A method for monitoring glacier mass balance using satellite albedo measurements:: application to Vatnajokull, Iceland

We compare satellite albedo images of Vatnajokull, Iceland, with mass-balance measurements for the years 1991-99. We find that the equilibrium line is mostly not visible when it is located above its position of the previous year(s). Equilibrium-line detection is further hindered by clouds and a gradual transition between ice and firn or snow. Consequently, firn-line elevation at the end of the melting season is not particularly useful for estimating the annual mass balance. Instead, we propose to study the mean albedo of the entire ice cap throughout the melting season so that all available information about the surface albedo is taken into account. The mean net potential global radiation which can be estimated from the mean surface albedo alone, both depends on and influences summer melt. It also depends on winter precipitation and, integrated over the melting season, is found to relate linearly to the specific mass balance B (r = 0.87 and 0.94 for different outlets of Vatnajokull). B can be estimated quantitatively when this relation is known and qualitatively when it is not. The uncertainty in the satellite-derived value of B is 0.5-0.8 in w.e., which for Vatnajokull corresponds to about 27% of the interannual variability of B.