Showing papers on "Hydraulic conductivity published in 2002"

Electrical-hydraulic relationships observed for unconsolidated sediments

Abstract: [1] We use complex conductivity measurements to predict the hydraulic conductivity (K) of unconsolidated materials. The samples include natural sediments and artificial sand/clay mixtures. We apply the Borner et al. [1996] model, which is based on the Kozeny-Carman equation and incorporates electrical estimates of formation factor (F) and specific surface area per unit volume-to-porosity ratio (Spor), from the real (σ′) and imaginary (σ″) conductivity components respectively. We find that K correlates with σ″ but shows no correlation with F, which we attribute to the wide range in grain size for these materials. The Borner model appears primarily dependent on the K - σ″ relation. The relationship between σ″ and Spor is nonlinear and appears to depend upon material type. Further examination shows that σ″ is well correlated with effective grain size (d10) and is relatively independent of the material type. We propose a simple Hazen-type equation in which the effective grain size is estimated from σ″. This simple model provides order of magnitude estimates of K for a range of unconsolidated sediments.

Hydraulic redistribution in a stand of Artemisia tridentata: Evaluation of benefits to transpiration assessed with a simulation model

Abstract: The significance of soil water redistribution facilitated by roots (an extension of "hydraulic lift", here termed hydraulic redistribution) was assessed for a stand of Artemisia tridentata using measurements and a simulation model. The model incorporated water movement within the soil via unsaturated flow and hydraulic redistribution and soil water loss from transpiration. The model used Buckingham-Darcy's law for unsaturated flow while hydraulic redistribution was developed as a function of the distribution of active roots, root conductance for water, and relative soil–root (rhizosphere) conductance for water. Simulations were conducted to compare model predictions with time courses of soil water potential at several depths, and to evaluate the importance of root distribution, soil hydraulic conductance and root xylem conductance on transpiration rates and the dynamics of soil water. The model was able to effectively predict soil water potential during a summer drying cycle, and the rapid redistribution of water down to 1.5 m into the soil column after rainfall events. Results of simulations indicated that hydraulic redistribution could increase whole canopy transpiration over a 100-day drying cycle. While the increase was only 3.5% over the entire 100-day period, hydraulic redistribution increased transpiration up to 20.5% for some days. The presence of high soil water content within the lower rooting zone appears to be necessary for sizeable increases in transpiration due to hydraulic redistribution. Simulation results also indicated that root distributions with roots concentrated in shallow soil layers experienced the greatest increase in transpiration due to hydraulic redistribution. This redistribution had much less effect on transpiration with more uniform root distributions, higher soil hydraulic conductivity and lower root conductivity. Simulation results indicated that redistribution of water by roots can be an important component in soil water dynamics, and the model presented here provides a useful approach to incorporating hydraulic redistribution into larger models of soil processes.

Influence of Microbial Growth on Hydraulic Properties of Pore Networks

Abstract: From laboratory experiments it is known that bacterial biomass is able to influence the hydraulic properties of saturated porous media, an effect called bioclogging. To interpret the observations of these experiments and to predict possible bioclogging effects on the field scale it is necessary to use transport models, which are able to include bioclogging. For these models it is necessary to know the relation between the amount of biomass and the hydraulic conductivity of the porous medium. Usually these relations were determined using bundles of parallel pore channels and do not account for interconnections between the pores in more than one dimension. The present study uses two-dimensional pore network models to study the effects of bioclogging on the pore scale. Numerical simulations were done for two different scenarios of the growth of biomass in the pores. Scenario 1 assumes microbial growth in discrete colonies clogging particular pores completely. Scenario 2 assumes microbial growth as a biofilm growing on the wall of each pore. In both scenarios the hydraulic conductivity was reduced by at least two orders of magnitude, but for the colony scenario much less biomass was needed to get a maximal clogging effect and a better agreement with previously published experimental data could be found. For both scenarios it was shown that heterogeneous pore networks could be clogged with less biomass than more homogeneous ones.

