Essay: Spatial and temporal variations within the critical zone

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  • Spatial and temporal variations within the critical zone
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Review the approaches to characterising the spatial and temporal variations within the critical zone. How do these variations impact critical zone function?
The critical zone is defined as ‘a heterogeneous, near surface environment in which complex interactions involving rock, soil, water, air and living organisms regulate the natural habitat and determine availability of life sustaining resources’ by US NRC, 2001. The critical zone is also defined as the most heterogeneous and complex region of the Earth spans from the vegetation top to the aquifer bottom, with a highly variable thickness globally and a yet-to-be clearly defined lower boundary of active water cycle (Hess, 2010). From these 3 definitions we can come up with a definition that we will use for this paper, the critical zone is a living, breathing constantly evolving boundary layer where rock, soil, water, air and living organisms interact. These complex interactions regulate the natural habitat and determine the availability of life-sustaining resources, including our food production and water quality.
The critical zone provides most of the ecosystem services on which societies depend. Its intrinsic resilience, natural evolution, and fate in the face of human land use and climate change needs to be understood and predicted in order to inform our strategies for sustaining a wide range of human activities. Unprecedented pressures are being placed on the critical zone, and understanding the interrelated processes, system dynamics, sensitivities, and thresholds in this zone is of vital importance for informing human decisions (Hess, 2010).
Understanding the complex web of physical, chemical, and biological processes of the Critical Zone requires a systems approach across a broad array of sciences: hydrology, geology, soil science, biology, ecology, geochemistry, geomorphology, and more. It is no secret that too little is known about how physical, chemical, and biological processes in the Critical Zone are coupled and at what spatial and temporal scales. Many of these processes are highly nonlinear and can range across scales from atomic to global, and from seconds to aeons (CZO, 2014).
In this essay we will look at some of the approaches that seek to quantify variations of the critical zone due to environmental change either spatially or temporally. Some of these will involve remote sensing and some will involve land surface or catchment modelling. Some will go as far as involving field work studies.
Remote sensing has shown significant potential in measuring the changes occurring on the earth’s surface because it allows for the capture of sequential imagery in the same season during which the spectral differences between surfaces are distinct. It does have its resolution restrictions when it comes to analysing small areas.
Most studies nowadays use Hyperspectral Remote Sensing (HRS), which is an advanced tool that provides high spatial/spectral resolution data from a distance, with the aim of providing near-laboratory-quality radiance for each picture element (pixel) from a distance. This information enables the identification of targets based on the spectral behaviour of the material in question.
This encompasses all spectral regions, that is VIS (visible), NIR (near infrared), SWIR (shortwave infrared), MWIR (midwave infrared) and LWIR (longwave infrared)], all spatial domains and platforms (microscopic to macroscopic; ground, air and space platforms) and all targets (solid, liquid and gas).
Along with HRS, there are applications of Reflectance spectroscopy, which is a technique that measures the amount of sunlight absorbed or reflected by a surface at specific wavelengths. The spectra represent mixtures of spectra from individual minerals on the surface along with contributions from absorption lines in the solar spectrum and the atmosphere. By separating out each of these contributions, scientists can compare the resulting spectra to laboratory spectra of known minerals to determine the probable identity and abundance of individual minerals on the surface (Papp et al, 2002).
As photons enter a mineral, some are reflected from grain surfaces, some pass through the grain, and some are absorbed. Those photons that are reflected from grain surfaces or refracted through a particle are said to be scattered. Scattered photons may encounter another grain or be scattered away from the surface so they may be detected and measured. Photons are absorbed in minerals by several processes. The variety of absorption processes and their wavelength dependence allow information to be derived about the chemistry of a mineral from its reflected light (Papp et al, 2002).
There are various lab and field studies that focus on hydrological, geomorphological and biogeochemical variations which can be used for generating models. GIS has also been used through digital elevation models (DEMs) in order to look at variations that occur due to topography. In this paper will look at how all these approaches and how they can be coupled and used in order to measure the spatial and temporal variations that occur in the critical zone. With each study we will look at how the variations can alter the critical zone function (Hess, 2010).
