W1188: Characterizing Mass and Energy Transport at Different Scales
Statement of Issues and Justification
Citizens, our ultimate stakeholders, consider that affordable, high-quality food and fiber are central to their quality of life. At the same time, they overwhelmingly desire a clean and safe environment in which to live. Soils are critically important components of the earth's biosphere. They are central to food and fiber production, and to global environmental quality (Doran and Parkin, 1994). Balancing societal goals for a clean environment with profitable and sustainable agricultural systems requires sound fundamental understanding of the complex interrelationships between mass (e.g., water, nutrients, pesticides) and energy (e.g., heat, light) transport in soils and exchanges with the adjacent atmosphere. These processes are highly variable in space and time, and the transfer of our understanding derived at given scales to applications at greater or smaller scales has not always been successful. Identifying relevant core processes, developing new means for quantifying, measuring or simulating essential components, and understanding the interrelationships between agricultural and environmental goals or problems will lead to more powerful management of our natural resources.Because of the broad and interrelated nature of the problems associated with mass and energy transport in soils and atmosphere, collaborative multidisciplinary approaches are necessary to substantially enhance our abilities to manage agricultural landscapes. No single individual or institution has sufficient talents or resources to undertake the required activities. A synergism of multidisciplinary scientists working collaboratively through the auspices of our multistate research committee can have a significant impact on immediate as well as longer-term new technologies that will provide our society with safe, low-cost food supplies, while simultaneously reducing the environmental impacts of food production and related practices.
Management strategies that utilize the unsaturated soil (vadose) zone are widely considered superior to those involving groundwater, due to long chemical residence times and compounded access problems once contaminants enter groundwater reservoirs. In the latter case, damages are often irreversible, so prevention and remediation of soil and groundwater contamination should begin with informed management of the unsaturated zone (van Genuchten, 1994). The transport of mass and energy define many of the most relevant soil processes. Knowledge concerning the behavior of soil water and soil-applied chemicals including nutrients, pesticides and contaminants greatly expanded during the era following World War II, and has continued at an increasing pace to the present. Transport of energy in the form of heat impacts all biogeochemical reactions taking place in soil environments. Water, heat, and chemical transport are often coupled. Greater understanding has led to increased appreciation for the complexity and myriad interrelationships of mass and energy transport phenomena in soils, and of the necessity to achieve greater knowledge and technology to gainfully manage these processes. Members of this committee and their colleagues both here and abroad are leaders in fields most closely aligned with these topics.
The multistate efforts of this project are consistent with USDA national program goals, including protection of surface and ground waters, development of new instrumental and analytical techniques, and supporting a secure and sustainable agriculture. Efforts are also consistent with the goals of other programs and initiatives, including Sustainable Agriculture (SARE).
The multiple-collaborations that characterize the proposed and past project activities can provide increased benefits relative to more focused yet restrictive approaches such as common field sites or identical experimental approaches carried out at different locations. While the proposed activities do in fact include some of these endeavors, committee members have a productive history of working together in small teams to develop complementary knowledge, tools, or approaches, sharing findings with each other, then incorporating and applying these new resources in re-configured teams to further advance the states of knowledge and technology. We view this as a more flexible, more synergistic, and (importantly) a more productive approach to multi-state, multi-agency, multi-investigator collaborative research. W-188 members have willingly worked very productively with each other and with those outside the committee, without the necessity to closely proscribe the nature of the interactions. The multistate committee structure provides an official and convenient means for periodic formal professional interactions among the entire group, which we all find extremely valuable.
Objective 1 - To develop an improved understanding of the fundamental soil physical properties and processes governing mass and energy transport, and the biogeochemical interactions these mediate.
Despite our best effort some physical properties and processes have eluded our understanding, especially at differing scales. The dynamics of water and solute transport are well understood at the lab or column scale, but we are unable to fully translate these processes to the field scale. Many current approaches are limited because of the enormous amount of data required to characterize flow of mass and energy at scales larger than a few square centimeters or meters. We will work to increase our understanding in several areas related to water, gas and temperature fluxes in near surface soils. These include, but are not limited to, preferential flow of water and/or solute, carbon sequestration, transport of reactive chemicals, scale- and time-dependent hydraulic properties, coupled heat and mass transport, effects of micro-gravity on plant growth systems for space exploration, root water uptake, and colloid transport or colloid assisted transport, and the transport of biologically active chemicals (hormones and other pharmaceuticals).
Preferential and Unstable Flow
Transport in highly heterogeneous materials such as structured soils and coarse sediments is often associated with preferential or unstable flow pathways. Transport models using the advection-dispersion and Richards equations often fail to simulate reality because large portions of the infiltration fluid may flow in discrete open pathways such as fractures or unstable fingers driven primarily by gravity (Kampf et al., 2002). Preferential water flow plays an important role in accelerated transport of contaminants toward ground water, resulting in unexpected concentrations or species found in drinking water and other wells. Preferential flow is not only found in natural soils, but also poses difficulties relative to heap leach mining operations common in Nevada and other western states (Kampf et al., 2002). In this process, irrigation of a reactive solution, typically cyanide, onto gold-bearing ore piled 10-100 m tall results in leaching of residual gold for extractive recovery. At mine closure, large amounts of rock material remain saturated with cyanide solution and effective rinsing of this material is an important step in environmental remediation.
