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Modeling water distribution and flow in partially saturated soils requires knowledge of the soil water characteristic (SWC). However, measurement of the SWC is challenging and time-consuming and, in some cases, not feasible. This study introduces two predictive models (FW-model and AW-model) for the SWC, derived from readily available soil properties such as texture and bulk density. A total of 46 undisturbed soils from different horizons at 15 locations across Denmark were used for model evaluation. The FW-model predicts the volumetric water content as a function of volumetric fines content (organic matter and clay). It performed reasonably well for the dry-end of SWC (above a pF value of 2.0; pF = log(|ψ|), where ψ is the matric potential in cm), but did not do as well closer to saturated conditions. The Aw-model predicts the volumetric water content as a function of volumetric content of different particle size fractions (organic matter, clay, silt, and fine and coarse sands). The volumetric content of a particular soil particle size fraction was considered if it contributed to the pore size fraction still occupied with water at the given pF value. Hereby, the Aw-model implicitly assumes that a given particle size fraction creates an analogue pore size fraction and further this pore size fraction filled with water is corresponding to a certain pF value according to the well-known capillary rise equation. The AW-model was found to be quite robust, and it performed exceptionally well for pF values ranging from 0.4 to 4.2 for different soil types. For prediction of the continuous SWC, it is recommended to parameterize the van Genuchten model based on the SWC data points predicted by the Aw-model. [ABSTRACT FROM AUTHOR]

A unified measurement system with Suction control (UMS—SC) was developed for measuring soil water characteristics curves (SWCC) and gas transport parameters under alternating drying and wetting cycles. The new system consists of a diffusion chamber, sample ring, porous plate, tensiometer, moisture sensor, oxygen electrodes, and air pressure gauges. The SWCC and gas transport parameters [gas diffusion coefficient (Dp) and air permeability (ka)] for two different porous materials, Toyoura sand and granulated slag, were simultaneously measured under drying and wetting cycles. The SWCC and gas transport parameters measured by UMSSC were consistent with recent models and independently measured data on exactly the same materials using standard experimental setups from literature. For an applied water suction head of less than 50 cm and corresponding water saturation of around 0.3-0.5, the UMS—SC data documented hysteretic (nonsingular) behavior in both measured Dp and κa under repeated drying and wetting cycles. The hysteretic behavior was insignificant for water and air contents but large for both gas transport parameters when applying suction, and hysteretic effects were larger for air permeability than for gas diffusivity. Additionally, hysteresis in the percolation threshold (soil-air content where gas transport ceases due to interconnected water-filled pores) for both gas diffusion and air permeability was insignificant for both materials. These findings should be taken into account when developing models for diffusive and convective gas transport and their parameters in variably saturated porous media. [ABSTRACT FROM AUTHOR]

Abstract: Landfill sites are emerging in climate change scenarios as a significant source of greenhouse gases. The compacted final soil cover at landfill sites plays a vital role for the emission, fate and transport of landfill gases. This study investigated the effects of dry bulk density, ρb , and particle size fraction on the main soil–gas transport parameters – soil–gas diffusivity (Dp /Do , ratio of gas diffusion coefficients in soil and free air) and air permeability (ka ) – under variably-saturated moisture conditions. Soil samples were prepared by three different compaction methods (Standard and Modified Proctor compaction, and hand compaction) with resulting ρb values ranging from 1.40 to 2.10gcm−3. Results showed that Dp and ka values for the ‘+gravel’ fraction (<35mm) became larger than for the ‘−gravel’ fraction (<2mm) under variably-saturated conditions for a given soil–air content (ε), likely due to enhanced gas diffusion and advection through less tortuous, large-pore networks. The effect of dry bulk density on Dp and ka was most pronounced for the ‘+gravel’ fraction. Normalized ratios were introduced for all soil–gas parameters: (i) for gas diffusivity Dp /Df , the ratio of measured Dp to Dp in total porosity (f), (ii) for air permeability ka /ka ,pF4.1, the ratio of measured ka to ka at 1235kPa matric potential (=pF 4.1), and (iii) for soil–air content, the ratio of soil–air content (ε) to total porosity (f) (air saturation). Based on the normalized parameters, predictive power-law models for Dp (ε/f) and ka (ε/f) models were developed based on a single parameter (water blockage factor M for Dp and P for ka ). The water blockage factors, M and P, were found to be linearly correlated to ρb values, and the effects of dry bulk density on Dp and ka for both ‘+gravel’ and ‘−gravel’ fractions were well accounted for by the new models. [Copyright &y& Elsevier]

