Measurement Of Unsaturated Hydraulic Conductivity Research Articles

Hydropedological Characterization of a Coal Mining Waste Deposition Area Affected by Self-Burning

Coal mining often produces severe environmental effects, including impacts on the soil system and, specifically, on hydropedological conditions that control the leaching of significant ions and Potentially Toxic Elements (PTEs). The research objective is to assess changes in the hydropedological conditions in an area with a coal mining waste pile that underwent self-burning. An integrative approach was implemented, starting with the definition of hydropedological zoning based on field observations of soil formation factors (namely, parent material, relief, biological activity, anthropic influence, and time). The soil profile in each hydropedological zone was characterized regarding morphological features. The upper mineral horizons were sampled and characterized in terms of mineralogy and PTE geochemistry. Field measurements of unsaturated hydraulic conductivity, soil water content, and hydrophobicity were performed. Afterwards, the hydrogeochemistry of leachates was determined, and the soil leaching potential was evaluated. The research outcomes express substantial differences regarding the hydropedological zones: development of different soil profiles, diverse mineralogy and PTE geochemistry, higher unsaturated hydraulic conductivity and leaching of major ions, and PTEs in soils affected by coal mining activities. Finally, a Principal Component Analysis confirmed the existence of significant contrasts according to hydropedological zoning.

The importance of unsaturated hydraulic conductivity measurements for green roof detention modelling

Characterising the unsaturated hydraulic conductivity of a green roof substrate is essential for accurately modelling runoff detention in response to rainfall events. In this paper, the unsaturated hydraulic conductivities for four representative green roof substrates were determined in an infiltration column using steady state and transient techniques. The conventional Durner-Mualem Hydraulic Conductivity Function (HCF) model, for which parameters were calibrated based on the measured Soil Water Release Curve (SWRC) data, was shown to provide a poor fit to the experimental data. A new three-segment HCF was, therefore, proposed to fit measured unsaturated hydraulic conductivity data. Detention tests were carried out on 100 mm and 200 mm deep substrates using four simulated storm events. The runoff and moisture content data collected during the detention tests was used to validate the HCFs using the Richards Equation. The new three-segment HCF resulted in simulated runoff and moisture content profiles that closely matched the measured data (with mean Rt2 = 0.754 for modelled runoff), in contrast to predictions made using the conventional Durner-Mualem model (with mean Rt2 = 0.409 for modelled runoff). It was also demonstrated that further simplification of the HCF to a function defined by moisture content at just two points – the saturated hydraulic conductivity and at an unsaturated hydraulic conductivity of 0.1 cm/min – provides a model that is fit-for-purpose for green roof runoff estimation (with mean Rt2 = 0.629 for modelled runoff).

A study on the geotechnical characterization and water retention characteristic curve of pond ash.

The understanding of the engineering behaviour of unsaturated soil is totally dependent on the water retention characteristic curve (WRCC). In this paper, a comprehensive study of the WRCCs of pond ash along with the ash's geotechnical behaviour has been made. The WRCC has been drawn experimentally using a Fredlund device based upon the pressure plate technique for both wetting and drying cycles. Further, an investigation was carried out to study WRCC hysteresis of pond ash. There exists a considerable hysteresis in drying and wetting curves of pond ash sample. The different WRCC models were used to fit the experimental WRCC data. The effect of compaction on WRCC was also studied. The air entry value in the case of a loose sample is low and the sample gets nearly desaturated at low soil suction as compared to a dense sample. Also, the wetting WRCC is predicted using the Feng and Fredlund model as it is difficult and time consuming to measure the whole hysteresis. The predicted results are compared with the measured wetting WRCC. Since the direct measurement of unsaturated hydraulic conductivity is difficult to obtain in engineering practices, the unsaturated hydraulic conductivity function is predicted using the measured WRCC as the input parameter using SEEP/W software.

