LANDSAT TM AND ETM+ DERIVED EQUILIBRIUM LINE ALTITUDES AND THEIR IMPLICATIONS, CORDILLERA HUAYHUASH, PERU, 1986-2005

Glaciers throughout much of the tropics are thinning and shrinking. Satellite images were used to document changes in the Cordillera Huayhuash, a glacial range located at 10º 15'S, 76º 55'W, ~65 km south of the Cordillera Blanca in the Andes Mountains. Equilibrium line altitudes (ELA), derived from Landsat Thematic Mapper (TM) and Enhanced Thematic Mapper (ETM+) images acquired during the tropical dry season, approximating minimum snow cover, were used to document the last 20 years of glacier change. The ELA is defined by the minimum elevation where glacial ice is continuously covered by snow, mapped as the visible snowline. Images from 1986 – 2005 with 28.5 m resolution are from the Eros Data Center and Global Land Cover Facility and were selected based on time of year, lack of cloud cover, and the absence of recent snowfall. Images were registered, calibrated, and classified to determine snowlines. Snowline vectors were overlain on a 15m ASTER-derived DEM to extract ELAs. Overall, ELAs from 1986-2005 range from 4941 to 5291 meters with an average change of +14 meters. Eleven glaciers have good coverage throughout this time period. Increases ranged from 2 to 115 meters. Glaciers with large changes occur throughout the range, indicating no obvious spatial trend in ELA rise. ELA changes may reflect a combination of factors such as position, aspect, slope, elevation. Since 1986, the ELAs on 7 glaciers rose by an average of 63 meters, while 4 other glacial ELAs decreased by 27 meters. However, since 1989, all ELAs rose by an average of 52 meters. The average ELA rise may be related to both higher temperatures and lower precipitation. Although both variables are important to glacial mass balance, modern changes in precipitation are assumed to be minimal relative to the influence of temperature on ablation rates. Using a lapse rate of -0.6ºC/100m, and assuming moisture was not a factor, ΔT=+0.4ºC to +0.7ºC over the 20 year study period. Significant ELA rises may impact local hydrology, supplying fresh water and hydro power to both local and coastal communities, particularly during the dry season. However, if ELAs continue to increase and mass balances decrease, glacial volume will decrease resulting in less available meltwater during the dry season. Annual meltwater contributions from rising ELAs can be estimated using ELA change to estimate volume changes.