Temperature index modeling of the Kahiltna Glacier: comparison to multiple field and geodetic mass balance datasets

Abstract

Glaciers of Alaska, USA, and Northwestern Canada are shedding mass at one of the highest rates of any mountain glacier system, with significant impact at the global and local scales. Despite advances in satellite and airborne technologies, fully characterizing the temporal evolution of glacier mass change in individual watersheds remains a challenge. Temperature index modeling is an approach that can be used to expand on sparse ground observations, and that can help bridge the gap between regional and individual watershed estimates of the time series of glacier mass change. Here we present a study on temperature index modeling of glacier-wide mass balance for the large Kahiltna Glacier (502 km�_, 270 to 6100 m in elevation) in the Central Alaska Range, using a combination of ground observations and past climate data products. We reproduce mass changes from 1991 to 2011, and assess model performance by comparing our results to several field and remote sensing datasets. First, we compare our results to a 20-year record of mass balance measurements at a National Park Service index site at the glacier's equilibrium line altitude. We find low correlation between index site measurements and modeled glacier-wide balances (R�_ = 0.24), indicating that the index site may not be representative of the glacier-wide mass balance regime. We compare next to glacier-wide mass balances derived from airborne laser altimetry, to assess the model's long-term mass change estimates. We find disagreement between the mean annual balances for 1995 to 2010 (-0.95 �0.49 m w.e. yr����_ from the model versus -0.69 +0.07/-0.08 m w.e. yr����_ from laser altimetry). To validate the laser altimetry methods, we then compare estimates from 1951 to 2011 from laser altimetry and digital elevation model differencing, finding close agreement (-0.48 +0.08/-0.09 m w.e. yr����_ and -0.41 �0.26 m w.e. yr����_, respectively), and lending strength to the laser altimetry centerline extrapolation techniques. We also examine estimates derived from regionally-downscaled satellite gravimetry. While gravimetry likely underestimates long-term mass loss for this glacier (-0.36 �0.13 m w.e. yr����_ for 2003 to 2010), it correlates well to individual modeled annual balances (R�_ = 0.72) and to the time series of mass balance at an ablation stake location (R�_ = 0.81). Given ongoing refinements to gravimetry downscaling and geodetic techniques, our results point to the potential for integrating multiple methods to obtain the most information on subannual and long-term mass changes at the basin scale for remote sites such as the Kahiltna Glacier.