Threshold Water Content Research Articles

Experimental Analyses of the Evaporation Dynamics in Bare Soils under Natural Conditions

In spite of recent progresses in evaporation estimate through advanced models and laboratory experiments, the drying process of bare soils through its successive stages remains difficult to predict. A study which addresses evaporation modeling in natural bare soils is presented. It relies upon hydro-meteorological measurements performed in a plot with a bare silt loam soil maintained in natural conditions. The following steps are involved: estimate of daily actual evaporation, Ea, through the hydrological balance, scaling with pan evaporation measurements, Epan, and analysis of the relation of Ea/Epan with the soil moisture vertical profile. The results enable us (1) to check the occurrence of the first stage of the evaporation process, characterized by not limited-soil water supply and high evaporative rates, and (2) to identify the transition from the first to the second stage, with decreasing soil water-limited Ea values. The last point requires the introduction of a soil water content threshold at 5 cm depth, that is associated with the soil field capacity. The adopted procedure provides insights on the soil water dynamics at depths differently involved through the successive stages of the evaporative process. Finally, indications on the use of pan evaporation measurements in evaporative rate estimate at least during the first stage of the process are also given.

Methane Production Explained Largely by Water Content in the Heartwood of Living Trees in Upland Forests

AbstractMost forests worldwide are located in upland landscapes. Previous studies have mainly focused on ground methane (CH4) flux in upland forests, and living tree stem‐based CH4 processes and fluxes are thus relatively poorly understood. This study investigated the relationship between CH4 concentration and water content in the heartwood of living trees in midtemperate, warm temperate, and subtropical upland forests and also measured seasonal changes of in situ stem CH4 flux and the CH4 concentration and water content in the heartwood of Populus davidiana Dode in a warm temperate upland forest. We found that approximately 4–13% of tree stems or approximately 8–31% of tree species had a substantial CH4 concentration of ≥10,000 μL L−1 in their heartwood across the three types of upland forests. The heartwood CH4 concentration was related to water content by a power function. A threshold of water content occurred beyond which CH4 was produced at high levels in the heartwood. The CH4 emissions from the breast height stems of P. davidiana ranged from 202.1 to 331.6 μg m−2 h−1 on a stem surface area basis during July–October 2016 and were significantly linearly correlated with the CH4 concentration or water content in the heartwood throughout the experimental period, but the linear correlation was not significant at daily and monthly scales. Temperature was not a limiting factor for CH4 production during July–October 2016, and thus, most of the CH4 production may be explained by water content in the heartwood of living trees in upland forests.

Transient Liquid Water Distributions in Polymer Electrolyte Membrane Fuel Cell Gas Diffusion Layers Observed through In-Operando Synchrotron X-ray Radiography

In this work, in-operando synchrotron X-ray radiography was used to capture the changes in the liquid water saturation of gas diffusion layers (GDLs) during changes in operating current density. Through in-operando visualizations at high temporal and spatial resolutions, we observed that the liquid water saturation increased with increasing current density. Eventually, a threshold water content in the GDL was reached despite further increases in current density. A time lag between the change in current density and the onset of increasing GDL water content was also observed. Current density consistently reached a steady state value before the GDL water content reached steady state, and the trends in liquid water distributions in the MPL were distinct from, yet influential to the accumulation in the substrate. We present a logarithmic growth function that describes the dynamic changes in GDL water content. The formulation of the GDL liquid water content transient response to changes in operating conditions provides a new metric for designing next generation fuel cell powertrains with fast dynamic responses. Through plane water thickness profiles were also used to show that the water accumulation patterns continued to evolve for up to 15 minutes at the low current density.

