Field measured mobile water fraction for soils of contrasting texture

Soil Research ◽  
2000 ◽  
Vol 38 (6) ◽  
pp. 1131 ◽  
Author(s):  
A. E. A. Okom ◽  
R. E. White ◽  
L. K. Heng
Keyword(s):  
Hydraulic Conductivity ◽  
Solute Transport ◽  
Water Content ◽  
Time Scale ◽  
Solute Transfer ◽  
Water Fraction ◽  
Mobile Water ◽  
Immobile Water ◽  
Mobile Fraction ◽  
Fertiliser Application

For the purpose of modeling solute transport, soil water has often been simply divided into an essentially mobile fraction, q m , which is active in solute transport, and an apparently immobile fraction, q im . Distinction between q m and q im was sought using the disc permeameter technique. This study examines unsaturated estimates of mobile water content at suction heads, h, of 20, 40, 80, and 120 mm for several soils ranging in texture from sand to clay. Following infiltration of 35 mm depth of 0.01 M KBr into initially dry soils, soil samples were collected from below the base of the disc permeameter and analysed for tracer concentrations which enabled partitioning of mobile and immobile water. Hydraulic conductivity and sorptivity were also derived from the infiltration data. The results show the expected non-linearity of hydraulic conductivity and sorptivity with suction. The mobile water expressed as a fraction, f, of the volumetric water content q (f = q m / q ) was generally found to range from 0.7 to 0.95, with an average of 0.85. The exception was one site for which f was ª 0.50. These values of f are comparable to those derived from leaching studies reported in the literature. An important finding of this work is that within the range of suctions measured, the mobile fraction was independent of suction. A possible explanation for this observation is that the soil capillary forces were dominant during the time scale of the experiment and therefore rapidly drew the invading solution. This finding could have important implications for fertiliser application. Furthermore, this result suggests that the assumption of a negligible solute transfer coefficient, a , between the mobile and immobile domains may be valid within the time scale of this method of measuring the mobile water content.

Parameterisation of physically based solute transport models in sandy soils

Soil Research ◽  
2003 ◽  
Vol 41 (4) ◽  
pp. 771 ◽  
Author(s):  
Y. M. Oliver ◽  
K. R. J. Smettem
Keyword(s):  
Solute Transport ◽  
Water Content ◽  
Mass Exchange ◽  
Granular Media ◽  
Sandy Soils ◽  
Exchange Coefficient ◽  
Water Fraction ◽  
Immobile Water ◽  
Mass Exchange Coefficient ◽  
Tracer Experiments

Immobile water fractions of up to 40% had been reported in sands, and it was therefore relevant to determine if the convection-dispersion equation (CDE) or mobile-immobile model (MIM) should be used as the basic physical model for field studies of solute transport in sandy soils. A review of literature data for granular media indicated that for steady state flow, the dispersion coefficient could be estimated from the grain Peclét number and the mass exchange coefficient from the pore water velocity, but the immobile water fraction was poorly predicted from soil properties.In this study, performed on a sandy soil at Moora, Western Australia, the choice of model was determined after analysis of column effluent breakthrough curves (BTC), sequential tracer experiments, and single tracer experiments on the same core. The latter two methods have recently been introduced in an attempt to independently measure some of the MIM parameters (immobile water content and mass transfer exchange coefficient) in situ. Low immobile water content was found in this sand, with very rapid exchange between the mobile and immobile regions. All 3 techniques gave measured immobile water contents around 10%, which was consistent with most literature values for granular media. The single tracer experiment does not give the mass exchange coefficient α (h–1), but α determined by the sequential tracer technique could not be confirmed by the BTC technique due to the wide 95% confidence interval of the fitted parameter. Although the MIM behaviour was minor and inconsistent in the Moora sand, the choice of model may depend on the problem under consideration. For short column experiments, the CDE and MIM produced similar solute transport behaviour. However, for leaching below the root-zone, the MIM is recommended.

scholarly journals Spatial Variability of Physical Parameters and Processes in Two Field Soils

1991 ◽  
Vol 22 (5) ◽  
pp. 275-302 ◽  
Author(s):  
K. Høgh Jensen ◽  
J. C. Refsgaard
Keyword(s):  
Hydraulic Conductivity ◽  
Solute Transport ◽  
Water Content ◽  
Numerical Solutions ◽  
Sandy Loam ◽  
Solute Concentration ◽  
Coarse Sand ◽  
Richards Equation ◽  
Physical Parameters ◽  
Field Site

