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Showing papers in "Vadose Zone Journal in 2009"


Journal ArticleDOI
TL;DR: In this paper, the authors revisited the Vereecken database, which has been used to derive pedotransfer functions (PTFs) to match the soil hydraulic parameters of Belgian soils.
Abstract: We revisited the Vereecken database, which has been used to derive pedotransfer funcƟ ons (PTFs) to esƟ mate the soil hydraulic parameters of Belgian soils. We developed new PTFs based on the Mualem-van Genuchten model, constrain- ing m = 1 − 1/n and using fewer parameters. The goodness-of-fi t was similar to the one originally obtained by Vereecken. We used a one-step procedure that allows direct quanƟ fi caƟ on of the correlaƟ on matrix and the uncertainƟ es of the esƟ mated parameter values. The coeffi cients of the new PTFs were esƟ mated using a global search algorithm and they were validated against independent data. The PTFs have a wider range of applicability since: (i) they allow the use of the closed-form soluƟ on of the unsaturated hydraulic conducƟ vity in the Mualem-van Genuchten model; and (ii) they consider the eff ect of macroporosity. We determined that the hydraulic conducƟ vity measured close to saturaƟ on could not be esƟ mated based on the available esƟ mators; however, the hydraulic conducƟ vity in the matrix domain was predicted with high accuracy.

179 citations


Journal ArticleDOI
TL;DR: In this paper, the authors demonstrated the formation and dynamics of air gaps around roots through x-ray tomography and image analysis, and found that when the soil dries to a water content of 0.025 m 3 m −3, gaps occurred around the taproot and the lateral roots of lupin ( Lupinus albus L.).
Abstract: It has been speculated that during periods of water deficit, roots may shrink and lose contact with the soil, with a consequent reduction in root water uptake. Due to the opaque nature of soil, however, this process has never been observed in situ for living plants. Through x-ray tomography and image analysis, we have demonstrated the formation and dynamics of air gaps around roots. The high spatial resolution required to image the soil–root gaps was achieved by combining tomography of the entire sample (field of view of 16 by 16 cm, pixel side 0.32 mm) with local tomography of the soil region around the roots (field of view of 5 by 5 cm, pixel side 0.09 mm). For a sandy soil, we found that when the soil dries to a water content of 0.025 m 3 m −3 , gaps occur around the taproot and the lateral roots of lupin ( Lupinus albus L.). Gaps were larger for the taproot than the laterals and were caused primarily by root shrinkage rather than by soil shrinkage. When the soil was irrigated again, the roots swelled, partially refilling the gaps; however, large gaps persisted in the more proximal, older part of the taproot. Gaps are expected to reduce water transfers between soil and roots. Opening and closing of gaps may help plants to prevent water loss when the soil dries, and to restore the soil–root continuity when water becomes available. The persistence of gaps in the more proximal parts is one reason why roots preferentially take up water from their more distal parts.

133 citations


Journal ArticleDOI
TL;DR: In this paper, the accuracy and variability of four EM sensors and the NMM, compared with gravimetric measurements, in transects of 10 to 20 access tubes during three fi eld seasons, using soil-specifi c calibraƟ ons.
Abstract: AU›o®ƒa®EAÝ : EM, electromagnetic; ET, evapotranspiration; NMM, neutron moisture meter. OU®¦®Aƒ½ R›Ý›ƒU‘« Since the late 1980s, electromagneƟ c (EM) sensors for determinaƟ on of soil water content from within nonmetallic access tubes have been marketed as replacements for the neutron moisture meter (NMM); however, the accuracy, variability and physical signifi cance of EM sensor fi eld measurements have been quesƟ oned. We studied the accuracy and variability of four EM sensors and the NMM, compared with gravimetric measurements, in transects of 10 to 20 access tubes during three fi eld seasons, using soil-specifi c calibraƟ ons. The three capacitance EM sensors produced water content readings for which SD values were up to an order of magnitude larger than those from the NMM. The EM sensor based on travel Ɵ me (waveguide) principles produced SD values up to six Ɵ mes larger than those of the NMM or gravimetric sampling. The EM sensors would require from two to 72 Ɵ mes as many access tubes to obtain a mean profi le water content to a given precision than would the NMM or gravimetric sampling, with more tubes required for drier condiƟ ons. The NMM exhibited spaƟ al variaƟ on of similar magnitude and paƩ ern as that of gravimetrically sampled profi le water contents. The EM methods poorly reproduced the spaƟ al and temporal behavior of NMM and gravimetric sampling and implied spaƟ al variability of profi le water content that was not evident in either the NMM or gravimetric data, even though EM sensing volumes were larger than the ?75-cm 3 volume of the gravimetric samples. We infer that EM sensors were infl uenced not only by the mean water content in the sampling volume but by the smaller scale structure of soil electrical properƟ es.

116 citations


Journal ArticleDOI
TL;DR: In this paper, the impact of soil depth, soil water content, and soil electrical conductivity on the signal transmission strength of SoilNet was determined, and it was shown that sufficient power will remain to ensure data communication over longer distances for most soil conditions.
Abstract: Wireless sensor network technology allows real-time soil water content monitoring with a high spatial and temporal resolution for observing hydrological processes in small watersheds. The novel wireless soil water content network SoilNet uses the low-cost ZigBee radio network for communication and a hybrid topology with a mixture of underground end devices each wired to several soil sensors and aboveground router devices. Data communication between the end and router devices occurs partially through the soil, and this causes concerns with respect to the feasibility of data communication due to signal attenuation by the soil. In this study, we determined the impact of soil depth, soil water content, and soil electrical conductivity on the signal transmission strength of SoilNet. In a first step, we developed a laboratory experimental setup to measure the impact of soil water content and bulk electrical conductivity on signal transmission strength. The laboratory data were then used to validate a semi-empirical model that simulates signal attenuation due to soil adsorption and reflection and transmission at the soil boundaries. With the validated model, it was possible to show that in the case of a soil layer of 5 cm, sufficient power will remain to ensure data communication over longer distances for most soil conditions. These calculations are fairly simplified and should be considered as a first approximation of the impact of attenuation. In actual field situations, signal transmission may be more complex. Therefore, a field evaluation of signal attenuation is a crucial next step.

