University of Nevada, Reno The Ecohydrology of Devils Hole, Death Valley National Park A dissertation submitted in partial fulfillment of the requirements for the degree Doctor of Philosophy in Hydrogeology by Mark Blanchard Hausner Dr. Scott W. Tyler/Dissertation Advisor May, 2013 Copyright by Mark Blanchard Hausner, 2013 All Rights Reserved THE GRADUATE SCHOOL We recommend that the dissertation prepared under our supervision by MARK BLANCHARD HAUSNER entitled The Ecohydrology Of Devils Hole, Death Valley National Park be accepted in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Scott W. Tyler, Ph.D., Advisor Thomas Torgersen, Ph.D., Committee Member Sudeep Chandra, Ph.D., Committee Member Kevin P. Wilson, Ph.D., Committee Member W. Patrick Arnott, Ph.D., Graduate School Representative Marsha H. Read, Ph. D., Dean, Graduate School May, 2013 i Abstract Devils Hole, a water-filled fracture in the carbonate aquifer underlying the Mojave Desert, is home to the only extant population of Devils Hole pupfish (Cyprinodon diabolis). In the mid to late 1990s, the population of C. diabolis began an unexplained decline. A number of different hypotheses have been advanced to explain this decline, including the impacts of climate change. This study combines field observations and computational fluid dynamic (CFD) modeling to address the effects of climate on the physical processes in Devils Hole, relating those physical processes to the conservation of Devils Hole pupfish. Fiber-optic distributed temperature sensors (DTS) were deployed in Devils Hole to observe temperatures at high spatial and temporal resolution, and the DTS data were used in conjunction with previously recorded temperatures to calibrate and validate FLUENT-based CFD models of convection in Devils Hole. Seasonal convection cycles in the deep pool of Devils Hole are controlled by surface temperatures and the geothermal gradient in the area, and occur primarily during the food-limited winter. Diurnal convection cells controlled by daily meteorological variation occur on the ecologically critical shallow shelf throughout the year, limiting the temporal window during which C. diabolis can successfully spawn. Simulations of the shallow shelf under past climate conditions and future projections show that climate change has likely already impacted the population of C. diabolis, contributing to their recent decline. Future simulations indicate that the window of annual recruitment is likely to occur earlier and earlier in the year, resulting in a shift of more than two weeks in the timing of annual recruitment cycle. This shift drives a considerable reduction of the food available to larval pupfish, reducing the likelihood of successful recruitment and exacerbating the ii conditions that led to the recent decline of the C. diabolis population. Finally, potential mitigation strategy of raising the water level in Devils Hole is examined, and future research into this strategy is recommended. iii For my family, whose love, support, and patience helped to make this possible. iv Acknowledgments This work was made possible by funding from the Nevada Department of Wildlife, the Death Valley Natural History Association, the United States National Park Service, and the National Science Foundation. Thanks to my friend and advisor, Dr. Scott Tyler for his constant support during my time at the University of Nevada. Thanks also to my committee members Sudeep Chandra, Kevin Wilson, Tom Torgersen, and Pat Arnott for their time and support on this work. Their advice and support helped to shape this work, and I greatly appreciate it. Francisco Suárez, Lucas Williamson, Chris Sladek, Jazmín Aravena, and Margaret Shanfield provided much needed support throughout my time at the University of Nevada. Field assistance from Darrick Weissenfluh, D. Bailey Gaines, and the Devils Hole Dive Team was invaluable in collecting these data. Gayton Scoppetone and Paul Barrett helped to formulate the project and offered assistance during the research phase. Thanks also to John Selker, Nick van de Giesen, and Michael Mondanos for their thoughtful advice throughout the project. Thanks, finally, to my family, without whom I never would have done this. v Table of Contents 1: Introduction 1 2: Previous Work 4 2.1: The Devils Hole pupfish (Cyprinodon diabolis) 4 2.2: The geology and hydrology of Devils Hole 8 2.3: Management and conservation efforts 13 3: Study goals, materials and methods 16 3.1: Components of this dissertation 16 3.1.1: Seasonal convection in the deep pool of Devils Hole 16 3.1.2: Diurnal temperature dynamics on the shallow shelf 17 3.1.3: Simulation of the shallow shelf under different scenarios 17 3.2: Materials and methods 18 3.2.1: Field data collection 18 3.2.2: