Development of soil water characteristic equations for various soil texture classes

Different approaches were investigated for estimating the parameters in the Campbell soil water characteristic (SWC) equation from soil attributes such as particle size distribution (PSD), bulk density, and organic matter content. Predicted soil water characteristics were compared with measured values for soils of the wheatbelt of south-eastern Australia. A method of prediction is proposed incorporating an empirical relationship for estimating the slope of the SWC from the slope of the cumulative PSD. A power-law form is assumed for both the SWC and PSD functions. One measured SWC point is then used to locate and thus define the SWC curve. When SWC points predicted with this 'one-point' method were compared with measured values, the mean absolute value of the difference between each measured and predicted SWC point was 0.016 m3/m3 for the Geeves data and 0.027 m3/m3 for the Forrest data. Eight sets of predictive equations, previously developed using multiple regression analysis, were also evaluated. Whilst the equations predicted the slope of the SWC curves reasonably well, predictions of the air entry potential were poor. Although less accurate, the equations developed by multiple regression are less demanding in data requirement compared with alternative SWC prediction methods. The one-point method gave better predictions than the multiple regression approach but was less accurate than the 'two-point' method proposed in the first paper in this series. The one-point method should be considered where PSD data and 1 measured SWC point are available. In most other circumstances it will be more accurate and cost-effective to measure 2 SWC points to define the soil water characteristic function (the two-point method). * Part I, Aust. J. Soil Res., 1996, 34, 195–212.

Individual contributions of capillarity and adsorptive surface forces to the matric potential are seldom differentiated in determination of soil water characteristic (SWC) curves. Typically, capillary forces dominate at the wet end, whereas adsorptive surface forces dominate at the dry end of a SWC where water is held as thin liquid films. The amount of adsorbed soil water is intimately linked to soil specific surface area (SA) and plays an important role in various biological and transport processes in arid environments. Dominated by van der Waals adsorptive forces, surface‐water interactions give rise to a nearly universal scaling relationship for SWC curves at low water contents. We demonstrate that scaling measured water content at the dry end by soil specific surface area yields remarkable similarity across a range of soil textures and is in good agreement with theoretical predictions based on van der Waals interactions. These scaling relationships are important for accurate description of SWC curves in dry soils and may provide rapid and reliable estimates of soil specific surface area from SWC measurements for matric potentials below ‒10 MPa conveniently measured with the chilled‐mirror dew point technique. Surface area estimates acquired by fitting the scaling relationship to measured SWC data were in good agreement with SA data measured by standard methods. Preliminary results suggest that the proposed method could provide reliable SA estimates for natural soils with hydratable surface areas smaller than 200 m2/g.

The engineering behavior of natural untreated and compacted lime-treated expansive soils, which typically exist in a state of unsaturated condition, can be better-explained using concepts from unsaturated soil mechanics. The soil water characteristics curve (SWCC) is the key unsaturated soil property for obtaining unsaturated soil property functions (USPFs). However, there are limited studies on the effect of lime on the SWCC of lime-treated expansive soils. This study investigated the effect of lime on the SWCC of lime-treated expansive soils. The drying portion of SWCCs for untreated natural soil and lime - treated soil samples with three different lime contents (3 %, 6 % and 9 %) with 7 days of curing were studied. The SWCCs were determined by using pressure plate apparatus in the suction range of 0 – 1400 kPa. The shrinkage curve (SC) was also determined to evaluate the change in volume of the different soil samples. The experimental results indicate that SWCC is affected by lime treatment and there is a change in the SWCC parameters and in the shape and position of SWCC as the percentage of lime is changed. The SWCC of the lime-treated soil samples show a higher rate of desaturation as the lime content increases. The Air Entry Value (AEV) and residual water content of lime-treated soil decreases with increase in percentage of lime and the SWCC shifts towards the left side as the AEV decreases. The differences in AEV obtained from gravimetric- water-content-based SWCC (w-SWCC) and degree-of-saturation-based SWCC (S-SWCC) for the lime-treated soil samples were small when compared to the untreated natural soil sample.Keywords: Air-entry value, Expansive soil, Lime treatment, Shrinkage curve, Soil–water characteristic curve, Unsaturated soil.

Soil physical properties, including infiltration, strength, water storage and aeration, affect the productivity of agricultural land. This paper investigated the stability of soil structure to chemical disruption, using the soil water characteristic curve. The soil water characteristic was determined twice on intact soil cores of an acidic mesotrophic, red Ferrosol (Gn4.11) and an acidic, eutrophic, grey Dermosol (Gn4.51). Saturated NaCl or deionised water was used to wet the soil cores between each determination. Despite the variability associated with undisturbed soil cores, significant differences were detected in the soil water characteristic curves both within and between soil types. The ability to detect differences in the soil structure of the Ferrosol and Dermosol, both Emerson class 8, suggests this technique could be used to determine the chemical stability of soils with minimal mechanical disruption of the structure.

