Effective Hydraulic Properties Research Articles

Stiff, Smooth, and Solid? Complex Fracture Hydraulics' Imprint on Oscillatory Hydraulic Testing

AbstractFractured bedrock aquifers, especially deep aquifers, represent increasingly common targets for waste storage and alternative energy development, necessitating detailed quantitative descriptions of fracture hydraulic properties, geometry, and connectivity. Yet, multi‐scale characterization of the physical properties that govern fluid flow through and storage in fractured bedrock remains a fundamental hydrogeologic challenge. Oscillatory hydraulic testing, a novel hydraulic characterization technique, has been showing promise in field experiments to characterize the effective hydraulic properties of bedrock fractures. To date, these characterization efforts utilize simplified diffusive analytical models that conceptualize a non‐deforming, parallel‐plate fracture embedded within impermeable host rock, and have found that the returned fracture hydraulic parameter estimates exhibit an apparent period‐dependence. We conduct synthetic experiments using three different numerical models to examine proposed mechanisms that might contribute to the observed period‐dependence including heterogeneous flow and storage within the fracture (i.e., aperture heterogeneity), fracture‐host rock fluid exchange, and fracture hydromechanics. This work represents the first systematic analysis that seeks to understand the process(es) occurring within a bedrock fracture that might be contributing to this apparent period‐dependence. Our analysis demonstrates that all investigated mechanisms generate period‐dependent effective hydraulic parameter estimates, each with their own potentially diagnostic trends; however, fracture hydromechanics is the only explored mechanism that consistently reproduces period‐dependent trends in parameter estimates that are consistent with existing field investigations. These results highlight the need to develop more complex numerical modeling approaches that account for this hydromechanical behavior when characterizing fractured bedrock aquifers.

Calibration of groundwater seepage against the spatial distribution of the stream network to assess catchment-scale hydraulic properties

Abstract. The assessment of effective hydraulic properties at the catchment scale, i.e., hydraulic conductivity (K) and transmissivity (T), is particularly challenging due to the sparse availability of hydrological monitoring systems through stream gauges and boreholes. To overcome this challenge, we propose a calibration methodology which only considers information from a digital elevation model (DEM) and the spatial distribution of the stream network. The methodology is built on the assumption that the groundwater system is the main driver controlling the stream density and extension, where the perennial stream network reflects the intersection of the groundwater table with the topography. Indeed, the groundwater seepage at the surface is primarily controlled by the topography, the aquifer thickness and the dimensionless parameter K/R, where R is the average recharge rate. Here, we use a process-based and parsimonious 3D groundwater flow model to calibrate K/R by minimizing the relative distances between the observed and the simulated stream network generated from groundwater seepage zones. By deploying the methodology in 24 selected headwater catchments located in northwestern France, we demonstrate that the method successfully predicts the stream network extent for 80 % of the cases. Results show a high sensitivity of K/R to the extension of the low-order streams and limited impacts of the DEM resolution as long the DEM remains consistent with the stream network observations. By assuming an average recharge rate, we found that effective K values vary between 1.0×10-5 and 1.1×10-4 m s−1, in agreement with local estimates derived from hydraulic tests and independent calibrated groundwater model. With the emergence of global remote-sensing databases compiling information on high-resolution DEM and stream networks, this approach provides new opportunities to assess hydraulic properties of unconfined aquifers in ungauged basins.

Numerical assessment of morphological and hydraulic properties of moss, lichen and peat from a permafrost peatland

