The soil water density is defined as the ratio of soil water mass to soil water volume. It is a cornerstone in defining thermodynamic states of either saturated or unsaturated soils for quantifying ...water storage and movement in the subsurface and for mechanical stability of landscape. So far, it has been widely treated as identical to the free water density, that is, a constant of 0.997 g/cm3, but can be remarkably different from this value as it is subject to a wide range of variation in energy levels. Some experimental and theoretical evidence indicate that it can be as high as 1.680 g/cm3 and as low as 0.752 g/cm3. However, to date, there is no unanimous agreement upon a reliable experimental method to measure the soil water density or a unified theory to explain why and how the soil water density can deviate remarkably from the free water density. Consequently, the understanding of the soil water density is controversial and elusive, or some theories are contradictory to each other. In this review, the authors will (1) conduct critical reviews on the experimental and theoretical methodologies to identify their limitations, flaws, and uncertainties, (2) synthesize some recent findings on intermolecular forces, interfacial interactions, and soil water retention mechanisms to clarify molecular‐scale physicochemical mechanisms governing the soil water density, and (3) propose a unified model to quantify soil water density variation. It is found that capillarity associated with surface tension tends to generate tensile stress in soil water and thereby decreases the soil water density, whereas adsorption stemmed from cation hydration, surface hydration, and interlamellar cation hydration tends to produce compressive stress thus increases the soil water density. Furthermore, the abnormally high water density greater than 1.15 g/cm3 is a result of cation and surface hydration that involves significant water structure change around exchangeable cations and mineral surface hydroxyls. The unified soil water density model, explicitly quantifying adsorptive and capillary water, could potentially reconcile the unresolved controversies. The critical reviews and the unified model also would allow us to further confine the upper and lower bounds of the soil water density. The upper bound is theoretically inferred to be around 1.872 g/cm3, whereas the lower bound is around 0.995 g/cm3; both are higher than that reported in the literature. With the unified model and measured soil water retention curves, it is demonstrated quantitatively that the soil water density significantly impacts the magnitude of various fundamental soil properties such as matric potential, specific surface area, and volumetric water content. The abnormally high soil water density has significant implications to the conventional concepts of matric potential and pore water pressure in soils and other earthen porous materials.
Plain Language Summary
We conduct critical reviews on the experimental and theoretical methodologies on soil water density to identify their limitations, flaws, and uncertainties. We synthesize some recent findings on intermolecular forces, interfacial interactions, and soil water retention mechanisms to clarify molecular‐scale physicochemical mechanisms governing the soil water density. We propose a unified model to quantify soil water density variation.
Key Points
We conduct critical reviews on the experimental and theoretical methodologies on soil water density to identify their limitations, flaws, and uncertainties
We synthesize some recent findings on intermolecular forces, interfacial interactions, and soil water retention mechanisms to clarify molecular‐scale physicochemical mechanisms governing the soil water density
We propose a unified model to quantify soil water density variation
Spatio‐temporal heterogeneity in soil water content is recognized as a common phenomenon, but heterogeneity in the hydrogen and oxygen isotope composition of soil water, which can reveal processes of ...water cycling within soils, has not been well studied. New advances are being driven by measurement approaches allowing sampling with high density in both space and time. Using in situ soil water vapour probe techniques, combined with conventional soil and plant water vacuum distillation extraction, we monitored the hydrogen and oxygen stable isotopic composition of soil and plant waters at paired sites dominated by grasses and Gambel's oak (Quercus gambelii) within a semiarid montane ecosystem over the course of a growing season. We found that sites spaced only 20 m apart had profoundly different soil water isotopic and volumetric conditions. We document patterns of depth‐ and time‐explicit variation in soil water isotopic conditions at these sites and consider mechanisms for the observed heterogeneity. We found that soil water content and isotopic variability were damped under Q. gambelii, perhaps due in part to hydraulic redistribution of deep soil water or groundwater by Q. gambelii in these soils relative to the grass‐dominated site. We also found some support for H isotope discrimination effects during water uptake by Q. gambelii. In this ecosystem, the soil water content was higher than that at the neighbouring Grass site, and thus, 25% more water was available for transpiration by Q. gambelii compared with the Grass site. This work highlights the role of plants in governing soil water variation and demonstrates that they can also strongly influence the isotope ratios of soil water. The resulting fine‐scale heterogeneity has implications for the use of isotope tracers to study soil hydrology and evaporation and transpiration fluxes to improve understanding of water cycling through the soil–plant–atmosphere continuum.