Drainage mechanisms in porous media: From piston‐like invasion to formation of corner flow networks

AbstractWater drainage from porous media is a highly dynamic process often marked by rapid piston‐like air invasion events at the front and other rapid interfacial reconfigurations. Liquid phase entrapped behind the moving front drains at significantly slower rates often via gravity driven flow through corners and crevices. This distribution of slowly draining residual water phase determines the plant available water and biological functioning of soils. The study aims to determine the conditions for the flow regime transition from piston‐like invasion at a drainage front to slower corner dominated flow at the pore and sample scale. This transition was observed experimentally for sand and glass beads with fast X‐ray tomography, revealing water fragmentation into clusters of full pores interconnected by water corner films. The observed liquid morphology at the transition from piston to corner flow was reproduced by a quasi‐static pore network model and predicted by percolation theory. The amount of capillary‐retained water at flow transition controlling the subsequent drainage dynamics could be reproduced by an idealized star shaped pore whose geometry is deduced from macroscopic properties of the porous medium. Predictions of water content thresholds at flow transitions were in agreement with other critical saturation values associated with cessation of solute diffusion and of internal drainage (at field capacity) highlighting the criticality of water phase continuity disruption for formation of relatively stable unsaturated conditions controlled by slow corner flow that support life in soil.

Timer versus moisture sensor-based irrigation control of soilless lettuce: Effects on yield, quality and water use efficiency

The study compares the effects of: timer ('Timer') and soil moisture sensor-controlled irrigation on soilless lettuce; two volumetric water content (Θ) thresholds for irrigation (0.30 ('Θ = 0.3') and 0.40 m3/m3 ('Θ = 0.4')). The most nutrient solution (NS) was applied in 'Timer' where the lowest water use efficiency was observed, with 17 and 42% less NS used in 'Θ = 0.4' and 'Θ = 0.3', respectively. Irrigation volumes followed the plant water needs in the sensor-controlled treatments, with little or no leaching, while 18% of leaching was recorded in 'Timer'. Plants in 'Timer' and 'Θ = 0.4' had higher fresh weights (24%) and leaf area (13%) than plants in 'Θ = 0.3'. Similar dry weight was observed among treatments but percentage of dry matter was 20% higher in 'Θ = 0.3'. Gas exchanges and leaf tissues chemical composition were similar in all treatments, but nitrate concentration was lower in the 'Θ = 0.3' plants. Precision sensor-controlled irrigation based on Θ measurements is an effective tool to increase the overall water use efficiency and to improve the quality of soilless-grown lettuce by acting on the substrate moisture level.

Variations in internal water distribution and leaf anatomical structure in maize under persistently reduced soil water content and growth recovery after re-watering

Episodes of regional drought are increasing and are frequently associated with increased duration and intensity. However, relatively little is known about effects of long-lasting drought on leaf microscopic structure and physiological metabolism of plants. In this study, we investigated internal water re-distribution and leaf anatomical structure of maize (Zea mays L.) grown under persistently reduced soil water content. Meanwhile, the threshold of soil water content at which maize cannot recover growth vitality after re-watering was determined. Our results showed that during persistent reductions in the field water capacity from 75 to 25 %, plant growth declined, while the water content in maize decreased following the order from the lower to upper leaves and their leaf sheathes to the stem and roots. At 20 % of field water capacity, the volume of bulliform cells declined, accompanied by an inward shrinkage of cell walls. Under field water capacity below 20 %, the number of chloroplasts in bundle-sheath cells decreased, chloroplasts in mesophyll cells deformed from oval to round, concomitant of a near to zero net photosynthetic rate. It was demonstrated that the growth vitality of maize plants could be recovered by re-watering even if field water capacity reduced to 15 %, but not to 10 %.

Land–atmosphere carbon and water flux relationships to vapor pressure deficit, soil moisture, and stream flow

Climatic change is exerting considerable influence on the hydrologic and biogeochemical cycles of snow- dominated montane forest ecosystems. Growing season drought stress is a common occurrence after snowmelt-derived soil water content (WC) and stream flow (Q) have declined, leading to an increase in atmospheric water demand (i.e., vapor pressure deficit, VPD). Here, we analyzed a 6-year record (2006–2011) of H2O and CO2 fluxes from the Tenderfoot Creek Experimental Forest, a montane forest in the northern Rocky Mountains to examine (1) how growing season evapotranspiration (ET), net ecosystem production (NEP), and water-use efficiency (WUE, NEP/ET) respond to changing WC and VPD, (2) how stream flow (Q), an integrated measure of catchment-level water availability, relates to NEP, and (3) how annual NEP is related to annual precipitation and the temperature-defined growing season length (GSL). Growing season NEP exhibited a linear relationship with WC and a log-linear relationship with Q, indicative of persistent water limitations when streamflow and soil moisture reach their annual minima late in the growing season. Nevertheless, years with long GSLs had relatively higher NEP, with a small net carbon sink maintained even at low levels of WC and Q, suggesting that trees are able to obtain water from deeper portions of the soil profile (>30cm) during droughts. However, the warmer, drier climate projected for this region could bring this system closer to a critical threshold of GSL, WC, and VPD, introducing vegetation water stress that could alter the current relationship between GSL and annual NEP.