New automatic minidisk infiltrometer: design and testing

Abstract Soil hydraulic conductivity is a key parameter to predict water flow through the soil profile. We have developed an automatic minidisk infiltrometer (AMI) to enable easy measurement of unsaturated hydraulic conductivity using the tension infiltrometer method in the field. AMI senses the cumulative infiltration by recording change in buoyancy force acting on a vertical solid bar fixed in the reservoir tube of the infiltrometer. Performance of the instrument was tested in the laboratory and in two contrasting catchments at three sites with different land use. Hydraulic conductivities determined using AMI were compared with earlier manually taken readings. The results of laboratory testing demonstrated high accuracy and robustness of the AMI measurement. Field testing of AMI proved the suitability of the instrument for use in the determination of sorptivity and near saturated hydraulic conductivity

Spatial structure of hydraulic properties from canopy to interspace in the Mojave Desert

Desert piedmonts are a mosaic of interspersed vegetation and bare soil, previously considered to be a binary system of canopy and interspace microsites. The spatial structure of soils and hydraulic properties in these microsites affects our ability to upscale or downscale observed processes. We determined variability and spatial correlation of hydraulic properties using high‐resolution measurements of unsaturated hydraulic conductivity [K(ψ)] along linear transects radiating from canopies of perennial shrubs common to the Mojave Desert into bare soil interspaces. Hydraulic conductivity from −6 < ψ < 0 cm soil water pressures was lower under canopy. Spatial correlations to organic matter, particle‐size, and K(ψ) were observed to approximately 1.4 times canopy diameter, beyond which random processes dominated. Results illustrate that a predictable spatial structure in K(ψ) exists between canopy and interspace microsites and suggest that scaling from the plant to field plot could exploit this relationship.

Comparison of unconfined and confined unsaturated hydraulic conductivity

The field tension infiltrometer (TI) and the laboratory unit hydraulic gradient (UHG) methods are widely applied to determine the near-saturated soil hydraulic conductivity, K. Comparison between the two methods is relevant given that they differ in the explored soil volume (undetached or detached) and in the flow process (unconfined or confined). The objective of this investigation was to compare unconfined and confined measurements of unsaturated hydraulic conductivity. Twenty TI experiments were conducted in a relatively coarse-textured soil having an appreciable hysteretic behavior by using two different dry-to-wet-to-dry (DTWTD) sequences of pressure head, h 0, values that differed by the highest h 0 value imposed within the sequence (i.e. h 0 = − 150, − 75, − 30, + 5, − 30, − 75, − 150 in site A or h 0 = − 150, − 75, − 30, − 10, − 30, − 75, − 150 in site B). The same pressure head sequences were applied on twenty undisturbed soil cores, collected at the exact location of the TI measurements, to perform the laboratory UHG measurements. Regardless of the type of experiment (i.e. unconfined or confined) and the applied pressure head sequence (i.e. site A or B), higher K 0 values were obtained with a drying sequence of h 0 values ( K 0,d) than with a wetting one ( K 0,w)and the discrepancies between K 0,w and K 0,d decreased as the imposed h 0 value increased, as it was expected due to hysteresis. A tendency of the UHG method to overestimate the K 0 values was detected (ratios of mean K 0,1D to mean K 0,3D values ranging from 0.93 to 4.35), but the statistical significance of the observed differences varied with the considered sequence of pressure head values. It was concluded that both the TI and the UHG methods were effective in detecting hysteresis effects on K 0, but the laboratory method resulted in K 0 values that were higher and more variable probably as a consequence of a more substantial effect of macropore flow on the measured flow rates.

