Initial Water Table Research Articles

Developing a new unsteady‐state equation for the design of subsurface drainage systems suitable for the Delta region, Egypt

AbstractThe main goal of this research is to obtain a new unsteady‐state equation for the design of subsurface drainage systems that provides a more combatable solution under conditions in the Delta region, Egypt, while simultaneously avoiding all obstacles, difficulties and points of argument in other unsteady‐state equations. The point of argument in unsteady‐state assumptions is the initial water table shape. Then, the study reached a constant called Csh, which represents the initial water table shape and was proven to depend on drain depth (d.d.). Moreover, the new equation avoids using the drainable porosity of soil (f), which is the greatest restriction in using unsteady‐state equations. The new equation replaces f with another constant called Cs, which can be easily determined by using tables or graphs depending on drain depth (d.d.) and hydraulic conductivity (k). Another new important variable is also used in the new equation: the hydraulic head after 6 days of irrigation (h). Using this parameter with the initial water table (h0) and the water table after t days (ht) ensures more accurate results. Then, the new equation, which can be called the Safaa equation, can result in an optimum solution for the Delta region, Egypt, because it provides spacing between laterals (L), which results in the desired performance with the least cost.

Implementation of a slope stability method in the CRITERIA-1D agro-hydrological modeling scheme

AbstractThis paper presents the implementation of a slope stability method for rainfall-induced shallow landslides in CRITERIA-1D, which is an agro-hydrological model based on Richards’ equation for transient infiltration and redistribution processes. CRITERIA-1D can simulate the presence and development of roots and canopies over space and time, the regulation of transpiration activity based on real meteorological data, and the evaporation reduction caused by canopies. The slope can be considered composed of a multi-layered soil, leading to the possibility of simulating the bedrock and of setting an initial water table level. CRITERIA-1D can consider different soil horizons characterized by different hydraulic conductivities and soil water retention curves, thus allowing the simulation of capillarity barriers. The validation of the proposed physically based slope stability model was conducted through the simulation of the collected water content and water potential data of an experimental slope. The monitored slope is located close to Montuè, in the north-eastern sector of Oltrepò Pavese (northern Apennines—Italy). Just close to the monitoring station, a shallow landslide occurred in 2014 at a depth of around 100 cm. The results show the utility of agro-hydrological modeling schemes in modeling the antecedent soil moisture condition and in reducing the overestimation of landslides events detection, which is an issue for early warning systems and slope management related to rainfall-induced shallow landslides. The presented model can be used also to test different bioengineering solutions for slope stabilization, especially when data about rooting systems and plant physiology are known.

Numerical Modelling of Matric Suction in Unsaturated Soil under Shallow Foundation Under Varying Soil and Hydrological Conditions

In recent times, extreme hydrological events have disrupted the performance of various structures, particularly the structural foundations responsible for transferring the superstructure's weight to the natural ground. This disruption underscores the significance of matric suction and soil saturation, which are influenced by hydrological conditions like precipitation, soil shear strength, and foundation settlement. These factors are essential when designing structures in specific locations with distinct geotechnical parameters. To address these challenges, our research employs Plaxis 2D numerical modeling to investigate the dynamic changes in matric suction within soil beneath shallow foundations under varying rainfall conditions. Our approach involves a fully-coupled flow method, incorporating the Van Genuchten hydraulic model. In recognition of practical constraints, we utilize fundamental soil classification parameters for model configuration. Our findings reveal the substantial impact of matric suction fluctuations during rainfall on soil deformation, as indicated by displacement patterns. This highlights the critical importance of matric suction in comprehending soil behavior. Furthermore, we observe that higher initial water table levels correlate with reduced variations in matric suction and soil deformation during rainfall, emphasizing the regulatory role of water table depth. In conclusion, this study emphasizes the necessity of considering matric suction and water table depth in structural design and geotechnical analysis, particularly when faced with extreme hydrological events. By comprehending these factors, we can enhance our understanding of soil behavior, improve foundation stability, and develop more effective design strategies for structures in various environmental conditions.