104 citations


Journal ArticleDOI
TL;DR: In this article, the authors developed and tested a simplifi ed apparatus and procedure for characterizing the fi eld-saturated hydraulic conducability (K fs ) over broad areas with extreme spa- al variability and ordinary limita- ons of ƒ me and resources.
Abstract: To provide an improved methodology for characterizing the fi eld-saturated hydraulic conducƟ vity (K fs ) over broad areas with extreme spaƟ al variability and ordinary limitaƟ ons of Ɵ me and resources, we developed and tested a simplifi ed apparatus and procedure, correcƟ ng mathemaƟ cally for the major defi ciencies of the simplifi ed implementaƟ on. The methodology includes use of a portable, falling-head, small-diameter (~20 cm) single-ring infi ltrometer and an analyƟ cal formula for K fs that compensates both for nonconstant falling head and for the subsurface radial spreading that unavoidably occurs with small ring size. We applied this method to alluvial fan deposits varying in degree of pedogenic maturity in the arid Mojave NaƟ onal Preserve, California. The measurements are consistent with a more rigorous and Ɵ me-consuming K fs measurement method, produce the expected systemaƟ c trends in K fs when compared among soils of contrasƟ ng degrees of pedogenic development, and relate in expected ways to results of widely accepted methods.

100 citations


Journal ArticleDOI
TL;DR: In this paper, the authors compare different methods of multimodel simulation of the field soil water regime using pedotransfer functions (PTFs) using HYDRUS-1D with parameter sets derived from 19 published PTFs and compared different methods combining the simulation results from the 19 individual models by (i) using only the best model, (ii) using equal weights, (iii) regressing measured values to the results of the individual models, (iv) using singular-value decomposition (SVD) in the regression, (v) using Bay
Abstract: Calibration of variably saturated flow models with field monitoring data is complicated by the strongly nonlinear dependency of the unsaturated flow parameters on the water content. Combining predictions using various independent models, often called multimodel prediction , is becoming a popular modeling technique. The objective of this study was to compare different methods of multimodel simulation of the field soil water regime using pedotransfer functions (PTFs). We solved the Richards flow equation using HYDRUS-1D with parameter sets derived from 19 published PTFs and compared different methods of combining the simulation results from the 19 individual models by (i) using only the best model, (ii) using equal weights, (iii) regressing measured values to the results of the individual models, (iv) using singular-value decomposition (SVD) in the regression, (v) using Bayesian model averaging, and (vi) using weights derived from Akaike criteria. Data on soil water contents and basic soil properties at five depths along a 6-m transect in a layered loamy soil were used to calibrate the Richards equation and to develop the input for the PTFs. The SVD multimodel was the best method, with an accuracy of about 0.01 m 3 m −3 at the 35-cm depth and about 0.005 m 3 m −3 at greater depths for 30 d of monitoring and 13 mo of testing. This indicates that multimodeling in combination with monitoring of the soil water regime can be a viable approach to simulating water flow in the vadose zone.

96 citations


Journal ArticleDOI
TL;DR: In this article, a physics-based, distributed, fully coupled, second-order accurate, upwind cell-centered, constrained unstructured mesh based finite-volume modeling framework (FIHM) is presented.
Abstract: Surface water, the vadose zone, and groundwater are linked components of a hydrologic conƟ nuum. In order to capture the interacƟ on between diff erent components of a hydrologic conƟ nuum and to use this understanding in water management situaƟ ons, an accurate numerical model is needed. The quality of model results depends on accurate representaƟ on of the physical processes and the data describing the area of interest, as well as performance of the numerical formulaƟ on implemented. Here we present a physics-based, distributed, fully coupled, second-order accurate, upwind cell-centered, constrained unstructured mesh based fi nite-volume modeling framework (FIHM) that simultaneously solves two-dimensional unsteady overland fl ow and three-dimensional variably saturated subsurface fl ow in heterogeneous, anisotropic domains. A mulƟ dimensional linear reconstrucƟ on of the hydraulic gradients (surface and subsurface) is used to achieve second-order accuracy. Accuracy and effi ciency in raster data and vector-boundary representaƟ ons are facilitated through the use of constrained Delaunay meshes in domain discreƟ zaƟ on. The experiments presented here (i) explore the infl uence of iniƟ al moisture condiƟ ons, soil properƟ es, anisotropy, and heterogeneity in determining the pressure head distribuƟ ons in the vadose and saturated zones, (ii) show the existence of localized “fl ux rotaƟ on” phenomenon due to heterogeneous anisotropy, leading to the creaƟ on of convergence–divergence zones, (iii) show the infl uence of verƟ cal drainage from unsaturated zone on the response of an unconfi ned aquifer to pumping, and (iv) show the eff ects of capillarity, saturaƟ on excess, infi ltraƟ on excess, and iniƟ al water table locaƟ on on determining the overland fl ow generaƟ on.

