The effects of basal water beneath Vatnajökull, Iceland, observed by SAR interferometry
Eyjólfur Magnússon(1) , Helmut Rott(1)
, Helgi Björnsson(2)
, and Finnur Pálsson(2)
University of Innsbruck,
(2) University of Iceland, Dunhagi 3, IS-107 Reykjavík, Iceland
We analysed InSAR data from the ERS1/2 tandem mission, obtained within the ERS AO projects VECTRA and AO3.239, to study special ice dynamic phenomena on the ice cap Vatnajökull, Iceland. In these data unusually high velocities are observed on Skeiðarárjökull, an outlet of Vatnajökull. Two case studies are shown where single tandem pairs from an ascending orbit are used to derive 3-dimensional velocity fields by combining mass continuity and the horizontal flow direction that was derived using interferograms from ascending and descending orbits acquired in December 1995. The first analysis is from 27-28 March 1996 during a jökulhlaup (glacier outburst) from Grímsvötn which flows into Skeiðará, the main glacier river of Skeiðarjökull. The effects of the jökulhlaup are clearly observed by 2-3 times faster surface velocities over large areas, compared to what should be expected from a normal winter scene. The water also seems to accumulate at some location in the path of the jökulhlaup below or within the glacier. The observations are made from an interferogram during the early stage of the jökulhlaup when only slight increase had been measured in Skeiðará water discharge compared to what was observed in later stages. Classical jökulhlaup theory where the water flows via single semi-cylindrical channel, can alone not explain the observed behaviour. The second case study is from 23-24 October 1996 where autumn rainfall of medium intensity (~10 mm/day) seems to trigger multiple average movements of Skeiðarárjökull. The effects of this rainfall are observed as well over large parts of Vatnajökull and even on Hofsjökull (central Iceland), evident as both velocity increase and accumulation of water at the glacier basin concluded from vertical motion observed in the interferogram. An interferogram from a descending orbit on a rainy day two days earlier shows the same on Skeiðarárjökull and lower parts of Mýrdalsjökull (S-Iceland). This along with the fact that measured ice-quakes on Skeiðarárjökull, caused by fracture of moving ice, are most frequent during rainfall, suggest high variability in the velocity of temperate glaciers. This means that estimating the mass fluxes of temperate glaciers over longer time periods from InSAR data of short time intervals may result in a considerably underestimate since most usable image pairs are acquired under dry conditions.