Estimation of Soil Hydraulic Properties from Numerical Inversion of Tension Disk Infiltrometer Data

Many applications involving variably saturated flow and transport require estimates of the unsaturated soil hydraulic properties. Numerical inversion of cumulative infiltration data during transient flow, complemented with initial or final soil water content data, is an increasingly popular approach for estimating the hydraulic curves. In this study, we compared Mualem–van Genuchten (MVG) soil hydraulic parameters obtained from direct laboratory and in situ unsaturated hydraulic conductivity measurements with estimates using numerical inversion of tension infiltration data of four coarse‐ to medium‐textured soils in Alentejo (Portugal). The laboratory methods used were suction tables, pressure plates, and the evaporation method as applied to undisturbed soil samples collected from the surface horizons of four different soil profiles. Field measurements were taken with a tension disk infiltrometer using consecutive supply pressure heads of −15, −6, −3, and 0 cm. Six MVG parameters (residual soil water content [θr], saturated soil water content [θs], empirical shape factorsα, η, and ℓ, and saturated hydraulic conductivity [Ks]) were estimated from the field data by numerical inversion using the HYDRUS‐2D software package, and compared with values estimated from the laboratory data. Macroporosity was also determined. The laboratory‐ and field‐measured water retention curves were found to agree closely for most experiments as reflected by relatively high values of the coefficient of determination, the modified coefficient of efficiency, and the modified index of agreement (always >0.9949, 0.8412, and 0.8931, respectively). The unsaturated hydraulic conductivity curves were predicted less accurately, although good estimates of Ks were obtained.

Quasi‐steady centrifuge method for unsaturated hydraulic properties

We have developed the quasi‐steady centrifuge (QSC) method as a variation of the steady state centrifuge method that can be implemented simply and inexpensively with greater versatility in terms of sample size and other features. It achieves these advantages by somewhat relaxing the criterion for steadiness of flow through the sample. This compromise entails an increase in measurement uncertainty but to a degree that is tolerable in most applications. We have tested this new approach with an easily constructed apparatus to establish a quasi‐steady flow of water in unsaturated porous rock samples spinning in a centrifuge, obtaining measurements of unsaturated hydraulic conductivity and water retention that agree with results of other methods. The QSC method is adaptable to essentially any centrifuge suitable for hydrogeologic applications, over a wide range of sizes and operating speeds. The simplified apparatus and greater adaptability of this method expands the potential for exploring situations that are common in nature but have been the subject of few laboratory investigations.

Measurement of Unsaturated Hydraulic Conductivity of Unfrozen and Frozen Soils by Centrifuging Method

Measurement of Unsaturated Hydraulic Conductivity of Unfrozen and Frozen Soils by Centrifuging Method

Improved design for an automated tension infiltrometer

Automated measurements of water infiltration rates are commonly done using two gage transducers to measure water level changes in the reservoir of an infiltrometer. Previous studies have evaluated and described in detail infiltrometers automated with two gage-transducers and have shown that measurement precision and accuracy of soil hydraulic properties are improved. A previous study has also suggested the use of a single differential transducer to automate an infiltrometer to eliminate measurement error associated with air bubbles in the infiltrometer reservoir. In this study, the automation of a tension infiltrometer using a differential transducer was developed, evaluated, and applied. A single differential transducer was installed at the bottom of an infiltrometer reservoir and the other end was connected by tubing to the head-space in the reservoir. Calibration of the reservoir height measurements vs. transducer voltage output was simplified over previous methods and was even demonstrated in situ. Measurement precision was also improved by two orders of magnitude over previous methods. Measurements were also done to demonstrated the use of the single differential transducer set-up to obtain field measurements of unsaturated hydraulic conductivity and sorptivity. Unlike previous methods though, this method does not allow for the determination of the imposed potential at the soil surface unless adaptations are made.

Improved design for an automated tension infiltrometer

Automated measurements of water infiltration rates are commonly done using two gage transducers to measure water level changes in the reservoir of an infiltrometer. Previous studies have evaluated and described in detail infiltrometers automated with two gage‐transducers and have shown that measurement precision and accuracy of soil hydraulic properties are improved. A previous study has also suggested the use of a single differential transducer to automate an infiltrometer to eliminate measurement error associated with air bubbles in the infiltrometer reservoir. In this study, the automation of a tension infiltrometer using a differential transducer was developed, evaluated, and applied. A single differential transducer was installed at the bottom of an infiltrometer reservoir and the other end was connected by tubing to the head‐space in the reservoir. Calibration of the reservoir height measurements vs. transducer voltage output was simplified over previous methods and was even demonstrated in situ. Measurement precision was also improved by two orders of magnitude over previous methods. Measurements were also done to demonstrated the use of the single differential transducer set‐up to obtain field measurements of unsaturated hydraulic conductivity and sorptivity. Unlike previous methods though, this method does not allow for the determination of the imposed potential at the soil surface unless adaptations are made.