Groundwater Flooding on Atolls Caused by Storm Surges: Effects of the Dual‐Aquifer Configuration

AbstractStorm surges associated with tropical cyclones endanger atolls through groundwater flooding, where groundwater is discharged from the land surface as the sea level rises. Atolls are characterized by a “dual‐aquifer” configuration, where recent Holocene sediments unconformably overlie highly permeable Pleistocene limestone, creating an interface called a “Thurber discontinuity.” This study aimed to quantitatively analyze how the dual‐aquifer configuration of atolls controls the temporal dynamics of groundwater flooding caused by storm surge. To this end, we ran surface‐subsurface coupled synthetic numerical simulations using HydroGeoSphere and compared 12 scenarios with different Thurber discontinuity elevations and hydraulic conductivities of the Pleistocene aquifer (KP). The results showed that the shallower the Thurber discontinuity and the higher the KP value, the higher the maximum water depth in the freshwater swamp on the atoll during the storm surge and the longer the flooding duration. Despite the effects of the different dual‐aquifer configurations, the initial water table elevation and salinity distribution were almost identical in all the simulation cases. These findings suggest that accurate information on the dual‐aquifer configuration is necessary to evaluate the potential risk of groundwater flooding on atolls accompanying storm surges. Furthermore, the results indicate that groundwater flooding caused by storm surges substantially contributes to cyclone‐driven flooding on atolls, and hence, it should not be neglected in flood predictions to avoid underestimation.

Laboratory studies of the temporal evolution process of the riparian groundwater flow system related to rainfall

Comprehensive investigations of the riparian groundwater flow system response to rainfall based on laboratory experiments and numerical simulation are presented and discussed. The 1D sand column experiment results showed that the capillary fringe of the sand sample was 25 cm, and the supporting capillary water height was about 65 cm. The 2D sandbox was constructed to investigate the changes of hydraulic heads and groundwater flow system in the hypothetical riparian zone under different initial water tables and rainfall intensities. When the initial water table was shallow, a small amount of rainfall infiltration resulted in a rapid and large rise of the water table due to the effect of capillary fringe. When the initial water table was equal to the riverbed elevation, groundwater flow pattern changed once after the rainfall infiltration. In the early rainfall period, a groundwater divide and two opposite local flow systems occurred; one flowed to the upslope unsaturated zone (counter directional local flow system), the other flowed to the river (codirectional local flow system). As the rain continued, the groundwater divide kept moving to the upslope and finally disappeared by forming a stable codirectional regional flow system. When the initial water table was lower than the riverbed, groundwater flow pattern was more complex and changed twice. At the beginning, all the rainfall infiltration recharged the thick unsaturated zone and there was only a counter directional regional flow system. When the water table rose to the riverbed, a groundwater divide and two opposite local flow systems were formed. The groundwater divide was moving to the upland and finally formed a steady codirectional regional flow system. More fine characterization of hydrological variation in a short-time scale in the riparian zone is necessary in the future.

Centrifuge modeling of the run-out behavior of fly ash deposits subjected to a loss of confinement

Numerous industries are concerned by the humanitarian, environmental, and economic consequences of debris flows, landslides, and material run-outs. However, the run-out behavior following a loss of confinement, e.g., dam failure, is not well understood. The material’s complex constitutive behavior means multiple mechanisms contribute to the deformation, e.g., slope instability, seepage forces, erosion, or static liquefaction. In this study, centrifuge models of fly ash deposits, with varying initial deposit density and water table height, were subjected to a rapid loss of lateral confinement. Deformation, pore pressure, and water content measurements were used to investigate and separate the mechanisms governing the run-out. Static liquefaction was observed in deposits initially looser than the critical state and led to a rapid material outflow. Slope instability was the initial failure mechanism for denser deposits or those with reduced water tables, with transient stability due to dilation-induced negative excess pore pressures followed by progressive failures caused by seepage pressures from drainage and pore pressure dissipation. Cone penetration tests, performed before the loss of confinement at different penetration rates, were used to characterize the material run-out and volume change tendencies and demonstrate a practical tool for assessing the run-out risk of deposits in the field.