Indirect estimation of soil thermal properties and water flux using heat pulse probe measurements: Geometry and dispersion effects

Abstract: from 1.0 to >10 m d � 1 . We also demonstrate the general application of inverse modeling to estimate soil thermal properties and their functional dependence on volumetric water content in a separate numerical experiment. We suggest that inverse modeling of HPP temperature data may allow simultaneous estimation of soil water retention (when combined with matric potential measurements) and unsaturated hydraulic conductivity (through water flux estimation) from simple laboratory experiments. INDEX TERMS: 1866 Hydrology: Soil moisture; 1875 Hydrology: Unsaturated zone; 1894 Hydrology: Instruments and techniques; KEYWORDS: Soil water flow; soil heat flow; inverse modeling; dispersivity

Practical pedotransfer functions for estimating the saturated hydraulic conductivity

Abstract: The saturated hydraulic conductivity k is one of the most important and widely used geotechnical parameters, commonly involved in a diversity of applications. The value of k depends on many factors, which can be divided into three classes: properties of the fluid, pore size distribution, and characteristics of the solid surfaces. Because the latter two are not necessarily constant within a given deposit, the hydraulic conductivity may vary significantly in space. Engineers and scientists need indications about how changing factors may affect the actual k value. In this paper, the authors propose some simple expressions, based on pedologic properties, to estimate the value of k. Using experimental results of their own and taken from the literature, it is shown that the proposed pedotransfer functions can be used for quickly estimating the k value for granular and plastic/cohesive soils. Such expressions can be employed, with a useful chart format, for the preliminary design phase of a project, and also for estimating the range of k values to be anticipated within a given deposit.

Effectiveness of hydraulic tomography: Sandbox experiments

Abstract: [1] Two sandbox experiments were conducted to evaluate the performance of a sequential geostatistical inverse approach for hydraulic tomography in characterizing aquifer heterogeneity. One sandbox was packed with layered sands to represent a stratified aquifer, while the other was packed with discontinuous sand bodies of different shapes and sizes to represent a more complex and realistic heterogeneous aquifer. Parallel to the sandbox experiments, numerical experiments were conducted to assess the effects of measurement errors and uncertainties associated with laboratory data, and to diagnose the hydraulic conductivity estimates obtained from sandbox experiments. Results of this study show that our sequential inverse approach works well under realistic conditions, in spite of measurement errors and uncertainties associated with pumping rates, boundary conditions, pressure head measurements, and other parameters required by our model. The tomography was found to be ineffective if abundant head measurements were collected at closely spaced intervals in a highly stratified aquifer. On the other hand, it was found to be beneficial when pressure head measurements were limited and the geological structure was discontinuous.

Hydraulic and Physical Properties of Stony Soils in a Small Watershed

Abstract: The presence of rock fragments in soil layers can have a profound impact on measured hydraulic properties. Variation of surface soil hydraulic properties influences the amount, distribution, and routing of overland flow. The objective of this study was to assess the effect of rock fragments and soil texture on infiltration, hydraulic conductivity, and related physical properties in soils of a small watershed in northwestern Arkansas. Single-ring and tension infiltrometer measurements at three pressure heads (h = -0.03, -0.06, and -0.12 m) were completed on the surface soil layer at 42 sites along three transects crossing the watershed. Upland (Nixa, loamy-skeletal, siliceous, active, mesic Glossic Fragiudults) and side slope (Clarksville, loamyskeletal, siliceous, semiactive, mesic Typic Paleudults) soils had significantly less rock fragments, lower infiltration rates (i), and lower hydraulic conductivities (K) at and near saturation compared with the valley bottom soil (Razort, fine-loamy, mixed, active, mesic Mollic Hapludalfs). Average infiltration rate at h = -0.03 m for all soils was only 9% of the ponded value suggesting that pores >1 mm in diameter dominated water flow under saturated conditions. At saturation, hydraulic properties tended to increase with rock fragment content while, at h = -0.12, the opposite was true. It is hypothesized that the source of rock fragments (weathering In place vs. colluvial and alluvial origin) and contact with the surrounding fine-earth fraction influence water flow by affecting hydraulic continuity near fragment surfaces. These relatively subtle morphological factors may have a disproportionate impact on water flow under near-saturation conditions in these soils.

