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Spatial and Temporal Variability of Glacier Melt in the Mcmurdo Dry

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Spatial and Temporal Variability of Glacier Melt in the Mcmurdo Dry Spatial and Temporal Variability of Glacier Melt in the McMurdo Dry Valleys, Antarctica by Matthew James Hoffman A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Environmental Sciences and Resources: Geology Dissertation Committee: Andrew G. Fountain, Chair Scott Burns Christina Hulbe Martin Lafrenz Glen Liston Alan Yeakley Portland State University © 2011 Abstract In the McMurdo Dry Valleys, Victoria Land, East Antarctica, melting of glacial ice is the primary source of water to streams, lakes, and associated ecosystems. To better understand meltwater production, three hypotheses are tested: 1) that small changes in the surface energy balance on these glaciers will result in large changes in melt, 2) that subsurface melt does not contribute significantly to runoff, and 3) that melt from 25-m high terminal cliffs is the dominant source of baseflow during cold periods. These hypotheses were investigated using a surface energy balance model applied to the glaciers of Taylor Valley using 14 years of meteorological data and calibrated to ablation measurements. Inclusion of transmission of solar radiation into the ice through a source term in a one-dimensional heat transfer equation was necessary to accurately model summer ablation and ice temperatures. Results showed good correspondence between calculated and measured ablation and ice temperatures over the 14 years using both daily and hourly time steps, but an hourly time step allowed resolution of short duration melt events and melt within the upper 15 cm of the ice. Resolution of short duration melt events was not important for properly resolving seasonal ablation totals. Across the smooth surfaces of the glaciers, ablation was dominated by sublimation and melting was rare. Above freezing air temperatures did not necessarily result in melt, and low wind speed was important for melt initiation. According to the model, subsurface melt between 5 and 15 cm depth was i extensive and lasted for up to six weeks in some summers. The model was better able to predict ablation if some subsurface melt was assumed to drain, lowering ice density, consistent with observations of a low density weathering crust that forms over the course of the summer on Dry Valley glaciers. In extreme summers, drainage of subsurface melt may have contributed up to half of the observed surface lowering through reduction of ice density and possibly through collapse of highly weathered ice. When applied spatially, the model successfully predicted proglacial streamflow at seasonal and daily time scales. This was despite omitting a routing scheme, and instead assuming that all melt generated exits the glacier on the same day, suggesting refreezing is not substantial. Including subsurface melt as runoff improved predictions of runoff volume and timing, particularly for the recession of large flood peaks. Because overland flow was rarely observed over much of these glaciers, these model results suggest that runoff may be predominantly transported beneath the surface in a partially melted permeable layer of weathered ice. According to the model, topographic basins, particularly the low albedo basin floors, played a prominent role in runoff production. Smooth glacier surfaces exhibited low melt rates, but were important during high melt conditions due to their large surface area. Estimated runoff contributions from cliffs and cryoconite holes was somewhat smaller than suggested in previous studies. Spatial and temporal variability in albedo due to snow and debris played a dominant role in flow variations between streams and seasons. In general, the model supported the existing assumption that snowmelt is insignificant, but in extreme melt years snowmelt in the accumulation area may contribute significantly to runoff in some locations. ii Acknowledgments This dissertation would not have been possible without the input and guidance of many people. First and foremost, I thank my advisor, Andrew Fountain, for his support, patience, and tireless good humor. He has shown me how to synthesize disparate data and analyses, helped me clarify my thoughts on numerous areas of study, and inspired me to become a better scientist. I also offer my gratitude to Christina Hulbe for her devoted instruction and the perspective she has given me on modeling, and to Glen Liston for his keen insight. I would also like to thank the rest of my committee, Scott Burns, Martin Lafrenz, and Alan Yeakley, for their time and assistance. My experiences with coworkers in the field and in the office will be fondly remembered always, and, in particular, I am truly grateful to Hassan Basagic and Thomas Nylen for the myriad skills they have taught me and the countless enjoyable hours spent with each of them. Additionally, they both have provided endless explanation and interpretation of the data collected in Taylor Valley over the last decade. I thank Erin Pettit, Liz Bagshaw, Josh Carmichael, and Rae Spain and countless others for their noteworthy support in the field, and I acknowledge the contributions Keith Jackson, Shaun Marcott, Pete Sniffen, and Kristina Thorneycroft made to my time at Portland State. I also must thank Jon Ebnet for the pioneering work he did applying this model to Taylor Valley. I am indebted to Tom Neumann for his tremendous support and understanding during the last two years. iii I am deeply thankful for the continued support that I have received from my family, and in particular, my parents who have always provided endless encouragement. Chapters 3 and 5 were largely completed thanks to the hospitality of Craig and Judy Barry. My daughter Penelope came along midway through this dissertation and altered the equation, and I am grateful to her for the patience she has shown me beyond her years. Finally, I must acknowledge the amazing support of my wife, Amanda. It was her enthusiastic encouragement that convinced me to tackle a project with polar field work in the first place, her experience that helped me navigate the ups and downs of a doctoral program, her companionship and humor on adventures around the globe and at home that has given me peace, and her accommodation of my demanding schedule during the last two years that has ultimately allowed me to complete my work. iv Table of Contents Abstract ………………………………………………………………………………i Acknowledgments................................................................................................................iii Table of Contents .................................................................................................................. v List of Tables ...................................................................................................................... vii List of Figures ....................................................................................................................viii Chapter 1: Introduction ..................................................................................................... 1 1.1 Setting ............................................................................................................. 3 1.2 Significance. ................................................................................................... 6 1.3 Background ..................................................................................................... 7 1.4 Description of melt model ............................................................................ 14 1.5 Organization of the dissertation .................................................................... 15 Chapter 2: Data Summary .............................................................................................. 17 2.1 Description of Datasets ................................................................................. 18 2.2 Climate Summary ......................................................................................... 31 Chapter 3: Development of Gridded Weather Data ....................................................... 55 3.1 Solar radiation procedure .............................................................................. 58 3.2 Longwave radiation procedure ..................................................................... 68 3.3 MicroMet Results: Glacier surface domain .................................................. 78 3.4 MicroMet Results: Glacier cliff Domain ...................................................... 83 Chapter 4: Initial Melt Model Application: Surface Energy Balance and Melt Thresholds Using a Daily Time-Step Point Model ....................................... 90 4.1 Introduction and Background ....................................................................... 90 4.2 Model Description ........................................................................................ 93 4.3 Results and Analysis ................................................................................... 100 4.4 Discussion ................................................................................................... 110 4.5 Conclusions ................................................................................................. 117 Chapter 5: Hourly Model Calibration: Importance of Internal Melting on Dry Valley Glaciers ....................................................................................................... 120 5.1 Introduction and Background ..................................................................... 120 5.2 Site Description ..........................................................................................
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