95 citations


Journal ArticleDOI
TL;DR: In this article, the authors provide guidance for field scientists who are not instrumentation experts but who wish to determine soil water content as part of their work, targeted at those conducting projects in developing countries.
Abstract: The purpose of this manual is “to provide guidance for field scientists who are not instrumentation experts but who wish to determine soil water content as part of their work.” It is targeted at those conducting projects in developing countries. The first chapter covers basic information on

90 citations


Journal ArticleDOI
TL;DR: In this paper, the fractal dimensions and microstructures of porous media were reviewed and discussed in two and three dimensions, and the specifi c surface areas of fractal porous media in 2D and 3D dimensions were derived.
Abstract: AU›o®ƒa®EAÝ : DLA, diff usion-limited aggregation; SSA, specifi c surface area. SO›‘®ƒ½ S›‘a®EA : FUƒ‘aƒ½Ý In this study, we summarized some basic characters of fractal porous media, including the fractal pore or parƟ cle size distribuƟ on, pore or parƟ cle density funcƟ on, the fractal dimensions for the pore and solid phases, and their relaƟ ons. The geometric porosiƟ es vs. the fractal dimensions and microstructures of porous media were reviewed and discussed in two and three dimensions. The specifi c surface areas of fractal porous media in two and three dimensions were derived and were expressed as a funcƟ on of the fractal dimensions and microstructural parameters. The fl uid velociƟ es in fractal porous media were also derived and found to be a funcƟ on of the fractal dimensions and microstructural parameters of the medium. The parameters presented are the fundamental ones and may have potenƟ al in analysis of transport properƟ es in fractal porous media.

88 citations


Journal ArticleDOI
TL;DR: In this paper, the authors used integrated hydrogeophysical inversion of time-lapse, proximal ground penetrating radar (GPR) data to remotely infer the unsaturated soil hydraulic properties of a laboratory sand during an infiltration event.
Abstract: We used integrated hydrogeophysical inversion of time-lapse, proximal ground penetrating radar (GPR) data to remotely infer the unsaturated soil hydraulic properties of a laboratory sand during an infiltration event. The inversion procedure involved full-waveform modeling of the radar signal and one-dimensional, vertical flow modeling. We combined the radar model with HYDRUS-1D. The radar system was set up using standard, handheld vector network analyzer technology. Significant effects of water dynamics were observed on the time-lapse radar data. The estimated hydraulic parameters were relatively consistent with direct characterization of undisturbed sand samples. Significant differences were particularly observed for the saturated hydraulic conductivity, which was underestimated by two orders of magnitude. Nevertheless, the use of soil hydraulic parameters derived from reference measurements failed to correctly predict water dynamics, whereas GPR-based predictions captured some of the major features of time domain reflectometry measurements and better agreed with visual observations. These results suggest that the proposed method is promising for noninvasive, effective hydraulic characterization of the shallow subsurface and hence, monitoring of water dynamics at the field scale. © Soil Science Society of America.

85 citations


Journal ArticleDOI
TL;DR: In this article, the role of surface roughness in governing colloid transport in the vadose zone was investigated, and it was shown that more particles are retained at lower water contents, smoother grain surfaces retain fewer colloids in the porous media under both saturated and unsaturated conditions, and the effects of surface surface rougheness diminish with increasing grain surfaces roughness.
Abstract: Work to date has raised awareness regarding the importance of transport of colloids in the vadose zone, as it is the critical connection between shallow contaminant sources and the deeper groundwater. Existing models do not predict accurately the physical conditions under which such particle transport occurs. Recent theory on capillary and friction forces acting at the air-water meniscus-solid (AWmS) interface suggests that grain roughness is an important factor in colloid retention for unsaturated media. Our main objective was to investigate the role of surface roughness in governing colloid transport in the vadose zone. Unsaturated flow cell experiments with sands of different grain roughness were performed to determine colloidal retention at the postulated AWmS interface. Two rectangular acrylic vertical flow cells (2 by 2 by 5 and 2 by 2 by 10 cm) were built to collect visual and numerical data under the effects of gravity. Colloid behavior was visualized in situ with digital bright field microscopy, effluent concentrations of colloids were measured concomitantly with spectrophotometry, and retention was quantified with mass balance analysis. From visual analysis, significant retention was observed at apparent AWmS interfaces and in zones of immobile water. An equilibrium deterministic convective-dispersion model was used and fitted to our data with fair accuracy. This study';s visual, quantitative, and statistical results show that: more particles are retained at lower water contents, smoother grain surfaces retain fewer colloids in the porous media under both saturated and unsaturated conditions, and the effects of surface roughness diminish with increasing grain surface roughness.

Journal ArticleDOI
TL;DR: In this article, the spatial variability of heterotrophic soil respiration at the plot scale has been investigated, showing that the spatiotemporal variability of respiration could be explained by the state variables soil temperature and water content.
Abstract: Soil respiration is known to be highly variable with time. Less is known, however, about the spatial variability of heterotrophic soil respiration at the plot scale. We simultaneously measured soil heterotrophic respiration, soil temperature, and soil water content at 48 locations with a nested sampling design and at 76 locations with a regular grid plus refinement within a 13- by 14-m bare soil plot for 15 measurement dates. Soil respiration was measured with a closed chamber covering a surface area of 0.032 m 2 . A geostatistical data analyses indicated a mean range of 2.7 m for heterotrophic soil respiration. We detected rather high coefficients of variation of CO 2 respiration between 0.13 and 0.80, with an average of 0.33. The number of observations required to estimate average respiration fluxes at a 5% error level ranged between 5 and 123. The analysis of the temporal persistence revealed that a subset of 17 sampling locations is sufficient to estimate average respiration fluxes at a tolerable root mean square error of 0.15 g C m −2 d −1 . Statistical analysis revealed that the spatiotemporal variability of heterotrophic soil respiration could be explained by the state variables soil temperature and water content. The spatial variability of respiration was mainly driven by variability in soil water content; the variability in the soil water content was almost an order of magnitude higher than the variability in soil temperature.