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•Biocrusts decrease soil water diffusivity by 24%–71% from dry to wet.•Biocrusts increase soil water content by 8%–53% across whole soil matric potential.•Biocrusts result in 15%–45% ...higher surface water content than bare soil in the field.•Effects of different biocrusts are ranked in the order of moss > mixed > cyano crusts.•Biocrusts would induce a complex influence on surface soil water balance in drylands.
As a “living skin” of soil in drylands, biocrusts possibly change the retention and movement of soil water, and thus critically influence hydrological and erosion processes as well as soil moisture regime. Contrary to vertical infiltration, little attention was paid to horizontal infiltration, which may provide important information on soil water diffusivity and sorptivity. In this study, the one-dimensional horizontal infiltration experiments for bare soil and three types of biocrusts (cyanobacterial, cyanobacterial-moss mixed, and moss crusts) were conducted with disturbed soil samples in a semiarid dryland. Undisturbed soil samples were also taken for each treatment, and their soil water retention capacity were determined in the laboratory. The substantial differences that were found were analyzed, and the differences were further validated through monitoring and comparing their in situ soil moisture regimes at depth of 0–20 cm. Our results showed that the three types of biocrusts had significantly lower constant infiltration rate (0.11 vs. 0.18 cm min−1) and infiltration water amount (6.75 vs. 10.01 cm) than bare soil, and their wetting front moved much slower (0.60 vs. 1.14 cm min−1) in comparison to bare soil, with bare soil > cyanobacterial crusts > cyanobacterial-moss mixed crusts > moss crusts. Total sorptivity of cyanobacterial, mixed, and moss crusts were 21%, 32%, and 53% lower than that of bare soil, respectively. More importantly, at the same level of water content, all crust types had lower water diffusivity, sorptivity, and conductivity than the bare soil, and exhibited a higher water content than bare soil at a same level of matric potential (across the range of −15,000–0 hPa), indicating an increase in water-holding capacity and water availability of the biocrusts in contrast to bare soil. Furthermore, the in-situ soil water content of all biocrusts increased on average by 30% at depth of 0–10 cm but decreased by 23% at 10–20 cm. Confirming the lower infiltrability and higher water-holding capacity of the biocrusts in comparison to bare soil, biocrusts increase surface soil water retention capacity and water availability at the surface, thus playing a vital role in reshaping the surface soil water balance, which subsequently may affect surface hydropedological and biochemical processes in drylands.
Extreme drought events occur more frequently due to climate change. Soil water loss through evaporation is therefore significantly intensified. This study introduces an environment‐friendly and ...sustainable bio‐mediated technique, known as microbially induced calcite precipitation (MICP), for water evaporation suppression in clayey soils. Through lab‐scale evaporation tests, we investigate the effects of cementation solution concentration (0.5, 1.0, and 1.5 mol/L) and MICP treatment procedure (one‐phase and two‐phase MICP methods) on both macroscale (e.g., water loss, desiccation cracking) and microscale (e.g., microstructure variations) behaviors of soils. Experimental results show that MICP is capable of improving water retention capacity and enhancing the inter‐particle bonding of clayey soils. Both water evaporation rate and total water loss decrease with the increasing concentration of cementation solution and the number of MICP treatment cycle. For most testing samples, both one‐phase and two‐phase MICP treatment methods have a similar influence on soil properties. Further microstructure characterizations reveal four key factors contributing to the improved soil response under drying, including dense surface crust, remediated desiccation cracks, smaller pore size and residual solutes. Dense structure of surface crust suppresses the migration of water vapor into the atmosphere. Calcite crystals tend to reduce the evaporation surface if precipitated within cracks, and clog the movement of pore water if precipitated within the soil pore space. This study is expected to improve the fundamental understanding of soil‐atmosphere interactions under MICP treatment and provide insights into the potential application of bio‐mediated technologies as a nature‐based solution for drought mitigation in arid and semi‐arid region.
Plain Language Summary
Drought‐induced log‐term water scarcity in the soil system can have a substantial impact on the ecosystem and environment. For instance, excessive evaporation of soil water caused by drought results in land degradation, salinization, desertification and subsidence. It is clear that maintaining the soil moisture profile by reducing evaporation is vital to the mitigation of negative drought effects on ecosystem in dry areas. This study introduces an environment‐friendly and sustainable bio‐mediated technique, known as microbially induced calcite precipitation (MICP), for water evaporation mitigation in clayey soils. Experimental results show that MICP is capable of improving water retention capacity and enhancing the inter‐particle bonding of clayey soils. Both water evaporation rate and total water loss are significantly decreased after MICP treatment. A relationship between soil moisture evaporation rate and suction is established. This study is expected to improve the fundamental understanding of soil‐atmosphere interactions under MICP treatment and provide insights into the potential application of bio‐mediated technologies as a nature‐based solution for drought mitigation in arid and semi‐arid region.
