Soil Hydraulic Behaviour Research Articles

Effects of soil heating changes on soil hydraulic properties in Central Chile

Wildfires are natural phenomena for most ecosystems on Earth. Many soil properties are impacted by fire, including soil hydraulic properties. We used a laboratory experiment to replicate the temperatures reached by a natural wildfire and documented the effects on soil hydraulic properties. This study hypothesizes that the impact of heating on soil hydrological properties can be explained by the interaction of a number of variables especially organic matter content (OM), cation exchange capacity (CEC), texture, pH, and electrical conductivity (EC). The main objective of this study is to explore the interconnections between soil hydraulic, chemical, and physical properties, focusing on understanding how these relationships change across different ecoregions and temperatures. Sixteen soils were collected across 16 sites susceptible to forest fires in the Central Zone of Chile and heated to 100 °C and 300 °C for two hours. These sites were representative of two distinct ecoregions: the Chilean Matorral (CM) and the Valdivian Temperate Forests (VTF). Chemical, physical, and hydraulic soil properties were measured before and after heating. At 100 °C, there were no significant changes in chemical, physical, or hydraulic soil properties. At 300 °C, significant changes were observed in most soil properties in soils from both ecoregions. The OM content and CEC decreased, whereas pH and electrical conductivity increased. In addition, clay content and water aggregate stability (WSA) decreased, while all hydraulic properties increased their values. The aforementioned results demonstrate that infiltration increased after the soil was heated. This can be attributed primarily to decreases in clay content. At the same time, the water repellency (R) index decreased, allowing water to more easily wet the soil particles. Correlations revealed that CEC and clay are the main factors ruling soil hydraulic properties at all temperatures. Clay mineralogy also contributes to the soil hydraulic behavior observed. Nonlinear models were developed to estimate hydraulic properties at 100 °C and 300 °C, using the main soil properties. The models illustrated that the soils of the CM ecoregion, which are characterized by lower OM and influence of clay/CEC ratio, would be less affected by fire compared to the soils of VTF. The water holding capacity would decrease in both ecoregions. However, due to the greater changes in OM and clay in VTF, the impact would be greater than in CM.11Ca2+ = Calcium cations (meq/ 100g); CEC = Cation exchange capacity (meq/ 100g); CM = Chilean Matorral; EC = Electrical conductivity (μS/cm); I = Cumulative infiltration (cm); K = Unsaturated hydraulic conductivity (mm/h); K+ = Potassium cations (meq/ 100g); Ks = Saturated hydraulic conductivity (mm/h); Mg2+ = Magnesium cations (meq/100g); Na+ = Sodium cations (meq/100g); OM = Organic matter (g/100g); R index = Water repellency index; Se = Ethanol sorptivity (cm/s05); Sw = Water sorptivity (cm/s05); VTF = Valdivian Temperate Forest; WSA = Water-stable aggregates (%).

Effects of vegetation growth on soil microstructure and hydro-mechanical behaviour

Literature studies are presenting counterposed effects of vegetation on soil hydraulic behaviour. Besides, root impacts at the micro-scale are rarely correlated to phenomenological observations due to the complexity of adopting up-scaling factors. In this study, the microstructural causes behind observed changes in vegetated compacted clayey sand’s hydraulic and volume change behaviour were explored. Laboratory experiments were carried out on compacted samples seeded with Cynodon dactylon. Significant changes in soil water-saturated permeability, water retention and shrinkage upon drying were observed after root growth. Laboratory measurements were complemented by the quantification of root morphological features and observations at the micro-scale for different soil hydraulic states. X-ray scans and mercury intrusion porosimetry allowed to cover seven orders of magnitude of pore sizes, shedding light on soil fabric changes at the soil–root interface and the clay aggregate scale. The effects of roots on the soil pore size distribution consisted of Multiphysics phenomena: fissure generation, void clogging and soil aggregation due to roots’ chemical interaction. Furthermore, a good correlation was found between the normalised volume of roots and the micro-pores volume. This new expression was included in a framework to predict soil water retention behaviour considering the aggregated structure and soil–root hydro-chemical interactions.