In the loess region, the hydraulic properties of the loess, used as either surrounding rock, backfilling or geoplomer material, are significant for engineering construction and agriculture development projects. This work investigated the soil-water characteristic curves (SWCC) of the undisturbed and remolded loess during the drying process using the tensiometer and psychrometer method. Based on the test results, SWCC was fitted using the Van Genuchten, and Fredlund and Xing models. Moreover, the permeability was comparatively calculated by the Childs and Collis-George, Van Genuchten, and Fredlund models, respectively. Results revealed that the SWCC of both the undisturbed and remolded loess exhibited three-stage characteristics in the relationship between the logarithmic matric suction and moisture, including the boundary effect zone, transition zone, and residual zone. The corrected Fredlund and Xing model provided an optimal calculation for the SWCC of the loess, while the Van Genuchten model showed suction deviations of about 103 kPa. Meanwhile, the undisturbed loess had a low water retention at the low (<103 kPa) suction range, which was attributed to the large pore structure of the undisturbed loess that reduces the air-entry value. This research clarified the differences in the water retention and permeability properties of the loess, providing a theoretical foundation for evaluating the hydraulic properties of the loess.

The unsaturated behaviour of residual soil in Malaysia has been studied, yet the impact of particle size distribution (PSD) on soil-water characteristics still needs to be more adequately explored. Previous research highlights the significance of fine particles in controlling geotechnical properties. This study investigates how the PSD of five soil samples influences the Soil-Water Characteristic Curve (SWCC) parameters, water retention, storage properties and permeability. The study involved five samples, four samples from Ibrahim et al., [1] research, such as reserved forest, gravelly sand, silty sand, kaolin and silty clay soil from the Garinono Formation soil. A pressure plate extractor determined the samples' Soil-Water Characteristic Curve (SWCC) and bulk density. The van Genutchen (1980) model fitted experimental data and the hydraulic response was studied on the five soil samples. The results show that Kaolin and silty clay have higher volumetric water content than gravelly and silty sand due to a higher percentage of silty and clay particles. However, the reserve forest sample has volumetric water content like kaolin and silty clay due to its natural location. Reserved forest, kaolin and silty clay samples have higher water storage, i.e., effective soil porosity (ESP) values than gravelly sand due to higher clay and silty contents. Silty sand, reserve forest and gravelly sand have higher shape parameter (n) values due to their higher sand and gravel content. Clay-rich samples had smaller (n) values, indicating a more uniform distribution. Hydraulic conductivity function (K(θ)) is influenced by scale parameter (α), Air-Entry Value (AEV) and Saturated Hydraulic Conductivity K_s. Kaolin and silty clay samples exhibit lower α values, resulting in higher suction at AEV than other soils as they contain high silty and clay contents. Saturated conductivity values directly affect hydraulic conductivity function, while AEV inversely affects the unsaturated flow. Understanding unsaturated hydraulic responses and SWCC characteristics could assist in anticipating slope instability and ground stabilisation in residual soil and soil behaviour during and after rainfall.

Red soils with white net-like veins occur widely along the south of middle to lower reaches of the Yangtze River in China. Up to the present few studies paid attention to hydraulic and unsaturated mechanical aspect in spite of many investigations focusing on age, distribution, and climate significance. However, the soil-water characteristics are critical for understanding the engineering features. Herein, microstructure features of undisturbed red soils with white net-like veins taken from Chenshan laterite section in Xuancheng, China were analyzed by scanning electron microscope (SEM). Also, the soil-water characteristics curves (SWCCs) were discussed by means of unsaturated GDS triaxial stress path system and numerical simulation. The SWCC test of axial pressure 510kPa and radial pressure 505kPa was conducted. The Gardner model, Van Genuchten model, and Fredlund model were used to simulate the measured drying cycle of SWCC. The results of SEM tests showed that microstructures and micropores are of obvious anisotropy, and the measured SWCC of the net-like red soil during wetting process behaves hysteretic characteristics compared to that during drying process. The computed results indicated that the simulation value of Fredlund model has good agreement with measured value.

