Measurement and Modeling of Reduced-Gravity Fluid Distribution and Transport in Unsaturated Porous Plant-Growth Media

The effect of reduced gravity on the balanced management of liquid, gaseous and ionic fluxes in unsaturated porous media remains a central challenge for plant-based bio-regenerative life support systems needed for long-duration space missions. This research investigated how shifting capillary and gravitational forces alter the sample-scale transport and distribution of fluids in mm-sized porous ceramic aggregates. Measurements in variably saturated media conducted on the International Space Station in microgravity ($sim1cdot10^{-3} g_{earth}$) and measurements during parabolic flight in variable gravity encompassing microgravity, terrestrial gravity and hypergravity ($sim1.8 g_{earth}$) were supported by numerical modeling based on fundamental, earth-derived soil-physical relationships. Measurements of water fluxes in rigid saturated media suggested Darcian flow unaffected by gravity. Observations of hydraulic potential and sample water content were used to estimate the primary draining and wetting water-retention characteristic (WRC). Terrestrial parameterizations of the WRC were largely unaffected by reduced gravity. However, because the WRC is hysteretic, heterogenous water-content distributions resulted within the confines of the primary draining and wetting characteristics. Ensuing distributions were fundamentally different from terrestrial observations and were stable in the absence of a significant gravity gradient. We showed that these distributions, though unexpected, could be predicted using the Richards equation. One consequence of altered water distribution could be the reduction in, and increased tortuosity of, continuous gas-filled pathways for diffusive transport compared to terrestrial estimates. Measurements of oxygen diffusion in microgravity suggested reduced diffusivities during draining. These observations, particularly for the smaller particle-sized media, were suggestive of the delayed formation of critical air-filled pathways at lower water contents. This dissertation further uses a case history of a stratified root-zone developed based on water-retention characteristics of different particle-sized media. The root-zone design provided a more uniform water-content distribution at terrestrial gravity suggested to provide more optimal conditions for root growth. Additionally, the design and testing of a novel integrated sensor for measurements of water content based on the dissipation of heat and estimation of nutrient status based on electrical resistivity are discussed. These results should provide insights into microgravity fluid distribution and transport contributing to the design and implementation of controllable plant-growth systems for use in microgravity and future planetary habitats

Towards Using Time-Lapse Electrical Resistivity Imaging for Improved Subsurface Snowmelt Characterization

In the intermountain west, snowmelt accounts for the majority of the annually available water. Understanding the fate of snow packs in the melting phase and particularly the infiltration of snowmelt is therefore central for maximizing the harvestable water and for maintaining the quality of snowmelt. Some of the open questions at the field-scale regard the localized infiltration of water and the nature and patterns of subsurface flow in the soil overlying bedrock. Electrical resistivity imaging (ERI) is known to be able to noninvasively image processes if state variables (i.e. water content) associated with the processes give rise to contrasts in the electrical conductivity. In the Vadose zone, the interpretation of such images is hampered by the equivalency of different model solutions. However, in time-lapse measurements, changes in electrical resistivity can be more directly related to changes in water content. In this way, flow processes in the subsurface can be characterized with high spatial and temporal resolution. The presented study was conducted within the well characterized and instrumented TWDEF experimental watershed in the Cache National Forest close to Logan, UT. To monitor the spatio-temporal evolution of snow melt, we installed 72 Electrodes with 5 m spacings on a sloping profile in the previous autumn. Automated measurements at this remote site were collected daily at 5 p.m. over several weeks using a Wenner/Schlumberger acquisition array. Measured data were inverted to produce apparent resistivity images of the subsurface. Subsequent temperature correction and differential comparison of temporal changes in subsurface resistivity were accompanied by soil surveys and depthto-bedrock soundings that guided in the interpretation and calibration of the measurements. Results suggest localized infiltration of snowmelt and subsurface flow bounded by non-conductive layers. Such behavior is consistent with visual observations of localized snowpack decreases and surface runoff patterns

Improving Root Zone Performance Physical and Numerical Modeling of a Layered Plant-growth Medium

Growing media in greenhouses and nurseries are selected based on gas exchange, control of the liquid phase and nutrient holding capability. In an effort to maintain favorable aeration and to avoid nutrient/salinity buildup, irrigated and free draining root zones operate under suboptimal water and nutrient use efficiencies. A novel layered plantgrowth medium was designed to improve the efficiency of water and nutrient application and promote more uniform root density compared to conventional containerized plant growth media. Our objectives were to (1) design and model an optimized root zone system using layered media, (2) instrument the root zone to monitor the water content distribution and track nutrient release and transport, and (3) compare the layered media system to conventional, non-layered plant growth media. The root-zone system is comprised of layered Ottawa sand, where watering is achieved by maintaining a shallow saturated layer at the bottom of the column and allowing capillarity to draw water upward. Coarser particle sizes form the bottom layers with finer particles sizes forming the layers above. The depth of each layer was chosen to optimize water content based on the wetting water retention curves retaining saturation between 50 and 85 percent. The saturation distribution was verified by dual-probe heat-pulse sensors, while the nutrient concentration was sensed by in-situ electrical conductivity measurements. Hydrus-1D was used to model the dynamic response of nutrient transport and root water uptake during diurnal cycling of transpiration. This design should provide a more optimal rootzone environment than conventional potting soils by maintaining a uniform water content profile and on-demand water supply