Pit Lake Slope Stability under Water Level Variations

This paper presents the results of a geotechnical investigation regarding the slope stability in a pit lake, emphasizing the impact of water level variations. Advanced analysis techniques were utilized for this study. The research was performed by using fully coupled flow-deformation analyses. For the fully coupled approach, Bishop’s effective-stress equation was used, and for the description of soil hydraulic behavior, the Van Genuchten’s model was applied. The analysis of slope stability associated with reservoir water level changes revealed that the slope tended to become unstable as the water level decreased; the stability factor was negatively related to the rate of water level reduction. Concerning the water level fluctuations, the analyses revealed that the soil mass seemed to become less stable as the rate of water level change increased. Under a specific range of rates of water level variation, the safety factor became higher as the number of fluctuations increased. Additionally, the simulation results concerning the water level rising indicate that the pressure due to the external water level acts on the slope surface with a positive impact on the stability factor. The results obtained reflect the effects under a specific site condition, but they can be used as a reference for evaluating slope stability in a pit lake design.

Influences of ink-bottle effect evolution on water retention hysteresis of unsaturated soils: An experimental investigation

The ink-bottle effect, resulting from the non-uniformity of pore structure, plays a significant role in the hydraulic behaviour of soils. In this paper, a series of laboratory tests including standard mercury intrusion porosimetry (MIP), intrusion-extrusion cyclic MIP (IEC-MIP), and scanning electron microscopy (SEM) were conducted on two soils (a Nanyang expansive clay and a Kaolin mixture) to examine the evolution pattern of ink-bottle effect in soils with different initial void ratios. To evaluate the impact of such evolution on the soil-water retention behaviour, the axis translation and vapour equilibrium techniques were utilised to measure the water retention curve of the tested soils (SWRC). A key part of the analysis involved a separation between pore bodies (or ink-bottle pores) and pore throats to facilitate a more thorough examination of the pore structure. The test results indicated that the reduction in soil volume during compression is mainly attributed to the reduction in pore body size, whereas the pore throat size remains relatively constant. Therefore, the ink-bottle effect would weaken with a decreasing initial void ratio, which will reduce the hysteresis potential of soils, manifesting as a contraction of the SWRC hysteresis loop. Additionally, under a fixed void ratio but with decreasing pore size, the ink-bottle effect will also weaken, which will reduce the distance between the drying and wetting curves with increasing matric suction. Finally, the mechanisms regarding how the evolution of ink-bottle effect during soil deformation influences water retention hysteresis were addressed.

Preliminary insights from hydrological field monitoring for the evaluation of landslide triggering conditions over large areas

Rainfall-induced landslides represent a major threat to human activities, and thus an improved understanding of their triggering mechanisms is needed. The paper reports some preliminary inferences on this topic, based on the data recorded over a 2-year period by a multi-parametric monitoring station located on one of the slopes of the Monterosso catchment (Cinque Terre, north-western Italy). This catchment has experienced multiple, concurrent shallow landslides after intense rainfall events. After defining a soil hydraulic model through data interpretation and numerical simulations, slope stability analyses were performed to elucidate several aspects related to shallow landslide occurrence. Both long-term climate conditions and single rainfall events were simulated via physically based approaches. The findings from these simulations enabled us to assume the pattern of infiltration and quantify the impact of soil hydraulic behavior on landslide triggering conditions. In this regard, various analyses were carried out on the same triggering event both at local scale and in the overall catchment, with a view to highlighting the role of initial soil moisture and soil hysteretic behavior in slope stability.