Journal ArticleDOI
TL;DR: In this paper, the effects of average particle size (D 50 ) and dry bulk density (ρ b ) on D p and k a for six sandy soils under variably saturated conditions were investigated.
Abstract: The soil gas diffusion coefficient ( D p ) and air permeability ( k a ) and their dependency on soil air content (e) control gas diffusion and advection in soils. This study investigated the effects of average particle size ( D 50 ) and dry bulk density (ρ b ) on D p and k a for six sandy soils under variably saturated conditions. Data showed that particle size markedly affects the effective diameter of the drained pores active in leading gas through the sample at −100 cm H 2 O of soil water matric potential (calculated from D p and k a ) as well as the average pore diameter at half saturation (calculated from the water retention curve), both exhibiting similar and exponential relationships with D 50 . Under variably saturated conditions, higher D p and k a in coarser sand (larger D 50 ) were observed due to rapid gas diffusion and advection through the less tortuous large-pore networks. In addition, soil compaction (larger ρ b ) simultaneously caused reduced water blockage effects and a reduction of large-pore space, resulting in higher D p (e) but lower k a (e). Two recent models for D p (e) and k a (e) were evaluated: the water-induced linear reduction (WLR) model for D p , and the reference-point power law (RPL) model for k a , with reference point k a set at −100 cm H 2 O. The performance of both models for the sandy soils (particle size range 0.02–0.9 mm) was improved if the pore connectivity–tortuosity factor and water blockage factors were assumed to be functions of D 50 and ρ b . Water blockage factors, N for the WLR D p (e) model and M for the RPL k a (e) model, showed a strong nonlinear relationship ( R 2 = 0.95) that seems promising for predicting D p (e) from the more easily measureable k a (e).

Journal ArticleDOI
TL;DR: In this article, the authors used waveguides and tensiometers to obtain in situ measurements of soil moisture in unsaturated soils and applied three scaling methods to improve the performance of these measurements.
Abstract: Models of water movement in unsaturated soils require accurate representaƟ ons of the soil moisture retenƟ on and hydraulic conducƟ vity curves; however, commonly used laboratory methods and pedotransfer funcƟ ons (PTFs) are rarely verifi ed against fi eld condiƟ ons. In this study, we invesƟ gated the eff ects of using soil hydraulic property informaƟ on obtained from diff erent measurement and esƟ maƟ on techniques on one-dimensional model predicƟ ons of soil moisture content. Pairs of Ɵ me domain refl ectometry waveguides and tensiometers were installed at two depths in the side of a soil pit face to obtain in situ measurements. Undisturbed soil samples were taken near the instruments and subjected to parƟ cle size analysis, mulƟ step ouƞ low (MSO), and falling-head permeability tests to obtain esƟ mates of the soil moisture retenƟ on curves. Three scaling methods were then applied to improve the fi t of the various esƟ mates to the fi eld data. We found that soil hydraulic property esƟ mates obtained from inverse methods lead to the best simulaƟ ons of soil moisture dynamics, and that laboratory MSO tests or commonly used PTFs perform poorly. These laboratory and PTF esƟ mates can be dramaƟ cally improved, however, by simply constraining the range of possible moisture contents to the minimum and maximum measured in the fi eld. It appears that this method of scaling PTF results can be used to obtain soil hydraulic property inputs of suffi cient accuracy for plot-scale modeling eff orts without requiring expensive laboratory or in situ tests.

Journal ArticleDOI
TL;DR: Water percolation and tracer migration through the vadose zone underneath an ephemeral channel were studied using a VMS and application of a multitracer test in this paper.
Abstract: Water percolation and tracer migration through the vadose zone underneath an ephemeral channel were studied using a vadose zone monitoring system (VMS) and application of a multitracer test. The VMS included flexible time-domain reflectometry (FTDR) probes for continuous tracking of water content profiles, and vadose zone sampling ports (VSPs) for frequent sampling of the deep vadose pore water at multiple depths. The VMS was installed directly under an infiltration pond with several infiltration rings containing a traceable solution. Water content measurements by the FTDR probes allowed detailed visualization of the vadose wetting process; VSP samples allowed the establishment of tracer breakthrough curves at various depths. Flow velocities and fluxes were calculated from both the wetting process and the tracer breakthrough curves. The multitracer experiment revealed an unsteady flow pattern strongly affected by the layered structure of the sediments. The tracer breakthrough curves indicated domination of a mobile–immobile flow mechanism controlling contaminant migration across the vadose zone. The experimental setup demonstrated the ability of the VMS to provide real-time monitoring of water flow and contaminant transport in the vadose zone.

Journal ArticleDOI
TL;DR: The end of an academic term is always a good time to sit back, review one's notes, and decide if the time has come to revise the choice of text and content.
Abstract: The end of an academic term is always a good time to sit back, review one's notes, and decide if the time has come to revise the choice of text and content. In 2007, Paul and Clark's Soil Microbiology and Biochemistry was reissued as a third edition under the title Soil Microbiology, Ecology, and

