Investigating Hydrologic Loading in Southeast Alaska Using GRACE and GPS Data

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This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).The glaciers and icefields of coastal southeast Alaska and the adjacent areas of Canada are among the most dynamic on the planet, and they are shedding mass at a dramatic rate due to climate warming and the dynamics of tidewater glacier retreat. Deglaciation of this region began with the end of the Little Ice Age, and continues today, raising sea level and producing dramatic uplift and gravity change. Today?s spectacular Glacier Bay was created by the rapid tidewater retreat of glaciers beginning around 1800; glaciers originally well over 1 km thick in places disappeared in a few decades, with their ice discharged into the ocean. The land rises up as the weight of the ice is removed, causing uplift easily measured by precise surveying using the Global Positioning System (GPS), and causing changes in Earth's gravity field that can be measured at the surface or from space. However, post-glacial uplift does not occur at a steady rate; the ground subsides slightly throughout the winter before uplifting rapidly from the onset of the spring melt until subsidence begins again the next winter. These variations are dominated by hydrological loading, mainly due to the seasonal accumulation and melting of snow and ice. The seasonal variations here are larger than predicted by large-scale hydrologic models, which do not account for snow and ice well. The investigators of this study will look at variations in height of the land surface and of gravity, and use these measurements to develop regional models of the changes in mass of water, snow and ice in the region and carry out these measurements and modeling in collaboration with scientists at Tohoku University (Japan), and Scott Luthcke at NASA Goddard Space Flight Center. Including absolute gravity measurements, made by the Japanese colleagues, they will test and refine regional GIA models, and test whether the uplift rates are accelerating with the recent increase in ice mass loss rate. They will continue operation of 6 GPS sites installed with our Japanese colleagues, analysis and modeling of the GPS (including PBO) and GRACE data, and will develop a regional model of hydrological load variations for the time span of GRACE data, and extend the model back in time using the longer GPS time series. The hydrological load model will likely represent a large improvement over large-scale hydrologic models such as NASA's Global Land Data Assimilation System, which are difficult to constrain in data-poor regions such as Alaska.This project will improve our understanding of the hydrology of southeast Alaska. Freshwater discharge into the Gulf of Alaska affects marine habitat and may alter the patterns of the Alaska coastal current. The model will provide new information on the distribution and timing of runoff, which are important for oceanographic and biological studies, and will be of value as additional input into ocean salinity models and habitat distribution models. The methods for combining GRACE, GPS data and hydrological models will provide a template for application of these approaches to other regions. Ultimately these methods will provide more accurate estimates of the glacier contribution to rising sea level. They will continue to inform the public and National Park Service personnel of research results, and this work will provide new information to them about the magnitude and distribution of snow accumulation and melt in the region.