Abstract. Due to its insulating and draining role, assessing ground vegetation cover properties is important for high-resolution hydrological modeling of permafrost regions. In this study, morphological and effective hydraulic properties of Western Siberian Lowland ground vegetation samples (lichens, Sphagnum mosses, peat) are numerically studied based on tomography scans. Porosity is estimated through a void voxels counting algorithm, showing the existence of representative elementary volumes (REVs) of porosity for most samples. Then, two methods are used to estimate hydraulic conductivity depending on the sample's homogeneity. For homogeneous samples, direct numerical simulations of a single-phase flow are performed, leading to a definition of hydraulic conductivity related to a REV, which is larger than those obtained for porosity. For heterogeneous samples, no adequate REV may be defined. To bypass this issue, a pore network representation is created from computerized scans. Morphological and hydraulic properties are then estimated through this simplified representation. Both methods converged on similar results for porosity. Some discrepancies are observed for a specific surface area. Hydraulic conductivity fluctuates by 2 orders of magnitude, depending on the method used. Porosity values are in line with previous values found in the literature, showing that arctic cryptogamic cover can be considered an open and well-connected porous medium (over 99 % of overall porosity is open porosity). Meanwhile, digitally estimated hydraulic conductivity is higher compared to previously obtained results based on field and laboratory experiments. However, the uncertainty is less than in experimental studies available in the literature. Therefore, biological and sampling artifacts are predominant over numerical biases. This could be related to compressibility effects occurring during field or laboratory measurements. These numerical methods lay a solid foundation for interpreting the homogeneity of any type of sample and processing some quantitative properties' assessment, either with image processing or with a pore network model. The main observed limitation is the input data quality (e.g., the tomographic scans' resolution) and its pre-processing scheme. Thus, some supplementary studies are compulsory for assessing syn-sampling and syn-measurement perturbations in experimentally estimated, effective hydraulic properties of such a biological porous medium.

Modeling lost-circulation in natural fractures using semi-analytical solutions and type-curves

Drilling is a requisite operation for many industries to reach a targeted subsurface zone. During operations, various issues and challenges are encountered, particularly drilling fluid loss. Loss of circulation is a common problem that often causes interruptions to the drilling process and a reduction in efficiency. Such incidents usually occur when the drilled wellbore encounters a high permeable formation such as faults or fractures, leading to total or partial leakage of the drilling fluids.In this work, a semi-analytical solution and mud type-curves (MTC) are proposed to offer a quick and accurate diagnostic model to assess the lost-circulation of Herschel-Bulkley fluids in fractured media. Based on the observed transient pressure and mud-loss trends, the model can estimate the effective fracture conductivity, the time-dependent cumulative mud-loss volume, and the leakage period. The behavior of lost-circulation into fractured formation can be quickly evaluated, at the drilling site, to perform useful diagnostics, such as the rate of fluid leakage, and the associated effective fracture hydraulic properties. Further, derivative-based mud-type-curves (DMTC) are developed to quantify the leakage of drilling fluid flow into fractures. The developed model is applied for non-Newtonian fluids exhibiting yield-power-law (YPL), including shear thickening and thinning, and Bingham plastic fluids. Proposing rigorous dimensionless groups generates the dual type-curves, MTC and DMTC, which offer superior predictivity compared to traditional methods. Both type-curve sets are used in a dual trend matching, which significantly reduces the non-uniqueness issue that is typically encountered in type-curves.Usage of numerical simulations is implemented based on finite elements to verify the accuracy of the proposed solution. Data for lost circulation from several field cases are presented to demonstrate the applicability of the proposed method. The semi-analytical solver, combined with Monte Carlo simulations, is then applied to assess the sensitivity and uncertainty of various fluid and subsurface parameters, including the hydraulic property of the fracture and the probabilistic prediction of the rate of mud leakage into the formation. The proposed approach is based on a robust semi-analytical solution and type-curves to model the flow behavior of Herschel-Bulkley fluids into fractured reservoirs, which can serve as a quick diagnostic tool to evaluate lost-circulation in drilling operations.