Journal ArticleDOI
TL;DR: In this paper, Vanclooster et al. proposed an approach to upscaling predictions from local-scale data based on the assumption that soil mapping units (soil series or associations, characterized by benchmark pedons, i.e., typical sequences of recognized soil horizons in a profi le) are characterized by information relevant to solute transport and can be considered as macroscopically homogeneous structural units.
Abstract: (shrinkage cracks, interaggregate voids, earthworm channels, and root holes) strongly infl u-ence water fl ow in structured soils and therefore patterns of solute displacement. Macropore fl ow increases the risk of leaching of surface-applied contaminants to groundwater, since infi ltrating water can be quickly channeled through only a very small fraction of the total pore volume, bypassing much of the adsorption and degradation capacity of the chemically and biologically reactive topsoil. Th e signifi cance of macropore fl ow has long been recog-nized (e.g., Th omas and Phillips, 1979), and in recent decades this has stimulated a major experimental research eff ort aimed at improving understanding of its causes, controlling factors, and consequences (Jarvis, 2007). Th is research eff ort has gone hand in hand with the development of many models that attempt to synthesize knowledge of the relevant processes in mathematical form (Gerke, 2006). Th ese models can be used as tools in research to test the limits of our understanding and generate new hypoth-eses, and as management tools to support policy development and decision making (Vanclooster et al., 2004).At well-investigated sites, input parameters to macropore fl ow models can be either directly measured or derived by calibra-tion against depth profi les of resident concentrations and solute breakthrough curves to further minimize prediction uncertainty (Larsbo and Jarvis, 2005). Many model users (e.g., agricultural advisors, water managers, and regulatory authorities), however, are also required to make predictions for less well characterized sites or at larger scales, for example, to map and quantify diff use pollution risks at farm, catchment, regional, and even national scales. Models that account for macropore fl ow must then be used predictively, without any direct measurements of input param-eters or site data for calibration, although it can be assumed that “soft” information in the form of soil and topographic maps and land use statistics will generally be available. Soil maps have long been used as a basis for catchment- and regional-scale vulnerabil-ity and risk assessments (Vanclooster et al., 2004). Th is approach to upscaling predictions from local-scale data is based on the assumption that soil mapping units (soil series or associations, characterized by benchmark pedons, i.e., typical sequences of recognized soil horizons in a profi le) are characterized by infor-mation relevant to solute transport and can be considered as macroscopically homogeneous structural units (e.g., Vogel and Roth, 2003; Vereecken et al., 2007). Th is assumption may be

Journal ArticleDOI
TL;DR: In this paper, an automated data logging system designed to measure gas pressures within a 15m-high waste rock test pile was installed at a diamond mine site in the Northwest Territories, Canada.
Abstract: An automated data logging system designed to measure gas pressures within a 15-m-high waste rock test pile was installed at a diamond mine site in the Northwest Territories, Canada. Data collected from 12 Aug. 2007 to 15 Oct. 2007 shows distinct gas pressure gradients within the waste rock pile. The magnitude of the gradients within the pile shows a clear response to wind speed external to the pile. The direction of the gradients shows a response to the wind direction. The results demonstrate the ability to measure wind-induced gas pressure gradients within a waste rock pile or other similar porous structures. The general pattern of the observed gradients is inconsistent with the results of numerical modeling assuming homogeneous permeability within the pile. This inconsistency suggests that heterogeneity within the pile and an irregular landscape surrounding the pile affect the way in which the wind flows around and air flows through the rock pile. Calculations of O 2 fluxes using the observed gradients show that wind-induced air flow through the pile has the potential to be a significant mechanism of O 2 transport, similar in magnitude to other mechanism such as diffusion and convection. These results suggest that wind-driven advection may be an important process in waste rock piles where the supply of oxygen limits the overall rate of sulfide oxidation.

Journal ArticleDOI
TL;DR: In this article, a coupled model that integrates a subsurface moisture and energy transport model with a land surface model of the land surface energy balance was studied using a coupled approach.
Abstract: In land surface models, which account for the energy balance at the land surface, subsurface heat transport is an important component that reciprocally influences ground, sensible, and latent heat fluxes and net radiation. In most models, subsurface heat transport parameterizations are commonly simplified for computational efficiency. A simplification made in all models is to disregard the sensible heat of rain, Hl, and convective subsurface heat flow, qcv, i.e., the convective transport of heat through moisture redistribution. These simplifications act to decouple heat transport from moisture transport at the land surface and in the subsurface, which is not realistic. The influence of Hl and qcv on the energy balance was studied using a coupled model that integrates a subsurface moisture and energy transport model with a land surface model of the land surface energy balance, showing that all components of the land surface energy balance depend on Hl. The strength of the dependence is related to the rainfall rate and the temperature difference between the rain water and the soil surface. The rain water temperature is a parameter rarely measured in the field that introduces uncertainty in the calculations and was approximated using the either air or wet bulb temperatures in different simulations. In addition, it was shown that the lower boundary condition for closing the problem of subsurface heat transport, including convection, has strong implications on the energy balance under dynamic equilibrium conditions. Comparison with measured data from the Meteostation Haarweg, Wageningen, the Netherlands, shows good agreement and further underscores the importance of a more tightly coupled subsurface hydrology–energy balance formulation in land surface models

Journal ArticleDOI
TL;DR: In this paper, apparent electrical conductivity (EC a ) was used for the spatial characterization of soil organic C (SOC) within a long-term tillage experiment and its predictive potential for mapping SOC was evaluated.
Abstract: Ancillary information, such as apparent electrical conductivity (EC a ), can improve the spatial and temporal estimation of soil properties. The purpose of this study was to determine if EC a could be used for the spatial characterization of soil organic C (SOC) within a long-term tillage experiment. Apparent electrical conductivity was measured using an electromagnetic induction sensor, the EM38DD, and its predictive potential for mapping SOC was evaluated. The EC a maps showed clear differences between the conventional tillage and direct drilling plots, with higher EC a and SOC in the direct drilling plots. A normalized EC a difference (ΔEC a ), calculated as the difference between the normalized vertical and horizontal dipole EC a values (EC aV and EC aH , respectively) successfully classified the SOC observations according to their corresponding management systems. Maps of ΔEC a (FKM1) and EC aV and EC aH (FKM2) classified by fuzzy k -means accounted for 30% of the total SOC variability, whereas the individual plots and management strategy explained 44 and 41%, respectively. Simple kriging with local varying means using either FKM2 or plot-average SOC as secondary information reduced the RMSE by 8% and increased the efficiency index by about 70% compared with ordinary kriging. Despite the low point-to-point correlation between EC a and SOC, EC a was shown to be useful for the spatial estimation of SOC.