Natural field systems exhibit a large degree of soil heterogeneity which affects the movement of water and solutes and thus leads to highly varying observations of water content and solute concentration. To investigate this problem comprehensive field investigation programs were carried out at two field sites in Denmark representing two different soil types, a coarse sand and a sandy loam, respectively. The field investigations included collection of soil samples for analysis of textural composition, retention, and hydraulic conductivity, measurements of water content and suction, and measurements of radioactive tracer concentration, all carried out at a number of positions within the two field sites. Models for one-dimensional vertical unsaturated flow and solute transport were applied to the two field sites, and the simulation results were compared to field measurements of water content, suction and solute concentration. This paper describes results from model simulations in individual soil profiles, while the variability issues at field scale are described in the two accompanying papers. The modelling approach was based on numerical solutions to Richards' equation for water flow and the convection-dispersion equation (CDE) for solute transport. The model results from the coarse sand field site compared relatively well to measurements of water content, suction, and concentration except for the upper soil layer (∼ 10 cm depth) where the measured water contents appeared to be somewhat uncertain. Due to the neglecting of hysteresis and macropore flow (by-pass) in the model the measured retention curves (drainage based) and the hydraulic conductivity functions at the sandy loam field site had to he modified empirically through the calibration procedure in order to match the measurements.

scholarly journals Estimation of soil hydraulic and solute transport properties from field infiltration experiments

2021 ◽  
Vol 10 (14) ◽  
pp. e195101421764
Author(s):  
André Maciel Netto ◽  
Suzana Maria Gico Lima Montenegro ◽  
Ademir de Jesus Amaral
Keyword(s):  
Solute Transport ◽  
Transport Properties ◽  
Numerical Models ◽  
Three Dimensional ◽  
Solute Concentration ◽  
Water Fraction ◽  
Mobile Water ◽  
Single Ring ◽  
Long Time ◽  
Soil Hydraulic

To model water flow and solute transport in soils, hydrodynamic and hydrodispersive parameters are required as input data in the mathematical models. This work aims to estimate the soil hydraulic and solute transport properties using a ponded axisymmetric infiltration experiment using a single-ring infiltrometer along with a conservative tracer (Cl-) in the field. Single ring infiltration experiments were accomplished on an Oxisol in Areia in the state of Paraíba, Brazil (6o 58' S, 35o 41' W, and 645 m), in a 50 x 50 m grid (16 points). The unsaturated hydraulic conductivity (K) and the sorptivity (S) were estimated for short or long time analysis of cumulative three-dimensional infiltration. The single tracer technique was used to calculate mobile water fraction (Ф) by measuring the solute concentration underneath the ring infiltrometer at the end of the infiltration. Two solute transfer numerical models based on the mobile-immobile water concept were used. The mobile water fraction (Ф), the dispersion coefficient (D), and the mass transfer coefficient (a) between mobile and immobile were estimated from both the measured infiltration depth and the Cl- concentration profile underneath the infiltrometer. The classical convection-dispersion (CD) and the mobile-immobile (MIM) models showed a good agreement between calculated and experimental values. However, the lowest standard errors to the fitted parameters were obtained by the CD model.

Inverting stop-flow leaching experiments and detection of physical and chemical non-equilibria

2020 ◽  
Author(s):  
Lian Zhou ◽  
Laurent Lassabatere ◽  
Khalil Hanna
Keyword(s):  
Experimental Data ◽  
Breakthrough Curves ◽  
Water Fraction ◽  
Mobile Water ◽  
Immobile Water ◽  
Leaching Experiments ◽  
Water Fractions ◽  
Physical And Chemical ◽  
Flow Experiments ◽  
Stop Flow

<p>Flow heterogeneity strongly impacts mass transport. In particular, the presence of water fractionation into mobile and immobile water fractions may affect pollutant sorption to soil particles. Indeed, before sorbing, the pollutants need to diffuse from mobile water to immobile water fractions. In a previous study, we investigated the possibility of stop-flow experiments for the detection of physical and chemical non-equilibria. A sensitivity analysis proved that it was possible to detect the two types of non-equilibria. The effect of parameters related to physical  (mobile water fraction and solute exchange rate)  and chemical (chemical kinetics) non-equilibria were varied and related impacts on the shape of the breakthrough curves were characterized for stop-flow experiments. However, the feasibility of inverting procedures was not investigated at all. In particular, the estimation of these parameters by fitting the model to real experimental data (with noise) may be feasible but may also bring some uncertainty with biased and non-unique estimates. In this study, using both numerically generated data and experimental data, we characterize the estimate uncertainty and equifinality. This study will help in optimizing the inverting procedure for the design of more robust and less biased estimates and the quantification of physical and chemical non-equilibria parameters.</p>

The measured mobile-water content of an unsaturated soil as a function of hydraulic regime

Soil Research ◽  
1995 ◽  
Vol 33 (3) ◽  
pp. 397 ◽  
Author(s):  
BE Clothier ◽  
L Heng ◽  
GN Magesan ◽  
I Vogeler
Keyword(s):  
Solute Transport ◽  
Water Content ◽  
Sandy Loam ◽  
Pure Water ◽  
Solute Concentration ◽  
Petroleum Engineering ◽  
Spatial Distance ◽  
Water Fraction ◽  
Hydraulic Regime ◽  
Single Mass