Cosmic ray neutron sensing (CRNS) has become a promising method for soil water content (SWC) monitoring. Stationary CRNS offers hectare‐scale average SWC measurements at fixed locations ...maintenance‐free and continuous in time, while car‐borne CRNS roving can reveal spatial SWC patterns at medium scales, but only on certain survey days. The novel concept of a permanent mobile CRNS system on rails promises to combine the advantages of both methods, while its technical implementation, data processing and interpretation raised a new level of complexity. This study introduced a fully automatic CRNS rail‐borne system as the first of its kind, installed within the locomotive of a cargo train. Data recorded from September 2021 to July 2022 along an ∼9 km railway segment were analyzed, as repeatedly used by the train, supported by local SWC measurements (soil samples and dielectric methods), car‐borne and stationary CRNS. The results revealed consistent spatial SWC patterns and temporary variation along the track at a daily resolution. The observed variability was mostly related to surface features, seasonal dynamics and different responses of the railway segments to wetting and drying periods, while some variations were related to measurement uncertainties. The achieved medium scale of SWC mapping could support large scale hydrological modeling and detection of environmental risks, such as droughts and wildfires. Hence, rail‐borne CRNS has the chance to become a central tool of continuous SWC monitoring for larger scales (≤10‐km), with the additional benefit of providing root‐zone soil moisture, potentially even in sub‐daily resolution.
Key Points
The first rail‐borne Cosmic ray neutron sensing system for automatic and continuous soil water content monitoring at the hectare scale is presented
The system provided almost uninterrupted data from September 2021 to July 2022 along a 9 km railway track in the Harz low mountains, Germany
Results showed spatial pattern, related to surface features, seasonal change, and individual responses of railway parts to wetting and drying
The soil water retention curve is the fundamental soil hydraulic property to characterize soil water movement and solute transport. Many efforts have been devoted in the past decades to developing ...models to describe soil water retention curves. However, most of them are empirical equations or assume that soil pore size distributions conform to a lognormal distribution. Yet, few effects have been undertaken to systematically propose and compare a series of possible alternative probability density functions to describe the sigmoid retention curves with parameters physically explainable. Here, we proposed a family of five soil water retention models based on sigmoid functions with parameters of clear physical implications coinciding with the statistical measures of soil pore size distribution. Compared with the widely used models (i.e., Brooks & Corey, 1964; Kosugi, 1996; van Genuchten, 1980), the proposed models have somewhat improved performances to characterize water retention data for a wide range of soil textures without introducing additional model parameters. Two of the proposed models are capable of characterizing the observed two local extrema in the moisture capacity curves. The associated unsaturated hydraulic conductivity models of the proposed soil water retention models are also derived, which show superior performance in characterizing the observed hydraulic conductivities compared with competing models, especially in macropore regimes. Additionally, we analyzed the parameter‐equivalent conversion between the proposed and the existing models, and a simple linear regression equation can be used to derive the parameters of the proposed models from the existing and other alternative different proposed models.
Key Points
A family of soil water retention models based on sigmoid functions and related relative hydraulic conductivity functions are proposed
Parameters have statistical implications against pore size distributions and can be converted from existing or alternative new models
Characterization of hydraulic properties is improved using new models, especially for macropore properties, without additional parameters
•Fiber-optic sensing technique is innovatively applied in the field of hydrology and geotechnical engineering.•Fiber Bragg grating (FBG) is utilized to continuously monitor soil water ...potential.•FBG-suction method can measure a wide range of total suction accurately and efficiently.•The geophysical method holds significant potential for characterizing dynamic surface processes.
Accurate measurement of soil water potential or suction is crucial for hydrology and geotechnical engineering. Inspired by recent advances in fiber-optic sensing techniques, we develop an innovative geophysical method for measuring the soil suction by using fiber Bragg grating (FBG), named FBG-suction method. In this study, working principles and theoretical background associated with the FBG-suction method are discussed. For verification purposes, a series of laboratory-scale tests including calibration, suction measurement, and evaporation monitoring are performed on two clayey soils of low to high plasticity under different degrees of saturation. The calibration and measurement results demonstrate the efficacy and accuracy of the developed method for measuring a wide range of soil suction. The curves of soil saturation with suction obtained by WP4C and FBG-suction method are comparable and reflect the difference in the water retention capacity of the tested soils. Additionally, the evaporation monitoring results show that the FBG-suction method can measure the low suction during the drying process and accurately obtain the high suction above the measuring range of the PST-55 psychrometer. The findings affirm the scientific viability of the fiber-optic sensing technique for acquiring the spatial distribution of water potential or suction in unsaturated soils and provide a novel perspective for future investigations into the spatiotemporal evolution characteristics of in-situ unsaturated soil under climate change. As interdisciplinary collaborations continue to advance, it becomes increasingly apparent that methods based on fiber-optic sensing have the potential to revolutionize traditional approaches to characterizing dynamic subsurface processes.
