W2188: Characterizing Mass and Energy Transport at Different Vadose Zone Scales
Statement of Issues and Justification
Knowledge about physical transformations occurring in the vadose zone is crucial for understanding, predicting and managing biotic and abiotic processes occurring in Earths terrestrial critical zones. Because they form an interface with the atmosphere, near-surface soils within the vadose zone are particularly important for controlling mass fluxes and transforming energy, nutrients, and organic materials. The near-surface environment of the vadose zone also sustains plants with water and essential microbiological communities. Although public awareness of the role of soils - and by extension the vadose zone - is meager, good stewardship of vadose zone functions should be among the highest priorities of our society. Meanwhile, changing societal food and energy demands, land use and climatic conditions, and introduction of man-made substances are imposing ever greater stresses on the vadose zone. The protection and sustainability of this crucial resource can only be assured through a better understanding of vadose zone processes at different spatio-temporal scales.Soil and vadose zone physics plays a critical role in managing soil resources, and outstanding progress in understanding this role has been made in recent years, though only at limited spatio-temporal scales. Indeed, storage, redistribution, transport and transformation processes of water, heat and chemicals are understood for relatively small-scale systems, and an overriding challenge for the scientific (soil physics) community has been to apply our understanding across scales. In 1997, Nielsen stated with respect to the vadose zone, it is there but nobody cares. Much has been learned and documented since then in the Vadose Zone Journal and other esteemed outlets, though knowledge gaps still remain in measurement and modeling, transfer across spatio-temporal scales, and multidisciplinary integration of results.
Non-destructive imaging for understanding sub-visible scale processes--Non-destructive imaging methods such as X-Ray Computed Tomography (CT) yield high-resolution (2 microns), 3-D representations of soil pore space that can be used in conjunction with advanced simulation techniques such as smoothed particle hydrodynamics (SPH) or the lattice Boltzmann (LB) model to significantly advance our knowledge about fluid distribution, interfacial phenomena, and flow and transport processes in agricultural soils. Ongoing and future research areas include improving scanning procedures and developing image segmentation and pore-space analysis tools for quantitatively characterizing soil pore space. A number of CT systems are used today by multi-state researchers, ranging from benchtop scanners to synchrotron microtomographs. They primarily differ in x-ray source and energy, detector geometry, and sample manipulation capabilities. Comprehensive reviews about fundamentals of computed tomography are provided in Stock (1999), Ketcham and Carlson (2001), and Wildenschild et al. (2002). Some applications of CT in porous media research include pore space characterization with respect to variables such as bulk density (Rogasik et al., 1999), volumetric water content (Hopmans et al., 1992; Rogasik et al., 1999), phase distributions (Wildenschild et al., 2002), breakthrough of solutes in porous media (Clausnitzer and Hopmans, 2000; Perret et al., 2000), and pore-scale configuration of immiscible organic fluids in multiphase systems (Schnaar and Brusseau, 2005, 2006a, b).
Transport and Transformations of Colloids and Compounds--Emerging contaminants, such as hormones and pharmaceuticals, are widespread in the environment (Kolpin et al., 2002). Pathogens, which include viruses, bacteria or other microorganisms, are also creating major challenges as water resources dwindle and wastewater is reused. These emerging contaminants, pathogens and metabolites present unique challenges in understanding their fate and transport in the soil-water environment. Many emerging contaminants are potent at very low concentrations and labile, associating strongly with soil solids and undergoing rapid and complex transformations. Specialized laboratory experiments together with modeling and field observations are required to fully understand their fate and transport in the vadose zone (Fan et al., 2007a, b). Often these compounds (pesticides, radionuclides, and metals) can strongly associate with soil colloid particles (10mm in size), which can significantly enhance immobility and persistence (McGechan and Lewis, 2002; Bradford et al., 2003; Bradford and Torkzaban, 2008). Colloids can be mineral particles, organic entities, or small living organisms, like bacteria or viruses, and engineered nanoparticles. They are ubiquitous in soils, and play an important role in soil formation and contaminant fate and transport. Understanding colloidal processes in soils and sediments is important for environmental quality and human health. We need to understand the mechanisms of colloid retention at different interfaces to make accurate predictions of colloid transport and to design effective management and remediation practices to prevent soil and water contamination.