Accurate description of the soil water retention curve (SWRC) at low water contents is important for simulating water dynamics and biochemical vadose zone processes in arid environments. Soil water retention data corresponding to matric potentials of less than −10 MPa, where adsorptive forces dominate over capillary forces, have also been used to estimate soil specific surface area (SA). In the present study, the dry end of the SWRC was measured with a chilled-mirror dew point psychrometer for 41 Danish soils covering a wide range of clay (CL) and organic carbon (OC) contents. The 41 soils were classified into four groups on the basis of the Dexter number (n = CL/OC), and the Tuller-Or (TO) general scaling model describing water film thickness at a given matric potential (<−10 MPa) was evaluated. The SA estimated from the dry end of the SWRC (SA_SWRC) was in good agreement with the SA measured with ethylene glycol monoethyl ether (SA_EGME) only for organic soils with n > 10. A strong correlation between the ratio of the two surface area estimates and the Dexter number was observed and applied as an additional scaling function in the TO model to rescale the soil water retention curve at low water contents. However, the TO model still overestimated water film thickness at potentials approaching ovendry condition (about −800 MPa). The semi-log linear Campbell-Shiozawa-Rossi-Nimmo (CSRN) model showed better fits for all investigated soils from −10 to −800 MPa and yielded high correlations with CL and SA. It is therefore recommended to apply the empirical CSRN model for predicting the dry part of the water retention curve (−10 to −800 MPa) from measured soil texture or surface area. Further research should aim to modify the more physically based TO model to obtain better descriptions of the SWRC in the very dry range (−300 to −800 MPa). [ABSTRACT FROM AUTHOR]

Soil water repellency (WR) occurs worldwide and affects hydrologic processes such as infiltration, preferential flow, and surface erosion. The degree of WR varies with soil organic C (SOC) and water contents. In this study, we measured WR (by ethanol molarity) as a function of moisture conditions for two soil profiles (17 layers, of which 13 exhibited WR), representing different vegetation and SOC between 0.6 and 14%. Generally, WR was found at SOC =2%. Based on measured data, a two-region water repellency (TRWR) model was developed. The model assumes two linear regions in a WR vs. pF (=log[-ψ], where ψis the soil water matric potential in centimeters of H2O) plot, with linear increase in WR from the moisture content where WR first occurs during drying to the maximum WR at pFWR-max, and a linear decrease from pWR-max until ambient air-dried conditions. The van Genuchten soil water retention model was used to convert WR-θ (where θis the volumetric water content) to WR-pF. The TRWR model fitting parameters, slopes, and intercepts, were all highly correlated with SOC (R² > 0.8). The TRWR model was tested against an independent data set for five soils with 2 to 12% SOC and predicted well the measured WR-θ and WR-pF relations. For high-SOC surface soils, the TRWR model seems promising to predict WR from fully wettable to ambient air-dried conditions, i.e., within the interval where WR-induced fingered water flow probably occurs. Finally, our data imply that clay saturation by SOC (quantified by the so-called Dexter index) is useful for predicting if soils are likely to exhibit WR. [ABSTRACT FROM AUTHOR]

The soil gas diffusion coefficient (Dp) and its variations with soil air content &varepsilon; and soil water matric potential ψ control vadose zone transport and emissions of volatile organic chemicals and greenhouse gases. This study revisits the 1904 Buckingham power-law model where Dp is proportional to &varepsilon;X, with X characterizing the tortuosity and connectivity of air-filled pore space. One hundred years later, most models linking Dp&varepsilon; to soil water retention and pore size distribution still assume that the pore connectivity factor, X, is a constant for a given soil. We show that X varies strongly with both and matric potential [given as pF = log(-ψ, cm H2O)] for individual soils ranging from undisturbed sand to aggregated volcanic ash soils (Andisols). For Andisols with bimodal pore size distribution, the X-pF function appears symmetrical. The minimum X value is typically around 2 and was observed close to ψ of -1000 cm H2O (pF 3) when interaggregate voids are drained. To link Dp with bimodal pore size distribution, we coupled a two-region van Genuchten soil water retention model with the Buckingham Dp&varepsilon; model, assuming X to vary symmetrically around a given pF. The coupled model well described Dp as a function of both and for both repacked and undisturbed Andisols and for other soil types. By merely using average values of the three constants in the proposed symmetrical X-pF expression, predictions of Dp were better than with traditional models. [ABSTRACT FROM AUTHOR]