AIMS: This study aims to (i) determine the effects of incorporating 47 Mg ha⁻¹ acacia green waste biochar on soil physical properties and water relations, and (ii) to explore the different mechanisms ...by which biochar influences soil porosity. METHODS: The pore size distribution of the biochar was determined by scanning electron microscope and mercury porosimetry. Soil physical properties and water relations were determined by in situ tension infiltrometers, desorption and evaporative flux on intact cores, pressure chamber analysis at −1,500 kPa, and wet aggregate sieving. RESULTS: Thirty months after incorporation, biochar application had no significant effect on soil moisture content, drainable porosity between –1.0 and −10 kPa, field capacity, plant available water capacity, the van Genuchten soil water retention parameters, aggregate stability, nor the permanent wilting point. However, the biochar-amended soil had significantly higher near-saturated hydraulic conductivity, soil water content at −0.1 kPa, and significantly lower bulk density than the unamended control. Differences were attributed to the formation of large macropores (>1,200 μm) resulting from greater earthworm burrowing in the biochar-amended soil. CONCLUSION: We found no evidence to suggest application of biochar influenced soil porosity by either direct pore contribution, creation of accommodation pores, or improved aggregate stability.
Soil water is a critical factor closely related to hydrological and ecological processes. Owing to the complex surface conditions with heterogeneous soil thickness and abundant underlying fissures, ...soil water in the karst region has been a complicated issue. In this study, the dynamic changes of soil water in the vertical profile of selected grassland, farmland and bare land on a karst yellow soil hillslope in southwest China were monitored at five depths including 20, 40, 60, 80 and 100 cm with an interval of 15 min. Results showed that (1) there were spatial differences in the response to rainfall of the soil water content at different depths. When the rainfall amount was similar, the soil water replenishment amount and migration depth under the three land uses decreased with the rainfall intensity. In the case of light rainfall, the soil water content at 20 cm was the most sensitive to rainfall, and the response of soil water to rainfall mainly occurred at 0–40 cm. In the case of moderate and heavy rainfall, soil water could migrate down to 100 cm on grassland but less than 100 cm on farmland and bare land under heavy intensity rainfall. (2) The variation in the soil water content had interlayer differences over time. The response of soil water to rainfall in different soil layers showed multipeak fluctuations. In general, when the rainfall intensity was the same, the soil water fluctuation on grassland and farmland at the same depth was larger than that on bare land; however, the peak value of soil water decreased with soil depth. (3) Land use and the antecedent soil water content had important effects on soil water loss during the dry period. Soil water loss was faster at the beginning and before slowing down. The soil water loss rate on grassland and farmland increased with the length of the dry period, but decreased gradually on bare land. These results can support the utilization and protection of soil and water resources on karst yellow soil slopes and help to understand the temporal and spatial dynamics of soil water under natural rainfall.
Form and function are two major characteristics of hydrological systems. While form summarizes the structure of the system, function represents the hydrological response. Little is known about how ...these characteristics evolve and how form relates to function in young hydrological systems. We investigated how form and function evolve during the first millennia of landscape evolution. We analyzed two hillslope chronosequences in glacial forelands, one developed from siliceous and the other from calcareous parent material. Variables describing hillslope form included soil physical properties, surface, and vegetation characteristics. Variables describing hydrological function included soil water response times, soil water storage, drainage, and dominant subsurface flow types. We identified links between form and hydrological function via cluster analysis. Clusters identified based on form were compared in terms of their hydrological functioning. The comparison of the two different parent materials shows how strongly landscape evolution is controlled by the underlying geology. Soil pH appears to be a key variable influencing vegetation, soil formation and subsequently hydrology. At the calcareous site, the high buffering capacity of the soil leads to less soil formation and fast, vertical subsurface water transport dominates the water redistribution even after more than 10,000 years of landscape evolution. At the siliceous site, soil acidification results in accumulation of organic material, a high water storage capacity, and in podsolization. Under these conditions water redistribution changes from vertical subsurface water transport at the young age classes to water storage in the organic surface layer and lateral subsurface water transport within 10,000 years.
Key Points
The underlying geology controls landscape evolution in glacial forefields
After 10,000 years of evolution, hillslope form and hydrological functioning differ between the calcareous and siliceous sites
Soil pH is a key variable indicative of differences in soil evolution and hydrological response between the two forefields
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