The impact of cyanobacteria inoculation on soil hydraulic properties at the lab-scale experiment

Ecosystem-based approaches, including soil inoculation, have been considered to improve the soil water behavior, for the purpose of ensuring better irrigation management on farms. This study was, therefore, conducted to assess the responses of the moisture characteristic curve (MCC) and saturated hydraulic conductivity (SHC) to soil inoculation with cyanobacteria. To this end, the most suitable existing cyanobacteria (i.e., Nostoc sp. and Oscillatoria sp.) from the original soil were inoculated on clay, loam and sand textures at a small cylinder scale. After 35 days, the MCC and SHC of the treatments (six control and inoculation treatments in three repetitions) were measured using the sandbox and pressure plate, and constant head method, respectively. The results showed that, in the control treatment, the volumetric water content of the clay, loam and sand textures was 38–69%, 28–58% and 8–24%, respectively. Meanwhile, the inoculation of cyanobacteria led to a significant (P < 0.05) increase in the MCC of the loam and sand soils; their values were 36–65% and 16–31%, respectively. In the control, the SHC of the clay, loam and sand textures was 0.42, 0.54 and 1.21 cm min−1, respectively; meanwhile, this was 0.37, 0.50 and 0.67 cm min−1 for the inoculated treatments, respectively. On the other hand, the inoculated cyanobacteria reduced the SHC of the sand soils by 44%. We also found that the inoculated cyanobacteria perceptibly improved the soil surface physical and chemical properties, which could play a key role in determining the soil hydraulic and hydrologic behavior. Overall, we found that soil inoculation with cyanobacteria could be an effective approach in improving the soil water behavior, especially in coarse texture soils; thus, it could serve as a suitable irrigation and drainage management method in agricultural lands.

Soil-water characteristic curve of polypropylene fibrereinforced sandy soil

Fibre reinforcement is considered a good alternative for improving the geotechnical properties of soil. However, studies that investigate its behaviour, accounting for the unsaturated condition, and the hydraulic behaviour of soil mixtures with fibre, are limited. Therefore, the current study evaluates the impact of the inclusion of polypropylene fibres on the hydraulic behaviour of soil through geotechnical characterisation, scanning electron microscopy (SEM), macroporosity and microporosity tests, and filter paper tests. The soil-water characteristic curve (SWCC) of different mixtures of fibre-reinforced soil was adjusted by the models enshrined in the literature, using polypropylene fibres of length 6 mm, diameter 18 μm, and fibre contents 0.25% (SF025), 0.75% (SF075), 1.0% (SF100), and 1.25% (SF125) relative to the dry weight of the soil. The results indicated a transition from unimodal to bimodal shape in the SWCC for the polypropylene fibre-reinforcement, suggesting that their inclusion altered the soil structure. The same bimodal behaviour of SWCC was observed in all reinforced samples that produced similar values of air-entry suction and residual volumetric water content, but with increased water retention for the same level of suction for higher fibre content. The results of the tension table test indicated an increase in the volume of macropores with an increase in fibre content and a decrease in micropore volumes. These results agree with the compaction tests, which showed a decrease in the dry maximum density with an increased fibre content, whereas the optimum water content increased.

Soil moisture forecast for smart irrigation: The primetime for machine learning

The rise of the Internet of Things allowed higher spatial–temporal resolution soil moisture data captured through in situ sensing. Such abundance of data enables machine learning-based soil moisture forecast as an alternative to traditional mechanistic approaches for irrigation water need estimation. This paper develops a guideline for soil moisture forecast modeling based on machine learning, tested in a real case analysis comprehending eight crop types in twelve fields from four farms distributed over diverse climatic scenarios in Brazil. Instead of a single value, we predict the following days' minimum and maximum values as targets to monitor risks of extreme soil moisture values. Furthermore, modeling soil moisture directly in volumetric water content (VWC) is better than modeling soil matric potential (SMP) to later convert in soil moisture VWC. We test several algorithms and find out that LightGBM outperforms linear regression, decision tree, random forest, multilayer perceptron, LSTM, and StemGNN. Also, blending predictions via algorithm ensemble provides an additional accuracy gain. For model training and accuracy measurement, we use weighted datasets to privilege rare but critical data points. We show that soil moisture forecast reaches its maximum performance considering only past soil moisture, a context-aware index, and a precipitation forecast. Finally, we demonstrate that traditional domain-knowledge features - such as evapotranspiration, crop phenology, and soil hydraulic behavior - are not relevant to improving SM forecast performance. Consequently, our paper suggests full data-driven approaches for irrigation water need estimation, observed some care regarding data quality.

