Glacier-Wide Mass Balance and Compiled Data Inputs

Since the late 1950s, the USGS has maintained a long-term glacier mass-balance program at three North American glaciers. Measurements began on South Cascade Glacier, WA in 1958, expanding to Gulkana and Wolverine glaciers, AK in 1966, and later Sperry Glacier, MT in 2005. The Juneau Icefield Research Program has measured surface mass balance on Lemon Creek and Taku Glacier since the mid-1940s, with USGS providing complimentary seasonal measurements of Lemon Creek beginning in 2014 (JIRP; Pelto and others, 2013). Direct field measurements of point glaciological data are combined with weather and geodetic data to estimate the seasonal and annual mass balance at each glacier in both a conventional and reference surface format (Cogley and others, 2011). The analysis framework (O'Neel and others, 2019; prior to v 3.0 van Beusekom and others, 2010) is identical at each glacier to enable cross-comparison between output time series. Vocabulary used follows Cogley and others (2011) Glossary of Glacier Mass Balance. This portion of the data release includes glacier wide mass balance, as well as the refined inputs used in these calculations. Input data are of three types: 1) time-variable area altitude distribution (AAD); 2) time series of point water balance at long term sites (with secondary sites given in recent years); 3) weather data from nearby stations, either installed along the glacier margins or taken from a nearby site if continuous glacier-adjacent data is unavailable. The USGS runs a coded analysis to transform the three input data types to the output glacier-wide data. Output data represent surface mass balance estimates. The output solution is a geodetically calibrated, conventional glacier-wide mass balance, which represents our preferred solution. Conventional glacier-wide mass balance from direct observations without calibration can be easily derived by using the geodetic calibration coefficients provided, if desired. We do not explicitly account for basal or englacial accumulation or ablation. Mass balances are reported in water equivalent (w.e.) units, and often represent integration of multiple field measurements. Whenever possible, we average multiple field measurements to account for surface roughness and measurement errors. These raw point measurements and other mass-balance related data are included in the larger USGS Benchmark Glacier Project Comprehensive Data Collection, available at https://doi.org/10.5066/P9AGXQSR. Preliminary mass balance estimates for the current calendar year are provided, but do not include direct measurements of ablation after the date of the fall visit. Preliminary estimates of mass balance model this winter ablation for the current year. During subsequent field visits in the following calendar year, any ablation that occurred over the winter season is measured and used to revise the previously modeled estimate of mass balance.