Mass balance investigations at Glaciar Chico, Southern Patagonia Icefield, Chile
The Southern Patagonia Icefield (SPI) is the largest temperate ice mass at mid-latitudes in the Southern Hemisphere, and an area where most glaciers have been retreating during the last 50 years. Based on field data, vertica aerial photographs and satellite images, the changes experienced by Glaciar Chico, are presented. These glacier variations are compared to climatic
changes recorded in Patagonia during recent decades through modelling of the glacier mass balance, using a degree-day factor model forced with climatic and field data. The main aim of this work is to analyse the mass balance of Glaciar
Chico, in order to distinguish climatically driven glacier behaviour from tha which could be related to ice dynamics. Between the end of the Little Ice Age and the middle of the 20`h century the glacier retreated at relatively low rates,
but in recent decades there has been an acceleration in the rate of ice thinning and area loss of up to 0.33 ± 0.22 km2 a"1. In total, the volume lost due to frontal retreat and area change between 1975 and 2001 is -1.73 ± 1.43 km3 water equivalent (w. eq. ). Although the glacier was slowly thinning before 1975, during recent decades the rate of thinning has accelerated to reach a maximum of -5.4 ± 0.55 m a-1 at the glacier front. From GPS data, ice thinning was also observed at the lower elevations of the accumulation area, at an average rate of -1.9 ± 0.14 m a"' between 1998 and 2001. Interestingly,
the thinning rate in the lower accumulation zone is three times higher than the mass balance estimated for that part of the glacier. Analysis of climatic data showed a warming trend of up to +0.033°C a-' between 1969 and 1990, and a trend of decreasing precipitation of up to -27 mm a"2 between 1961 and 1990. The mass balance model was run several times in order to obtain the best-fit parameters which were calibrated with direct observations of accumulation and ablation from glaciers across Patagonia. The resulting net mass balance for Glaciar Chico obtained from the model yielded an average of -0.165 ±
0.059 km3 a-' w. eq. between 1975 and 2001, but this only accounts for three quarters of the volume lost by the glacier due to ice elevation changes (-0.226 ± 0.081 km3 a"1 w. eq. ). It is therefore concluded that in addition to the response of Glaciar Chico to climatic changes, a dynamic component also
affects its behaviour, which could possibly be related to enhanced sliding generated by an increase in melt water, and/or a long term change in the glacier and its neighbours in the SPI.