Drainage is important to golf and athletic facilities trying to avoid lost play time. Native soil containing clay is sometimes incorporated into sand profiles with the intent to increase water and nutrient holding capacities. However, mixes high in silt and/or clay often have drainage problems. Research was conducted on soil physical properties from incremental 10% v/v additions of silt and clay (fines) to a U.S. Golf Association (USGA)-specification sand. Soils were evaluated based on volumetric water retention from 0 to 50 cm matric potential, saturated hydraulic conductivity (K sat ), porosity, and bulk density. The soil water characteristic (SWC) for 100:0 (sand:fines) had lower volumetric water content (θ v ) throughout the profile than any other mixture. Addition of 10% fines increased θ v to more than 0.17 cm 3 ·cm –3 throughout the 0- to 50-cm matric potential range, whereas 20% fines increased θ v to more than 0.26 cm 3 ·cm –3 . The 70:30 mixture had greater θ v throughout the profile than mixtures containing more than 70% sand. Mixtures with less than 70% sand produced similar SWCs. Increasing sand content increased bulk density, which altered saturated volumetric water content. K sat was reduced from more than 265 cm·h –1 in 100:0 mixtures to 43 cm·h –1 for 90:10 mixtures, and to less than 5 cm·h –1 with ≥20% fines. The addition of ≥20% by volume of fines to a USGA sand increased water content in the soil to the point it was rendered unacceptable for trafficked turf sites. This research illustrates the influence fine particles, even in small amounts, can have on a USGA sand, and why they should not be added without prior evaluation.

As metropolitan areas expand, the municipal and industrial uses of freshwater increase. Therefore, water resources for irrigation become limited and wastewater reuse for irrigation becomes a good alternative. For this purpose, the effects of suspended solids in wastewater on the soil physical properties, i.e., saturated hydraulic conductivity, Ks, have to be considered. The objectives of this research were to study the effects of applying freshwater and differently treated wastewater on Ks in the surface and subsurface layers of sandy‐loam, loam, and clay‐loam soils. This effect was studied by investigating the ratio of Ks for wastewater to Ks for fresh water in soil surface as Kr1 and in soil subsurface as Kr2. The results showed that the application of freshwater did not reduce the Kr1 considerably. However, the reduction in Kr1 mainly occurred in soil depth of 0–50 mm due to the application of wastewater. This effect is more pronounced in clay‐loam soil than in loam and sandy‐loam soils. It is concluded that application of wastewater with TSS (total suspended solid) of ≥ 40 mg L–1 resulted in Kr1 reduction of >50% in different soil textures. However, the Kr2 reduction at soil depth of 100–300 mm is not considerable by application of wastewater for different soil textures. Further, it is concluded that less purified wastewater can be used in light‐texture soils resulting in less reduction in Kr1. Empirical models were developed for predicting the value of Kr1 as a function of amounts of wastewater application and TSS for different soil textures that can be used in management of wastewater application for preventing deterioration of soil hydraulic conductivity.

Considerably long periods of time are required to measure soil-water characteristic curves using conventional equipment such as pressure plate apparatus or a Tempe cell. A commercially available, small-scale medical centrifuge with a swinging type rotor assembly was used to measure the soil-water characteristic curves on statically compacted, fine-grained soil specimens. A specimen holder was specially designed to obtain multiple sets of water content versus suction data for measuring the soil-water characteristic curve at a single speed of rotation of the centrifuge. The soil-water characteristic curves were measured for three different types of fine-grained soils. The three soils used in the study were processed silt (liquid limit, wL = 24%; plasticity index, Ip = 0; and clay = 7%), Indian Head till (wL = 35.5%, Ip = 17%, and clay = 30%), and Regina clay (wL = 75.5%, Ip = 21%, and clay = 70%). The soil-water characteristic curves for the above soils were measured in 0.5, 1, and 2 days, respectively, using the centrifuge technique for suction ranges from 0 to 600 kPa. Time periods of 2, 4–6, and 16 weeks were required for measuring the soil-water characteristic curves for the same soils using a conventional pressure plate apparatus. There is reasonably good agreement between the experimental results obtained by the centrifuge and the pressure plate methods. The results of this study are encouraging as soil-water characteristic curves can be measured in a reduced time period when using a small-scale centrifuge.Key words: unsaturated soils, soil-water characteristic curve, centrifuge technique, soil suction, matric suction, water content.

The soil water characteristics curve (SWCC) represents the relation between soil water potential and soil water content. The shape and range of SWCC according to the relation could vary depending on soil characteristics. The objective of the study was to estimate SWCC depending on soil types and layers and to analyze the trend among them. To accomplish this goal, the unsaturated three soils were considered: silty clay loam, loam, and sandy loam soils. Weighable lysimeters were used for exactly measuring soil water content and soil water potential. Two fitting models, van Genuchten and Campbell, were applied. Two models entirely fitted well the measured SWCC, indicating low RMSE and high R2 values. However, the large difference between the measured and the estimated was found at the 30 cm layer of the silty clay loam soil, and the gap was wider as soil water potential increased. In addition, the non-linear decrease of soil water content according to the increase of soil water potential tended to be more distinct in the sandy loam soil and at the 10 cm layer than in the silty clay loam soil and at the lower layers. These might be seen due to the various factors such as not only pore size distribution, but also cracks by high clay content and plow pan layers by compaction. This study clearly showed difficulty in the estimation of SWCC by such kind of factors. Parameters obtained from the Campbell and van Genuchten models and goodness-of-fit results.