Vadose Zone Role in Quantifying Basin-scale Responses--A traditional soil physicists approach for explaining environmental processes is to examine small scale (~1 m3 or smaller) behaviors and then apply those results to larger, basin-scales using statistical upscaling techniques. This approach has provided significant mechanistic understanding of mass and energy balances, but it has limitations when using the results to explain basin-scale processes (surface runoff, soil moisture estimates for regional scale atmospheric models, large-scale water budgets). A review of the current status and research opportunities (Harter and Hopmans, 2004) in this area implied a need to better link small-scale physics with larger-scale hydrology through upscaling and downscaling approaches, so that soil property variability and dominant factors influencing water exchange can be placed into the appropriate context. For example, Seyfried and Wilcox (1994) identified deterministic length scales that could be applied to scale-dependent influences on hydrologic processes and models. They showed that shrub effects were limited to about 10 m and soil depth became important at length scales from 10 m to 10,000 m (for elevations below 1300 m). This has implications for hydrological response units that are used in large-scale rainfall-runoff and other models. This research program provides an excellent venue for soil physicists to participate in upscaling research, which would provide opportunities to collaborate with climate modelers and hydrologists.
Soil Hydraulic and Thermal Properties--Soil hydraulic properties are key to quantitatively describing soil water flow and chemical transport. Hydraulic properties of natural soils are scale dependent, time dependent, and spatially variable. In agricultural soils, temporal changes in soil hydraulic properties are primarily caused by tillage (Or et al., 2000), clay content and clay mineralogy, and water and soil quality. Changes in soil volume and pore space induced by clay swell-shrink processes present a challenge to developing predictive models for flow and transport, in particular to develop constitutive hydraulic functions. Such functions are important not only for design of man-made hydraulic barriers such as clay liners constructed for waste isolation, but also for fluid flow predictions in porous media (Mitchell, 1993; Benson et al., 1994). Recent advances in pore scale modeling of fluid flow and liquid distribution in rigid angular pores have been developed through our collaborations. These consider both capillarity and adsorption (Tuller et al., 1999; Or and Tuller, 1999; Masad et al., 2000; Tuller and Or, 2001; Tashman et al., 2003) and provide the basis for a proposed multiscale-modeling framework in soils. Other methods for deriving hydraulic properties include pore-scale network models (Vogel, 2000; Vogel et al., 2005; Li et al., 2005) and lattice-Boltzmann methods (Vogel et al., 2005; Zhang et al., 2005; Schaap et al., 2007).
The process of mass diffusion in porous media is important for understanding migration of volatile gas from contaminated sites, transport of gases through the root zone of vegetated soils, and the interaction of aqueous and gaseous chemical constituents with the solid soil matrix. Many methods have been developed for relating the mass diffusion coefficient to phase fractions (solid, liquid, gas) of porous media, most of which have been empirical in form. In recent years and especially in the last decade, the mass diffusion coefficient was related to porous media properties such as the water retention curve (Moldrup et al., 2000; Moldrup et al., 2005; Resurreccion et al., 2008). The dependence of mass diffusion on phase distribution at the pore scale has been examined with pore scale models (Steele and Nieber, 1994a, b) and lattice-Bolzmann models (Chau et al., 2005).
Thermal properties of porous media are important in many environmental and industrial applications. For instance, the thermal conductivity and heat capacity of soils greatly affect the partitioning of solar radiation into components of soil heating, the transfer of long-wave radiation, and sensible and latent heat transfer. The method of DeVries (1963) is commonly used for predicting the thermal properties from texture and phase content of porous media. More recent methods have used pore-scale modeling methods to relate the core-scale thermal conductivity to phase distributions (Hu et al., 2001; Ewing and Horton, 2007).