Geophysical Assessment of Groundwater Protective Layers

Layers covering aquifers can play a crucial role in protecting groundwater from infiltrating contaminants pending on their spatial distribution and hydrological properties. Non-invasive geophysical characterizations of groundwater protective layers provide important subsurface information at low cost and are highly desirable for the management of risks associated with groundwater contamination and runoff prediction. Geophysics ultimately employs measuring soil physical properties, and yields the prospect of gaining information beyond the point scale usually employed by hydrologists, therefore greatly reducing the uncertainty, especially in highly heterogenic geological settings.Soil electrical resistivity is one important soil physical property that depends primarily on structure and volume of pore space, soil water content, mineralization and clay content, thus making resistivity methods a natural choice for hydrologic surveys. Two technologies, which are applicable in the field, are High Resolution Multielectrode Geoelectrics and GeoRadar. The added value of combining the two methods offers the perspective of developing a survey technology applicable under wider conditions than any of the two methods on its own. However, a simple conversion of geophysical data into hydrological information is not commonly practicable. In order to improve the understanding of the relationships between geophysical measurements and soil physical and hydrological properties, correlation functions were derived, which effectively demanded the measurement of both hydrological and geophysical parameters.The protective potential of groundwater covering layers against contaminants is quantified by the infiltration time a contaminated volume of water would need to percolate through a unit area. In this context both the coefficient of permeability and the thickness of such layers must be identified with high accuracy. While thickness and structure determination of layers is a common application with a high expertise, the determination of soil hydrologic properties proofs to be a new ground. The rational is based on the idea that the hydraulic conductivity depends significantly on the grain size distribution and in particular on the clay content of fine-grained soils; consequently special interest was deployed in clay content variations.The results completely meet the expectations and support the field observations. A linear correlation between field measured resistivity and lab measured clay content was found, manifesting a trend of lower resistivities with an increase in clay content. Moreover, we were able to substantiate a dependence of resistivity from the lab measured saturated hydraulic conductivity. Although the uncertainty of estimating the saturated hydraulic conductivity is significant, results will be shown that support the practical use of the method established in accurately predicting variations with a precision that could be in the order of magnitude. Cost effective Geoelectric and Georadar surveys provide additional fundamental information required by hydrologists and engineers by greatly reducing the uncertainty accompanied with point scale measurements at a comparable precision, thus ultimately providing a tool for assessing ground water hampering sediments for a variety of tasks

Organic Materials Used in Agriculture, Horticulture, Reconstructed Soils, and Filtering Applications

International audienceOrganic material has historically been used in growing media and has traditionally been harvested from peatlands. Deposits of organic material in peatlands may also be very productive when cultivated. This editorial brings attention to different sources of organic material, including peatlands, and their use as growing media and farmland. The editorial identifies new fields for which organic material may be used like green roof and filtering appli cations. Concepts developed in these fields are widespread in different journals and, as a result, soil scientists are somewhat unfamiliar with recent developments in the characterization of growing media and their different uses. This special issue illustrates concepts used in growing media science, and brings attention to these new fields of investigation to benefit soil scientists and horticulturists

Water Content and Electrical Conductivity Assessment Using Small-Scale Multifunctional Heat-Pulse Sensors

Water content and electrical conductivity are key parameters for predicting transport of water and solutes in hydrological sciences. Measurements of these dynamic physical characteristics are often needed at a small-scale that is masked by field-scale surveys. For example, measurements in the first few centimeters of the soil surface (e.g., remotely sensed measurement calibration) or within a root system (e.g., where plant uptake is critical) necessitate small-scale sensors that provide soil water content and electrical conductivity estimates within a comparable sampling volume. Under saline conditions, common in the Great Salt Lake Basin, measurements with state-of-the-art electromagnetic methods fail because of signal attenuation associated with the high electrical conductivity of the soil. Under these conditions sensors that predict water content using principles of heat dissipation are insensitive to salinity. Our objectives were to design, construct and test a novel sensor providing estimates of water content and electrical conductivity. The heat-pulse sensor is constructed of dual parallel needles, one of which serves as a heater while the other contains a thermistor for temperature measurement. We evaluated direct current electrical conductivity measurements in a four-electrode configuration formed by integrating two dual-probe heat-pulse sensors. The experiments monitored water content and electrical conductivity in samples of baked clay media. We will show data from the heat-pulse and electrical conductivity measurements using convective and diffusive solute transport experiments. Specifically, we advocate the use of the multifunctional sensors for non-invasive tomographical studies. Combining the heat-pulse water content determination with electrical conductivity measurements will provide improved environmental assessment capabilities

Measurements and Modeling of variable gravity effects on water distribution and flow in unsaturated porous media

Liquid behavior under reduced gravity conditions is of considerable interest for various components of life-support systems required for manned space missions. High costs and limited opportunities for spaceflight experiments hinder advances in reliable design and operation of elements involving fluids in unsaturated porous media such as plant growth facilities. We used parabolic flight experiments to characterize hydraulic properties under variable gravity conditions deduced from variations in matric potential over a range of water contents. We designed and tested novel measurement cells that allowed dynamic control of water content. Embedded time domain reflectometry probes and fast-responding tensiometers measured changes in water content and matric potential. For near-saturated conditions, we observed rapid establishment of equilibrium matric potentials during the recurring 20-s periods of microgravity. As media water content decreased, the concurrent decrease in hydraulic diffusivity resulted in limited attainment of equilibrium distributions of water content and matric potential in microgravity, and water content heterogeneity within the sample was influenced by the preceding hypergravity phase. For steady fluxes through saturated columns, we observed linear and constant hydraulic gradients during variable gravity, yielding saturated hydraulic conductivities similar to values measured under terrestrial gravity. Our results suggest that water distribution and retention behavior are sensitive to varied gravitational forces, whereas saturated hydraulic conductivity appears to be unaffected. Comparisons between measurements and simulations based on the Richards equation were in reasonable agreement, suggesting that fundamental laws of fluid flow and distribution for macroscopic transport derived on Earth are also applicable in microgravity