Comparison of geodetic and glaciological mass balance on Gulkana Glacier, Alaska

Abstract

The net mass balance on Gulkana Glacier has been measured since 1966 by the glaciological method, in which seasonal balances are measured at three index sites and extrapolated over large areas of the glacier. Systematic errors accumulate through time in this method; therefore the geodetic balance, in which errors are independent of time, was calculated for comparison to and possible calibration of the glaciological method. Digital elevation models (DEMs) of the glacier in 1974, 1993, and 1999 were prepared and geodetic balances computed, giving -6.0+̲0.5 m of water equivalent (weq) from 1974 to 1993 and -11.8+̲0.5 m weq from 1974 to 1999. These were compared to the glaciological balances over the same intervals, which were -5.8+̲0.9 m weq and -11.2+̲1.0 m weq, respectively; both balances show a tripling in thinning rate in the 1990s. These cumulative balances differ by less than 6%. For this, the glaciological method on Gulkana Glacier must be largely free of systematic errors and use a changing area altitude distribution. Relatively good contrast in the accumulation area of the glacier increased accuracy in the geodetic method.