Journal ArticleDOI
TL;DR: In this paper, the authors used a variety of characterization methods to derive conceptual understanding and estimates of hydraulic properties of a coarse alluvial gravel vadose zone in New Zealand.
Abstract: Understanding the physical and hydraulic properties of the vadose zone is important for modeling land use effects on groundwater quality. This study used a variety of characterization methods to derive conceptual understanding and estimates of hydraulic properties of a coarse alluvial gravel vadose zone in New Zealand. Sandy gravel (SG) material constituted approximately 90% of the vadose zone, with the remainder comprising sand lenses and open-framework gravels. The gravel content of the SG material was approximately 70% (v/v) (range 68–73%). The water content of the bulk SG material (43 samples) ranged from 3.5 to 13.9%. The average bulk density of the SG material was 2.20 g/cm 3 (range 2.00–2.33 g/cm 3 ) giving an average calculated porosity of 17%. The average porosity of the open-framework gravels was 34% and these gravels were often coated with 2- to 3-mm-thick deposits of amorphous Fe and Mn oxides. Neutron probe (NP) depth profiles indicated unsteady conditions, with variable water contents with depth and time reflecting the vertical heterogeneity and the variably saturated state of the vadose zone. Time series NP data to 3 m indicated water content in the alluvial gravels responded quickly to soil drainage events, and saturation variability was greater in the sand lenses and the SG material immediately underlying. When compared with derived water retention curves, variability in the water content equated to significant fluctuation in unsaturated hydraulic conductivity ( K unsat ). Tension infiltrometer measurements were variable but were within the range of the K unsat estimates from site-average particle size distribution data. The gravel-transformed, texture-based models used to estimate saturated water content values in this study appeared to underestimate the measured values.

Journal ArticleDOI
TL;DR: In this paper, the geologic and hydraulic properties of a 16m-deep, alluvial vadose zone were characterized using sediment cores from the Eastern San Joaquin Valley, California.
Abstract: StaƟ sƟ cal analysis and interpretaƟ on of heterogeneous sediment hydraulic properƟ es is important to produce reliable forecasts of water and solute transport dynamics in the unsaturated zone. Most fi eld characterizaƟ ons to date have focused on the shallow 2-m root zone. We characterized the geologic and hydraulic properƟ es of a 16-m-deep, alluvial vadose zone consisƟ ng of unconsolidated sediments typical of the alluvial fans of the eastern San Joaquin Valley, California. The thickness of individual beds varies from 2.5 m for large sandy deposits associated with buried stream channels. Eight major geologic units (lithofacies) have been idenƟ fi ed at the site. Unsaturated hydraulic properƟ es were obtained from mulƟ step ouƞ low experiments on nearly 100 sediment cores. MulƟ variate analysis of variance and post hoc tesƟ ng show that lithofacies and other visual- and texture-based sediment classifi caƟ ons explain a signifi cant amount of the spaƟ al variability of hydraulic properƟ es within the unsaturated zone. GeostaƟ sƟ cal analysis of hydraulic parameters show spaƟ al conƟ nuity of within-lithofacies variability in the horizontal direcƟ on in the range of 5 to 8 m, which is approximately an order of magnitude larger than spaƟ al conƟ nuity in the verƟ cal direcƟ on. Low nugget/sill raƟ os suggest that 1- to 10-m sampling intervals are adequate for detecƟ on of horizontal spaƟ al structure. The existence of thin clay or silt layers within lithofacies units results in only moderate spaƟ al conƟ nuity in the verƟ cal direcƟ on, however, suggesƟ ng inadequate sampling frequency for hydraulic parameter variogram development in that direcƟ on.

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TL;DR: In this article, the authors used spin-lattice NMR relaxometry at different Larmor frequencies to investigate water dynamics in the pore space of natural porous media, yielding so-called relaxation dispersion curves (1/ T 1 vs log ν).
Abstract: This study used nuclear magnetic resonance (NMR) relaxometry at different Larmor frequencies to investigate water dynamics in the pore space of natural porous media. Spin-lattice NMR relaxation times ( T 1 ) were determined in purified fine sand and two natural soils, Kaldenkirchen sandy loam and Merzenhausen silt loam, by means of fast field cycling relaxometry. This technique investigates relaxation processes as a function of the Larmor frequency ν in the range between 0.005 and 20 MHz, yielding so-called relaxation dispersion curves (1/ T 1 vs. log ν). The data were further analyzed by means of inverse Laplace transformation to calculate the T 1 relaxation time distribution functions. Only the fine sand was characterized by monomodal distribution with T 1 of about 1 s at 20 MHz, whereas the natural soil samples showed multimodal distribution functions in the range between 2 and 70 ms. With decreasing Larmor frequency, all distribution functions kept their shapes but were shifted to faster relaxation times. The corresponding relaxation dispersion curves indicate predominance of two-dimensional diffusion of water in the soils, whereas in the macroporous sand, diffusion behaved like unrestricted three-dimensional diffusion. In terms of the Brownstein–Tarr model, the decrease in the T 1 relaxation times with increasing silt and clay content can be explained by an increase of the surface/volume ratios ( S / V ) of these porous media, i.e., by a decrease in the pore sizes. Finally, distribution functions of the pore size parameter V / S were obtained from the spin-lattice relaxation time distributions by normalizing on the specific surface area. They ranged from submicrometers in the Merzenhausen soil to micrometers and submillimeters in the Kaldenkirchen soil and fine sand, respectively.

