Laboratory Method for Estimating Water Retention Properties of Unsaturated Soil (original) (raw)
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Comparing the Applicability of Soil Water Retention Models
2016
Recently, the public has become more aware of the environmental problems facing our planet today. Underground water pollution is one of these problems, and it requires serious attention. As a result, many studies have been performed to examine the movement of water within soil. These studies present many new models for determining soil water retention rates. It is essential that these models be investigated for accuracy and applicability. This study attempts to analyze three of these well-known water retention models: Campbell, Saxton, and Huston-Cass. To analyze these models, the pressure plate test (ASTM C199-09) was performed on soil samples with different soil textures and from different geographical locations within the country of Iran. Using these results, the general applicability of each model was measured using the 1:1 slope line correlation method. In addition, the accuracy and applicability of each model was measured by calculating the Root Mean Square Error (RMSE) and th...
Estimation of the van Genuchten Soil Water Retention Properties from Soil Textural Data
2010
The van Genuchten (vG) function is often used to describe the soil water retention curve (SWRC) of unsaturated soils and fractured rock. The objective of this study was to develop a method to determine the vG model parameter m from the fractal dimension. We compared two approaches previously proposed by van Genuchten and Lenhard et al. for estimating m from the pore size distribution index of the Brooks and Corey (BC) model. In both approaches we used a relationship between the pore size distribution index of the BC model and the fractal dimension of the SWRC. A dataset containing 75 samples from the UNSODA unsaturated soil hydraulic database was used to evaluate the two approaches. The statistical parameters showed that the approach by Lenhard et al. provided better estimates of the parameter m. Another dataset containing 72 samples from the literature was used to validate Lenhard's approach in which the SWRC fractal dimension was estimated from the clay content. The estimated SWRC of the second dataset was compared with those obtained with the Rosetta model using sand, silt, and clay contents. Root mean square error values of the proposed fractal approach and Rosetta were 0.081 and 0.136, respectively, indicating that the proposed fractal approach performed better than the Rosetta model.
Vadose Zone …, 2008
Th e objective of this work was to evaluate eight closed-form unimodal analytical expressions that describe the soil-water retention curve across the complete range of soil water contents. To meet this objective, the eight models were compared in terms of their accuracy (RMSE), linearity (coeffi cient of determination, R2, and adjusted coeffi cient of determination, R2 adj), and prediction potential. Th e latter was evaluated by correlating the model parameters to basic soil properties. Retention data for 137 undisturbed soils from the Unsaturated Soil Hyudraulic Database (UNSODA) were used for the model comparison. The samples showed considerable diff erences in texture, bulk density, and organic matter content. All functions were found to provide relatively realistic fi ts and anchored the curve at zero soil water content for the coarse-textured soils. Th e performance criteria were similar when averaged across all data sets. Th e criteria were found to be statistically diff erent between the eight models only for the sandy clay loam soil textural class. An analysis of the individual data sets separately showed that the performance criteria were statistically diff erent between the models for 17 data sets belonging to six diff erent textural classes. We found that the Khlosi model with four parameters was the most consistent among diff erent soils. Its prediction potential was also relatively good due to signifi cant correlation between its parameters and basic soil properties.
Water retention in unsaturated soils subjected to wetting and drying cycles
2011
The suction is an essential parameter to describe and understand the behavior of unsaturated soils. The ability of unsaturated soils to retain water is quantified by determining the water retention curves (WRC), which express the hydraulic behavior of porous materials such as soil. These curves are determined by subjecting samples to several drying and wetting cycles. The curve during drying path is located above the wetting curve, developing a hysteresis phenomenon [1], and value of content water at a given suction value depends on the path used to reach this point. The aim of this paper is to present a study on the hydraulic behavior of soil, water retention capacity due to drying and wetting cycles, pointing out the hydro-mechanical behavior of unsaturated soils. In the first part, the effect of physical and mechanical properties of soil [32] (initial void ratio, particle size, cohesion, density...) on the water retention is presented. In the second part, a complete numerical model was developed, based on the empirical model of Van Genuchten [18], to model the two boundary curves, and the experimental scanning data were bestfitted using the same theory of Mualem model [13]. This complete model requires 4 parameters.
Review of Soil Water Retention Characteristic (SWRC) Models between Saturation and Oven Dryness
Soil water retention characteristic is the key soil property used in many applications in the fields of irrigation, hydrology, geotechnical engineering and soil science in general. Since the advent of digital soil mapping and digital soil assessment paradigms, there has been an upsurge of development of soil inference models and the need to increase accurate application of soil mapping products. All soils can be partially saturated with water and also near oven dryness. Therefore, constitutive models for soils should ideally represent the soil behaviour over entire range. This paper reviewed commonly used SWRC models. In order to stem potential for biasness, the models were grouped into three categories depending on the number of fitting parameters, namely, fiveparameter, four-parameter and three-parameter categories. The evaluation used correlation and residual standard error statistics to choose the best overall performing model and in each category. Its results serve as a guide f...