Case Study 1: Spatial and temporal variations across a reef platform
Coral reef islands are recognised as low-lying land surface accumulations of unconsolidated sediments deposited on reef platforms. A substantial amount of variation exists in the size and elevation of such islands. The textural characteristics of their sediments and numerous classification schemes have been developed to account for these differences. Therefore it is assumed that islands formed in low-energy reef settings are comprised predominantly of sand-size sediments, referred to as sand cays. The sediment which comprises these islands is bioclastic in origin and is generated solely from the adjacent reef system. Due to this, island growth, maintenance and change are dependent on the transport of sediments from the reef source to the island sink. Thus we can assume that the most important factor controlling sediment transfers is the composite interaction of waves and currents in operation on the reef surface (Brander et al, 2004).
Brander et al denotes that there is a lot known about the major reef-top processes that control sediment dispersal, but there is very little is known regarding the specific hydrodynamic controls on sediment entrainment, transport and deposition across reef surfaces that eventually control the development and stability of island shorelines. In this research of hydrodynamic studies the paper focussed on those examining the interaction and transformations of incident swell energy against reef platforms. This study exhibits data from a detailed investigation of wave processes across a 2.7 km reef platform on Warraber Island, a coral sand cay situated in Torres Strait, Australia, with the aims of describing spatial and temporal variations in wave characteristics and energy across the reef platform and quantifying the period of time that segments of the reef system are influenced by different types of wave activity over both individual and neap-spring tidal cycles (Brander et al, 2004).
Approach
Measurements of water depth were obtained using five pressure sensors deployed across a 2.7-km section of reef flat from July 3 ‘ 5, 2001 (. The reef surface was classified as uneven and consisted of an outer reef flat, a central reef flat depression, an inner reef ramp, a palaeo-reef surface and the shoreline. The water levels around these areas decreased as they moved landward across the platform with tide ranges at the shoreline being almost 50% lower than at the outer reef flat. Rising and falling tides were characterised by a bimodal energy distribution with both short-period (0 ‘ 3 s) and wind (3 ‘ 8 s) waves present. Higher water levels were dominated by wind waves. The highest waves occurred at high tides associated with nocturnal tidal cycles with Hs decreasing from 0.5 to 0.2 m from the outer reef flat to the shoreline. Reef geometry and changes in water level determine the magnitude of wave energy on the reef platform (Brander et al, 2004). Up to 85 ‘ 95% of incident wave energy is said to have been weakened by the central reef flat depression at high and low tide, respectively, and thus strong linear relationships could be seen between wave energy and height at all locations. Insinuating that both wave height and wave type are strongly depth dependent. Critical reef rim depths required to produce Hs of a given size vary spatially across the reef rim due to variations in reef topography. A distinct depth related threshold exists at which short-period and wind wave dominance reverses. Over a 14-day spring-neap tidal cycle, the time of occurrence of wave action diminishes across the reef platform to the shoreline. Larger waves (Hs = 0.2 m) occur for only 9% of time at the outer reef flat and for less than 0.5% over the remaining reef platform. This implies that on mesotidal (2- 4metres) reef platforms, sediment entrainment and transport are severely constrained under normal wave energy conditions and significant change is likely restricted to extreme events (Brander et al, 2004).
Thus the findings of this study have numerous important implications towards the geomorphology of coral reef platforms and islands such as Warraber. Water level controls across the reef rim severely constrain the time that waves of sufficient energy are able to perform geomorphic work on the reef platform. Temporal representation of the time that waves of a given significant wave height (Hs = 0.05, 0.10, 0.15 and 0.20 m) occur across the reef platform for the spring tidal cycles measured over the experimental period and the entire 14-day spring-neap tidal cycle were recorded. This allowed the potential for sediment entrainment across the reef platform to be viewed as wave energy decreases landward and transport was likely to be most active under highest water levels exhibiting bi-weekly temporal peaks associated with spring high tide conditions. This study also allowed for the assumption to be made that, under normal energy conditions, and for the vast majority of time, the reef platform surface is geomorphologically inactive. Ultimately this meant that for significant changes to occur to sediment production, transport rates and island sediment budgets, extreme waves or storm conditions needed to be existent (Brander et al, 2004).
