Using atmospherically-corrected Landsat imagery to measure glacier area change in the Cordillera Blanca, Peru from 1987 to 2010

The dynamic, tropical glaciers of the Peruvian Cordillera Blanca are rapidly changing and these changes are expected to affect water availability, especially during the dry season. In this study, we quantify recent changes to these water reservoirs, providing estimates of glacier area in the Cordillera Blanca and sub-watersheds of the Rio Santa for the following years 1987, 1996, 2004, and 2010. We explore the effects of atmospheric and topographic corrections by comparing debris-free glacier area estimates generated using raw scenes and corrected scenes. Our results suggest that these corrections can have a significant impact on debris-free glacier area estimates when the same threshold is applied. Debris-free glacier area estimates derived from uncorrected scenes are approximately 5% less than debris-free glacier area estimates derived from atmospherically-corrected scenes. We determined that debris-free glacier area estimates are most sensitive to the choice of threshold and topographic effects. To map glacier area change, we used high-resolution satellite imagery to calibrate our selection of a single threshold for the Normalized Difference Snow Index (NDSI). This threshold value was applied to all NDSI images, which were derived from four carefully selected and atmospherically-corrected Landsat Thematic Mapper (TM) scenes acquired at the end of the dry season. In order to calculate total glacier area, we manually mapped debris-covered glaciers, because automated methods were unsuccessful in this region. As of August 2010, the Cordillera Blanca had a total glacier area of 482 km2, which amounts to a 25% decrease since 1987. Glaciers in the southern portions of the Cordillera Blanca, which have lower median elevations on average, lost a greater percentage of their area from 1987 to 2010, relative to their northern counterparts. Overall, glacier area change in the Cordillera Blanca appears to be accelerating. Between 2004 and 2010, glaciers in the Cordillera Blanca lost area at a rate that was approximately 3.5 times the average rate of area loss from 1970 to 2003.