated at 54, 800 km 2 across the Greenland ice sheet