Field-scale assessment of the unsaturated hydraulic properties of residual soils in southeastern Brazil

Abstract Field tests were carried out to estimate effective unsaturated soil hydraulic properties of layered residual soils in Rio de Janeiro, southeastern Brazil. Data of this type are important for understanding the initiation of rainstorm-induced soil landslides, which often occur in the state of Rio de Janeiro as well as other areas having similar geologic settings and climate conditions. Tests were carried out using a simplified field approach, referred to as the Monitored Infiltration Test, which requires only a tensiometer to measure pressure heads below the wetting front, triggered by flow from a Mariotte bottle which maintains a constant pressure at the top edge of the soil profile. The data can then be analyzed by numerical inversion using the HYDRUS-2D software package. The test is relatively fast since no steady-state flow conditions are needed, and versatile since the test can be carried out quickly on steep slopes with the help of a manual auger. Soil water retention and the unsaturated hydraulic conductivity functions were obtained for a range of young, mature and saprolitic residual soils. The effective hydraulic properties of the distinct residual soil layers can be quite large, reflecting a need to provide a careful analysis of field-scale hydraulic heterogeneity in geotechnical analyses.

Effective hydraulic properties of 3D virtual stony soils identified by inverse modeling

Abstract. Stony soils that have a considerable amount of rock fragments (RFs) are widespread around the world. However, experiments to determine the effective soil hydraulic properties (SHPs) of stony soils, i.e., the water retention curve (WRC) and hydraulic conductivity curve (HCC), are challenging. Installation of measurement devices and sensors in these soils is difficult, and the data are less reliable because of their high local heterogeneity. Therefore, effective properties of stony soils especially under unsaturated hydraulic conditions are still not well understood. An alternative approach to evaluate the SHPs of these systems with internal structural heterogeneity is numerical simulation. We used the Hydrus 2D/3D software to create virtual stony soils in 3D and simulate water flow for different volumetric fractions of RFs, f. Stony soils with different values of f from 11 % to 37 % were created by placing impermeable spheres as RFs in a sandy loam soil. Time series of local pressure heads at various depths, mean water contents, and fluxes across the upper boundary were generated in a virtual evaporation experiment. Additionally, a multistep unit-gradient simulation was applied to determine effective values of hydraulic conductivity near saturation up to pF=2. The generated data were evaluated by inverse modeling, assuming a homogeneous system, and the effective hydraulic properties were identified. The effective properties were compared with predictions from available scaling models of SHPs for different values of f. Our results showed that scaling the WRC of the background soil based on only the value of f gives acceptable results in the case of impermeable RFs. However, the reduction in conductivity could not be simply scaled by the value of f. Predictions were highly improved by applying the Novák, Maxwell, and GEM models to scale the HCC. The Maxwell model matched the numerically identified HCC best.

Hydraulic transport properties of unsaturated cementitious composites with spheroidal aggregates

Understanding of the coupled effects of aggregates and interfacial transition zones (ITZs) on hydraulic transport properties of unsaturated cementitious composites (CC) is of great importance for the design of CC durability. In this work, a comprehensive micromechanical theoretical framework is devised for the accurate predictions of the effective hydraulic conductivity, diffusivity and sorptivity of saturated and unsaturated CC, in which CC are regarded as a three-phase structure consisting of spheroidal aggregates with a certain gradation, their surrounding permeable ITZs with a specific thickness, and homogeneous cement paste at a mesoscopic level. Comparisons against experimental, numerical and analytical data suggest that the proposed framework is a reliable means to predict these effective hydraulic transport properties. The micromechanical framework can be regarded as a generic model that suits for predicting other hydraulic-like transport properties of multiphase composites not limited to cementitious composites involving spheroidal inhomogeneities and their surrounding interphase. This framework sheds light on the coupled effects of properties of aggregates (shape, gradation, mean size, and volume fraction) and ITZ (volume fraction, thickness, and hydraulic properties) on the effective hydraulic transport properties of CC. The results elucidate the complex relationship of components-structures-transport properties, which in turn can provide insights for understanding degradation of concrete in practical applications.

