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Chapter 1. Introduction

Chapter 1. Introduction

1.1. Background

Nearly all terrestrial life on the planet depends on the ecosystem services provided by the Critical Zone, a dynamic system of earth’s near surface acting as the nexus of the pedosphere, atmosphere, biosphere, and hydrosphere (Brantley et al., 2007; Anderson et al., 2008). Unfortunately, anthropogenic forces are now applying significant stresses and pressures around the global, generating an urgent need for a better understanding of this complex system (National Research Council, 2001). Further complicating the scenario, the dynamics of Critical Zone exist over a large range of spatial and temporal scales, from tectonic activity, chemical weathering and drainage network development over large regions taking millennia to fluid transport and ion exchange in soil pore spaces acting on the order of seconds or microseconds. Often associated with these processes are feedbacks and process interactions, which can be difficult to understand without interdisciplinary efforts. Progress has been made by various disciplines including soil scientists (Wilding and Lin, 2006), hydrologists (Kirchner, 2003; Lin et al., 2006a; McDonnell et al., 2007), geochemists (Anderson et al., 2007), and ecologists (Brooks et al., 2009) on various processes yet further work is required to fully understand the complexities of the Critical Zone. In 2006, the Susquehanna-Shale Hills Critical Zone Observatory (SSHCZO) was established to further the understanding of Critical Zone processes and to ultimately understand regolith formation, evolution and function including hydrologic flow paths and timescales (Brantley et al., 2007; Anderson et al., 2008). SSHCZO provided an excellent opportunity to further environmental research due to its proximity to the Penn State campus and because of the previous hydrologic work accomplished at the site (Nutter, 1964; Lynch, 1976; Duffy, 1996). Located in the Ridge and Valley Physiographic Province of eastern United States, SSHCZO has the added benefit of being a classic headwater basin of the Chesapeake Bay watershed (Figure 1-1), with work potentially contributing to the Chesapeake Bay Program and other related projects. Figure 1-1: Location of Susquehanna-Shale Hills Critical Zone Observatory in relation to the Chesapeake Bay watershed. Numerous hydrologic studies at the catchment allowed researchers to build off previous findings (Nutter, 1964; Lynch, 1976; Duffy, 1996). Early work established the importance of antecedent soil moisture to quickflow and total storm runoff (Lynch, 1976). More recent works have made substantial progress with respect to soil moisture dynamics (Lin, 2006; Lin et al., 2006b), preferential flow pathways (Lin and Zhou, 2008; Graham and Lin, 2011), and solute transport (Jin et al., 2010, 2011; Andrews et al., 2011; Kuntz et al., 2011; Jin and Brantley, 2011). Additionally extensive distributed hydrologic modeling using the Penn State Integrated Hydrologic Model (PIHM) has successfully detailed the importance of antecedent soil moisture, topographic controls of connectivity, saturated/unsaturated storage dynamics, and isotopic age distributions (Duffy, 1996; Qu and Duffy, 2007; Bhatt, 2012). To better grasp the driver of many complex environmental processes, research has focused on development of a conceptual hydrologic model. Jin et al. (2011) used water stable isotopes and Magnesium concentrations on one planar hillslope in SSHCZO to better understand surface and subsurface hydrologic processes. Their findings suggest the prevalence of three types of water: low flow waters in the A and B horizons with high Mg concentrations, high flow waters with low Mg concentrations, and groundwater with high Mg concentrations. They also found residence times within the catchment were relatively short, typically less than one year. The work of Holmes (2011) further refined the estimates of residences times within SSCHZO by using a piecewise constant input isotopic age model. His findings suggest the mean age of the water in the catchment reached a peak of 9 months during the summer drought and a minimum of 4.5 months during the winter with an average of 5-6 months. Bhatt (2012) further refined the estimates of isotopic age at SSHCZO by using a spatially distributed, multi-process numerically based model equipped with transport equations. His results showed the space-time average isotopic age for the catchment was 217 days or 7.2 months and varied from 85 days to 347 days. His estimates correspond to an age for the integrated depth of water over all grid cells in the catchment. All results of the previous studies demonstrate the transient nature and complexity of hydrology at SSHCZO.