Saturated Hydraulic Conductivity and Its Impact on Simulated Runoff for Claypan Soils

Abstract: Saturated hydraulic conductivity (K sat ) is an essential parameter for understanding soil hydrology. This study evaluated the K sat of in situ monoliths and intact cores and compared the results with other studies for Missouri claypan soils. These K sat values were used as runoff-model inputs to assess the impact of K sat variation on simulated runoff. Lateral in situ K sat of the topsoil was determined on 250 by 500 by 230 mm deep monoliths. These values were compared with the K sat of 76 by 76 mm diam. intact cores with and without bentonite to seal macropores. Mean (± SD) lateral in situ K sat was 72 ± 0.7 mm h -1 and mean intact core K sat without bentonite was 312 ± 58 mm h -1 . The mean intact core K sat without bentonite was significantly larger than the lateral in situ K sat (P = 0.03). The lateral in situ K sat was not different from core K sat with bentonite (71 ± 1.1 mm h -1 ). The intact core K sat with bentonite differed from previous studies by 10 times. This was attributed to the variations in soil depth to claypan, macropore presence, and methodology. The impact of using an effective hydraulic conductivity (K off ) computed from measured K sat on intact cores without bentonite underestimated the Water Erosion Prediction Project (WEPP) simulated runoff by 28% for a measured runoff event of 40 mm. The core K sat with bentonite was correlated with measured runoff from long-term erosion-runoff plots. A quadratic regression explained 95% of the variability between measured and simulated runoff.

A hydrological model dedicated to topography-based simulation of nitrogen transfer and transformation: rationale and application to the geomorphology-denitrification relationship

Abstract: A new integrated hydrological and nitrogen model, called TNT2 (topography-based nitrogen transfer and transformation), has been developed to study nitrogen fluxes in small catchments. This model, process-based and spatially distributed in order to take spatial interactions into account, has been kept as simple as possible. Here, only the hydrological module is discussed. The two main hypotheses of the hydrological model are taken from the TOPMODEL concept (constant hydraulic gradient equal to slope and hydraulic conductivity decreasing exponentially with depth). The model is based on a daily water balance for each cell of a regular square grid and computes an explicit cell-to-cell routing. Transfer through the vadose zone is simulated using a conceptual, layer-based algorithm analogous to the Burns model, except that a drainage water reservoir has been added to simulate mobile/immobile water processes and variations of the water table within the soil. The crop growth and nitrogen transformations are simulated using the equations of a generic plant-soil model, STICS. As an example, a preliminary study of the effect of the catchment geomorphology on denitrification is presented. The study was performed on theoretical catchments with contrasted slope shapes and pathway patterns. Results show that the whole-catchment denitrification depends on catchment geomorphology, although not directly through the extent of saturated areas. It is concluded that TNT2 seems to be a powerful tool to explore catchment processes, both by application to actual cases and by exploration on simple scenarios.

Ohmic Conductivity of a Compacted Silty Clay

Abstract: In its natural state, loess can be considered as an unstable soil, which develops large deformations when moistened. In Argentina, loess is used in most Geotechnical constructions, including embankments and liners. The interest of this work to evaluate the potential application of electrical conductivity measurements for monitoring the effects introduced by remolding and compaction in the soil. Samples of loess were compacted at varied densities and mixed with electrolytes of different concentrations. Electrical conductivity was measured with a two electrode cell. The effects introduced on the measured conductivity by frequency, degree of saturation, soil density, temperature, and electrolyte type and concentration are addressed. Additionally, hydraulic permeability tests were performed on compacted specimens of loess and the relationship between electrical and hydraulic conductivity was determined. It is concluded here that the ohmic conductivity of compacted specimens depends mainly on the salt concentr...

Hydraulic tests with direct-push equipment.

Abstract: The potential of direct-push technology for hydraulic characterization of saturated flow systems was investigated at a field site with a considerable degree of subsurface control. Direct-push installations were emplaced by attaching short lengths of screen (shielded and unshielded) to the bottom end of a tool string that was then advanced into the unconsolidated sediments. A series of constant-rate pumping tests were performed in a coarse sand and gravel aquifer using direct-push tool strings as observation wells. Very good agreement (within 4%) was found between hydraulic conductivity (K) estimates from direct-push installations and those from conventional wells. A program of slug tests was performed in direct-push installations using small-diameter adaptations of solid-slug and pneumatic methods. In a sandy silt interval of moderate hydraulic conductivity, K values from tests in a shielded screen tool were in excellent agreement (within 2%) with those from tests in a nearby well. In the coarse sand and gravel aquifer, K values were within 12% of those from multilevel slug tests at a nearby well. However, in the more permeable portions of the aquifer (K > 70 m/day), the smaller-diameter direct-push rods (0.016 m inner diameter [I.D.]) attenuated test responses, leading to an underprediction of K. In those conditions, use of larger-diameter rods (e.g., 0.038 m I.D.) is necessary to obtain kappa values representative of the formation. This investigation demonstrates that much valuable information can be obtained from hydraulic tests in direct-push installations. As with any type of hydraulic test, K estimates are critically dependent on use of appropriate emplacement and development procedures. In particular, driving an unshielded screen through a heterogeneous sequence will often lead to a buildup of low-K material that can be difficult to remove with standard development procedures.

