Evaluation of remote-sensing techniques to measure decadal-scale changes of Hofsjökull ice cap, Iceland

Abstract Dynamic surficial changes and changes in the position of the firn line and the areal extent of Hofsjökull ice cap, Iceland, were studied through analysis of a time series (1973–98) of synthetic-aperture radar (SAR) and Landsat data. A digital elevation model of Hofsjökull, which was constructed using SAR interferometry, was used to plot the SAR backscatter coefficient ( σ °) vs elevation and air temperature along transects across the ice cap. Seasonal and daily σ ° patterns are caused by freezing or thawing of the ice-cap surface, and abrupt changes in σ ° are noted when the air temperature ranges from ∼−5° to 0°C. Late-summer 1997 σ ° (SAR) and reflectance (Landsat) boundaries agree and appear to be coincident with the firn line and a SAR σ ° boundary that can be seen in the January 1998 SAR image. In January 1994 through 1998, the elevation of this σ ° boundary on the ice cap was quite stable, ranging from 1000 to 1300 m, while the equilibrium-line altitude, as measured on the ground, varied considerably. Thus the equilibrium line may be obscured by firn from previous years. Techniques are established to measure long-term changes in the elevation of the firn line and changes in the position of the ice margin.