Watershed‐Scale Effective Hydraulic Properties of the Continental United States

AbstractIn land surface models (LSMs), the hydraulic properties of the subsurface are commonly estimated according to the texture of soils at the Earth's surface. This approach ignores macropores, fracture flow, heterogeneity, and the effects of variable distribution of water in the subsurface on effective watershed‐scale hydraulic variables. Using hydrograph recession analysis, we empirically constrain estimates of watershed‐scale effective hydraulic conductivities (K) and effective drainable aquifer storages (S) of all reference watersheds in the conterminous United States for which sufficient streamflow data are available (n = 1,561). Then, we use machine learning methods to model these properties across the entire conterminous United States. Model validation results in high confidence for estimates of log(K) (r2 > 0.89; 1% < bias < 9%) and reasonable confidence for S (r2 > 0.83; −70% < bias < −18%). Our estimates of effective K are, on average, two orders of magnitude higher than comparable soil‐texture‐based estimates of average K, confirming the importance of soil structure and preferential flow pathways at the watershed scale. Our estimates of effective S compare favorably with recent global estimates of mobile groundwater and are spatially heterogeneous (5–3,355 mm). Because estimates of S are much lower than the global maximums generally used in LSMs (e.g., 5,000 mm in Noah‐MP), they may serve both to limit model spin‐up time and to constrain model parameters to more realistic values. These results represent the first attempt to constrain estimates of watershed‐scale effective hydraulic variables that are necessary for the implementation of LSMs for the entire conterminous United States.

Watershed-Scale Effective Hydraulic Properties of the Continental United States

Watershed-Scale Effective Hydraulic Properties of the Continental United States

Multiscale approach to characterize effective mechanical, hydraulic and acoustic properties of a new bio-based porous material

This paper is a multi-aspect study which has undertaken essential numerical characterizations of not only mechanical and hydraulic properties but also acoustic behaviour of a new bio-based porous epoxy resin obtained by a ‘green’ adapted combination of the cationic photopolymerization and the porogen leaching technique. This new kind of material generally possesses interconnected fillet-edge cubic pores which lead to more complex morphology than in the case of spherical or cylindrical pores. In order to characterize the effective properties of the material, a multiscale approach using the asymptotic homogenization method has been applied. Such a method has induced cell problems whose resolutions have been conducted on the geometrical configuration defined from the experimental data of the samples by using scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP). The sound absorption behaviour of a plate made of the studied material has been subsequently characterized by solving a normal incidence acoustic problem with an assumption of rigid impervious backing. The mechanical model has been validated by balancing numerical results of the sound absorption coefficient, reflection coefficient, surface impedance, equivalent dynamic density and equivalent dynamic bulk modulus with corresponding experimental results obtained by conducting the three-microphone impedance tube testing. As a consequence, the mechanical model has been applied to investigate the influence of microstructural characteristics on effective properties and acoustic performance of the material. To the authors’ knowledge, the features of the microstructure obtained from an elaboration process using the porogen leaching technique have rarely been studied in the literature. Four types of ordered pore arrangements together with systematic variations of the porosity and the pore size have been taken under consideration. Based on the results of these investigations, the subtle relation between microstructures and properties has been established. The processing parameters of material elaboration could be adjusted so that the obtained porous material would possess the best sound absorption performance.

Gas Permeability of Salt Crusts Formed by Evaporation from Porous Media

Soil salinization in irrigated croplands is a key factor in soil degradation and directly affects plant growth and soil hydrological processes such as evaporation and infiltration. In order to support the development of appropriate irrigation strategies, it is important to understand the impact of salt crusts that form during evaporation from saline soils on water flow. The determination of the effective hydraulic properties of salt crusts that control evaporation is still a challenge due to the lack of suitable measurement techniques. In this study, we propose an approach using gas flow to determine the permeability of salt crusts obtained from evaporation of unsaturated saline solutions of three different salt types and investigate the impact of the crust permeability on evaporation. For this, sand columns saturated with initial solutions of sodium chloride (NaCl), magnesium sulfate (MgSO4), and sodium sulfate (Na2SO4) at concentrations corresponding to 33% of the solubility limit were prepared and allowed to evaporate in order to induce crust formation. The results demonstrated that the intrinsic permeability of the dry salt crusts was similar for the different types of salts (≈4 × 10−12 m2), whereas the evaporation of the prepared columns differed significantly. We conclude that the intrinsic crust permeability only partly explains the impact of the crust on evaporation. Other effective crust properties such as porosity or unsaturated hydraulic properties may provide additional information on how evaporation is affected by salt crust formation.