Using previous measurements as a reference, correction factors are used to analytically estimate seasonal changes in the soil resistivity and seasonal changes in the resistance of a driven-rod earth electrode. These correction factors are based on weather or seasonal data obtained from meteorological stations in the form of rainfall and soil frost depth. When applying this weather data, it is necessary to add their effect on different soil compositions, textures, and site topographies and consider the water infiltration, water runoff, or water retention for each earth electrode site. Site topography influences water runoff and infiltration, while soil texture and structure influence its retention or release. The water content and the corresponding effect on soil resistivity can be determined using the hydraulic properties of soil (water permeability and water retention characteristics). The resistivity of a finite volume of soil around the earth electrode and the changes within this volume dominate the resistance of the earth electrode. This volume is called the Region of Influence (ROI). As soil suction or soil water pressure head changes, the saturated and unsaturated portions within the ROI change, suggesting changes to the volumetric water content and the corresponding soil resistivity. The cumulative effect of weather on soil can be expressed in terms of soil suction. The unique Soil Water Characteristic Curve (SWCC) for each soil texture may be used to evaluate the degree of saturation or water content within the ROI for any soil suction. Based on the preceding deductions, this paper presents analytical formulas for estimating the resistance of driven-rod earth electrodes in variably saturated homogeneous soil. Although not illustrated in this paper, when compared with the simulation result for a driven rod in silty soil modelled in COMSOL multi-physics, the values predicted with these formulas were found to be within 10% for volumetric water content, soil resistivity, and resistance of the earth electrodes over a range of soil suctions. This approach can potentially improve the prediction capability of existing weather and seasonal-based techniques.

The management standard on soil conditions in 'Fuji' apple to produce high quality fruit were not yet made. Therefore, this study was carried out to investigate the optimum soil environmental conditions of ten contents on production of high quality fruit in 'Fuji' apple. The soil and fruit characteristics were analyzed at total 60 orchards in major apple producing areas such as Chungju, Moonkyeung, yeongju, andong, yeosan and yeongcheon (10 orchards an area). The soil environmental factors affected fruit weight was highest relative contribution in saturated hydraulic conductivity of 36.5%. The bulk density and soil pH were low as relative contribution. The fruit weight was influenced by soil physical properties more than soil chemical properties. The soil environmental factors affected sugar content were hydraulic conductivity of 28.3% and organic matter content of 18.2%. The cultivation layer depth and soil pH were low as relative contribution. The fruit coloring was highest relative contribution in saturated hydraulic conductivity 55.9%. while soil pH, cation and soil texture were low. Fruit coloring was high influenced over 70% by soil physical properties. Finally, relative contribution on fruit quality related with sugar content, fruit weight, and coloring were high influenced by saturated hydraulic conductivity of 21.% and organic matter content of 18.7% but bulk density and cultivation layer depth were low relative contribution. The fruit growth and saturated hydraulic conductivity in 'Fuji' apple were very closely related. Therefore, orchard soil management to produce high quality fruit was very importance drainage management and organic matter application. We concluded that scientific soil management is possible by qualifiable of soil management factors.

Trade-off between soil pH, bulk density and other soil physical properties under global no-tillage agriculture

Cover crop (CC) impacts on soil ecosystem services including erosion control, C sequestration, soil productivity, and others have been reviewed. However, CC impacts on soil physical properties, which affect the soil's ability to perform a variety of soil ecosystem services, have not been synthesized. We reviewed 98 peer‐reviewed publications up to 10 June 2020 on CCs and soil physical properties. Our review indicates that, in most studies, CCs reduce soil penetration resistance or compaction by 0–29% (average, 5%). They improve wet aggregate stability by 0–95% (average, 16%) and cumulative infiltration by 0–190% (average, 43%) but have negligible impacts on bulk density, dry aggregate stability, saturated hydraulic conductivity ( K sat ), unsaturated hydraulic conductivity ( K unsat ), volumetric water content at −10 to −33 kPa matric potential (field capacity), and plant available water. Soils under CCs can be cooler in daytime and warmer at nighttime, and warmer in winter and cooler in the rest of year. Volumetric heat capacity increases and thermal diffusivity decreases, indicating that CCs could moderate heat transport rate. Soil texture, tillage system, CC‐derived soil C concentration, and CC duration affect CC impacts on physical properties. For example, the positive impacts from CCs can increase as CC management duration increases and combining CCs with no‐till can improve water infiltration more than with tilled soils. Long‐term data on hydraulic properties, thermal properties, dry aggregate stability, and pore‐size distribution are scant. In general, CCs improve most soil physical properties, but the magnitude of improvement is highly site‐ and management specific.