1.2. Water Stable Isotope Overview

One promising tool at SSHCZO is the use of stable isotopes of water (oxygen-18 and deuterium (²H)) as natural tracers to determine water flowpaths and hydrologic process timescales. Because stable isotopes move with the water molecule itself, they allow for nondestructive, long-term monitoring of subsurface hydrologic processes (Vanclooster et al., 2005; Leibundgut et al., 2009). Over the past decades, water stable isotopes have been used successfully to determine groundwater recharge rates (Gat, 1971; Darling and Bath, 1988; Davisson and Criss, 1993; Criss and Davisson, 1996; Winograd et al., 1998), hydrograph separation (Sklash et al., 1976; Sklash and Farvolden, 1979; Pearce et al., 1986; Rice and Hornberger, 1998), preferential flow paths (McDonnell, 1990; Leaney et al., 1993; Kirchner, 2003; Vogel et al., 2010), soil water evaporation (Zimmermann et al., 1968; Barnes and Allison, 1983, 1988; Gazis and Feng, 2004), and sources of transpired water (Dawson and Ehleringer, 1991; Ehleringer and Dawson, 1992; Brooks et al., 2009; Gierke, 2012). Stable isotope hydrology dates back nearly 80 years with the discovery of seawater being isotopically heavier than fresh waters (Gilfillan, 1934). Unfortunately, soon after this discovery, World War II forced researchers to devote efforts towards more defense-minded endeavors, including the development of mass spectrometry. After the war, researchers equipped with these new laboratory methods began to analyze natural abundances of oxygen-18 and deuterium in meteoric waters around the globe and started to explain the underlying physics controlling the abundances (Epstein and Mayeda, 1953; Friedman, 1953; Dansgaard, 1954). Efforts to summarize and standardize global results (Craig, 1961a; b) laid the framework for future isotopic studies by formally establishing a linear relationship between concentrations of oxygen-18 and deuterium and defining a standard for use globally. In 1961, Craig was the first to propose development of Standard Mean Ocean Water (SMOW) which then allowed samples to be compared against each other (Craig, 1961a), simplifying analysis and promoting world wide investigation of isotope hydrometeorology. Standard isotope notation follows the form: !^! (1) !_!(!”#$%&) = _!_!”#$−1 × 1000 where R is the ratio of deuterium (²H) to hydrogen (H) or ¹⁸O to ¹⁶O in the standard (X) or Standard Mean Ocean Water (SMOW). Delta values are multiplied by 1000 for convenient comparisons of samples. Over time SMOW has undergone revisions to more precisely establish a world wide standard for isotopic composition of ocean water, the most recent revision occurring in 2009 with the development of V-SMOW2 or Vienna Standard Mean Ocean Water 2 (International Atomic Engery Agency, 2009). The International Atomic Energy Association (IAEA) was also influential in the coordination of an observation network intended to monitor oxygen-18, deuterium, and tritium concentration spatial and temporal patterns across the globe. In 1964, Dansgaard thoroughly detailed the preliminary results of the IAEA’s network and outlined the geochemical processes governing them (Dansgaard, 1964). He found empirical relationships relating precipitation isotope compositions to physical and environmental parameters such as surface temperatures, latitude, distance from coast, and amounts of precipitation. Later studies supported his findings (Gat and Gonfiantini, 1981; Rozanski et al., 1993). The empirical relationships can be thought of as representing the degree of rainout and associated isotopic depletion as the air mass is transported from its source area to the site of precipitation. That is to say, the isotopic composition of precipitation contains information regarding the history of a given air mass and the associated atmospheric circulation patterns. The isotopic composition of precipitation is controlled by the interaction of four main factors: the meteorological (e.g. temperature, humidity) conditions at the source area, the fraction of available moisture, snow formation or evaporation of liquid from falling droplet, and mixing of different air masses (Gat, 1996; Araguás-Araguás et al., 2000). Much of these factors may vary seasonally leading to seasonal variation in isotopic compositions for a given site. Deuterium and oxygen-18 compositions of precipitation have been found to vary linearly at the global (Craig, 1961b; Rozanski et al., 1993; Araguás-Araguás et al., 2000) and regional scales (Fritz et al., 1987; IAEA, 1992). Refining Craig’s (1961b) initial estimate of the Global Meteoric Water Line (GMWL: !”=8 δ^!”O+10 ‰), Rozanski et al. (1993) found the GMWL based on long term amount weighted means of 205 observation points to be: !”=8.17 ±0.07 δ^!”O+11.27 ±0.65 ‰ (2) The GMWL can be though of as a composite of many local meteoric water lines (LMWL) from different locations or regions. Deviations from the GMWL are due to evaporative processes which influence the d-excess value (d-excess = !”− 8 δ^!”O, originally defined by Dansgaard (1964)), and is commonly found in arid or semi-arid environments. Typically LMWLs for arid or semi-arid regions have slopes less than 8. Slopes larger than 8 are possible in locations where depleted precipitation values fall below the GMWL and relatively enriched precipitation has high d-excess values. Locations with large LMWL slopes typically have two or more distinct sources of precipitation (Araguás-Araguás et al., 2000).