Effects of internal erosion on the cyclic and post-cyclic mechanical behaviours of reconstituted volcanic ash

Internal erosion is the transportation of soil particles from within or beneath the main structure inside the ground due to seepage flow. Internal erosion impacts the mechanical and hydraulic behaviour of soil. The present study investigates the impact of internal erosion on the cyclic and post-cyclic mechanical behaviours of volcanic ash sampled from Satozuka, Japan. A series of experiments using volcanic ash with loose, medium, and dense conditions have been performed using an erosion triaxial apparatus. Further, reconsolidation after application of cyclic loading was considered to study the impact of reconsolidation on the post-cyclic mechanical behaviour. The results show that the cyclic resistance of eroded specimens is improved regardless of the percentage of eroded fines and the initial relative density. The higher cyclic resistance for the eroded specimens is found to be due to a decrease in the intergranular void ratio after the erosion, change in fabric, and pre-strain history during erosion. Reduction in the critical state friction angle (φcri) and peak stress ratio (Rpeak) for the monotonic loading due to cyclic loading is greater for the eroded specimens. However, reconsolidated eroded specimens show increases in φcri and Rpeak, which are lower than those of the non-eroded and eroded specimens without cyclic loading. This could be due to the change in the soil fabric during cyclic loading where the positive impact of erosion is lost after cyclic loading. Such limited recovery of deviatoric stress with deformation is density-dependent.

Two-Scale Investigation of the Retention Behavior of a Well-Graded Mixed Soil

The hydraulic characterization of mixed compacted soils is helpful for the design of earthworks subjected to drying–wetting cycles. When the mixed soil is well-graded and made of both coarse and fine fractions, its matric suction may also be due to the short-range adsorption phenomena, as for the soil investigated in this research work. A silty–clayey sand was created by a mixing procedure and experimentally investigated at two different scales. Physical modeling of an infiltration process was performed, allowing an inverse numerical analysis to infer the water retention and the hydraulic conductivity functions of the soil, whereas element testing on soil specimens allowed direct determination of the same equations. In the article, problems related to the employed suction measurement techniques have been pointed out and discussed. By this two-scale combined strategy, features of the soil hydraulic behavior, such as the wetting collapse, the shrinkage during drying, and the loop of hysteresis, were also determined.

Undrained Spherical Cavity Expansion in Unsaturated Soils: Semianalytical Solution Coupling Hydraulic and Mechanical Behaviors

A coupled hydromechanical solution to undrained spherical cavity expansion problem in unsaturated soil is proposed with an elastoplastic model for unsaturated soil. The mechanical response and hydraulic behavior of soil during expansion are coupled together through a soil–water characteristic curve (SWCC) that evolves with the void ratio. A system of four first-order differential governing equations is derived based on the equilibrium equation, large stain theory and coupled hydromechanical constitutive relationship of soil. The present solution can reduce to unified hardening parameter model based solution under saturated condition, which validates the proposed solution. The effects of overconsolidation ratio and initial suction on expansion responses are explored based on extensive parametric studies. Special attention is paid to investigate the distributions of the principal stresses, suction, and saturated degree of soil around the expanded cavity; the stress trajectory of a representative element at the cavity surface; and cavity pressure–displacement curves. The proposed analytical solution can serve as a theoretical framework for modelling of some fundamental geotechnical problems in unsaturated soils using critical state soil models.