Measurement of unsaturated hydraulic conductivity using tension and drip infiltrometers

We investigated differences between constant flux and constant potential methods for determining unsaturated hydraulic conductivity in the laboratory. A cheap and robust method was required. The constant flux drip infiltrometer has been used with large intact cores on a wide range of Australian soils. However, the method can be simplified by replacing the drip infiltrometer with a constant potential tension infiltrometer (disc permeameter). We conducted a series of measurements using 9 soil cores to determine whether the measured hydraulic conductivity differed with each method at matric potentials of –10, –20, or –50 mm. Hysteresis effects were also examined because tension infiltrometer measurements are usually made on the adsorption curve of the hydraulic conductivity and matric potential [K(Ψ)] relationship. Drip infiltrometer measurements are often made on the desorption curve. The reproducibility of measurements on a single core was also examined. A large decline in K(Ψ ) was observed on some cores with repeated measurements and this effect was larger than differences between the methods. In the absence of evidence of slaking or dispersion, the suspected cause of the decline in K(Ψ) was clogging of pores from accumulation of microbial biomass and their by-products. The results support the view that K(Ψ) in some soils is a dynamic property. There were consistent differences between the constant flux and constant potential methods on those soil cores not exhibiting a large decline in K(Ψ) (the others were omitted from the method comparison). The tension infiltrometer method indicated greater hydraulic conductivity in soils with well-developed macrostructure when matric potential was greater than –50 mm. Hysteresis effects were significant with both methods and measurements made on desorption and adsorption curves are not considered comparable. Overall, we concluded that the tension infiltrometer method was more suited than the drip method for routine processing of large numbers of samples at low cost.

Effects of sodium-contaminated wastewater on soil permeability of two New Zealand soils

There is increasing anecdotal evidence from some land treatment sites in New Zealand that irrigating sodium-contaminated wastewaters onto soils may be causing soil structural problems and reduced permeability. In this study, the effect of irrigating such waste (derived from agricultural industries) on soil physical and chemical properties was investigated in an Allophanic Soil (Te Puninga silt loam) and a Gley Soil (Waitoa silt loam). Wastewater irrigation at the sites investigated had taken place for the previous 5 years, with sodium adsorption ratios (SAR) of the wastewater varying between 17 and 51 (mmol/L)0.5 . Increases in exchangeable sodium percentage (ESP) were recorded to 300 mm depth in both soils. At the soil surface (0–20 mm), ESP had increased to 31%, compared with 0.4% at control sites. In laboratory studies using soil from the 0–20 mm layer in non-irrigated sites, leaching distilled water through repacked columns of the soil pretreated with various SAR solutions caused saturated hydraulic conductivity (K sat) to decrease below 100% at SAR greater than 3.5 and 8.5 for the Waitoa silt loam and Te Puninga silt loam, respectively. The decreases in K sat coincided with an increase in dissolved organic carbon (DOC) in collected leachate samples, and no dispersed clay was observed in the leachate. The laboratory studies would predict that effects of past irrigation of industrial wastewater at the study site would be measurable in the field due to the large ESPs that were recorded. Saturated and unsaturated hydraulic conductivity measurements carried out at irrigated sites in the field showed no evidence of reduced conductivity in the surface soil until a pressure head of –120 mm was applied, the decrease being greater for the Te Puninga soil than the Waitoa. These results, along with the laboratory studies, suggest that whereas there may have been some structural deterioration in the soil matrix as a result of irrigation with the wastewater, macropore flow at higher moisture contents in the field was sufficient to overcome any adverse effects. It is suggested that laboratory studies using repacked soil may have limited use in predicting effects of Na-contaminated wastewater on soil hydraulic properties in structured soils. The results also further support suggestions that organic matter dissolution in Na-affected soils may affect soil physical properties.