Highly conducting composite hydrogels from gellan gum, PEDOT:PSS and carbon nanofibres

Conducting filler composite hydrogels were prepared by first dispersing vapour grown carbon nanofibers (VGCNF) in a dispersion of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), followed by mixing with a gellan gum (GG) solution prior to cross-linking with CaCl2. The gel transition temperature of GG was largely unaffected by the inclusion of PEDOT:PSS and/or VGCNFs. We demonstrate that electrical impedance spectroscopy coupled with percolation analysis can be used to identify the water content threshold at which the electrical behaviour of hydrogels becomes dominated by conducting fillers (rather then the cationic cross-linkers). These studies suggested that the electrical behaviour transfers from dominated by Ca2+ cross-liners to dominated by electrons (from the conducting fillers) at a water content of 87.2±1.2%. The composite hydrogels exhibited conductivity values of up to 107±6mS/cm for gels containing 81% water.

Total water content thresholds for shallow landslides, Outer Western Carpathians

Shallow landslides are fairly frequent natural processes which emerge as a result of both rainfall and rapid snowmelt in the Flysch Belt of the Outer Western Carpathians. We estimated the total water content thresholds for the previously defined seven phases of increased landsliding which took place between 1939 and 2010 around the Napajedla meteorological station. The time series were reconstructed on the basis of data from surrounding stations. Rainfalls with the highest intensities (>1 mm/min) were removed from the set. Rainfall of such an intensity primarily causes overland flow and soil erosion and does not contribute to landslide threshold. The snow water equivalent was computed on the basis of the snow height, and possible errors were evaluated as interval estimations. An interval of 10 days before a landslide phase was selected for the total water content threshold. The resulting lower boundary (67.0 mm/10 days) and upper boundary (163.3 mm/10 days) thresholds of water infiltrated into soil during an event shall be part of the prepared online warning system in this area.

Development and evaluation of an efficient soil-atmosphere model (FHAVeT) based on the Ross fast solution of the Richards equation for bare soil conditions

Abstract. In agricultural management, a good timing in operations, such as irrigation or sowing, is essential to enhance both economical and environmental performance. To improve such timing, predictive software are of particular interest. Optimal decision-software would require process modules which provide robust, efficient and accurate predictions while being based on a minimal amount of parameters easily available. The objective of this study is to assess the accuracy of a physically based model with high efficiency. To this aim, this paper develops a coupled model with climatic forcing based on the Ross fast solution for Richards' equation, heat transfer and detailed surface energy balance. The present study is limited to bare soil, but the impact of vegetation can be easily included. The developed model, FHAVeT (Fast Hydro Atmosphere Vegetation Temperature), is evaluated against the coupled model based on the Philip and De Vries (1957) description, TEC. The two models were compared for different climatic and soil conditions. Moreover, the model allows using various pedotransfer functions. The FHAVeT model showed better performance in regards to mass balance, mostly below 0.002 m, and generally improved computation time. In order to allow for a more precise comparison, six time windows were selected. The study demonstrated that the FHAVeT behaviour is quite similar to the TEC behaviour except under some dry conditions. The ability of the models to detect the occurrence of soil intermediate water content thresholds with a 1 day tolerance was also evaluated. Both models agreed in more than 90% of the cases.