Computational fluid dynamics modeling 24 3.2.3: Simulations of future conditions 27 3.3: Synthesis and conclusions 28 4: Seasonal convection in the deep pool of Devils Hole 29 4.1: Introduction 29 4.2: Study site, materials, and methods 34 4.2.1: The Devils Hole system 34 4.2.2: Fiber-optic distributed temperature sensing 36 4.2.3: Composite temperature profiles and statistical analyses 41 4.2.4: Numerical simulations and flow characterization 42 4.3: Results and discussion 46 4.3.1: DTS temperature profiles 46 4.3.2: Numerical simulations of mixing processes 51 4.3.3: Effects of divers on vertical mixing processes 55 4.3.4: Ecological impacts of seasonal convective mixing 57 4.4: Conclusions 60 5: The shallow thermal regime of Devils Hole 62 5.1: Introduction 62 5.2: Study site, materials, and methods 66 5.2.1: The Devils Hole system 66 5.2.2: Field data collection 68 5.2.3: Computational fluid dynamic modeling 70 vi 5.2.4: Sensitivity analysis 78 5.3: Results and discussion 79 5.3.1: Field data 79 5.3.2: Model simulations 85 5.3.3: Model sensitivity 93 5.3.4: Vulnerability to climate change 96 5.4: Significance to aquatic environments 99 6: The changing thermal regime of Devils Hole 101 6.1: Introduction 101 6.2: Methods 102 6.2.1: Numerical simulations 102 6.2.2: Simulations of varying water level 103 6.2.3: Present-day simulations 103 6.2.4: Past and future simulations 104 6.3: Results and discussion 109 6.3.1: Depth of water on the shallow shelf 109 6.3.2: Present and historical simulations 113 6.3.3: Projected simulations 116 6.4: Conclusions 121 7: Conclusions and recommendations 123 7.1: Summary of results 123 7.2: Management implications 125 7.3: Future research needs and opportunities 127 8: References 130 vii List of Tables 4-1: Dates and details of four 2009 DTS field deployments 39 4-2: Details of each 24-hour composite temperature profile 49 4-3: Pearson’s linear correlation matrix for the mean temperature profiles 50 from the four 2009 deployments 4-4: Numerical simulation results 54 5-1: Light attenuation parameters 77 5-2: Model parameterization 87 5-3: Previously reported water temperatures in Devils Hole 93 5-4: Model sensitivity 94 6-1: Delta change offsets for each simulated timeframe 108 viii List of Figures 2-1: Devils Hole pupfish (Cyprinodon diabolis) 5 2-2: Temperature ranges best suited to various stages of the life cycle of 7 various Cyprinodon species 2-3: Area map of Devils Hole 9 2-4: Orthogonal cross-sections of Devils Hole 10 3-1: High spatial resolution DTS cross sectional array 22 4-1: SW-NE and NW-SE orthogonal cross-sections of Devils Hole 35 4-2: Raw Raman spectra data and instrument-calibrated temperature 39 4-3: 24-hour temperature profiles and mean temperature profiles recorded 47 during the four 2009 field campaigns 4-4: Summer and winter temperature profiles observed during the course of 48 2009 4-5: Deviations from the mean temperature in time 48 4-6: Typical simulated water column temperatures and convection patterns 53 5-1: Seasonal cycles in Devils Hole 64 5-2: The shallow shelf of Devils Hole 67 5-3: Schematic of CFD model (not to scale) 70 5-4: DTS temperature observations for 23-24 June, 2010 81 5-5: Spatial variability in the DTS observations 83 5-6: Mean water column temperature calculated from raw and radiation- 84 corrected DTS observations 5-7: Mean observed and simulated water column temperatures and simulated 86 spatial variability of water column temperatures 5-8: Water temperature and circulation patterns recorded at different times 89 over a 24-hour simulation 5-9: Histograms of observed and simulated temperatures at two different depths 92 5-10: Sensitivity of the Devils Hole system to changes in air temperature 95 5-11: Simulated major heat fluxes for June over a section of the shallow shelf 97 6-1: Grid cell scaling illustration 106 6-2: Annual cycle of simulated shallow shelf water temperatures in a 10 cm 110 and 90 cm deep water column 6-3: Simulated peak daily water temperature vs. water column depth for spring 111 6-4: Simulated water temperature on the shallow shelf, compared to observed 114 water temperatures 6-5: Simulated historical and present water temperatures on the shallow shelf 115 6-6: Present and projected water temperature 117 6-7: Simulated peak daily temperature and carbon availability 120 1 1 Introduction Devils Hole, found at 36°25’ N, 116°17’ W, is a small geothermal spring in the Mojave Desert of southern Nevada, and provides a window into the carbonate aquifer system (Riggs and Deacon, 2002). Located within the boundaries of Ash Meadows National Wildlife Refuge (Nye County, Nevada), the spring and a 40 acre parcel surrounding it are managed as a disjunct portion of Death Valley National Park (Anderson and Deacon, 2001).