In geosciences, soil-water interactions are defined by soil water potential, which provides a quantitative estimate of the soil water thermodynamic state. Due to the interactions between water and soil particles, soil water has different physical properties than free water; hence, analyzing soil water may require different methods and approaches. Typically, soil water potential is defined as the sum of three independent functions: gravitational, osmotic, and matric. However, there is a problem with this definition because the osmotic and matric potentials exhibit coupling effects. Moreover, due to its high values, the matric potential dominates the total potential, whereas the gravitational potential may appear negligible. However, gravity may lead to different flow mechanisms altering the soil's mechanical behavior. As a result, it may not be valid to calculate the total water potential as the algebraic sum of the different potentials. There are also mathematical challenges in the common use of water potential; as soil saturation decreases, water potential can reach thousands of kPa, which requires mathematical balancing in the equations by multiplying it by a variable with a value near zero. However, multiples of numbers of different magnitudes are problematic from a mathematical perspective, especially when applied to numerical analysis. This paper discusses the strengths and limitations of the definitions and mathematical formulations of this variable.

Soil compaction has been found to deform soil structures and alter water flows. Although previous studies have suggested that a load exceeding the critical stress, determined by static load application, can be applied for a short duration without causing substantial damage to the soil structure, the immediate consequences of short loading times on structural integrity and the subsequent influence on soil water flow remain relatively underexplored. The principal objective of this research was to explore the effects of loading intervals, ranging from 0.1 to 2.5 s, commonly used by vehicles and machinery in the agricultural sector, on the changes in water-stable aggregates and saturated hydraulic conductivity (K-sat) associated with soil compaction, thereby enhancing our understanding of how transient external forces could affect the soil properties. Four distinct soils with varying soil organic matter (SOM) contents (13, 43, 77, and 123 g/kg) were collected from a typical Mollisol area in Northeast China, each characterized by different initial gravimetric soil water contents of 11%, 15%, 19%, and 24%, respectively. Under an applied load of 4.0 kg/cm(2), the short loading time resulted in an increase in small macroaggregates (SMAs) and a decrease in microaggregates within the distribution of water-stable aggregates, whereas it did not affect aggregate stability. K-sat decreased significantly (p < 0.05) as the loading time increased from 0.1 to 2.5 s. The effects of loading time and SOM on water-stable aggregates with particle sizes exceeding 0.25 mm, mean weight diameter, geometric mean diameter, and K-sat were identified as statistically significant or highly significant (p < 0.05 or p < 0.01). Notably, the initial soil water content remained unchanged during the short compaction period. A significant negative correlation was identified between SMAs and K-sat for each soil, with the loading time and initial soil water content (correlation coefficients ranging from -0.834 to -0.622). The results, combined with the structural equation modeling analysis, indicated that both a short loading time and SOM could directly increase SMA and decrease K-sat, with both factors influencing K-sat through SMA during the soil compaction process. This suggests that the loading time and SOM during a short duration under the same external force, rather than initial soil water content, can determine the potential degradation of the soil.

A series of laboratory tests were conducted to investigate the properties of fiber-reinforced underwater flowable solidified soil (UFSS) as a novel material for scour protection in marine structures. The tests included flowability, underwater anti-dispersion, unconfined compressive strength (UCS), and anti-scour resistance. Results showed that adding fibers reduced UFSS's flowability and significantly enhanced its underwater anti-dispersion, exhibiting a similar trend with increasing fiber content. Increasing fiber length initially decreased and then increased flowability, with the opposite trend for anti-dispersion. The least favorable fiber lengths for flowability were 6 mm for PVA fiber and 9 mm for both basalt and glass fibers, whereas these lengths were optimal for antidispersion. Fibers improved both UCS and anti-scour resistance of UFSS, with both properties first increasing and then decreasing as fiber content and length increased. Excessive fiber content or length reduced both properties. In this study, the optimal fiber content for improving UCS was 0.3% for PVA and 0.2% for basalt and glass fibers, with an optimal length of 6 mm for all three. An empirical exponential relationship between UCS, critical scour resistance velocity, and critical scour shear stress at typical times (t = 3 h, 5 h) was established for rapid prediction of UFSS's anti-scour resistance.

Reynolds number (Re), pore water pressure (P), and water flow shear force (tau) are primary indicators reflecting the characteristics of subsurface flow. Exploring the calculation of these parameters will facilitate the understanding of the hydrodynamic characteristics in different subsurface flows and quantify their differences. Hence, we conducted a study to monitor soil water content, matrix potential, and pore water pressure in two typical soil profiles (with and without fissures). The distribution of Re, P, and tau in both matrix flow (MF) and preferential flow (PF) were calculated with an improved calculation method, focusing on their energy changes. Results showed that these hydrologic parameters are quite different between MF and PF. Re values in MF remained below 0.1, indicating lower water flow velocities, while the Re values ranged from 0.8 to 2 in PF, indicating higher flow velocities. The P values in PF was tens to hundreds of times higher than that in MF, which is mainly due to the rapid accumulation and leakage of water within soil fissures. Additionally, the larger hydraulic radius and gradient in PF also resulted in higher tau values in PF (2 similar to 6 N m(-2)) than in MF (0 similar to 1.5 N m(-2)). In PF, the pressure potential was the significant factor for tau, while tau in MF was dominated by the matrix potential and varies with the magnitude of the matrix potential gradient. This study suggests that Re, P, and tau could be considered as the major indexes to reflect dynamic characteristics of subsurface flow.