Conservatives and Gamblers: Interpreting plant functional response to water stress in terms of a single indicator

Water availability has a decisive impact on plant growth, survival and distribution. Climate change is expected to alter both the amount and variability of precipitation. To predict and understand plant responses to water stress, efficient and robust mechanisms for describing their functional responses to water availability are needed. However, most ecohydrological processes and models which take into account these responses are inherently complex, difficult to understand and require large amounts of data. We develop a novel straightforward approach and hypothesize that: (1) Plants exhibit two archetypical response patterns under water stress, one typical to slow-growing plants and one typical to fast-growing ones, with most plants being situated between these two. (2) Differences within and between these functional types can be adequately described by a single parameter - the threshold of relative soil water content - at which plants reduce their transpiration in response to water stress. This indicator is straightforward and relies on data which is relatively easy to measure. Its effect has been previously described and it is already used in several models to simulate the effect of water stress on plants. In our approach, we combine this indicator with a description of reactions patterns. This combination provides a general and efficient way of classifying plant responses and allows the assessment of the impact of water stress on a wide variety of plants. Due to its simplicity, our approach offers the opportunity to include water relations of plants in a larger set of models and descriptions than it is possible with more complex ecohydrological descriptions. It also can be used to explain biodiversity in fluctuating environments.

Hydration and Mobility of Interlayer Ions of (Nax, Cay)-Montmorillonite: A Molecular Dynamics Study

In most of natural montmorillonites, Na+ and Ca2+ ions commonly coexist in the interlayer space as compensation ions. Molecular dynamics simulations have been performed to investigate the swelling properties, hydration behaviors, and mobility of the interlayer species of (Nax, Cay)-montmorillonites with different water contents. Nine montmorillonites with different Na+/Ca2+ ratio were selected as model clay frameworks, and the content of interlayer water was set within a range from 0 to 486 mgwater/gclay. The results show that the montmorillonites with coexisting of Na+ and Ca2+ present slightly different swelling curves, hydration energies, and immersion energies from Na- or Ca-montmorillonite. The double-layered hydrates are the thermodynamically stable states for all montmorillonites in the regime of crystalline swelling. A total of 170 mgwater/gclay is found as the threshold water content for the complexing modes of interlayer Ca2+ and Na+ ions switching from inner-sphere complexes to outer-sphere ones. The self-confusion coefficient of interlayer species obviously reveals the confining effects of clay surfaces. In all montmorillonites, the mobility of Na+ is always much greater than that of Ca2+ due to their different hydration shells. According to the water residence time in typical Na+ and Ca2+ hydration complexes, Ca2+ hydration complexes is pronounced more stable than those of Na+, and in montmorillonites with high Ca2+/Na+ ratio, the inhibitory effects of Ca2+ hydration complexes on the mobility of Na+ is clearly revealed.

DC conductivity percolation on drying moistened mesoporous silica SBA-15

DC conductivity percolation on drying to air in water-moistened SBA-15 was characterized. The percolation exponent value, μh≈1, is typical for hydrophilic fumed silica and biological porous materials. Somewhat surprisingly, we found that the percolation water content threshold depends linearly of the initial hydration of dry SBA-15 material. The finding is rationalized within the concept of morphological wetting transition at hydrophilic/phobic structured interface of moistened SBA-15.

Sensitivity analysis of SWAT nitrogen simulations with and without in-stream processes

The Soil and Water Assessment Tool (SWAT) has been widely used to estimate pollutant losses from various agricultural management practices. Although many studies have shown good performance in simulating total nitrogen (TN) and dissolved nitrogen (N), the model performed poorly in many other applications, particularly on dissolved N. Poor performance on dissolved N could be attributed to landscape (in-field) processes and/or in-stream N processes in the model. Therefore, the overall goal of this study was to evaluate SWAT N simulations with in-stream processes and without in-stream processes. Sensitivity analysis results showed that when in-stream processes were not simulated, denitrification threshold water content (SDNCO), N in rainfall (RCN) and N percolation coefficient (NPERCO) were the most sensitive parameters to dissolved N losses. However, when in-stream processes were simulated, the most sensitive parameters changed to initial organic N concentration in soil layers (SOLORGN) and organic N enrichment ratio (ERORGN); and the impact of SDNCO, RCN, and NPERCO was greatly decreased. Furthermore, fertilizer timing and amount had little impact on N simulations. SWAT under-estimated dissolved N, but over-estimated organic N and TN. Further calibration could improve the simulation of dissolved N, but would degrade the simulations of organic N and TN.

