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The Role of Fog in the Hydrological Functioning of Tropical Island Ecosystems

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The Role of Fog in the Hydrological Functioning of Tropical Island Ecosystems THE ROLE OF FOG IN THE HYDROLOGICAL FUNCTIONING OF TROPICAL ISLAND ECOSYSTEMS Sarah Rose Schmitt A dissertation submitted to the faculty at the University of North Carolina at Chapel Hill in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Geography (Ecohydrology) in the College of Arts & Sciences. Chapel Hill 2018 Approved by: Diego Riveros-Iregui Jia Hu Conghe Song Aaron Moody Lawrence Band © 2018 Sarah Rose Schmitt ALL RIGHTS RESERVED ii ABSTRACT Sarah Rose Schmitt: The Role of Fog in the Hydrological Functioning of Tropical Island Ecosystems (Under the direction of Diego Riveros-Iregui) Fog is a critical water source in many tropical ecosystems, especially those that are semi- arid, or seasonally dry. Patterns of fog water input to these ecosystems are poorly understood, and currently limited by a lack of in-situ data spanning both space and time. Large gaps exist in our understanding of the spatiotemporal variability and mechanisms driving fog water deposition, and how fog travels through tropical systems. Given the significance of fog to semi-arid ecosystems across the globe, I use stable isotopes, remote sensing and plant physiological analyses to examine the role of fog in the semi- arid ecosystems of San Cristóbal Island, Galápagos and Ascension Island, UK by utilizing data from four field campaigns. I first create a ground-based optical fog detection scheme to trace fine-temporal scale mechanisms driving fog formation, evolution and dissipation across varying hydroclimatic zones. I then establish the isotopic signature of fog and other environmental waters to assess the overall contribution of fog to different microclimatic zones and under different hydroclimatic regimes. And finally, I trace fog through the Galápagos ecosystem, specifically examining how native versus invasive flora utilize fog under varying hydroclimatic conditions. In this research, I create a simple model to predict degree-of-fogginess with commonly measured meteorological variables, and I show how different fog formation mechanisms can be taking place in tandem over a concentrated spatial scale. I also demonstrate that fog is a common phenomenon on San Cristóbal Island, especially during the dry season, and that fog is iii consistently enriched compared to co-collected rainfall. Finally, I utilize the isotopic signature of fog and other environmental waters to trace water sources through the Galápagos ecosystem, suggesting that invasive guava’s water use strategy (including fog water utilization) plays a key role in its competitive capacity versus co-occurring native plants. Through this island lens, I address the critical disconnect between the hydrological and ecological role that fog plays in tropical island ecosystems. Taken together, these findings will be critical in projecting future water resource availability across many seasonally arid ecosystems, especially those that may become drier under future regimes of climate change. iv ACKNOWLEDGEMENTS I am forever indebted to a number of people who have helped me in a number of ways. From making sure that I had a drink in hand at TRU when I needed to take a step back from my research to encouraging me to pursue research that I once thought was beyond my comprehension, you have all been incredible. I gratefully acknowledge the mentorship of my advisor, Diego. Your guidance has had an incredible impact on me, and you have challenged me to become a better scientist. I am so appreciative of the time you have invested in helping me develop my ideas into invigorating, “sexy” projects, and you have taught me that I am far more intelligent and capable than I ever thought possible. We have truly grown together over the past five years, and I hope that I have taught you at least a fraction of what you have taught me. I am also immensely grateful to my committee members: Jia, Conghe, Aaron and Larry. You are all incredible scientists. And I am beyond grateful that you all have provided amazing insights throughout this whole dissertation process that have played a pivotal role in my research and writing. I have genuinely enjoyed spending time with and getting to know each of you. I also gratefully acknowledge my colleagues and collaborators outside of my committee. Dr. Touchette, thank you for being an incredible mentor and fostering my passion for research. Coertney, Pepe, Giulianna, Pablo, Erin, Rebecca and Madelyn, thank you all for your support in the field. Geovanny and Angel, thank you for being such amazing collaborators on San Cristóbal. Fellow members of the Carbonshed Lab, thank you all for your feedback on my project, from its infancy to completion. Tong, Nuvan