To account for observations of preferential solute transport through soil, increasingly models are used in which the total water content of the soil (�, m3 m-3) is partitioned into an essentially mobile phase (8,) and an apparently immobile fraction (�im). However, few methods exist for measuring this separation in the field. Here we use the recently proposed, disc permeameter technique. Following infiltration with a tracer, measurements of the resident solute concentration directly under the disc are used to infer the �m-�im partitioning. For Manawatu fine sandy loam in situ, we applied this technique at the three pressure heads (ho) of -20, -40 and -150 mm, in order to deduce the influence of the hydraulic regime on the soil water fraction �m/� that appears to be actively involved in solute transport during infiltration. When a depth I of between 15 and 25 mm of KBr tracer was added to soil already wet by pure water to h,, the measured mobile fraction �m/� rose from 0.41 at -20 mm, through 0.50 at -40, to 0.64 at the most unsaturated head of -150 mm. Thus, less evidence of preferential solute transport was recorded with decreasing ho. The spatial distance between the preferential paths was observed to range from 20 to 150 mm, the separation increasing and the pathways becoming more diffuse with decreasing h,. Depthwise dispersion of the invading solute thus increased with ho. At the two higher heads, when I =80mm of KBr was allowed to infiltrate, the �m/�, inferred from the resident concentration observed directly under the disc, now also became 0.65. For ho = -40 mm, the measured rise in the resident concentration under the disc, with I, could be predicted using a dispersivity, �, of 20 mm in the approximation provided by the 1-D form of the convective dispersion equation. When 15-25 mm tracer was applied directly to initially dry soil (� = 0.3), capillary forces drew the invading solute from the disc with much less dispersion, such that the resident concentration under the disc rose more rapidly with I. Now �m/� was found to be virtually 0.65 at all the heads. In several experiments at h, = -40 mm, ethanol was used as the solvent for the tracer. No change in the measured �m/� was observed. Thus hydrophobicity was deemed not to be a factor in our measurement of 8,/B being consistently about 0.65. An attempt was made to parameterize the diffusive-exchange scheme that the Coats and Smith (1964) model, taken from petroleum engineering, proposes as the link between the mobile and immobile domains. However, our observations at ho = -20 mm suggest that no single mass-transfer coefficient 5 can describe this solute exchange. Over the first few days, a � value of 0.5 day-1 seemed reasonable, but over the next fortnight there appeared no further interdomain exchange of solute between the two domains.

Effect of water content on solute transport in a porous medium containing reactive micro-aggregates

1998 ◽  
Vol 33 (1-2) ◽  
pp. 211-230 ◽  
Author(s):  
Claudia Fesch ◽  
Peter Lehmann ◽  
Stefan B. Haderlein ◽  
Christoph Hinz ◽  
René P. Schwarzenbach ◽  
...  
Keyword(s):  
Porous Medium ◽  
Solute Transport ◽  
Water Content ◽  
Effect Of Water

Hyphal colonization of Rhizophagus irregularis increases unsaturated hydraulic conductivity of a loamy sand distant from roots

2021 ◽  
Author(s):  
Michael Bitterlich ◽  
Richard Pauwels
Keyword(s):  
Hydraulic Conductivity ◽  
Water Content ◽  
Hydraulic Properties ◽  
Volumetric Water Content ◽  
Rhizophagus Irregularis ◽  
Loamy Sand ◽  
Soil Hydraulic Properties ◽  
Unsaturated Hydraulic Conductivity ◽  
Soil Hydraulic ◽  
Root Exclusion

<p>Hydraulic properties of mycorrhizal soils have rarely been reported and difficulties in directly assigning potential effects to hyphae of arbuscular mycorrhizal fungi (AMF) arise from other consequences of AMF being present, i.e. their influence on growth and water consumption rates of their host plants that both also influence soil hydraulic properties.</p><p>We assumed that the typical nylon meshes used for root-exclusion experiments in mycorrhizal research can provide a dynamic hydraulic barrier. It is expected that the uniform pore size of the rigid meshes causes a sudden hydraulic decoupling of the enmeshed inner volume from the surrounding soil as soon as the mesh pores become air-filled. Growing plants below the soil moisture threshold for hydraulic decoupling would minimize plant-size effects on root-exclusion compartments and allow for a more direct assignment of hyphal presence to modulations in soil hydraulic properties.</p><p>We carried out water retention and hydraulic conductivity measurements with two tensiometers introduced in two different heights in a cylindrical compartment (250 cm³) containing a loamy sand, either with or without the introduction of a 20 µm nylon mesh equidistantly between the tensiometers. Introduction of a mesh reduced hydraulic conductivity across the soil volumes by two orders of magnitude from 471 to 6 µm d<sup>-1</sup> at 20% volumetric water content.</p><p>We grew maize plants inoculated or not with Rhizophagus irregularis in the same soil in pots that contained root-exclusion compartments while maintaining 20% volumetric water content. When hyphae were present in the compartments, water potential and unsaturated hydraulic conductivity increased for a given water content compared to compartments free of hyphae. These differences increased with progressive soil drying.</p><p>We conclude that water extractability from soils distant to roots can be facilitated under dry conditions when AMF hyphae are present.</p><p> </p>

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