The effects of air exposure on the desiccation rate and photosynthetic activity of Thalassia hemprichii and Enhalus acoroides

The effects of air exposure on the desiccation rate and photosynthetic activity of Thalassia hemprichii and Enhalus acoroides were studied using chlorophyll fluorescence. The desiccation rate of T. hemprichii was higher than that of E. acoroides, while the critical threshold of relative water content (RWCcritical) showed opposite trend. Neither the desiccation rate nor RWCcritical of T. hemprichii was significantly affected by temperature, while the desiccation rate of E. acoroides was affected significantly. For both species, the effective photochemical efficiency of PSII decreased gradually above the RWCcritical, while declined rapidly below the RWCcritical. After initial air exposure, the relative maximum electron transport rate of both seagrasses increased gradually to a maximum and then decreased to initial value near the RWCcritical. Therefore, short-term air exposure above the RWCcritical is beneficial for both seagrasses, and the RWCcritical should be considered when examining the response of seagrass photosynthesis to air exposure and selecting transplantation sites.

Threshold water content beyond which hydrophobic soils become hydrophilic: The role of soil texture and organic matter content

Hydrophobicity is a soil water repellency phenomenon, which results in difficulty of soil wetting. The objectives of this study were to determine the hydrophobicity index and water–soil contact angle for different soil classes and to define the threshold water content beyond which hydrophobic soils become hydrophilic. Soil samples were collected from nine soils in southern Brazil, where vegetation was composed mainly of natural grassland species. Undisturbed soil samples were collected in three soil layers (0–5, 5–10 and 10–20cm) for the determination of bulk density, total porosity, macroporosity, microporosity and water retention curve. In these same layers, additional samples were collected for the evaluation of repellency index and soil–water contact angle; while soil sorptivity was analyzed using a micro tension infiltrometer. Among the soils, Hapludert, Dystrudept, Haplaquents and Albaqualf showed high hydrophobicity index and soil–water contact angles in the air-dried medium at three sampled soil layers, ranging from 0 to 20cm depth. In hydrophobic soils, the repellency index and hydrophobicity persistence in the soil decreased with depth, reduction in organic content, and increase in water content. The Haplaquents and Endoaqualfs had greatest repellency index until the volumetric water content equilibrated reached tension of 10kPa. The threshold water content beyond which hydrophobic soils become hydrophilic (θh) varied from 0.36 to 0.57cm3cm−3. The reduction in organic matter content of soil promotes the reduction of the repellency index by reducing the threshold water content beyond which hydrophobic soils become hydrophilic as well as hydrophobicity persistence in the soil. In conclusion, the threshold θh can be very different depending on the soil type and properties (texture and organic matter).

Modulation of bud survival in Populus nigra sprouts in response to water stress-induced embolism

Understanding drought tolerance mechanisms requires knowledge about the induced weakness that leads to tree death. Bud survival is vital to sustain tree growth across seasons. We hypothesized that the hydraulic connection of the bud to stem xylem structures was critical for its survival. During an artificial drastic water stress, we carried out a census of bud metabolic activity of young Populus nigra L. trees by microcalorimetry. We monitored transcript expression of aquaporins (AQPs; plasma membrane intrinsic proteins (PIPs), X intrinsic proteins (XIPs) and tonoplast membrane intrinsic proteins (TIPs)) and measured local water status within the bud and tissues in the bearer shoot node by nuclear magnetic resonance (NMR) imaging. We found that the bud respiration rate was closely correlated with its water content and decreased concomitantly in buds and their surrounding bearer tissues. At the molecular level, we observed a modulation of AQP pattern expressions (PIP, TIP and XIP subfamilies) linked to water movements in living cells. However, AQP functions remain to be investigated. Both the bud and tree died beyond a threshold water content and respiration rate. Nuclear magnetic resonance images provided relevant local information about the various water reservoirs of the stem, their dynamics and their interconnections. Comparison of pith, xylem and cambium tissues revealed that the hydraulic connection between the bud and saturated parenchyma cells around the pith allowed bud desiccation to be delayed. At the tree death date, NMR images showed that the cambium tissues remained largely hydrated. Overall, the respiration rate (Rco2) and a few AQP isoforms were found to be two suitable, complementary criteria to assess the bud metabolic activity and the ability to survive a severe drought spell. Bud moisture content could be a key factor in determining the capacity of poplar to recover from water stress.