Our ability to predict the accelerated transport of contaminants through preferential flow paths is severely limited. We cannot yet forecast where, when, and at what velocity the contaminants will move. We can effectively predict accelerated movement only when we know that the pathways are present in the soil and at what matric potential(s) they are filled and flowing. We need new methods that can that can help us to predict where, when and at what velocity.
The overwhelming and compelling advantage to doing this important work as part of the multistate effort will be the combined expertise. A second justification is the diverse set of circumstances that are present in the region.
Biogeochemistry
Soil physical properties and processes influence the rate and extent of most soil biogeochemical processes (Smith et al., 2002). Carbon dioxide emissions from agricultural systems are partially responsible for globally increased greenhouse gases (Cole et al., 1997). Agricultural soils under reduced tillage may have the potential to sequester C (West and Post, 2002; Six et al., 2002), but measurements to support this claim are scarce. The dynamic interactions between soil CO2 concentration, temperature, soil water, and root versus soil respiration are not well-understood (Bouma et al., 1997). Improved measurement and modeling methods are desperately needed to advance our understanding of C-cycling and to predict future changes in CO2 emission from agricultural operations.
Seasonal transitions in arid climates control the form and mobility of nitrogen and can in turn impact terrestrial and aquatic ecology. There is a significant body of work indicating that seasonal transitions can play a critical role in affecting the mobility of nitrogen and eventual nitrogen status in seasonally dry ecosystems (Vitousek and Field, 2001; Vitousek and Field, 1999; Meixner and Fenn, 2003). Halogenated organic compounds are widely used chemicals. Some of these compounds such as the halogenated fumigants and pesticides, chlorinated solvents, polychlorinated benzenes, and chlorofluorocarbons have been found to contaminate the environment. These compounds are often resistant to natural environmental degradation, which has led to major efforts to prevent contamination and to remediate contaminated systems. There are relatively few safe and efficient methods to decontaminate porous systems such as soils. Research has being conducted using microorganisms to degrade these compounds, but such organisms are rare and it is often difficult to transport them to contaminated regions and to maintain their viability. Chemical oxidation reactions and physical extraction methods may be inefficient and/or cause additional harm to the environment. There is a great need to find new, more selective approaches to decontaminate halogenated organic compounds.
Soil Physical-Microbial Relationships
Despite the recognized importance of soil physico-chemical properties and processes to microbial community ecology, fundamental conceptual and experimental issues have hindered the close integration of soil physical principles with soil microbiology (Or, 2003). Members of the regional research committee will work together to quantify the primary physical influences on microbial habitats and activities in variably-unsaturated soils. Of particular interest are diffusional transport processes (Or, 2003; Qureshi et al., 2003), including diffusion within exopolymeric substances (EPS) and coupled diffusion among EPS and the soil matrix. The amount of soil water and its energy state are critical to soil microbial activity. Because different soil types will contain substantially different amounts of water at a given matric potential, and microbes congregate in aqueous habitats within soil pore space, it is the configuration of water rather than its energy state that will directly define the pathways and the extent of diffusion processes to and from soil organisms (Or, 2003). As soil pores drain and water films on solid surfaces become thinner, nutrients and wastes must follow a more tortuous path in diffusing to or from cells. Desaturation thus results in fragmentation of aqueous habitats, and increased tortuosity of liquid phase nutrient and metabolite mass transfer (Wraith et al., 2003). An ultimate goal is to infer controls on microbial community colonization of specific microsites in heterogeneous field soils. This information will be important to myriad agricultural and environmental applications.
Soil Hydraulic and Thermal Flux Properties
Research and quantification of soil hydraulic properties has been ongoing for decades. Recent advances in measurement abilities, consideration of spatial and temporal differences, and enhanced recognition concerning the critical importance of soil hydraulic properties to mass and energy transport advocate that this topic merits continued attention. Parameters describing soil hydraulic properties and root water uptake are important inputs for various models that simulate mass and energy transport in the vadose zone. Direct determination of these parameters in the laboratory or in the field is difficult and/or time consuming.
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 swell-shrink behavior of clays present a challenge to the development of predictive models for flow and transport, in particular to development of constitutive hydraulic functions. Such functions are important not only for design of man-made hydrologic barriers such as clay liners constructed for waste isolation but also for fluid flow predictions in porous media (Benson et al., 1994; Mitchell, 1993). Recent advances in pore scale modeling of fluid flow and liquid distribution in rigid angular pores have been developed through past collaboration of W-188 scientists. These consider both capillarity and adsorption (Tashman et al., 2003; Tuller and Or, 2001; Masad et al., 2000; Tuller et al., 1999; Or and Tuller, 1999) and provide the basis for a proposed multiscale-modeling framework in swelling soils. Pore scale processes (Tuller and Or, 2003) must be upscaled via appropriate schemes to represent sample and profile scale behavior while retaining the essential physical, chemical, and geometrical features operating at the microscale.
