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Seasonal components of freshwater runoff in Glacier Bay, Alaska: diverse spatial patterns and temporal change

Abstract. A high spatial resolution (250 m), distributed snow evolution and
ablation model, SnowModel, is used to estimate current and future scenario
freshwater runoff into Glacier Bay, Alaska, a fjord estuary that makes up
part of Glacier Bay National Park and Preserve. The watersheds of Glacier
Bay contain significant glacier cover (tidewater and land-terminating) and
strong spatial gradients in topography, land cover, and precipitation. The
physical complexity and variability of the region produce a variety of
hydrological regimes, including rainfall-, snowmelt-, and ice-melt-dominated
responses. The purpose of this study is to characterize the recent
historical components of freshwater runoff to Glacier Bay and quantify the
potential hydrological changes that accompany the worst-case climate
scenario during the final decades of the 21st century. The historical
(1979–2015) mean annual runoff into Glacier Bay is found to be 24.5 km3 yr−1, or equivalent to a specific runoff of 3.1 m yr−1, with a
peak in July, due to the overall dominance of snowmelt processes that are
largely supplemented by ice melt. Future scenarios (2070–2099) of climate
and glacier cover are used to estimate changes in the hydrologic response of
Glacier Bay. Under the representative concentration pathway (RCP) 8.5, the
mean of five climate models produces a mean annual runoff of 27.5 km3 yr−1, 3.5 m yr−1, representing a 13 % increase from historical
conditions. When spatially aggregated over the entire bay region, the
projection scenario seasonal hydrograph is flatter, with weaker summer flows
and higher winter flows. The peak flows shift to late summer and early fall,
and rain runoff becomes the dominant overall process. The timing and
magnitudes of modeled historical runoff are supported by a freshwater
content analysis from a 24-year oceanographic conductivity–temperature–depth
(CTD) dataset from the U.S. National Park Service's Southeast Alaska
Inventory and Monitoring Network (SEAN). The hydrographs of individual
watersheds display a diversity of changes between the historical period and
projection scenario simulations, depending upon total glacier coverage,
elevation distribution, landscape characteristics, and seasonal changes to
the freezing line altitude.