Journal ArticleDOI
TL;DR: In this paper, the problem of subsurface injection of CO 2 is modeled as a multiphase flow, reactive transport, and geomechanical processes, and the authors demonstrate the use of modeling in project design, site characterization, assessments of leakage, and site monitoring.
Abstract: Geologic sequestration of CO 2 is a component of C capture and storage (CCS), an emerging technology for reducing CO 2 emissions to the atmosphere, and involves injection of captured CO 2 into deep subsurface formations. Similar to the injection of hazardous wastes, before injection of CO 2 , operators of geologic sequestration projects may need to demonstrate nonendangerment of groundwater resources during the lifetime of the project. Future requirements related to CO 2 accounting and transfer credits may require operators to evaluate and quantify any surface releases. Subsurface fluid flow computational models have been advocated as an integral tool in predicting and tracking the migration of CO 2 or mobilized constituents. Modeling the injection and sequestration of CO 2 poses unique challenges, such as the need to properly characterize CO 2 transport properties across a large range of temperatures and pressures, and the need to couple multiphase flow, reactive transport, and geomechanical processes. In addition, the volumes of CO 2 that may be injected are largely unprecedented, and an appropriate amount of site characterization across the potentially impacted area will be difficult. In the last several years, there have been several research studies specifically modeling the problem of subsurface injection of CO 2 . Existing studies demonstrate the use of modeling in project design, site characterization, assessments of leakage, and site monitoring. Particularly informative components of existing modeling studies include parameter sensitivity analyses, evaluation of numerical artifacts, code comparison, and demonstrations of model calibration to site monitoring data.

Journal ArticleDOI
TL;DR: In this article, the authors evaluated the usefulness of multifractal analysis for characterizing N2 adsorption isotherms also used for SSA determination, and found that the results indicated that most of the measure concentrates in a small size domain.
Abstract: The specific surface area (SSA) of a soil is commonly estimated by the Brunauer-Emmett -Teller (BET) equation, which implies linearity between applied pressure and volume of adsorbate for a restricted scale range. Dinitrogen adsorption isotherms provide useful data for BET analysis, but they also contain additional information that may render useful a multifractal analysis. The objectives of this work were: (i) to find out if differences in soil use intensity cause changes in SSA assessed by the BET method, and (ii) to evaluate the usefulness of multifractal analysis for characterizing N2 adsorption isotherms also used for SSA determination. The study soils were a Vertisol and a Mollisol from Entre Rios, Argentina. Treatments included: (i) native land never previously cultivated; (ii) permanent pasture; (iii) crop-pasture rotation; and (iv) continuous cropping. Vertisols had significantly greater SSA than Mollisols. Continuous cropping resulted in significant losses of organic matter (OM) content and aggregate stability decay in both soil types. Losses of OM by land use intensification significantly increased SSA in the Mollisol, but this trend was reversed in the Vertisol. All N2 adsorption isotherms exhibited multifractal behavior. Singularity spectra showed strongly asymmetric concave parabolic shapes with a left-hand side much wider than the right-hand one. Entropy dimension, D1, values were in the range 0.357 to 0.558 for the Vertisol and 0.401 to 0.658 for the Mollisol, which indicates that most of the measure concentrates in a small size domain. Several multifractal parameters were significantly different between soil types and soil use intensities.

Journal ArticleDOI
TL;DR: In this article, the delivery of calcium polysulfide (CPS) to the vadose zone using foam and the immobilization of Cr(VI) via reduction by the foam-delivered CPS was studied in a series of batch and column experiments.
Abstract: The delivery of calcium polysulfide (CPS) to the vadose zone using foam and the immobilization of Cr(VI) via reduction by the foam-delivered CPS was studied in a series of batch and column experiments. Batch tests were conducted to select the foam-generating CPS–surfactant solutions, to determine the solution foamability and the reducing potential of CPS-containing foams, and to study the influence of foam quality, surfactant concentration, and CPS concentration on foam stability. Column experiments were performed to test the foam delivery of CPS to sediments under conditions similar to a field vadose zone, to study the foam transport and interaction with sediments, and to determine the extent of Cr(VI) immobilization using this novel delivery approach. Foams containing CPS with high reducing potential were prepared based on the batch tests. Sediment reduction by foam-delivered CPS was observed in the columns. Massive mobilization of Cr(VI) from sediments occurred when CPS was delivered in aqueous solution. The Cr(VI) mobilization was minimized in column tests when CPS was delivered by foams, resulting in significant Cr(VI) in situ immobilization. These results demonstrated for the first time that foam injection can be successfully used for CPS delivery to the vadose zone and that foam-delivered CPS can be applied for Cr(VI) immobilization in contaminated vadose zones.