THE ROLE OF COMPACTION ON PYHSICOCHEMICAL PROPERTIES AND CARBON EMISSIONS OF TROPICAL PEAT SOILS: A REVIEW

The peat compaction method is currently adopted by Malaysia oil palm companies to mitigate the uprising environmental issues. This method is claimed to be effective in minimising the risk of fire through enhancement of soil moisture due to capillary effect. In this review, the authors discussed on the peatland function in global perspective, the important of peat soil compaction, emergence of potential peat compaction in oil palm plantation establishment, peat compaction processes and the effects of compaction on physicochemical properties and carbon emission via peat surface and fire. Authors also found that compaction terminology on tropical peatland should be defined wisely, as it closely depends on the initial water table level and the peat quality. Thus, the retrieved information could serve as basic platform to further probe into the highlighted aspects and may as well function as a guide for management of these sensitive ecosystems, particularly in light of carbon loss mitigation.

Approximate solutions for flows in unconfined double porosity aquifers

Although flow in unconfined double porosity aquifers occurs in most part of the world such as in karst and crystalline aquifers, their hydrodynamic behavior is still poorly understood. Especially, analytical or semi-analytical solutions for flow in unconfined double porosity (DP) aquifers are scarce. In this paper analytical solutions are proposed for two configurations involving groundwater discharge to a stream adjacent to an unconfined aquifer, and flow induced by pumping with constant extraction rate, respectively. The analytical solutions are obtained based on an h2-linearization technique of the full mathematical model and using a Laplace transform. The above technique has not been used before to investigate the behavior in DP aquifers. The newly developed analytical solution is in agreement with numerical solutions of the full mathematical model, for small to moderate drawdowns which are as high as 25% of the initial water table (case of the well pumping problem). This confirms on one side the correctness of the analytical solution and on the other side the reliability of the developed h2-linearization technique. On the contrary the conventional confined aquifer equations approach can be used only if the drawdown does not exceed 5% of the initial head values. As potential application examples, the estimation of the flow-rate from an aquifer to an adjacent water body, and the interpretation of pumping tests in unconfined double porosity aquifers, allowing to estimate the values of the models parameters, are proposed.

Effects of groundwater fluctuation on migration characteristics and representative elementary volume of entrapped LNAPL

Understanding migration characteristics and the representative elementary volume (REV) variation of the entrapped light non-aqueous phase fluids (LNAPL) under dynamic groundwater conditions is significant in quantifying the effective parameters and formulating pollution treatment strategies. This paper investigates the effect of the initial water table height, the fluctuation intensity, and the fluctuation sequence on redistribution characteristics and REVs of LNAPL in a two-dimensional (2D) flow chamber. The migration behaviors of pollution plumes and associated REVs were quantified by spatial moments and the criterion χi. The results indicated that the redistribution location of residual pollutants was mainly determined by the fluctuation sequence. The REV gradually decreases as the rising water table, while the minimum So-REV frequency distribution becomes closer to a regular Gaussian distribution. These results shed light upon the effect of dynamic water tables on the morphology features and REVs of LNAPL, which can offer valuable guidance for designing remediation schemes.

Three-dimensional theoretical analysis of seepage field in front of shield tunnel face

To evaluate hydraulic head distribution in front of a shield tunnel in a saturated soil layer, theoretical analysis and numerical simulations are carried out in this study. Based on the partial differential equilibrium equation of seepage flow, a three-dimensional (3D) theoretical analytical model of the shield tunnel face and the seepage field in front of it is established using the eigenfunction and the Fourier series expansion methods, and the hydraulic head calculation formula is derived. Combined with engineering cases, the theoretical analysis results and the 3D numerical simulation results are compared and analyzed. The effect of the water pressure of the tunnel face on the hydraulic head distribution is also analyzed. The results of the proposed analytical solution are in agreement with those of the numerical simulation solutions; moreover, the proposed analytical solution requires less time to calculate the seepage hydraulic head than the numerical simulation. The ratio of the initial water table to the diameter (D) of tunnel face has a more significant impact on the hydraulic head distribution at a position 0.5D above the tunnel vault. When the water pressure on the tunnel face is not considered, the values of the hydraulic head are significantly underestimated.