Flow in unsaturated fractured porous media: Hydraulic conductivity of rough surfaces

The general trend in models for flow in unsaturated fractured porous media is to regard desaturated fractures as nonparticipating elements that impede flow. Mounting experimental and theoretical evidence shows that fractures retain and conduct liquid in the form of film and partially filled corner flow to a relatively low degree of saturation. A simple geometrical model for rough fracture surfaces is developed offering a tractable geometry for calculations of surface liquid storage due to adsorbed films and capillary menisci. Assuming that under slow laminar flow the equilibrium liquid configurations on the fracture surface are not modified significantly, the average hydraulic conductivities for film and corner flows were derived and used as building blocks for a representative fracture roughness element and an assemblage of statistically distributed surface roughness elements. Calculations for a single representative element yielded excellent agreement with surface storage and unsaturated hydraulic conductivity measurements of Tokunaga and Wan [1997]. A statistical representation of surface roughness using a gamma distribution of pit depths resulted in closed‐form expressions for unsaturated hydraulic conductivity averaged across the fracture length (transverse to flow) or weighted by the liquid cross section occupying the fracture surface. An important attribute of the surface roughness model is the direct link between fracture surface and matrix processes unified by the matric potential. The proposed model represents a first step toward development of a comprehensive approach for liquid retention and hydraulic conductivity of unsaturated fractured porous media based on details of liquid configuration for different matric potentials.

Measurement of Subsurface Unsaturated Hydraulic Conductivity

Measurement of unsaturated hydraulic conductivity is needed for precise control of water and solutes in the vadose zone. Because of the spatial variation of soils, a large number of surface and subsurface measurements are needed to characterize a field. In this work, permeameters were developed and tested for estimating subsurface unsaturated hydraulic conductivity. The permeameters apply water under tension; they are easy to use and have adequate accuracy. Unsaturated hydraulic conductivity was determined by measuring the steady flow rates for various values of negative pressure. Tests using a soil of known hydraulic conductivity showed that the permeameters provided valid measurements. Two types were used, a porous cloth model that was inflated against the soil and a porous ceramic cup that was rigid. The field testing determined that a rigid design using a ceramic cup coupled to the soil by a layer of fine sand was easier to use, was reliable, and provided good results.

Field Measurement of Unsaturat-ed Hydraulic Conductivity of an Andic Regosol by Using TDR

Field Measurement of Unsaturat-ed Hydraulic Conductivity of an Andic Regosol by Using TDR

Water transport through the unsaturated zone of the Middle Chalk: a case study from Fleam Dyke lysimeter

Abstract The role of fractures in transporting water in the unsaturated zone of the Chalk has been a source of controversy for many years. There is clearly a potential for fractures to transport water and contaminants rapidly to the saturated zone, although direct observations are rare. The evidence from well hydrographs, solute profiles and soil water measurements is ambiguous concerning the importance of by-pass flow. This paper examines drainage data from several years’ operation of a 5 m cube undisturbed monolith lysimeter at Fleam Dyke, Cambridgeshire, for evidence of by-pass flow and investigates the factors responsible for its onset. Evidence from measurements of unsaturated hydraulic conductivity of the Chalk at the site and in situ measurements of soil water are also presented. Both the lysimeter and soil water observations agree that flow through fractures accounts for approximately 30% of the total drainage from this site, significantly higher than has been inferred at other Chalk sites. By-pass flow was observed to occur above a threshold of approximately −50 cm H 2 O matric potential, or above a flow rate of approximately 1 mm day −1 . During periods of no soil water deficit, about 50% of drainage occurred via fractures. In general, there was a delay of three or four days between rainfally and drainage at the base of the lysimeter. On once occasion, intense summer rainfall satisfied the existing soil moisture deficit and initiated fracture flow, the peak in drainage through the lysimeter occurring within 24 hours. Drainage from the lysimeter was consistently higher than MORECS estimates of hydrologically effective precipitation during the period after the onset of soil water deficits.

