The challenge of monitoring glaciers with extreme altitudinal range : mass-balance reconstruction for Kahiltna Glacier, Alaska

Glaciers spanning large altitudinal ranges often experience different climatic regimes with elevation, creating challenges in acquiring mass-balance and climate observations that represent the entire glacier. We use mixed methods to reconstruct the 1991-2014 mass balance of the Kahiltna Glacier in Alaska, a large (503 km(2)) glacier with one of the greatest elevation ranges globally (264-6108m a. s.l.). We calibrate an enhanced temperature index model to glacier-wide mass balances from repeat laser altimetry and point observations, finding a mean net mass-balance rate of -0.74 mw.e. a(-1)(+/-sigma = 0.04, std dev. of the best-performing model simulations). Results are validated against mass changes from NASA's Gravity Recovery and Climate Experiment (GRACE) satellites, a novel approach at the individual glacier scale. Correlation is strong between the detrended model-and GRACE-derived mass change time series (R-2 = 0.58 and p << 0.001), and between summer (R-2 = 0.69 and p = 0.003) and annual (R-2 = 0.63 and p = 0.006) balances, lending greater confidence to our modeling results. We find poor correlation, however, between modeled glacier-wide balances and recent single-stake monitoring. Finally, we make recommendations for monitoring glaciers with extreme altitudinal ranges, including characterizing precipitation via snow radar profiling.