Rainfall Infiltration Process of a Rock Slope with Considering the Heterogeneity of Saturated Hydraulic Conductivity

In order to analyze the effects of rainfall events on the stability of an open-pit rock slope, with considering the spatial variability of saturated hydraulic conductivity, based on the unsaturated seepage theory and the random filed theory, modified functions of the unit saturation, the hydraulic conductivity (k), and the shear strength parameters are established for unsaturated slope, by using FISH and the non-intrusive stochastic method. A saturated-unsaturated seepage random field model is proposed. And then the impacts of the rainfall intensity, the rainfall duration, and the spatial variability of saturated hydraulic conductivity (ks) on the infiltration process and stability of the unsaturated rock slope are analyzed. The results show that the proposed model can estimate rainfall infiltration of rock slope accurately. Rainfall mainly affects the seepage field in the shallow layer of the slope, where a transient saturated zone can be formed. With the development of the rainfall duration, the weight of the rock mass increased, the matric suction reduced, the negative pore pressure, the degree of saturation, and the infiltration depth of the rock slope increased, and the water in the slope root connects with the initial water table gradually, the unsaturated zone shrinks, which causes the safety factor of the model decreases, but the trend of change slows down gradually. As the rainfall intensity strengthened, the infiltration depth increased and the safety factor of the slope reduced, while the changing rate increases first and then decreases. Increasing the correlation length of k can reduces the infiltration depth and safety factor of the slope. Increasing the variation coefficient of k will increase the infiltration depth, while the safety factor of the slope decreases. The infiltration depth and safety factor of the slope are most affected by rainfall duration, but its sensitivity to the variability coefficient of k will be strengthened when the rainfall intensity exceeds the infiltration capacity. This conclusion can provide reference significance for the risk estimation of slope geological hazards, which are induced by the rainfall infiltration.

Interaction of soil water and groundwater during the freezing–thawing cycle: field observations and numerical modeling

Abstract. Freezing-induced groundwater-level decline is widely observed in regions with a shallow water table, but many existing studies on freezing-induced groundwater migration do not account for freezing-induced water-level fluctuations. Here, by combining detailed field observations of liquid soil water content and groundwater-level fluctuations at a site in the Ordos Plateau, China, and numerical modeling, we showed that the interaction of soil water and groundwater dynamics was controlled by wintertime atmospheric conditions and topographically driven lateral groundwater inflow. With an initial water table depth of 120 cm and a lateral groundwater inflow rate of 1.03 mm d−1, the observed freezing and thawing-induced fluctuations of soil water content and groundwater level are well reproduced. By calculating the budget of groundwater, the mean upward flux of freezing-induced groundwater loss is 1.46 mm d−1 for 93 d, while the mean flux of thawing-induced groundwater recharge is as high as 3.94 mm d−1 for 32 d. These results could be useful for local water resources management when encountering seasonally frozen soils and for future studies on two- or three-dimensional transient groundwater flow in semi-arid and seasonally frozen regions. By comparing models under a series of conditions, we found the magnitude of freezing-induced groundwater loss decreases with initial water table depth and increases with the rate of groundwater inflow. We also found a fixed-head lower boundary condition would overestimate freezing-induced groundwater migration when the water table depth is shallow. Therefore, an accurate characterization of freezing-induced water table decline is critical to quantifying the contribution of groundwater to hydrological and ecological processes in cold regions.

