Spectral mixture analysis of Landsat thematic mapper images applied to the detection of the transient snowline on tropical Andean glaciers

The tropical glaciers in the central Andes are sensitive indicators of climatic variability. They are an important water resource, but are presently in a state of rapid retreat. Spectral mixture analysis using Landsat Thematic Mapper (TM) images was used to identify the ablation and accumulation zones and the transient snowline at two tropical sites: Zongo Glacier in the Cordillera Real, Bolivia, and the Quelccaya Ice Cap in Peru. Delineation of the accumulation and ablation zones is relatively insensitive to the endmembers selected to represent each zone. Endmembers selected from Zongo Glacier were successfully used to delineate accumulation and ablation zones on the Quelccaya Ice Cap. Spectral mixture analysis was found to be superior to a single band image or the TM4/TM5 ratio in discriminating the accumulation and ablation zones on these small tropical glaciers. The altitude of the transient snowline identified on Zongo Glacier at the end of the ablation season is consistent with the altitude of the equilibrium line determined from mass balance studies. This suggests that in the tropics, as in mid-latitudes, the highest altitude reached by the transient snowline during a hydrological year can be used as a proxy for the altitude of the equilibrium line.