Colloid transport
Colloids are ubiquitous in soils, and play an important role in soil formation and contaminant fate and transport. Many contaminants in the vadose zone including pesticides, radionuclides, and viruses can associate with colloids, affecting the migration of these contaminants through soils. Some contaminants like viruses, bacteria, or other microorganisms are colloids themselves. 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 schemes to prevent soil and water contamination.
Objective 2 - To develop and evaluate instrumentation and methods of analysis for characterizing mass and energy transport in soils at different scales.
Several key areas of instrumentation and analysis are needed to provide sufficient accuracy in our predictions of mass and energy transport. Key areas to be investigated under this proposal are given below.
Improved and Multifunction Measurement Devices
In agriculture, ecology, hydrology, global change, and related sciences, there is a pressing need for improved instrumentation to augment the general understanding of water flow, nutrient/chemical transport, and heat transport processes in rooting zones, the vadose zone, and ground water (Hopmans et al., 1999). As during past years, effective new laboratory and field techniques to measure and monitor soil environmental parameters will be developed as part of this project.
Evapotranspiration
Evapotranspiration (ET) is the water flux through soil and plant systems to the atmosphere. Understanding spatial and temporal variations of ET is important for resource management, precision irrigation, controlling water fluxes carrying potential pollutants to groundwater, precision agriculture, and on larger scales for weather modeling and forecasting, carbon sequestration, and other environmental studies (Bastiaanssen et al., 1998ab; Peters-Lidard et al., 1997, 2001). Unfortunately, ground truth technologies have not kept abreast of remote sensing for estimating and measuring variations in ET (Allen et al., 2002).
Hydraulic Property Instrumentation and Models
Accurate measurement of soil hydraulic conductivity requires careful and often time-consuming attention on the part of scientists or practitioners. Most of the labor involved with steady state methods consists of a feed-back between the operator, the experimental settings, and the degree of steady state attainted. By automating the actions necessary for attaining steady-state, the measurements become nearly operator-independent and potentially much faster. As a result, the number of measurements will increase thereby increasing the detail of the measured hydraulic conductivity characteristics.
Pedo-transfer functions (PTF) are often needed in applications where critical soil properties are not known or not reliable. A common example is where soil survey information is used in landscape-scale applications. Critical questions still remain about whether PTF predictions might exhibit regional bias and whether they can account for spatial variability and scale effects.
Objective 3 - To develop and evaluate scale-appropriate methodologies for the management of soil and water resources.
Computer capabilities have evolved to a point where it is now possible to use multi-dimensional physically based hydrologic models to study spatial and temporal patterns of mass and energy flow in the vadose zone. However, so far these models have received limited attention because of their computational, distributed input, and flux parameter requirements. Global optimization algorithms may enable exploration of the utility of various mechanistic models with differing complexities to analyze the transport of salts and nutrients in irrigated areas, at spatial scales as large as a watershed.
Numerical simulation of dissolution/precipitation reactions in soils and other porous media do not strongly couple these reactions to the fluid flow domain. For example, precipitation of calcite in soils can result in plugging of pores and significant drop in hydraulic conductivity. However, typical reactive transport models contain only a porosity adjustment when precipitation reactions occur and, at best, a simple relationship between porosity and saturated conductivity only. Researchers will use tools developed in multiple states to assist this effort by comparing the hydraulic properties of soils to those of an evolving (through precipitation reactions) in volcanic tuff matrix.
Surface and ground waters in southwestern states are often impaired by nutrient runoff and leaching from irrigated agriculture. While actions to enhance the quality of surface waters will be addressed through TMDLs, degradation of aquifers under agricultural lands is a growing problem. Adoption of best management practices (BMPs) is the most effective way to prevent surface and ground water degradation from agricultural operations. However, an assessment of potential management problems should be conducted before BMPs are chosen or implemented. Several nitrogen indices have been developed to help growers assess nitrate leaching potential, but they are not directly applicable to irrigated agricultural lands in the Southwest.
Zone soil sampling may make precision farming practical in the Northern Great Plains, but defining zones is currently subjective. Comparing and evaluating methods of zone nitrogen determination will make zone development more objective and automated. This should include evaluation of sampling methods, determining ways to combine information from different sampling methods, automation of nutrient zone boundary determination, evaluation of water quality impacts from precision agriculture, and exploration of economic and environmental consequences of different zone methods with respect to nitrogen fertilization. Field-scale water content and soil property mapping approaches are needed to provide linkages between in-field measurements and remotely sensed data for improved resource management.
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