Temporary Confined Water-Induced Landslide in the Binary Structure of a Gentle Slope: A Case Study of the Fanshantou Landslide

With the increase in rainfall, landslides occur on many gentle slopes in the mountainous areas of southeast China. Gentle slopes have a particular dual geological structure, i.e., the lower part is a gravel soil layer with good water permeability, and the surface layer is clay soil with relatively poor water permeability. Under conditions of heavy rainfall, a gentle slope with this structure is likely to collect temporary confined water. The intermittent creep of the upper slope is caused by the floating force of the temporary confined water, which causes landslide disasters. The conditions that bring about temporary confined water are related not only to the stratum structure, but also to the rainfall intensity and the initial height of the water table level. On the basis of the characteristics of the stratum of the gentle slope landslide on the front of Fanshantou Mountain, we constructed a hydrological model in GEO-STUDIO. We investigated the effect of different rainfall intensities and initial water table levels on confined water under continuous rainfall conditions and conducted a corresponding stability analysis. The results show that when both the initial water table level and the rainfall intensity are high, temporary confined water is formed rapidly, increasing the chance of a landslide disaster. The research results provide a theoretical basis for the treatment of landslides on similar gentle slopes in the mountainous areas of southeast China.

Water use of Salix in the variably unsaturated zone of a semiarid desert region based on in-situ observation

Soil water and groundwater are important water resources for vegetation survival and growth in arid and semiarid regions, and their contributions to water use over a whole growth period are difficult to quantify with limited observations. Additionally, the root water uptake (RWU) processes of groundwater-dependent xeric vegetation are specific and aggravate the complexity of these issues. In this paper, Salix is selected to study the interaction among atmosphere, soil water, groundwater, and RWU from the sprout to withered stage based on an observation within the ‘In situ Monitoring Lysimeter System of the Atmosphere-Plant-Unsaturated Zone-Groundwater Continuum’. Two lysimeters with a diameter of 2 m and depths of 1.2 m and 4.2 m were implemented with Salix. The initial water table depth was 0.7 m in case 1 and 2.2 m in case 2. The results show that: Water use of Salix is the main contribution to actual evapotranspiration (ETa), no less than 80%. ETa was 462.75 mm in case 1 with a shallower soil layer and limited groundwater, during the entire growth period, 2016. Contrastively, it was 619.70 mm in case 2 with relatively abundant soil water and groundwater. The soil water and groundwater contribution were 89.6% and 10.4%, 74.3% and 25.7% for both cases. The selective water use by Salix in different soil layers was subject to temporal moisture availability. Salix extended to utilize groundwater in drought period and shifted back to wetter upper layers in case of rainfall. The access to groundwater significantly alleviates water stress, although rainfall replenishment can also play a part. Salix can not only intercept rainfall infiltration in the root zone, but also cause the decrease of water level, implying that the replanting of Salix should be carried out in a reasonable mode to ensure rainfall infiltration and reduce the ineffective loss of groundwater. Our results provide a practical reference for groundwater management and ecology restoration in semiarid desert regions.

Intelligent approach to predict future groundwater level based on artificial neural networks (ANN)

To depict hydrogeological variables and understand the physical processes taking place in a complex hydrogeological system, artificial neural networks (ANNs) are widely used as a good alternative approach to tedious numerical models. The aim of this study was to predict the dynamic fluctuations in piezometric levels in Nebhana aquifers (NE Tunisia) using ANNs. A correlation analysis carried out between piezometry, evapotranspiration and rainfall during the period 2000 to 2018 revealed that piezometric levels were influenced by monthly rainfall, evapotranspiration and initial water table level. These informative variables were used as input variables to train the ANN to predict future monthly water table levels for four hydrogeological systems. The minimal and maximal computed relative errors were 0.01 and 19.00%, respectively; root mean square error (RMSE) varied between 0.41 and 2.06; the determination coefficient (R2) ranged between 0.93 and 0.99; and the Nash–Sutcliffe (NASH) efficiency coefficient ranged from 85.32 to 97.82%. To test the generalization capacity of the developed ANN models, we used the ANNs to predict monthly piezometric levels for the period September 2016 to August 2018. The results were satisfactory for all piezometers. Indeed, the minimal and maximal computed RE were − 12.00 and 0.03%, respectively; RMSE was between 0.44 and 1.74; R2 varied between 0.95 and 0.98; the NASH coefficient ranged from 60.00 to 98.99%. These models developed in this study can be adopted for future groundwater level prediction to accurately estimate trends in piezometric levels as well as water pumping costs.