1.3. Geospatial Data and Surveying at SSHCZO

Concurrently with much of the isotope hydrology studies performed over the past two year, substantial progress was made surveying and mapping instrumentation at SSHCZO. Because of the scope and complexity of SSHCZO, instrumentation is continually being installed, and thus surveying is required to precisely map relative locations within the catchment. To expedite all future survey efforts, semi-permanent control points were installed and marked throughout the catchment based off permanent monuments installed by a professional surveyor in the summer of

Control points consist of 0.75 inch diameter rebar emplaced in the soil with approximately 2 inches of rebar exposed at each control point to allow for removal if desired. These additional control points now accelerate surveyors ability to quickly setup and gather coordinate data as well as check accuracy in the field.

1.4. Research Objectives and Efforts

There are two general objectives of this thesis: 1) building a robust understanding of water stable isotopes at the SSHCZO and 2) developing a high-resolution dataset of isotopes in precipitation for future implementation into the Penn State Integrated Hydrologic Model (PIHM) and HydroTerre. Ideally, with the knowledge gained through work on the first objective, an improved data product will be developed for the second objective. Chapter 2 partially addresses the first objective through in-depth investigation of the spatiotemporal patterns of soil water isotope compositions at SSHCZO. The motivation behind investigating soil water isotope compositions was to to improve our understanding of subsurface hydrologic processes and supplement the overall conceptual hydrologic model. The relationship between soil water, precipitation, and groundwater isotope compositions is also discussed briefly in the second chapter. Chapter 3 continues to addresses the first general objective of building a robust understanding of water stable isotopes at SSHCZO by more closely examining temporal patterns of precipitation stable isotope compositions. The results from this work are intended to support the development of the data product in Chapter 4 as well as to support any future isotope hydrology studies at SSHCZO or nearby sites. The second main objective was addressed in Chapter 4. Knowledge from the previous chapters was applied to the comparison of observed isotopic composition of SSHCZO precipitation to two related isotope-incorporated atmospheric general circulation models. It is intended that with a valid comparison, the results from one isotope-incorporated atmospheric general circulation models would suffice as the initial data product for isotopes in precipitation for the Chesapeake Bay watershed. The comparison was performed using standard regressions and singular spectrum analysis. With the results of the comparison, a 32-year record of isotopes in precipitation was established for SSHCZO. Future work includes validation of the isotope incorporated atmospheric general circulation model at other spatial locations and the development of a usable data layer for PIHM implementation. SPATIAL AND TEMPORAL PATTERNS OF WATER STABLE ISOTOPE COMPOSITIONS AT THE SUSQUEHANNA-SHALE HILLS CRTICAL ZONE OBSERVATORY

SPATIAL AND TEMPORAL PATTERNS OF WATER STABLE ISOTOPE COMPOSITIONS AT THE SUSQUEHANNA-SHALE HILLS CRTICAL ZONE OBSERVATORY

Table of Contents Hide

Chapter 1. Introduction

1.1. Background

1.2. Water Stable Isotope Overview

1.3. Geospatial Data and Surveying at SSHCZO

1.4. Research Objectives and Efforts

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SPATIAL AND TEMPORAL PATTERNS OF WATER STABLE ISOTOPE COMPOSITIONS AT THE SUSQUEHANNA-SHALE HILLS CRTICAL ZONE OBSERVATORY

Table of Contents Hide

Chapter 1. Introduction

1.1. Background

1.2. Water Stable Isotope Overview

1.3. Geospatial Data and Surveying at SSHCZO

1.4. Research Objectives and Efforts

Tags:

Leave a Reply Cancel reply

How Do You Solve Limit Questions by Using Online Limit Calculator

IMPLEMENTATION OF CONTEMPORARY HONEY BEE PRODUCTION AND PROCESSING PRACTICES AMONG FARMERS IN KOGI STATE, NIGERIA

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