and Varun, thank you for tolerating my v endless questions about coding and statistics. John and Manny, thank you for being my work wives and reminding me that you can work hard and play hard. And thank you to my countless friends and supporters too numerous to name. I’d also like to thank the organizations and institutions that have provided me with funding and logistical support throughout my research. Funding for this research was provided by: the NSF Graduate Research Fellowship, NSF SAVI: Crossing the Boundaries of Critical Zone Science with a Virtual Institute, the UNC-USFQ Consortium, the Geological Society of America Graduate Student Research Grant, the National Geographic Young Explorer’s Grant, the Consortium of Universities for the Advancement of Hydrologic Science, Inc. (CUAHSI) Pathfinder Fellowship, the National Center for Atmospheric Research (NCAR) Visiting Researcher Award, the UNC Department of Geography, the UNC Institute for the Study of the Americas, the American Geophysical Union, the Thomas S. Kenan III Graduate Fellowship, and the Thomas S. and Helen Borda Royster & Snowden and Elspeth Merck Henry Dissertation Fellowship. Logistical support for this research was provided by the Galápagos Science Center, the Galápagos National Park, the Ascension Island Government Conservation Department, and the United States Air Force on Ascension Island. Lastly, I am especially grateful to my loved ones who have always had faith in me even when I didn’t have faith in myself. Mom, Dad, Marybeth, Emily and Sammy – your love and support for me has never wavered. Mom and Dad, your countless sacrifices and unconditional love have afforded me more opportunities than I ever thought possible. Krista and Alex, your empathy and humor were pivotal in helping me face the challenges of graduate school. And Sam, your love, kindness and support have been invaluable. Without your encouragement, this dissertation certainly wouldn’t be possible. vi TABLE OF CONTENTS LIST OF TABLES………………………………………………………………………………..xi LIST OF FIGURES……………..……………………………………………………………….xii LIST OF ABBREVIATIONS AND SYMBOLS………………………………………………..xv CHAPTER ONE: INTRODUCTION………………………………………………..……………1 CHAPTER TWO: CHARACTERIZING FOG INUNDATION ALONG A TOPOGRAPHIC GRADIENT WITH GROUND-BASED CAMERAS………………………....5 Introduction………………………………………………………...……………………...5 Methods………………………………………………………………………..…………..9 Study site……………………………………………………….………………….9 In-situ, camera-based data collection ……………………………………………11 Image preprocessing……………………………………………………………..12 Normalized Difference Fog Index (NDFI) & Degree-of- Fogginess Model (DOFmodel) …………………………………………………....12 Validation of degree-of-fogginess model (DOFmodel) …………………………...13 Predicting degree-of-fogginess via a random forest model……………………...14 Assessing the utility of DOFmodel relative to meteorological measurements…….15 Understanding spatiotemporal patterns of degree-of-fogginess…………………16 Results…………………………………………………………………………..………..16 DOFmodel as a proxy for degree-of-fogginess…………………………………….16 Validating DOFmodel as a model for degree-of-fogginess………………………..17 Understanding fog cover (DOFmodel) through random forest…………………….17 vii Utility of DOFmodel relative to meteorological measurements…………………...18 Diurnal- and elevation-based assessment of fog cover: relevant meteorological parameters in predicting fog persistence in near-real time……...18 Discussion……………………………………………………………………..…………20 Is DOFmodel a suitable proxy for degree-of-fogginess?..........................................20 Do fog formation mechanisms vary by elevation?................................................21 Conclusions………………………………………………………………………………23 Tables and Figures………………………………………………………….……………25 CHAPTER THREE: THE ROLE OF FOG, OROGRAPHY AND SEASONALITY ON PRECIPITATION IN A SEMI-ARID, TROPICAL ISLAND…………………………………..35 Introduction………………………………………………………...…………………….35 Materials and Methods………………………….……………………………..…………39 Characterizing the Galápagos hydroclimate……………………….…………….39 Fog collector design……………………………………………………………...41 Precipitation collection…………………………………………………………..41 Climate variables………………………………………………………………...42 Isotope analysis and isotope theory……………………………………...………42 Data processing…………………………………………………………………..45 Results…………………………………………………………………………..………..46 Seasonality in Galápagos………………………………………………………...46 Comparing seasonal isotopic values of fog, rain, and throughfall……………….46 Variability of isotopic composition of water sources (δ2H, δ18O, D-excess)……47 Discussion……………………………………………………………………..…………48 Does fog isotopic composition vary on event and seasonal timescales?...............49 viii What drives the seasonal variability of throughfall isotopic composition?...........51 Does the amount effect control rainfall isotopic composition?.............................52 Does moisture source region control
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