Modeling water fluxes through containerized soilless substrates using HYDRUS

AbstractContainerized crop production can enhance plant health and ensure environmental sustainability, yet proper management requires improved understanding of water fluxes and storage within soilless substrates. Numerical simulation tools developed to simulate water movement in porous media, such as HYDRUS‐3D, may help to quantify effective hydraulic properties of soilless substrates but have not yet been tested in this capacity. Therefore, this study had three main objectives: (a) to assess the accuracy of HYDRUS‐3D for simulating water flow through peat‐ and bark‐based soilless substrates by comparing measured and modeled drainage and water storage; (b) to determine sensitivity of model outputs to individual hydraulic parameters; and (c) to compare model parameterization using three laboratory characterization methods (instantaneous profile sorption, instantaneous profile desorption, and evaporation) vs. inverse modeling with HYDRUS. The results showed that the modeled water contents and drainage timing and amounts were most sensitive to saturated volumetric water content (θs) and least sensitive to saturated hydraulic conductivity (Ks). With regard to parameterization methods, the inverse modeling approach provided the most accurate water balance simulations for both substrates, followed by the sorption method. These two methods estimated lower peak water contents and greater drainage compared with simulations parameterized using desorption and evaporation measurements. Overall, the study results showed that the Richards equation, as calculated using HYDRUS, can provide accurate simulations of water flux through containers when properly calibrated, though sorption‐derived parameters may suffice when model optimization is impractical.

Efficient estimation of effective hydraulic properties of stratal undulating surface layer using time-lapse multi-channel GPR

Abstract. Multi-scale soil architectures in shallow subsurface are widespread in natural and anthropogenic depositional environments, and acquisition of the surface stratal structure and hydrological properties are essential in quantifying water cycling. Geophysical methods like ground-penetrating radar (GPR) can provide quantitative information like soil architecture and spatiotemporal soil water content distribution for the shallow layer. Concerning the informative multi-dimensional water flow in the surface layer with an undulating bottom at the plot scale, this study assesses the feasibility of efficiently estimating soil hydraulic properties using a few time-lapse multi-channel GPR observations, namely soil water storage and layer thickness of the surface layer, at reclamation land near an old river channel. We show that effective hydraulic properties of the surface layer can be obtained with a small number of time-lapse GPR measurements during a rainfall event. Additionally, we analyze the effect of some key factors controlling the informative lateral water redistribution on the results of the proposed approach using synthetic simulations.

Analytical models for effective hydraulic sorptivity, diffusivity and conductivity of concrete with interfacial transition zone

This paper presents analytical models for evaluating effective hydraulic conductivity, diffusivity and sorptivity of concrete considering the properties of the Interfacial Transition Zone (ITZ) and the aggregate size distribution. Results of the proposed models compare well with the experimental results and those obtained from rigorous numerical Finite Element (FE) analysis. It is also found that a change of Interfacial Transition Zone (ITZ) properties has more influence on the effective hydraulic conductivity than it has on the effective hydraulic sorptivity. The significance of the proposed models is that they can estimate the hydraulic sorptivity and diffusivity of concrete by only knowing the hydraulic properties of mortar, effectively extricating the need for sophisticated and time-consuming FE analysis required for estimation of the effective hydraulic conductivity of concrete at the meso-scale. Furthermore, these models offer scientific insight into the effect of different components of concrete on its effective hydraulic properties.