The groundwater flow and the transport of solutes and contaminants in fractured geologic media play a very important role in various hydrogeological and geological processes. Fractures are discontinuities that occur in practically all types of rocks, consolidated and semi-consolidated sediments, in which groundwater flows at different scales of space and time. This article reviews more than 20 years of research in the CGEO of different selected examples in Mexico, from local to regional scales, associated with 1) gravitational Groundwater Flow Systems, 2) hydrogeochemical interaction of groundwater with fractured rocks through which it circulates, 3) instrumentation and coupled numerical analysis of flow parameters and time -varying geomechanics, during consolidation associated with pumping, 4) analysis of fracture generation with the development and application of coupled flow and geomechanical equations, 5) formation of new minerals, 6) sustenance of ecosystems, 7) artificial fracturing of soils for their conservation and infiltration of rainwater improvement; and on the issue of transport of natural solutes, 8) used as a tracers, 9) toxic elements to health and environment, 10) spills of hydrocarbon derivatives in low permeability and double porosity media due to fracturing and 11) heat. The results show the importance of the physical -chemical interaction between fractured and granular geological media at both local and regional scales, where groundwater residence times range from a few days to thousands of years; which implies modifying the criteria for water management and the permanence of ecosystems in the country. The complexity of these processes requires different methodologies for their evaluation, among them the instrumentation and calibration of numerical models from 1D to 3D for analysis, predictions and the proposal of restoration, sustainability and management solutions; they also help to prevent, control and mitigate the negative impacts on health and the environment caused by the induction of geogenic elements and by various types of pollutants; fractured geologic media also support numerous terrestrial and marine ecosystems; in the case of damaged agricultural soils, artificial fracturing allows increasing rainwater infiltration and improving productivity in adaptation to climate change and reducing the extraction in aquifers where safe capacity has been exceeded; unfortunately, excessive extraction in closed basins is causing fracturing of the aquitards, both hydraulic and due to differential settlement, which favors the migration of pore water rich in elements harmful to human health and the environment, whose natural function was its protection. All this requires designing mechanisms for the transfer of scientific knowledge to society and decision makers to propose novel restoration and sustainability strategies, under the new paradigm of Gravitational Groundwater Flow Systems.

Frost heaving in soils is a primary cause of engineering failures in cold regions. Although extensive experimental and numerical research has focused on the deformation caused by frost heaving, there is a notable lack of numerical investigations into the critical underlying factor: pore water pressure. This study aimed to experimentally determine changes in soil water content over time at various depths during unidirectional freezing and to model this process using a coupled hydrothermal approach. The agreement between experimental water content outcomes and numerical predictions validates the numerical method's applicability. Furthermore, by applying the Gibbs free energy equation, we derived a novel equation for calculating the pore water pressure in saturated frozen soil. Utilizing this equation, we developed a numerical model to simulate pore water pressure and water movement in frozen soil, accounting for scenarios with and without ice lens formation and quantifying unfrozen water migration from unfrozen to frozen zones over time. Our findings reveal that pore water pressure decreases as freezing depth increases, reaching near zero at the freezing front. Notably, the presence of an ice lens significantly amplifies pore water pressure-approximately tenfold-compared to scenarios without an ice lens, aligning with existing experimental data. The model also indicates that the cold-end temperature sets the maximum pore water pressure value in freezing soil, with superior performance to Konrad's model at lower temperatures in the absence of ice lenses. Additionally, as freezing progresses, the rate of water flow from the unfrozen region to the freezing fringe exhibits a fluctuating decline. This study successfully establishes a numerical model for pore water pressure and water flow in frozen soil, confirms its validity through experimental comparison, and introduces an improved formula for pore water pressure calculation, offering a more accurate reflection of the real-world phenomena than previous formulations.

