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

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 km(3) 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 km(3) 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 con-tent 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.