Multi-scale Flow and Transport Including Impacts from Climate Change--The design of spatial and temporal sampling schemes is based on several questions. How do we sense variation of a soil physical and related state variable or property? How do we separate measurement noise from signal? What are ways of transferring scale-specific soil physical information to different domains while maintaining important variance characteristics? Do the spatial or temporal covariance behavior manifest that our measurement design was adequate to solve the problem? Typically, measurements taken at the instrument scale are used for spatial or temporal processes at some larger domain (Ellsworth and Boast, 1996). For this purpose, a good average of the derived property is often applied with a local-scale model to make a large-scale prediction (Cahill et al., 1999). Averages can be misleading (Stockton and Warrick, 1971). For example, when averages fail to represent the horizontal variability structure (Nielsen et al., 1973), predictions over time may be fallacious when the local status deviates from the mean. Nielsen (1987) questioned, How can we integrate information from the measurement scale to our goal scales?
Investigations of the spatial and temporal variability structure of relevant soil physical state variables (soil water content) and related soil hydraulic functional properties (Western et al., 2004; Comegna and Vitale, 1993; Ünlü et al., 1989; 1990; Shouse et al., 1995) reveal substantial changes of spatial correlation lengths of soil-water-related state variables with time and the magnitude of soil-water content (Roth, 1995; Wendroth et al., 1999; Vereecken et al., 2007; Green et al., 2007; 2009). Accordingly, the cross-correlation structure between soil water and related variables depends on the magnitude of soil water status (Nielsen et al., 1973; Greminger et al., 1985). Moreover, seasonal evolution of soil moisture variance and correlation length reoccurs during times of reoccurring water status (Western et al., 2004).
Soil Physics and Ecological Interactions--Although soil physics and biophysics have addressed soil-plant-climate continuum issues for many decades (Russell, 1960; Campbell, 1977; Wraith and Baker, 1991; Campbell and Norman, 1998; Kirkham, 2005), the mechanistic understanding of plant root response to changing soil environmental variables such as temperature, water/salinity and nutrient concentration is still limited. Whereas a conceptual modeling framework of root response was recently developed (Simunek and Hopmans, 2009), experimental studies need to be conducted to confirm the hypothesized root responses to water, temperature and nutrient stresses, especially to compensation and nutrient uptake mechanisms. Interdisciplinary research has created a growing need and desire within this multistate project to pursue greater collaboration with ecologists and plant scientists. A decade ago, the term ecohydrology was coined and predictions of major breakthroughs and intensive activity in understanding spatio-temporal soil-plant-climate interactions were made (Zalewski et al., 1997; Baird and Wilby, 1999; Rodriguez-Iturbe, 2000). Today ecohydrology is a thriving discipline with soil physics as a major constituent informing research across scales from soil-nutrient-root interactions to desertification (Wardle et al., 2004; Hopmans, 2006; Reynolds et al., 2007). Recent inroads have been made applying principles of soil physics to vegetation and soil patterns (Robinson et al., 2008b), desert ecosystems (Shafer et al., 2007; Wang et al., 2007), Pinion-Juniper woodlands (Lebron et al., 2007), soil microbial habitat (Or et al., 2007), soil biophysics (Smucker and Hopmans, 2007) and other topics.
Improved Multifunction Measurement Devices--Throughout large segments of the terrestrial sciences, including agriculture, ecology, and hydrology, there is a pressing need to improve instruments to better interrogate subsurface environments, especially those capable of providing data for multiple state variables, which affect water movement, nutrient dynamics, plant root behavior, and temperature profiles. Improvements have been highlighted (Mori et al., 2003; Ren et al., 2003), and the broader importance of the technologies toward answering multi-disciplinary questions (Ferré and Kluitenburg, 2003; Jones and Shenai, 2007). These new multi-function probes will be field and laboratory tested. Nonetheless, many of these approaches are still under development and will be improved upon.