The results of this study have therefore added significantly to our knowledge of spatial and temporal variations in critical zone characteristics and energy on natural coral reef platforms. Across broad reef platforms such as Warraber, critical water depths at the reef rim have been shown to be of more importance to both the types and time of occurrence of waves acting across the reef platform. The rapid decay of wave energy across the reef rim means that wave characteristics across the reef platform are more dependent on short-period and wind waves than incident swell. Furthermore, the study showed that topographic reef platform variability does exert an influence on reef platform wave characteristics (Brander et al, 2004). Most importantly, however, it is clear that under normal environmental conditions, the amount of time that significant wave energy acts across the reef platform is minimal. The geomorphological implication here is that little opportunity exists for sediment transport to occur under normal energy conditions and that significant change is likely associated with extreme events.
Case Study 2: Spatial Distribution of Tree Species Governs the SpatioTemporal Interaction of Leaf Area Index and Soil Moisture across a Forested Landscape
Vegetation is a crucial part of any ecosystem, due to its role in the transportation of water throughout the landscape by exchanging water between the soil and the atmosphere via change in surface albedo and roughness, canopy water interception, and evapotranspiration. Therefore the survival and distribution of plants on a landscape depend on spatial and temporal patterns of soil water availability. This produces a need for an increased understanding of the spatial and temporal patterns of vegetation water use and underlying mechanisms in order to better understand critical zone variations (Naithani et al, 2013).
Previous studies on arid ecosystems have looked at the strong influence of spatial and temporal patterns of vegetation on horizontal and vertical gradients of soil moisture, but the underlying processes that create spatial and temporal patterns of leaf area index and soil moisture remain poorly understood (Naithani et al, 2013). It is therefore imperative to understand the controlling factors of such an interaction which is vital for characterising carbon, water, and energy cycles at the landscape scale.
This study we looked at the leaf surface as the site of gaseous exchange. This meant that the leaf area controlled terrestrial water, energy and CO2 fluxes. Leaf area index (L), defined as half of the total intercepting leaf area (m2) per unit ground surface area (m2), and is used as a key input to a variety of ecosystem. These along with volumetric soil water content (h) were used as inputs for the hydrologic models that indicates the available soil water for plants. Both leaf area index (L) and soil water content (h) were estimated by ground-based measurements, remote sensing derivations, and simulation modelling (Chem et al, 1992).
Ground-based (field-based) methods are relatively accurate at the site level, but sometimes cumbersome, costly, and even destructive to conduct but remote sensing is a time and cost effective tool for the detection of spatial and temporal changes in L and h over a large (>10 km2 ) area. At small scales (http://www.sciencedirect.com/science/article/B7CSX-4FF2V9H-4/2/ ac05a69eecbfa9b6e4fe079444dbf7db.
Naithani, K., Baldwin, D., Gaines, K., Lin, H. and Eissenstat, D. (2013). Spatial Distribution of Tree Species Governs the Spatio-Temporal Interaction of Leaf Area Index and Soil Moisture across a Forested Landscape. PLoS ONE, 8(3), p.e58704.
Papp, E. and Cudahy, T. (2002). Hyperspectral Remote Sensing. Geophysical and Remote Sensing Methods for Regolith Exploration, pp.13-21.
United States Department of Agriculture, Natural Resources Conservation Service, National Soil Survey Center, (2004). Soil Survey Laboratory Methods Manual. Soil Survey Investigations Report No. 42, Version 4.0. Lincoln: United States Department of Agriculture.
National Research Council (NRC). 2001. Basic research opportunities in the earth sciences. National Academies Press, Washington, DC.
Yoo, K., Amundson, R., Heimsath, A., Dietrich, W. and Brimhall, G. (2007). Integration of geochemical mass balance with sediment transport to calculate rates of soil chemical weathering and transport on hillslopes. J. Geophys. Res., 112(F2).

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