Estimating hydraulic properties of soil aggregate skins from sorptivity and water retention

Abstract: Skins of soil aggregates often consist of clayey or clay-organic coatings which may affect preferential flow in aggregated soils. The objective was to determine hydraulic properties of samples with intact and removed (cut) skins and interior/skin hydraulic conductivity ratios for estimating mass transfer parameters in dual-permeability models. Soil aggregates from the C sd -horizon of a clay-loam glacial till soil (Stagnic Caicaric Regosol) were analyzed. A tension-imbibition apparatus was used for measuring water uptake of multiple aggregates at boundary matric potential heads of -1 and -5 cm. Sorptivities were used to calculate mean weighted water diffusivities and final water contents to fit wetting retention functions. Water retention and hydraulic conductivity functions for the skin layer were derived from differences in water contents and hydraulic resistances between intact and cut samples. Water absorption rates were generally smaller for intact than for cut aggregates. The water retention function of cut was shifted towards smaller water contents compared with intact samples. Mean water diffusivity of intact was 4.5 times smaller than that of cut samples. The interior/skin ratio in unsaturated hydraulic conductivity was about 12 in the measured matric potential head range. The ratio was up to 70 near water saturation and dropped below unity for soil water potentials smaller -1000 cm of water. Aggregate skins may be regarded as a separate porous domain whose hydraulic properties may control water transfer between inter- and intraaggregate pore domains in structured soils.

Understanding the Hydraulics of Porous Pipes: Tradeoffs Between Water Uptake and Root Length Utilization

Abstract: The water uptake region in roots is several hundred times longer than the root diameter. The distributed nature of the uptake zone requires that the hydraulic design of roots be understood by analogy to flow through a “porous pipe.” Here we present results of an analytical and experimental investigation that allowed an in-depth analysis of root hydraulic properties. Measurements on nodal maize roots confirm the nonlinear distribution of water uptake predicted by the porous pipe model. The major design parameter governing the distribution of water uptake along a porous pipe is the ratio between its axial and radial hydraulic resistance. However, total flow is proportional to the pipe's overall resistance. These results suggest the existence of a tradeoff between the effective utilization of root length and the total capacity for water uptake.

Hydraulic performance of untreated and polymer-treated bentonite in inorganic landfill leachates

Abstract: Short- and long-term exposure to inorganic solutions can cause significant degradation of the hydraulic properties of bentonite clay used in geosynthetic clay liners (GCLs). In particular, the increase in hydraulic conductivity due to cation exchange when Na-montmorillonite is subjected to leachates rich in Ca and Mg has caused problems in incinerator ash landfill liners located in wet environments, where large quantities of leachates are generated. Experimental results are presented to evaluate the immediate change in hydraulic conductivity of seven types of GCL clays upon permeation with leachate generated from three ash landfills. The composition of the ash, which is a by-product of the incineration of municipal solid waste (MSW), in turn influences the composition of the resulting leachate. Falling head permeability tests were performed on flexible-wall permeameter specimens, with back-pressure saturation. Chemical analysis shows that the three leachate products contain high, medium, and low concentration Ca and Mg cations. The clay component of GCL materials tested in this study consists of regular and polymer-treated bentonite. Polymer treatment is believed to render the clay non-reactive to many organic and inorganic chemicals. The results of this study indicate that: (1) polymer treatment is generally more beneficial if the clay is first saturated with water and not directly with the leachate; (2) high swell potential of the bentonite is more advantageous than polymer treatment, especially when low hydraulic conductivity is required in the short term and if the clay is pre-hydrated. Experiment setup and special specimen preparation procedures are also discussed.