Effect of Wetting on the Pullout Resistance of Grouted Soil Nails

In this research, laboratory pullout tests were conducted on grouted soil nails to study the effect of wetting on the interface shear resistance between nail and soil during pullout. Deformed bars with equal size to the true soil nails were used as model nails. The soil used was silty fine sand collected at the site and prepared to a very dense condition. Rainfall infiltration was simulated using duplicated soil nails inundated by water for different periods. Test results indicated that the peak pullout resistance strongly decreases upon wetting, with a reduction of about 60% after soaking for 28 days. However, the experiments showed that there is a threshold water content (or soaking time) beyond which the effect of infiltration on the pullout resistance is reduced. The laboratory protocols developed in this study offered an easy scheme for the prediction of the pullout capacity of a grouted soil nail upon wetting.

Transition from hormonal to nonhormonal regulation as exemplified by seed dormancy release and germination triggering

It is generally believed that seed dormancy release is terminated by germination and that this process is controlled by phytohormones. Most attention was paid to gibberellins (GAs) because treatment with GAs is most frequently applied for seed dormancy breaking. The review characterizes the hormonal regulation of seed dormancy and its release, as exemplified by arabidopsis seeds possessing non-deep physiological dormancy. Dormancy release occurs under the influence of low temperature and/or illumination with red light. Two main trends are typical of this process: (1) a decrease in ABA content and blocking of signal transduction from ABA, and (2) GA synthesis and activation of GA signaling pathway. Dormancy release ends with the GA-induced syntheses of some proteins, enzymes in particular, required for the start of germination. Quiescent seeds are capable of realizing the germination program without hormonal induction, due to nothing but seed hydration. In imbibing seeds, the triggering role of water lies in the successive activation of basic metabolic systems after attaining the water content thresholds characteristic of these systems and in preparing cells of embryo axial organs for germination. Thus, seed dormancy release is controlled by phytohormones, whereas subsequent germination manifesting itself as the initiation of cell elongation in embryo axes is controlled by water inflow.

Investigation on the Structure of Water/AOT/IPM/Alcohols Reverse Micelles by Conductivity, Dynamic Light Scattering, and Small Angle X-ray Scattering

We have systematically investigated the effect of alcohols (ethanol, propanol, butanol, and pentanol) on the structure of the water/AOT/IPM system using conductivity, dynamic light scattering (DLS), and small-angle X-ray scattering (SAXS) techniques. The results show that no percolation phenomenon is observed in the water/AOT/IPM system, whereas the addition of ethanol (propanol and butanol) induces apparently percolation. The threshold water content (W(p)) depends closely on the alcohol type and concentration. The effect of alcohols on the conductance behavior is discussed from the physical properties of alcohols, the interfacial flexibility, and the attractive interactions between droplets. The hydrodynamic diameter of droplets (d(H)) obtained from DLS increases markedly with the increase in water content (W(0)); however, it decreases gradually with increasing alcohol chain length and concentration. SAXS measurements display distinctly the shoulder, the low hump peaks, and the heavy tail phenomenon in the pair distance distribution function p(r) profile, which rely strongly on the alcohol species and its concentration. The gyration radius (R(g)) increases with increasing W(0), and decreases with the increase of alcohol chain length and concentration. Schematic diagram of the conductance mechanism of water/AOT/IPM/alcohol systems is primarily depicted. Three different phases of the discrete droplets, the oligomers, and the isolated ellipsoidal droplets existed in the different W(0) ranges correspond to three different stages in the conductivity-W(0) curve. Coupling the structure characteristics of reverse micelles obtained from DLS and SAXS techniques with conductivity could be greatly helpful to deeply understand the percolation mechanism of water/AOT/IPM/alcohols systems.