Characterising groundwater–surface water interactions in idealised ephemeral stream systems

AbstractTransmission losses from the beds of ephemeral streams are thought to be a widespread mechanism of groundwater recharge in arid and semi‐arid regions and support a range of dryland hydro‐ecology. Dryland areas cover ~40% of the Earth's land surface and groundwater resources are often the main source of freshwater. It is commonly assumed that where an unsaturated zone exists beneath a stream, the interaction between surface water and groundwater is unidirectional and that groundwater does not exert a significant feedback on transmission losses. To test this assumption, we conducted a series of numerical model experiments using idealised two‐dimensional channel‐transects to assess the sensitivity and degree of interaction between surface and groundwater for typical dryland ephemeral stream geometries, hydraulic properties and flow regimes. We broaden the use of the term ‘stream–aquifer interactions’ to refer not just to fluxes and water exchange but also to include the ways in which the stream and aquifer have a hydraulic effect on one another. Our results indicate that deep water tables, less frequent streamflow events and/or highly permeable sediments tend to result in limited bi‐directional hydraulic interaction between the stream and the underlying groundwater which, in turn, results in high amounts of infiltration. With shallower initial depth to the water table, higher streamflow frequency and/or lower bed permeability, greater ‘negative’ hydraulic feedback from the groundwater occurs which in turn results in lower amounts of infiltration. Streambed losses eventually reach a constant rate as initial water table depths increase, but only at depths of 10s of metres in some of the cases studied. Our results highlight that bi‐directional stream–aquifer hydraulic interactions in ephemeral streams may be more widespread than is commonly assumed. We conclude that groundwater and surface water should be considered as connected systems for water resource management unless there is clear evidence to the contrary.

Analytical solutions of space fractional Boussinesq equation to simulate water table profiles between two parallel drainpipes under different initial conditions

This work presents the first attempt to derive analytical solutions of a space fractional Boussinesq equation (SFBE) for parabola type 1, parabola type 2, and elliptical initial water table profiles, denoted as SFBE-P1, SFBE-P2, and SFBE-E, respectively. Laboratory and field data published in literature were used to evaluate the performances of the fractional models in homogeneous and heterogeneous soils. Besides, the performances of the proposed fractional models were compared with that of a fractional analytical solution developed in the literature for a flat initial water table condition (SFBE-F). According to the results of the sensitivity analysis, heterogeneity degree of soil (α) had a more effect on outputs of the fractional models. Also, the results indicated that the α parameter in the SFBE-P1, SFBE-E, and SFBE-F was capable of describing well the heterogeneity degree of soil. The α value was estimated almost 2 in relatively homogenous soil, while its values were respectively appraised almost 1.3 and 1.1 in soils with average and relative heterogeneity degrees. The fractional models reduced to their counterpart classical ones (BE-P1, BE-P2, BE-E, and BE-F) in the relatively homogenous soil. The measurement and prediction results demonstrated the best and similar performance for the SFBE-E and SFBE-F models, while the SFBE-P2 models was the weakest. This behavior was also observed by using the classical models. Compared to the BE-E, the SFBE-E provided better prediction results in the heterogeneous soils. Overall, the SFBE-E and SFBE-F can be applied as practical models to simulate water table profile between two parallel drainpipes for both homogeneous and heterogeneous soils.