A new equation for the soil water retention curve
European Journal of Soil Science
The soil water retention curve is a fundamental characteristic of unsaturated zone flow and transport properties. Recent studies show that an air-entry value is needed in a soil water retention equation in order to provide a better prediction of relative hydraulic conductivity. A new equation considering the air-entry value is proposed to describe the soil water retention curve. The performance of the proposed equation is contrasted with a well-supported equation by comparing measured and calculated data for 14 soils, representing various soil textures, which range from sandstone to clay. Results show that the proposed equation provides adequate characterization of the soil water retention curves. The equation for predicting relative hydraulic conductivity is derived from the proposed soil water retention equation. An empirical equation for relative hydraulic conductivity is also used. Our results show that the agreement between the predicted and measured relative hydraulic conducti...
A conceptual model of the soil water retention curve
Water Resources Research, 1998
A conceptual model based on the assumption that soil structure evolves from a uniform random fragmentation process is proposed to define the water retention function. The fragmentation process determines the particle size distribution of the soil. The transformation of particles volumes into pore volumes via a power function and the adoption of the capillarity equation lead to an expression for the water retention curve. This expression presents two fitting parameters only. The proposed model is tested on water retention data sets of 12 soils representing a wide range of soil textures, from sand to clay. The agreement between the fitted curves and the measured data is very good. The performances of the model are also compared with those of the two-parameter models of van Genuchten [1980] and Russo [1988] for the water retention function. In general, the proposed model exhibits increased flexibility and improves the fit at both the high and the low water contents range.
Modeling Soil Water Retention Curves in the Dry Range Using the Hygroscopic Water Content
Vadose Zone Journal, 2014
Accurate information of the dry end (matric potential < −1500 kPa) of soil water retention curve (SWRCs) is crucial for studying water vapor transport and evaporation in soils. The objectives of this study were to assess the potential of the Oswin model for describing the water adsorption curves of soils, and to predict SWRCs at the dry end using hygroscopic water content at a relative humidity of 50% (θ RH50 ). The Oswin model yielded satisfactory fits to dry-end SWRCs for soils dominated by both 2:1 and 1:1 clay minerals. Compared to the Oswin model, the CS-K model (Campbell and Shiozawa model combined with the Kevin equation) produced better fits to dry-end SWRCs of 2:1 clay-dominated soils, but provided poor fits for 1:1 clay-dominated soils. The shape parameter α of the Oswin model was dependent on clay mineral types, and approximate values of 0.29 and 0.57 were obtained for 2:1 and 1:1 clay-dominated soils, respectively.
Errors in water retention curves determined with pressure plates: Effects on the soil water balance
Pressure plates apparatus are very common experimental devices utilized to measure the soil water retention curve. Many studies have demonstrated the lack of reliability of pressure plates apparatus when they are used to measure the soil water retention curve in the dry range, due to low plate and soil conductance, lack of hydrostatic equilibrium, lack of soil-plate contact and soil dispersion. In this research, we investigated measurements of soil water retention curves obtained with a combination of Stackman's tables, pressure plates apparatus and the chilled-mirror dew point technique. Specifically, the aim of this research was: (a) to investigate the differences in the measured soil water retention curves by the different experimental methods, (b) evaluate relationships between the experimental differences and soil texture, (c) analyze the effect of experimental differences on hydraulic properties parameterization and (d) investigate the effects of the different parameters set on water transport computation. The results showed differences in measurements made by the combination of Stackman's tables and Richards' pressure plates apparatus as compared to the dew point method, for fine textured soils, while no significant differences were detected for coarse textured soils. Computed cumulative drainage and evaporation displayed lower values if soil water retention curves were obtained from data obtained with the Stackman's tables and Richards' pressure plates apparatus instead of the dew point method. In soils, where the soil water retention curve was measured with traditional methods (Stackman's tables and Richards' pressure plates apparatus) average cumulative drainage was 173 mm, with respect to a combination of methods including the dew point methods, where the average cumulative drainage was 184 mm. Average cumulative evaporation was 77 mm for the traditional methods, while it was 91 mm, for the combination of methods. Overall, when simulation models are used for studies related to solute transport, polluted soil remediation, irrigation management and others, erroneous measurement of the SWRC for fine textured soils, may lead to erroneous computation of the soil water balance.