Soil cracking propagation due to dryness and its relation to suction

Investigation of Crack Intensity Factor is essential as it affects the mechanical and hydraulic behaviour of soils. Soil water coming from the wet seasons or from the water table, is removed by evaporation during the driest season. The loss of water provokes a significant increase in suction. When it exceeds the tensile strength of the soil, cracks occur that can modify the mechanical and mainly hydraulic properties of the soil, creating preferred paths for water infiltration. Little research is conducted on quantifying cracking in soil relating it to its hydraulic properties. This research aims to investigate the cracking of soils with focus on analysing its relation to water content and soil suction. Soils from a specific region in Brazil with clay predominance are collected and characterized. Unsaturated soil specimens are prepared and subjected to environmental real conditions in order to progressively check the consequences caused by the environment in soils with different clay content during four weeks. The Crack Intensity Factor is measured along the time through image processing. The water content is monitored through volume water content sensors. The measured results are evaluated to correlate crack intensity factor as function of weather variables and soil water content.

The impact of landscape evolution on soil physics: evolution of soil physical and hydraulic properties along two chronosequences of proglacial moraines

Abstract. Soil physical properties highly influence soil hydraulic properties, which define the soil hydraulic behavior. Thus, changes within these properties affect water flow paths and the soil water and matter balance. Most often these soil physical properties are assumed to be constant in time, and little is known about their natural evolution. Therefore, we studied the evolution of physical and hydraulic soil properties along two soil chronosequences in proglacial forefields in the Central Alps, Switzerland: one soil chronosequence developed on silicate and the other on calcareous parent material. Each soil chronosequence consisted of four moraines with the ages of 30, 160, 3000, and 10 000 years at the silicate forefield and 110, 160, 4900, and 13 500 years at the calcareous forefield. We investigated bulk density, porosity, loss on ignition, and hydraulic properties in the form of retention curves and hydraulic conductivity curves as well as the content of clay, silt, sand, and gravel. Samples were taken at three depths (10, 30, 50 cm) at six sampling sites at each moraine. Soil physical and hydraulic properties changed considerably over the chronosequence. Particle size distribution showed a pronounced reduction in sand content and an increase in silt and clay content over time at both sites. Bulk density decreased, and porosity increased during the first 10 millennia of soil development. The trend was equally present at both parent materials, but the reduction in sand and increase in silt content were more pronounced at the calcareous site. The organic matter content increased, which was especially pronounced in the topsoil at the silicate site. With the change in physical soil properties and organic matter content, the hydraulic soil properties changed from fast-draining coarse-textured soils to slow-draining soils with high water-holding capacity, which was also more pronounced in the topsoil at the silicate site. The data set presented in this paper is available at the online repository of the German Research Center for Geosciences (GFZ; Hartmann et al., 2020b). The data set can be accessed via the DOI https://doi.org/10.5880/GFZ.4.4.2020.004.

Run duration effects on the hydrodynamic properties of a loam soil estimated by steady-state infiltration methods

Steady-state methods for the analysis of single-ring infiltration data are commonly applied. However, the duration of an infiltrometer experiment is often established quite subjectively based on the assumption that in general infiltration stabilizes rather quickly in the field. For a loam soil, the effect of the duration of a beerkan run on sorptivity, S, and saturated hydraulic conductivity, Ks, was tested by using the BEST (Beerkan Estimation of Soil Transfer parameters)-steady method and SSBI (Steady version of the Simplified method based on a Beerkan Infiltration run) method for data analysis. The standard experiment, based on a total of 15 water volumes each establishing an initial ponding depth of ~0.01 m (on average, 0.32 hours of infiltration), yielded approximately two and &gt;100 times higher S and Ks values, respectively, than a long run (117 water volumes or 8.1 hours). Stabilization was faster for S (approximately in 3 hours) than Ks (6 hours). Similar Ks values were obtained with BEST-steady (192-261 mm/h) and the SSBI method (177-184 mm/h) for the standard run but not for the long-duration run (1.5-2.1 and 20-21 mm/h, respectively). This discrepancy was due to the fact that more information on the infiltration process is used by BEST-steady (total duration, total infiltrated water, steady-state infiltration rate) than the SSBI method (only the latter variable). In conclusion, Ks estimates are very sensitive to the used water volume for the run. The run duration should not only depend on the attainment of near steadiness, but also on the possibility of capturing the soil hydraulic behaviour in a representative situation for the hydrological process under study. In the near future, the possibility of using the hydrodynamic soil properties determined with the tested methodology to simulate rainfall effects on soil structure should be investigated.