Journal ArticleDOI
TL;DR: Solute displacement experiments with the anormal bio-c sulfadiazine (SDZ) and its main transformaverage on products in pig manure were performed to inves- fure gate the infl uence of manure on SDZ transport as discussed by the authors.
Abstract: Solute displacement experiments with the anƟ bioƟ c sulfadiazine (SDZ) and its main transformaƟ on products in pig manure were performed to invesƟ gate the infl uence of manure on SDZ transport. Either pig manure containing 14 C-sulfadiazine (4-amino-N-2-pyrimidinyl-benzenesulfonamide), and its main transformaƟ on products 14 C-4-OH-SDZ and 14 C-N-Ac-SDZ, or a 14 C-SDZ soluƟ on was incorporated in the fi rst cenƟ meter of undisturbed and repacked soil columns, which were then irrigated. Breakthrough curves (BTCs) of 14 C, SDZ, 4-OH-SDZ, N-Ac-SDZ and 4-[2-iminopyrimidine-1(2H)-yl]-anilin were measured. The 14 C distribuƟ ons vs. depths were determined aŌ er the conclusion of the leaching experiments. An applicaƟ on of SDZ together with manure resulted in lower peak values of the 14 C BTCs and a slightly lower amount of eluted mass. In the experiments with manure, the 14 C concentraƟ ons in the uppermost layers of the soil columns were higher, probably due to the fi ltraƟ on of manure parƟ cles onto which SDZ or its transformaƟ on products were sorbed. The transformaƟ on products showed a relaƟ vely high leaching potenƟ al similar to SDZ. Cotransport with organic parƟ cles seemed to be of minor relevance for the eluted amounts of solutes. All BTCs were modeled using a numerical model that considered degradaƟ on chains from N-Ac-SDZ into SDZ and from SDZ into 4-OH-SDZ, as well as one reversible and one irreversible kineƟ c sorpƟ on site for each solute. The applied model fi Ʃ ed the BTCs of SDZ and its transformaƟ on products reasonably well. The fi ƫ ng process revealed a high mobility of both SDZ and its transformaƟ on products. While N-Ac-SDZ degradaƟ on into SDZ was fast and no extended tailing of N-AcSDZ was observed, the transport behavior of 4-OH-SDZ was similar to that of SDZ.

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TL;DR: In this article, the authors measured the saturated hydraulic conductivity, K s, and water retention characteristics of weakly to highly weathered Tanakami granite and Rokko granite core samples.
Abstract: Recent studies have emphasized the importance of bedrock in hydrologic processes occurring in headwater catchments. To understand water flow processes through variously weathered bedrock, we measured the saturated hydraulic conductivity, K s , and water retention characteristics of weakly to highly weathered Tanakami granite and Rokko granite core samples. On the basis of these core-scale properties, along with the core shape and in situ K s measurements, we defined two groups of bedrock: C M class (weakly weathered) and C L to D L class (moderately to highly weathered). The C M class bedrock cores had almost no effective porosity (i.e., the amount of porosity that effectively contributes to water flow) and therefore extremely small core-scale K s , indicating that the matrix could be regarded as essentially impermeable. The in situ K s was much larger than the core-scale values, however, and the core shape showed apparent fractures, suggesting that water did flow preferentially through the fractures. The volumetric water content of the C L – to D L –class bedrock water retention curves changed little in the dry range but changed gradually in the wet range, resulting in a moderate core-scale K s of 10 −5 to 10 −3 cm s −1 . The core-scale K s values were well explained by the parameters characterizing the water retention curve. The similarity of the in situ K s to the core-scale values, and the lack of fractures in the core shape, suggested that water flow could be characterized as matrix flow. The hydraulic properties of weathered granite at other sites followed the trends observed at our sites, implying wide applicability of the findings in this study to various types of weathered granite.

Journal ArticleDOI
TL;DR: In this paper, the effect of pedogenesis on soil moisture dynamics influencing the character and quality of ecological habitat was assessed by conducting infiltration and redistribution experiments on three alluvial deposits in the Mojave National Preserve.
Abstract: To assess the effect of pedogenesis on the soil moisture dynamics influencing the character and quality of ecological habitat, we conducted infiltration and redistribution experiments on three alluvial deposits in the Mojave National Preserve: (i) recently deposited active wash sediments, (ii) a soil of early Holocene age, and (iii) a highly developed soil of late Pleistocene age. At each, we ponded water in a 1-m-diameter infiltration ring for 2.3 h and monitored soil water content and matric pressure during and after infiltration, using probes and electrical resistivity imaging (ERI). Infiltration and downward flow rates were greater in younger material, favoring deep-rooted species. Deep-rooted species tend to colonize the margins of washes, where they are unaffected by sediment transport that inhibits colonization. The ERI results support important generalizations, for example that shallower than 0.5 m, infiltrated water persists longer in highly developed soil, favoring shallow-rooted species. Soil moisture data for the two youngest soils suggested that saturation overshoot, which may have significant but unexplored hydroecologic and pedogenic effects, occurred at the horizontally advancing wetting front. Spatial heterogeneity of soil properties generally increased with pedogenic development. Evidence suggested that some early-stage developmental processes may promote uniformity; the intermediate-age soil appeared to have the least heterogeneity in terms of textural variation with depth, and also the least anisotropy. Lateral heterogeneity was pronounced in older soil, having a multitude of effects on the distribution and retention of soil water, and may facilitate certain water-conserving strategies of plants over what would be possible in a laterally homogeneous soil.

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TL;DR: In this article, the impact of the local hydraulic conductivity drop on denser root architectures and in drier soil regions was investigated using a coarse-to-fine discretization of the soil grid.
Abstract: To understand how water uptake locally affects and is affected by the soil water distribution, three-dimensional soil models need to be developed. Nowadays, fully coupled three-dimensional soil-root flow models at the plant available that simulate water flow along water potential gradients in the soil-root continuum, but the arise by the coupling of soil and root have not been investigated thoroughly. In a previous work, we introduced validated a microscopic model to be used on a coarse numerical soil grid, describing the hydraulic drop between the bulk soil and the soil-root interface within a voxel of a three-dimensional soil-root model. study, the impact of the local hydraulic conductivity drop on denser root architectures and in drier soil regions When a coarse discretization of the soil grid is used, the local hydraulic conductivity drop has a significant effect on the water potential distribution at the soil-root interface and in the xylem, especially under conditions stress where the local soil conductivity is lower than the radial root conductivity regulating root water uptake. consequence, plant stress conditions will be reached earlier than if the local conductivity drop within a soil voxel In comparison with a fine soil discretization, the soil water potential gradient calculated by including the conductivity drop at a coarser discretization does not fit the soil water potential gradient resulting from the fine discretization. Estimation of accurate water potential gradients throughout the soil requires a fine soil discretization.