Extreme precipitation events (EPEs) are projected to become more frequent and intense due to global warming. Understanding how coastal groundwater levels respond to and recover from these severe events is important for estuarine ecosystems to adapt to global change. Numerical model and non-EPE scenario simulation were used to examine groundwater level recovery time (RT) after Super Typhoon Lekima, which triggered EPEs that resulted in groundwater rise and widespread flooding in the Yellow River Delta (YRD). The three-day rainfall during Lekima totaled 290.9 mm, equivalent to 50 % of the annual rainfall for 2019 (581.5 mm), leading to a general rise in groundwater levels. Groundwater recovery to EPE can be divided into two types: inland and coastal. The RT of groundwater levels in monitoring wells in inland areas ranged from 12 to 89 days, with an average of 56.2 days, and there was spatial variation. However, groundwater levels in monitoring wells close to the coast may not recover. Differences in recovery are reflected in the land-sea gradient, with RT gradually increasing from inland highlands to coastal depressions and lowlands. The results showed that inland aquifers were more resistant to EPEs, while coastal aquifers were less resistant. In addition, EPE can cause groundwater flooding, and areas at lower altitude and close to the sea are more sensitive to flooding. Estuarine groundwater and the ecological processes on which it depends are profoundly affected by the direct and legacy effects of EPEs, including salt contamination, widespread flooding, crop damage, and reduced biodiversity. The study of this event provides case support for the response of estuarine groundwater to EPEs, while highlighting the importance of continuous monitoring.

The mapping of water content at a landslide sensitive area is important in order to identify the potency of ground motion. In such an area a minuscule amount of movement may lead to a catastrophic event. Water, which may act as a precursor of ground motion, changes the mechanical properties of the land, hence, changing the ability of the ground to resist gravitational force. In order to identify the water containment, as well as the flow of groundwater, we apply a geophysical method, namely the Self Potential (SP) measurement. Based on the analysis of Darcy's law the measurement result is related directly to the flow velocity. Although the measurement was performed on top of soil, the measured quantity is a response due to the amount of water infiltration into the soil. The mapped profile of the measurement identifies the flow pattern of groundwater. The result can be used to estimate the soil instability and the potency of landslide events. Our data shows the distribution of the groundwater in the observed area, which can be used as a hint to design the drainage system, in order to divert water from the landslide prone areas. The main goal of this work is to minimize the risk to the community by preventing groundwater flow from targeting inhabited regions, thus ensuring the safety of the residents.

Air entrapment in soil-pores during cyclically hydraulic loading requires a physical insight by means of both modelling and experimental works. In the present study, transient drying and wetting tests are performed in a sand column device. The device enables to perform measurements of transient water content and pore water pressure in unsaturated soils. The measurements of water content as well as the pore pressure are directly linked to soil-water retention curve. The experimental results show that it exists an entrapped air together with hysteresis in soil water retention. Based on the experimental results, a new computer model of soil water retention is proposed. Finally, numerical simulation of air -water transport in unsaturated media is implemented using this model. The comparison between measured data and numerical simulation results shows that the proposed model can improve an accuracy in simulation of the water transport in unsaturated media.

Under a warming climate, permafrost degradation has resulted in profound hydrogeological consequences. Here, we mainly review 240 recent relevant papers. Permafrost degradation has boosted groundwater storage and discharge to surface runoffs through improving hydraulic connectivity and reactivation of groundwater flow systems, resulting in reduced summer peaks, delayed autumn flow peaks, flattened annual hydrographs, and deepening and elongating flow paths. As a result of permafrost degradation, lowlands underlain by more continuous, colder, and thicker permafrost are getting wetter and uplands and mountain slopes, drier. However, additional contribution of melting ground ice to groundwater and stream-flows seems limited in most permafrost basins. As a result of permafrost degradation, the permafrost table and supra-permafrost water table are lowering; subaerial supra-permafrost taliks are forming; taliks are connecting and expanding; thermokarst activities are intensifying. These processes may profoundly impact on ecosystem structures and functions, terrestrial processes, surface and subsurface coupled flow systems, engineered infrastructures, and socioeconomic development. During the last 20 years, substantial and rapid progress has been made in many aspects in cryo-hydrogeology. However, these studies are still inadequate in desired spatiotemporal resolutions, multi-source data assimilation and integration, as well as cryo-hydrogeological modeling, particularly over rugged terrains in ice-rich, warm (>-1 degrees C) permafrost zones. Future research should be prioritized to the following aspects. First, we should better understand the concordant changes in processes, mechanisms, and trends for terrestrial processes, hydrometeorology, geocryology, hydrogeology, and ecohydrology in warm and thin permafrost regions. Second, we should aim towards revealing the physical and chemical mechanisms for the coupled processes of heat transfer and moisture migration in the vadose zone and expanding supra-permafrost taliks, towards the coupling of the hydrothermal dynamics of supra-, intra- and sub-permafrost waters, as well as that of water-resource changes and of hydrochemical and biogeochemical mechanisms for the coupled movements of solutes and pollutants in surface and subsurface waters as induced by warming and thawing permafrost. Third, we urgently need to establish and improve coupled predictive distributed cryo-hydrogeology models with optimized parameterization. In addition, we should also emphasize automatically, intelligently, and systematically monitoring, predicting, evaluating, and adapting to hydrogeological impacts from degrading permafrost at desired spatiotemporal scales. Systematic, in-depth, and predictive studies on and abilities for the hydrogeological impacts from degrading permafrost can greatly advance geocryology, cryo-hydrogeology, and cryo-ecohydrology and help better manage water, ecosystems, and land resources in permafrost regions in an adaptive and sustainable manner.