Towards the representation of groundwater in the Joint UK Land Environment Simulator

AbstractGroundwater is an important component of the hydrological cycle with significant interactions with soil hydrological processes. Recent studies have demonstrated that incorporating groundwater hydrology in land surface models (LSMs) considerably improves the prediction of the partitioning of water components (e.g., runoff and evapotranspiration) at the land surface. However, the Joint UK Land Environment Simulator (JULES), an LSM developed in the United Kingdom, does not yet have an explicit representation of groundwater. We propose an implementation of a simplified groundwater flow boundary parameterization (JULES‐GFB), which replaces the original free drainage assumption in the default model (JULES‐FD). We tested the two approaches under a controlled environment for various soil types using two synthetic experiments: (1) single‐column and (2) tilted‐V catchment, using a three‐dimensional (3‐D) hydrological model (ParFlow) as a benchmark for JULES’ performance. In addition, we applied our new JULES‐GFB model to a regional domain in the UK, where groundwater is the key element for runoff generation. In the single‐column infiltration experiment, JULES‐GFB showed improved soil moisture dynamics in comparison with JULES‐FD, for almost all soil types (except coarse soils) under a variety of initial water table depths. In the tilted‐V catchment experiment, JULES‐GFB successfully represented the dynamics and the magnitude of saturated and unsaturated storage against the benchmark. The lateral water flow produced by JULES‐GFB was about 50% of what was produced by the benchmark, while JULES‐FD completely ignores this process. In the regional domain application, the Kling‐Gupta efficiency (KGE) for the total runoff simulation showed an average improvement from 0.25 for JULES‐FD to 0.75 for JULES‐GFB. The mean bias of actual evapotranspiration relative to the Global Land Evaporation Amsterdam Model (GLEAM) product was improved from −0.22 to −0.01 mm day−1. Our new JULES‐GFB implementation provides an opportunity to better understand the interactions between the subsurface and land surface processes that are dominated by groundwater hydrology.

An analysis of vapour transfer in unsaturated freezing soils

The effects of vapour on water content in different unsaturated frozen soils have not been systematically analyzed in the literature. Based on the thermodynamic equilibrium theory and coupled water-heat theory, a new method for calculating unfrozen water content and ice content is obtained. A new model is then built by importing this method to the coupled heat and mass transfer theory. Unfrozen water content and ice content in this new model are only related to hydraulic parameters and temperature, which means specific physical meaning. The comparisons between simulations and test results of sandy loam validate the new model. Simulated results also show that temperature is the major factor to vapour transfer instead of suction. And vapour transfer in silt and sand cannot be neglected with freezing except clay. Initial water content, freezing temperature, freezing time and ground water table can all influence vapour transfer in freezing soils. In a word, even though the water content increment is low, the total mass of the increased amount and the average increase of the total water content in the frozen area are about 40 kg and 0.033 within the unit area in susceptible frost heaving soils such as silt, respectively. Remarkable frost heave could then occur due to vapour. Therefore, the vapour in unsaturated frozen soils must be paid more attention to in practical engineering. This paper strengthens the understanding of canopy effect and also validates that canopy effect usually occurs in covered freezing silt instead of sand or clay.

Quantification of transient specific yield considering unsaturated-saturated flow

Specific yield is one of the most important hydrogeological parameters, and is a key factor connecting flow processes in the unsaturated and saturated zones. In this study, an innovative expression for the dynamic (or time-dependent) specific yield is proposed considering the coupled unsaturated-saturated flow process. The new specific yield equation includes parameters such as saturated water content, residual water content, pore characteristic parameter, initial depth of water table, time-dependent depth of water table, initial pressure head, and time. The involving parameters in this new specific yield equation reflect the impacts of lithology, initial water table depth and other factors. This new equation approaches an asymptotic (steady-state) value which is the same as reported previously for a shallow water table condition. Both advective and diffusive unsaturated flow processes are taken into consideration, which is in contrast to a previous study that ignored the diffusive unsaturated flow process. The model established in this study reveals the complete dynamic process of variation of water content and water head in the unsaturated zone. The newly developed specific yield equation can be incorporated into groundwater flow theory considering a dynamic water table reflective of a physically based unsaturated-saturated flow process.