VARIABILITY AND TRENDS IN GLACIER MASS BALANCE AND FRESHWATER RUNOFF FOR THE JUNEAU ICEFIELD FROM 1980 TO 2016

Meltwater from glaciers plays a crucial role in the hydrological and ecological regime of the Gulf of Alaska. Glaciers serve as freshwater reservoirs over diurnal, seasonal, and longer time scales; they act to modify flow patterns (delayed runoff and diminished variability) relative to ice-free basins. Streamflow measurements downstream of glaciers with negative net mass balance (mass loss) typically display initial increased flow due to higher mass loss, followed by decreased flow due to shrinking glacier volume relative to mass loss. One piece of information in glacier hydrology is whether a glacierized basin is currently within the early period of increased runoff or the later period of decreased runoff, because this information is relevant to future downstream ecosystem health. In precipitation-dominated coastal glacier systems, where interannual variability in snow and rain input are large, trends in glacier runoff are difficult to discern.

This study aims to identify glacier mass loss, total runoff trends, magnitude, and variability from 1980 to 2016 for the Juneau Icefield through integrating multiple methods. In particular, we seek to better partition runoff as sourced from ice, snow, and rain, in addition to trends in the timing and quantity of total freshwater discharge. We calculate runoff from a coupled energy balance and water routing model, SnowModel-HydroFlow, driven by reanalysis climate data from NASA's MERRA-2. We assimilate data on snow water equivalent from ground-based point observations and airborne ground-penetrating radar; calibrate our results to melt observations, geodetic mass loss estimates, and river discharge data; and validate our results against regional mass changes derived from satellite gravimetry data (NASA GRACE). We find that, even in this precipitation-dominated region, glacier melt exhibits a significant and increasing trend, alongside an increase in melt season length. This glacier melt trend, in turn, dominates over variability in precipitation, resulting in increasing total freshwater runoff and a decreasing overall terrestrial water balance, in agreement with GRACE. Altogether, these results highlight the changing nature of freshwater discharge from the Juneau Icefield, with implications for the health of downstream ecosystems.