Effects of gypsum slotting on the hydraulic properties of a red-brown earth

In situ measurements of unsaturated hydraulic conductivity ( k) and sorptivity ( S) were made on a transitional red-brown earth at two sites. The first site was a non-ameliorated soil (control). At the second site the soil had been modified by mixing the soil with gypsum to a depth of 0.4 m using a rotary digger (gypsum slot). Measurements were made using a disc permeameter (radius 100 mm) at the surface of both sites and at depths of 0.15, 0.4 and 0.6 m at the control site, at various potentials. At the control site, the values of k and S decreased with depth and potential, except at the depth of 0.6 m. The range of S measured was within one order of magnitude. The values of k ranged over two orders of magnitude. The values of k and S measured for the surface horizon were similar to previous measurements by other workers. However, the values of k for the subsoil (0.4 and 0.6 m depth) were greater than previously published values. The values of k and S for the slot soil were intermediate between values for the surface horizon and subsoil of the control soil, and similar to the values at 0.15 m depth of the control soil. The steady state infiltration rate into a slot was calculated using a model developed for vertical slot mulches and compared with values from previous infiltration experiments. The calculated and measured values of the steady state infiltration rate were in agreement. An equation was derived for predicting the increase in steady state infiltration rate, of a soil containing slots, from the slot depth and spacing. The slot width has only a small effect on the steady state infiltration rate. The maximum steady state infiltration rate occurs when the depth of the slot is 0.837 m. The relationship between the increase in the maximum steady state infiltration rate and the slot spacing is presented. This and the relationship between the increase in steady state infiltration rate, slot spacing and depth were used in examples to predict the increase in steady state infiltration rate for various slot spacings and dimensions.

ON THE DETERMINATION OF UNSATURATED HYDRAULIC CONDUCTIVITY FROM SOIL MOISTURE PROFILES AND FROM WATER RETENTION CURVES

When solving water flow in a soil using Darcy-Richards' equation, information about the unsaturated hydraulic conductivity of the soil is necessary. However, direct measurement of unsaturated hydraulic conductivity is time consuming and expensive. Therefore, instantaneous profile methods and/or models to estimate the unsaturated hydraulic conductivity from water retention data are preferable. However, results of the instantaneous profile methods are often influenced by how precise we express the relation of measured volume wetness with time and the relation of measured hydraulic heads with soil depth. Here, we introduce three regression functions for expressing these relationships. In addition, an empirical model relating the measured unsaturated hydraulic conductivity with volume wetness was also developed [...]

Simulation of field measurement of hydraulic conductivity in unsaturated heterogeneous soil

In this study, the procedures used in common methods for field measurement of unsaturated hydraulic conductivity in heterogeneous soil were simulated through numerical experiments. It was assumed that the heterogeneity of the field soil can be described by a scaling factor δ which is treated as a lognormally distributed, second‐order stationary random variable in space. After the hypothetical random field was generated using property variability and correlation length scales typical of those observed in field measurements, it was used to test various field measurement methods currently in practice. One detailed and three simplified unsteady drainage flux methods were used to estimate the hydraulic conductivity under plausible field measurement conditions. In addition, the soil water retention data were used to predict hydraulic conductivity from a pore‐size distribution model. It was found that even under the assumption that there were no reading or instrument errors, other sources of errors can still be created due to the limitations of the observation techniques. These error sources include limited number of readings in space and time, separation distances between the readings of water content and pressure head, and volume‐averaged readings rather than point estimates with common instrumentation. Under the conditions studied in the simulations, the detailed method of estimation produced reasonable estimates of hydraulic conductivity, but only when very dense measurements of water content and matric potential were used in the estimation. None of the approximate methods yielded accurate measurements of the true hydraulic conductivity of the field region measured.