Current and future freshwater runoff into Glacier Bay National Park

Hill, David; Beamer, Jordan; Crumley, Ryan

A comprehensive modeling study of the freshwater runoff into the waters of Glacier Bay National Park and Preserve (GBNP) has been conducted. The modeling was done at a high spatial (250 m and 1 km) and temporal (sub-daily) resolution to capture the rich structure of the runoff due to individual storm events and the seasonal variation in rainfall, snowmelt, and ice melt. The model framework involved several steps. First, weather data were downscaled from a coarse grid to the high spatial resolution of the model grid. This meteorological model additionally partitioned the precipitation into rainfall and snowfall. Second, a snow and ice evolution model was used to compute the storage and eventual melt of the annual snowpack as well as the melt of the glacier ice. Finally, a runoff routing model was used to transport water across the landscape to the coastal boundary. A hindcast simulation was carried out for the period 1979-2015. Based on this simulation, the mean annual runoff volume into the GBNP domain (excluding the Alsek River drainage) was 46.6 km3 with a standard deviation of 4.1 km3. This value excludes precipitation falling directly on the surface of the water, and this domain is defined by hydrologic catchment, not park boundaries. The runoff was partitioned into 24% rainfall, 60% snow-melt and 16% ice-melt. Considerable spatial variability was observed. Generally, regions in the northwest receive a greater fraction of snowfall than those in the southeast. Additionally, regions in the northwest contribute greater ice melt than those in the southeast. Forecast simulations were then carried out for the period 2071-2100. Several climate models and several emissions scenarios were considered. The simulations specifically considered the sensitivity of GBNP runoff to changes in meteorological forcing (precipitation and temperature) and to changes in landcover (glacier retreat). In general, there will be a significant ‘flattening’ of the mean annual hydrograph. The strong summer peak, presently attributable to ice-melt, will be diminished and there will be greater influence of the late spring snowmelt and the late autumn rainfall. These simulations provide an unprecedented look at the hydrologic budget of GBNP and reveal the complex mix of processes that contribute to coastal runoff. A second component of this project focuses on the development of informative and visually appealing graphics and animations relevant to hydrosphere and cryosphere processes in GBNP.

Current and future freshwater runoff into Glacier Bay National Park

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