Quantifying Near-surface Processes with Instruments and Analyses--Net ecosystem production and net ecosystem exchange are closely tied to soil properties. Researchers have a significant opportunity to assist the agricultural and ecological communities by addressing the importance of soil properties and processes. Heitman et al. (2008) showed an approach that uses heat pulse probes as a means to estimate latent heat flux, a critical component of the energy budget. Upscaling these point-scale values to basin-scale (Zhu et al., 2006) would allow better assessments of regional-scale water status, yet advancement of instrumentation and analyses has lagged the potential societal applications. In particular, remote sensing tools are becoming more sophisticated, but ground truthing is needed.
Computer capabilities have evolved to a point where multi-dimensional, physically-based hydrologic models can be used to study spatio-temporal patterns of mass and energy flow in the vadose zone. However, these models have so far received limited attention because of their computational, distributed input and flux parameter requirements. Global optimization algorithms may help to explore various mechanistic models with differing complexities to analyze the salt and nutrient transport in irrigated areas, at different spatial scales. For example, zone soil sampling may make precision farming practical in the Northern Great Plains, but defining zones is currently subjective. Zone determination could be automated using different scale-appropriate methods, like combining information from different sampling methods, automating nutrient zone boundary determination, and evaluating water quality impacts from precision farming. Field-scale water content, landscape topography, landform, and soil property mapping approaches could link in-field measurements and remotely sensed data to improve resource management. Future opportunities will require us to reach across disciplines and establish working relationships, proposals and integrated research with ecosystem and environmental scientists. We will continue developing interdisciplinary meetings (joint session at the Ecological Society of America meeting, August 2008) and special publications (Young et al., 2007). Experimental methodologies and instruments (large-scale soil property determination) and sampling designs targeted at geostatistics (Isaaks and Srivastava, 1989) and applied statistical time series analyses (Shumway and Stoffer, 2000) will be adapted and developed for agricultural and ecological applications (Nielsen and Wendroth, 2003).
This project seeks to fill these gaps by developing new technologies for measuring transport, transfer, rate and state variables using comprehensive experimental designs that will yield appropriate scaling approaches. We will develop new measurement tools and process statistical structures for both measurements and processes essential for investigating soil ecosystem processes. We will improve conceptual and numerical modeling approaches that couple interdependent processes and improve our ability to transfer measurement and model information between scales. We will use our skills as soil and environmental physicists to advise and participate in national and international multidisciplinary projects to impart the importance of soil resources and the knowledge we have gained through decades of studying this critical zone. And, we will achieve this by participating in activities, like establishing national research site observatories and measuring the spatial distribution of soil moisture across our Nation.
The collaborations created and fostered through the multistate research program have spanned generations of soil physicists and hydrologists, and it is the collective opinion of the participants that multi-institutional and multi-PI collaborations have been significantly enhanced because of the multistate program. Indeed maintaining the focus of such a large group would not be possible without this multistate program. Using these collaborations, significant benefits have been realized through understanding soil physics principles and applying them to environmental sustainability of soil resources, protecting ground and surface waters, improving agricultural production, only to name a few areas. This group has maintained a flexible organization of researchers and field sites, rather than on focused, yet restrictive, approaches like common field sites or identical experimental approaches at different locations. Members tend to form and re-form around new multi-investigator programs, while addressing critical questions. This flexible and synergistic approach has been extremely productive and it encourages a rich pollination of ideas and solutions to complex problems. The multistate committee structure is a convenient and efficient platform for establishing national research collaborations, validating approaches and techniques, pooling data, creating rigorous peer reviews, accessing unique equipment and developing the next generation of highly-educated soil scientists, environmentalists, and engineers. This proposal seeks to maintain the ties between this extremely productive and creative group that without the W1188 committee and its long line of predecessors would not be as focused on national needs research. The proposal also highlights our efforts to improve environmental monitoring, implement basic soil physics research, reach out to a broader scientific community, and educate and communicate to stakeholders and colleagues within and outside our traditional discipline.
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