The Influence of Hydraulic Nonequilibrium on Pressure Plate Data

Abstract: Pressure plates are used routinely to measure water-retention characteristics of soils. Plates of varying porosity are used, depending on the pressure range of interest. For applied pressures up to 1.5 MPa, 15-bar porous ceramic plates with fine porosity are used because of their high bubbling pressure (>1.5 MPa), which limits airflow through the plate. The typical saturated hydraulic conductivity of the 15-bar plate is −11 m s −1 . Low plate conductance coupled with decreasing soil hydraulic conductivities at high pressures strongly influence equilibrium times, which theoretically may extend to months or years. We measured the soil water pressures (suctions) for three soils, a sand, a silt loam, and a clay, placed on 15-bar pressure plates for 10 d or longer, with and without static loads and with and without using a kaolinite slurry to improve plate contact. Total matric suctions, inferred from peltier psychrometry data, were always

Unsaturated hydraulic conductivity of structured porous media: A review of liquid configuration based models

Abstract: Common approaches for modeling hydraulic functions of unsaturated structured porous media (SPM) rely on macroscopic continuum representation, where parameterization schemes and constitutive relationships originally developed for homogeneous porous media are extended to represent hydraulic behavior of dual (or multi) continuum SPM. Such models often result in inconsistencies due to lack of consideration of structural pore space geometry and the neglect of underlying physical processes governing liquid retention and flow under unsaturated conditions. We review a new framework that considers equilibrium liquid configurations in dual continuum pore space as the basis for calculation of liquid saturation and subsequent introduction of hydrodynamic considerations. The SPM pore space is represented by a bimodal distribution of pore sizes, reflecting two disparate populations of matrix and structural pores. Three steady-state and laminar flow regimes are considered to derive unsaturated hydraulic conductivity functions: (i) flow in completely filled pore spaces, (ii) corner flow in partially filled pores and grooves, and (iii) film flow on solid surfaces. Two key assumptions are used in deriving the average cross-sectional flow velocities in these regimes: (i) that equilibrium liquid–vapor interfaces remain stable under slow laminar flows and (ii) that flow pathways are parallel. Liquid–vapor interfacial configurations for different matric potentials are calculated and statistically upscaled to derive sample-scale saturated and unsaturated hydraulic conductivity from velocity expressions weighted by the appropriate liquid-occupied cross-sectional areas, neglecting three-dimensional (3-D) network effects. Similarly, the hydraulic functions for matrix and structural pores are derived separately and later combined by weighting the individual contributions by the porosities of the associated pore spaces. A parameter estimation scheme was developed to calculate liquid saturation and to predict sample-scale unsaturated hydraulic conductivity. Model evaluation using measured data for homogeneous porous media, fractured welded tuff, and macroporous and aggregated soils shows favorable agreement (within the limitations of model assumptions). Effects of nonequilibrium conditions between matrix and structural pore domains on the hydraulic conductivity and approximate consideration of 3-D network effects are discussed.

Ectomycorrhizas increase apoplastic water transport and root hydraulic conductivity in Ulmus americana seedlings

Abstract: Summary • The extent to which water channel transport is responsible for the observed increases in root water flow of ectomycorrhizal plants is reported here. • To examine the contribution of water channel transport to root hydraulic conductance, temperatures in the range 4–20°C and mercuric chloride (HgCl2) were used to study the kinetics of water transport in ectomycorrhizal and nonmycorrhizal roots of American elm (Ulmus americana) seedlings. • Hydraulic conductance declined with decreasing temperatures in both mycorrhizal and nonmycorrhizal seedlings. However, hydraulic conductance and conductivity were higher in the mycorrhizal than the nonmycorrhizal roots at all temperatures studied. Mercuric chloride had a relatively greater impact on root hydraulic conductance in nonmycorrhizal than mycorrhizal roots and activation energy for root hydraulic conductance was significantly higher in mycorrhizal than nonmycorrhizal plants. • The results suggest that ectomycorrhizal hyphae increase hydraulic conductance of roots by decreasing water flow resistance of the apoplast rather than by water channel-mediated transport. The high rates of hydraulic conductance at low root temperatures might be important to plants growing in cold soils and under other challenging environmental conditions that inhibit metabolism and limit water transport.