A Sequentially Coupled Catchment‐Scale Numerical Model for Snowmelt‐Induced Soil Slope Instabilities

AbstractThe frequency of snowmelt‐induced soil slope instabilities is increasing in some seasonally cold regions because of climate change. Reliable hazard assessment and risk mitigation of snowmelt‐induced landslides require physically‐based prediction models. However, existing models either apply only at the slope scale or assume precipitation as the sole landslide trigger. In doing so, they neglect the complexity and coupled nature of the thermo‐hydro‐mechanical processes leading to slope instability in seasonally cold regions (such as snow accumulation and melting, infiltration and surface runoff, soil saturation, pore water pressure buildup and dissipation). Here, we present a spatially distributed and sequentially coupled numerical model to simulate snowmelt‐induced slope instabilities at the catchment scale. The model accounts for temperature‐dependent changes in the soil hydraulic behavior related to changes in water state by means of a routine implemented in a geographic information system. We verified the performance of the model using a case study of spring snowmelt‐induced soil slope failures that occurred after the 2004 Mid‐Niigata earthquake in Japan. Considering limitations and simplifications, the model was able to predict the triggering condition, magnitude, and spatial distribution of the snowmelt‐induced landslides with a satisfactory degree of accuracy. We believe that the robustness and simplicity of our numerical approach make it suitable for implementation in early warning systems.

The effect of microbial calcite precipitation on the retention properties of unsaturated fine-grained soils: discussion of the governing factors

In recent years, biogeotechnology has been introduced as a novel and environmentally friendly technique for soil improvement. The need to address global warming and the adverse environmental effects of the chemical additives have led to the emergence and development of the techniques which use calcite producing microorganisms in order to improve soil mechanical properties. While the effects of microbial induced calcite precipitation (MICP) on the hydraulics and mechanics of saturated coarse-grained soils have been well examined and studied, there is not yet much information on the effects these microorganisms would have on the unsaturated soil mechanical behaviour. The first step, in this regard, is to understand the effect of the processes involved in the MICP on the soil retention properties. Soil water suction is a key factor controlling soil hydraulic and mechanical behaviour. In this study, the influence of MICP on the soil total suction in an unsaturated fine-grained soil sample has been explored using filter paper experiment. The results of this study revealed that by increasing the amount of bacterial solution, the soil saturation-total suction curves are significantly affected. The soil water retention changes are attributed to the change in double layer thickness as well as the precipitation of calcite crystals.

Critical state of polymer-coated sands

The water-repellent behaviour of polymers such as polydimethylsiloxane enables them to be utilised as coatings in soil particles to control wetting, with potential applications in barriers and fills. However, little information exists on the mechanics of coated soils. Here, the mechanical behaviour of a coated sand subjected to two mass ratios of dimethyldichlorosilane (0·05% and 3%) was investigated by triaxial tests. Dimethyldichlorosilane was mixed with the sand to produce polydimethylsiloxane coatings. Damage to the polymer coatings was investigated by interferometry. The results revealed that for a low mass ratio of dimethyldichlorosilane (0·05%), the shear strength and critical state locus are similar to those of natural sand; whereas for a high mass ratio of dimethyldichlorosilane (3%), there is a marked reduction of the shear strength, with the critical state locus showing a significant difference compared to that of natural sand. The quantification of roughness prior to and after the triaxial tests confirms the damage to the polymer coatings, with the effect of the coatings diminishing with the rise of stress level. The findings highlight the importance of the thickness of the coatings, with the 0·05% coated sand sufficient to manipulate the soil's hydraulic behaviour and the 3% coated sand opening new possibilities to manipulate the soil's mechanical behaviour.