Effective Elastic and Hydraulic Properties of Fractured Rock Masses with High Contrast of Permeability: Numerical Calculation by an Embedded Fracture Continuum Approach

In this work, the hydromechanical modeling of the fractured rock masses was conducted based on a new numerical simulation method named as embedded fracture continuum (EFC) approach. As the principal advantage, this approach allows to simplify the meshing procedure by using the simple Cartesian meshes to model the fractures that can be explicitly introduced in the porous medium based on the notion of fracture cells. These last elements represent the grid cells intersected by at least one fracture in the medium. Each fracture cell in the EFC approach present a continuum porous medium whose hydromechanical properties are calculated from ones of the matrix and ones of the intersected fractures, thanks for using the well‐known solution of the joint model. The determination of the hydromechanical properties of the fracture cells as presented in this work allows to provide the theoretical base and to complete some simple approximations introduced in the literature. Through different verification tests, the capability of the developed EFC approach to model the hydromechanical behavior of fractured rock was highlighted. An analysis of different parameters notably the influence of the fracture cell size on the precision of the proposed approach was also conducted. This novel approach was then applied to investigate the effective permeability and elastic compliance tensor of a fractured rock masses taken from a real field, the Sellafield site. The comparison of the results calculated from this approach with ones conducted in the literature based on the distinct element code (UDEC) presents a good agreement. However, unlike the previous studies using UDEC, which limits only in the case of fractured rock masses without dead‐end fractures, our approach allows accounting for this kind of fractures in the medium. The numerical simulations show that the dead‐end fractures could have a considerable contribution on the effective compliance moduli, while their effect can be neglected to calculate the overall permeability of the of fractured rock masses.

Impact of fracture clustering on the seismic signatures of porous rocks containing aligned fractures

The presence of fractures in a reservoir can have a significant impact on its effective mechanical and hydraulic properties. Many researchers have explored the seismic response of fluid-saturated porous rocks containing aligned planar fractures through the use of analytical models. However, these approaches are limited to the extreme cases of regular and uniform random distributions of fractures. The purpose of this work is to consider more realistic distributions of fractures and to analyze whether and how the frequency-dependent anisotropic seismic properties of the medium can provide information on the characteristics of the fracture network. Particular focus is given to fracture clustering effects resulting from commonly observed fracture distributions. To do so, we have developed a novel hybrid methodology combining the advantages of 1D numerical oscillatory tests, which allows us to consider arbitrary distributions of fractures, and an analytical solution that permits extending these results to account for the effective anisotropy of the medium. A corresponding numerical analysis indicates that the presence of clusters of fractures produces an additional attenuation and velocity dispersion regime compared with that predicted by analytical models. The reason for this is that a fracture cluster behaves as an effective layer and the contrast with respect to the unfractured background produces an additional fluid pressure diffusion length scale. The characteristic frequency of these effects depends on the size and spacing between clusters, the latter being much larger than the typical spacing between individual fractures. Moreover, we find that the effects of fracture clustering are more pronounced in attenuation anisotropy than velocity anisotropy data. Our results indicate that fracture clustering effects on fluid pressure diffusion can be described by two-layer models. This, in turn, provides the basis for extending current analytical models to account for these effects in inversion schemes designed to characterize fractured reservoirs from seismic data.

Quantification of the microstructure, effective hydraulic radius and effective transport properties changed by the coke deposition during the crude oil in-situ combustion

Coke deposition during crude oil in-situ combustion (ISC) is an important phenomenon that significantly impacts the pore topology and permeability. In this study, X-ray computed microtomography and a specific image processing procedure were used to reconstruct the micro-tomographic images of packed beds with coke deposition. From the reconstructed images, the microstructural parameters related to the transport were analyzed, such as the effective porosity, the constrictivity and the geodesic tortuosity. The Lattice Boltzmann method was used to simulate the species diffusion and fluid flow through the microstructures to quantify the mass diffusivity and permeability. The experimental work was performed to validate the digital microstructures and the simulated permeability. The effects of the coke deposition on the pore topology and the permeability were analyzed. The coke deposition pattern showed significant deposition in the pore throat. Analyses of the pore size distribution lead to a more reasonable geometric approach to measure the effective hydraulic radius for the better permeability prediction. Based on the effective transport properties and the microstructural parameters, a developed permeability relation was introduced by factorizing the permeability into two distinct contributions from the characteristic length and the microstructural effect. The permeability model describes the main influences of all the relevant geometric parameters on the permeability reduction by the coke deposition.