The soil-air permeability (ka) and its dependency on air-filled porosity (ε) govern convective air and gas transport in soil. For example, accurate prediction of ka(ε) is a prerequisite for optimizing soil vapor extraction systems for cleanup of soils polluted with volatile organic chemicals. In this study, we measured ka at different soil-water matric potentials down to 5.6-m depth, totaling 25 differently textured soil layers. Comparing ka and soil-gas diffusivity (Dp/D0) measurements on the same soil samples suggested an analogy between how the two soil-gas transport parameters depend on ε. The exponent in a power-law model for ka(ε) was typically smaller than for Dp(ε)/D0, however, probably due to the influence of soil structure and large-pore network being more pronounced for ka than for Dp/D0. In analogy to recent gas diffusivity models and in line with capillary tube models for unsaturated hydraulic conductivity, two power-law ka(ε) models were suggested. One ka(ε) model is based on the Campbell pore-size distribution parameter b and the other on the content of larger pores (ε100, corresponding to the air-filled porosity at -100 cm H2O of soil-water matric potential). Both new models require measured ka at -100 cm H2O (ka,100) as a reference point to obtain reasonably accurate predictions. If ka,100 is not known, two expressions for predicting ka,100 from ε100 were proposed but will cause at least one order of magnitude uncertainty in predicted ka. The ka(ε) model based on only ε100 performed well in the model tests and is recommended together with a similar model for gas diffusivity for predicting variations in soil-gas transport parameters in the vadose zone. [ABSTRACT FROM AUTHOR]

The effects of soil water repellency (SWR) on runoff and nutrient losses are difficult to isolate. Hydrophobic organic substances coating soil particles can severely delay water infiltration and enhance runoff. We used a portable run-on simulator to investigate the effect of SWR on runoff and nutrient loss. Intact soil slabs, 0.48 m long and 0.19 m wide, were collected from a severely water-repellent Andosol under pasture. One day before simulating 60-min long run-on events with an intensity of 60 mm h −1 , superphosphate was applied at a rate of 45 kg P ha −1 . The effects of SWR were quantified by comparing runoff volumes and nutrient losses from run-on events conducted with water and a fully wetting aqueous ethanol solution as run-on liquids. Further, through conducting multiple consecutive water run-on events with the same soil slab, the hypothesis, that SWR is lost through the washing off of hydrophobic materials, was tested. Finally, runoff dynamics were visualised by adding a dye to the run-on water. In the first run-on experiment, 88% of the water applied was captured as runoff, while no runoff was observed when aqueous ethanol was used as run-on liquid, providing strong evidence that SWR governed runoff generation from this Andosol. In consecutive water run-on experiments, approximately 23% of the applied P was recovered in the runoff from the first event, while the cumulative P loss over ten consecutive run-on events was around 30% of the applied P. This confirms that nutrient losses were associated with SWR and the occurrence of runoff. After ten consecutive run-on events, the persistence of both actual and potential SWR in areas of the slab that had been wetted were significantly ( p  < 0.5) reduced. But the persistence of potential SWR of the soil was still classified as severe, suggesting only minor losses of hydrophobic materials from the soil surface. The persistence of potential SWR and the degree of SWR of the dry areas remained more or less unchanged. In accordance with this, visualisation of the wetted areas showed that runoff occurred as rivulets guided by surface topography, rather than as sheet flow, with the wetted area increasing from approximately 20% of the slab in the first event to around one third of the total slab area in the final event. This pattern is reflected in the cumulative pattern of the P losses over the ten events. Consequently, we conclude that SWR should be considered as a factor in hydrological modelling and should be included in models to address appropriately the risk of surface water contamination by solutes exogenously applied to water-repellent soils. [ABSTRACT FROM AUTHOR]

Soil water repellency (SWR) of soils is a property with significant consequences for agricultural water management, water infiltration, contaminant transport, and for soil erosion. It is caused by the presence of hydrophobic agents on mineral grain surfaces. Soils were samples in different depths at three forest sites in Japan and three pasture sites in New Zealand, covering soil organic carbon (SOC) contents between 1 and 26%. The SWR was measured over a range of water contents by three common methods; the water drop penetration time (WDPT) test, the molarity of an ethanol droplet (MED) method, and the sessile drop method (SDM). The aim to (i) compare the methods, (ii) characterize the soil-water repellency characteristic curves (SWRCC) being SWR as a function of the volumetric soil-water content ( θ ) or matric potential ( ψ ), and (iii) find relationships between SWRCC parameters and SOC content. The WDPT, MED, and SDM generally agreed well in predicting the θ range where SWR occurred, and there was close agreement between SWR results determined by average MED and SDM at similar θ . Generally, SWR was only found within the top 20 cm of the soil profiles. Six SWR parameters were introduced: (i) the area under the curve ( S WR ( θ ) ); (ii) θ at the maximum SWR ( θ WR-max ), (iii) θ where SWR ceased ( θ non-WR ), (iv) the maximum SWR ( CA i -max ), (v) pF at the maximum SWR ( pF WR-max ) and (vi) pF where SWR ceased ( pF non-WR ). The relationship between the first three parameters and SOC content were best described with Langmuir type equations (r 2 of 0.5–0.7), while the other three parameters changed linearly with SOC contents. [ABSTRACT FROM AUTHOR]