Compacted Brazilian soil hydraulic behaviour during the percolation of petroleum-produced water

This paper analyses the results of column tests performed on samples of compacted soils permeated by petroleum-produced water from onshore areas in north-east Brazil. It is shown that in almost all of the tested samples, the soil hydraulic conductivity decreased over time. Bioclogging and biofilm formation, as well as clay dispersion and filtration, are analysed; clay dispersion is indicated as the probable main cause of the observed behaviour. Furthermore, particle dispersion seems to be dependent on the changes in soil porosimetry (lower fraction of macropores), which is caused by higher compaction energy. Breakthrough curves obtained for magnesium (Mg) and calcium (Ca) suggest that bivalent cations were replaced by sodium (Na) cations during the tests. Furthermore, for longer test times, a noticeable amount of solid-particle sediment was observed at the bottom of the fluid collection jars. The authors confirmed that bioclogging did not significantly influence the reduction in soil hydraulic conductivity, as this reduction occurred before the start of any microbial activity.

Studying the effect of desiccation cracking on the evaporation process of a Luvisol – From a small-scale experimental and numerical approach

Cracking due to desiccation of the soil surface is a common phenomenon related to the interaction between soil and the atmosphere. Indeed, during dry seasons, high evaporation of pore water near the soil surface leads to an increase in soil suction in this region. Consequently, the suction results in compressive effective stress on the soil structure and produces shrinkage, including cracking. As the crack network forms, the initial soil structure is strongly modified, which provides preferential flow pathways for solute-water and influences the soil hydraulic behaviour in general. The work aims to study the formation of cracks during evaporation process of a Cutanic Luvisol and evaluate how cracking affects the soil evaporation process. Laboratory experiments were performed on one undisturbed soil sample. To do that, a small-scale environmental chamber was designed and equipped with sensors for measuring the ambient temperature and relative humidity, and a digital camera for investigating the initiation and propagation of cracks on the soil surface. In addition to a HYPROP device (UMS GmbH, Munich, Germany), the hydraulic properties and the kinetics of evaporation of soil samples were also determined through the test. Finally, numerical simulations were carried out by using the finite element code LAGAMINE developed at the University of Liege to emphasize the effect of desiccation cracking on the soil hydraulic conductivity and the moisture transport mechanisms in the soil, as well as exchanges with ambient atmosphere.

Quantification of soil pore dynamics during a winter wheat cropping cycle under different tillage regimes

The water retention characteristic (WRC) and the hydraulic conductivity characteristic (HCC) vary in time due to tillage system, weather conditions and biological activity. These changes in WRC and HCC are a result of varying pore size distributions (PSD). Considering these alterations in soil hydrological models has been shown to improve simulations of water dynamics. An important prerequisite for such an approach is the periodic quantification of WRC and HCC, e.g., over a cropping cycle. Therefore, our study frequently quantified WRC and HCC together with other soil physical and chemical properties on a long-term (23 years) tillage experiment with a silt loam soil. The aim was to identify differences between the three treatments conventional tillage (CT) with a moldboard plow, reduced mulch tillage (RT) with a cultivator and no tillage (NT) with direct seeding. WRC and HCC were parameterized using the bimodal version of the well-known Kosugi retention model together with the Mualem conductivity model to account explicitly for both textural and structural pores. Consequently, bimodal PSD were inferred using the Kosugi parameters. The structural part of the bimodal Kosugi model clearly showed a shift in the PSD on CT and RT from larger to smaller pores throughout the winter wheat growing season with a recovery later in the season on RT. Saturated hydraulic conductivity was positively correlated with the abundance of transmission pores (diameter 50–500 μm) which has implications for infiltration processes under the influence of seasonal PSD changes. Overall, a frequent experimental quantification of PSD may be warranted for modeling soil water on short time scales, e.g., during a cropping cycle, while for longer time frames one to two measurement campaigns per year may be sufficient to describe soil hydraulic behaviour.