Fracture connectivity can reduce the velocity anisotropy of seismic waves

Abstract The degree of connectivity of fracture networks is a key parameter that controls the hydraulic properties of fractured rock formations. The current understanding is that this parameter does not alter the effective elastic properties of the probed medium and, hence, cannot be inferred from seismic data. However, this reasoning is based on static elasticity, which neglects dynamic effects related to wave-induced fluid pressure diffusion (FPD). Using a numerical upscaling procedure based on the theory of quasi-static poroelasticity, we provide the first evidence to suggest that fracture connectivity can reduce significantly velocity anisotropy in the seismic frequency band. Analyses of fluid pressure fields in response to the propagation of seismic waves demonstrate that this reduction of velocity anisotropy is not due to changes of the geometrical characteristics of the probed fracture networks, but rather related to variations of the stiffening effect of the fracture fluid in response to FPD. These results suggest that accounting for FPD effects may not only allow for improving estimations of geometrical and mechanical properties of fracture networks, but may also provide information with regard to the effective hydraulic properties.

Influence of rock fragments on hydraulic properties of Ultisols in Ratchaburi Province, Thailand

In Thailand, stony soils are mainly located in hillside areas. Even though they have physical limitations for agricultural use and they are exposed to a risk for soil erosion, they continue to be used for crop production. A good understanding of the hydraulic characteristics of soils containing an important fraction of rock fragments is crucial for soil and water management in these areas. The objective of this study was to investigate the influence of different rock fragment contents on effective hydraulic properties of skeletal soil with a clay content lower than 35% using measurements of the water retention curve and the hydraulic conductivity. A tension infiltrometer was used to determine the field hydraulic conductivity at four pressure heads (h) of 0, −30, −60 and −120mm. Soil water retention was determined on a pressure plate between −33 and −1500kPa. Finally, the Hydrus-1D was used to predict soil moisture dynamics using the obtained effective hydraulic parameters. The results show a decreasing water retention capacity with increasing rock fragment content. The saturated hydraulic conductivity decreased with increasing stone contents from 0 to 20%, but then increased for increasing stone content. Contradicting behavior can be observed using field and lab measurements, clearly exposing the need for a better understanding of the functioning of stony soils.

Water retention properties of a sandy soil with superabsorbent polymers as affected by aging and water quality

AbstractHydraulic properties, specifically the water holding capacity of soils, play a key role in the ability of soils to sustain plant growth. Additions of hydrophilic polymers (superabsorbents) can improve the water holding capacity of sandy soils. This has led to practical applications of these materials particularly in arid regions and countries, where water is the limiting factor for plant production. The objectives of this study are to investigate how effective hydraulic properties of polymer‐soil mixtures are affected by addition of absorbents in different concentrations. Novel aspects are the investigation of aging under repeated wetting–drying‐cycles over an appreciable time in the field and a systematic investigation of the salt influence on the water uptake of polymers. We added the polymer Super AB, A‐200 (Iran Polymer Institute), to dune sand in ratios of 0.3%, 0.6%, and 1% w/w. We found that the effective water retention characteristics of the soil–absorbent mixtures were improved with respect to plant‐available water compared to the pure sand, and the improvement was related to the respective amount of absorbent in the mixture. The plant available water content (PAW) increased from 0.005 for the untreated sand to 0.06, 0.20, and 0.28 g g−1, respectively, for the sand with the three polymer additions. Due to aging of the polymers, PAW decreased after 6 months of cyclic drying and wetting to about half of the value immediately after the initial treatment. We attribute this to the effect of salts. This is corroborated by the results from water uptake experiments by the pure polymers. Repeated cycles of water uptake showed that salts in the water greatly reduced the uptake capacity of the polymers after few cycles. The effect was strong for bivalent cations and less pronounced for monovalent cations.