Variability Of Soil Water Content Research Articles

The shear strength alteration on clay soil considering the plasticity index and the percentage of fine aggregates in tropical climate regions

Seasonal changes in tropical regions could cause variations of soil water content, especially in clay soil. Furthermore, there is a significance difference in shear strength between wet and dry clay soil which depends on the plasticity index and the percentage of fine aggregates. Laboratory experiments were conducted to investigate the effect of the plasticity index and the percentage of fine aggregates in clay soil on soil shear strength changes due to water content variation. Soil samples were obtained from five locations in Surabaya City, Indonesia. The soil characteristics were tested in a laboratory to obtain the initial water content, Atterberg limits, specific gravity, and soil size distribution. The next stage was drying the soil samples. Each one was stored at room temperature to reach the determined soil weight. The shear strength of the soil in initial and drying conditions was determined by an unconfined compression test. The results indicated that the lower the moisture contents of soil, the greater the shear strength of soil, and vice versa. Soil shear strength changes up to 52 times within the range of 36% to 72% of moisture content The results also show that the value of shear strength variation depends on the plasticity index and the percentage of fine aggregate.

Soil water content and net precipitation spatial variability in an Atlantic forest remnant

This study was carried out in an Atlantic forest remnant in Southeastern Brazil and aimed to spatially model the soil water content (SWC) and net precipitation (NP) on a monthly time scale and to assess the spatial behavior of these hydrological variables in the different seasons. NP is defined by summing throughfall and stemflow, which have been collected after each rain event and accumulated monthly. Soil moisture measurements were carried out monthly up to a depth of 1.00 m and then integrated to obtain the SWC. The exponential semivariogram model was fitted for both hydrological variables, and the goodness-of-fit was assessed by a cross-validation procedure, spatial dependence degree (SDD) and spatial dependence index (SDI). This model provided adequate performance for SWC and NP mapping according to the cross-validation statistics. Based on the SDD, both variables have been classified as a ‘strong spatial dependence structure’. Nevertheless, when the SDI was assessed, NP showed less spatial dependence, while the SWC maintained almost the same performance. Kriging maps pictured the regional climate seasonality due to higher values of both variables in spring and summer than in autumn and winter seasons. However, correlations between NP and SWC are not expressive in the studied period.

Influence of the Spacing of Steam-Injecting Pipes on the Energy Consumption and Soil Temperature Field for Clay-Loam Disinfection

Soil steam disinfection (SSD) technology is one of the effective means to eliminate soil-borne diseases, especially under the condition of clay-loam soil cultivation for facility agriculture in Yangtze River delta (China). With the fine particles, small pores and high density of the soil, the way of steam transport and heat transfer are quite different from those of other cultivation mediums, and when using SSD injection method, the diffusion of steam between pipes will be affected, inhibiting the heat transfer in the dense soil. Therefore, it is necessary to explore the influence of steam pipe spacing (SPS) on the energy consumption and soil temperature (ST) for clay-loam disinfection. The best results are to find a suitable SPS that satisfies the inter-tube steam that can be gathered together evenly without being lost to the air under limited boiler heating capacity. To this purpose, we first used a computational fluid dynamics model to calculate the effective SPS to inject steam into deep soil. Second, the ST, ST rise rate, ST coefficient of variation, and soil water content variation among different treatments (12, 18, 24, or 30 cm pipe spacing) were analysed. Finally, the heating efficiency of all treatments depending on the disinfection time ratio and relative energy consumption was evaluated. The result shows that in the clay-loam unique to Southern China, the elliptical shape of the high-temperature region obtained from the numerical simulation was basically consistent with the experiment results, and the ratios of short diameter to long diameter were 0.65 and 0.63, respectively. In the SPS = 12 and 18 cm treatments, the steam completely diffused at a 0–20 cm soil layer depth, and the heat transfer was convective. However, at an SPS = 12 cm, steam accumulation occurred at the steam pipe holes, causing excessive accumulation of steam heat. The relative energy consumptions for SPS = 30, 24, and 12 cm were above 2.18 kJ/(kg·°C), and the disinfection time ratio was below 0.8. Thus, under a two-pipe flow rate = 4–8 kg/h, the inter-tube steam was found to be completely concentrated with a uniform continuous high temperature distribution within the soil for an appropriate SPS = 18–22 cm, avoiding the unnecessary loss of steam heat, and this method can be considered for static and moving disinfection operations in the cultivated layer (−20–0 cm) of clay loam soil. However, for soil with higher clay contents, the SPS can be appropriately reduced to less than 18 cm. For soil with lower clay contents and higher sand contents, the SPS can be increased to more than 22 cm.

The Stable Isotopic Composition of Different Water Bodies at the Soil–Plant–Atmosphere Continuum (SPAC) of the Western Loess Plateau, China

Understanding the isotopic composition and interrelations of different water bodies at the soil–plant–atmosphere continuum (SPAC) is crucial to reveal the processes and mechanisms of regional water cycles. Rainfall, river water, plant, and soil samples from Lanzhou City, China, were collected from April to October 2016. The hydrogen (δ2H) and oxygen (δ18O) of the local precipitation, river water, soil water, plant xylem water, and leaf water were determined. We found that trees mainly uptake the middle (30–60 cm) and deep (60–100 cm) layer soil water during the growing season, and the shrubs mainly uptake the middle soil water. All herbs uptake the shallow soil water (0–30 cm) during the growing season. The δ18O of shallow soil water was found to be isotopic-enriched because of evaporation and exhibited a decline from the shallow soil layer towards the deeper layer. The variation of δ18O and soil water content (SWC) was remarkable in shallow soil, which was mainly due to evaporation and precipitation infiltration, while water in the middle and deep layer was less affected by these phenomena.

Soil Water Content and Temperature Dynamics under Grassland Degradation: A Multi-Depth Continuous Measurement from the Agricultural Pastoral Ecotone in Northwest China

The agricultural pastoral ecotone (APE) in Northwest China is an ecological transition zone in the arid area with a very fragile ecosystem. In recent years, the ecosystem has deteriorated sharply, and increasing desertification has made the regional ecosystem more vulnerable and sensitive. In this study, we analyzed (using classical statistical methods) spatial and temporal variations in soil water content (SWC) from 14 September 2016 to 22 April 2019 for high and low vegetation in two grassland sites in Yanchi County, Ningxia. The results showed that the largest average seasonal SWC occurred in autumn. The SWC of the first three layers (0 ÷ 15 cm) of the soil profile responded strongly to precipitation, whereas the SWC in deeper soil (30 ÷ 50 cm) could only be recharged markedly after continuous precipitation. Additionally, the growing process of plants proved to be a cause of variability in soil moisture profiles. Vegetation degradation sped up the course of desertification and decreased soil organic carbon content. These changes left the soil increasingly desiccated and enhanced soil variability. Meanwhile, vegetation degradation also prompted changes in soil temperature and shortened the soil’s frozen time in winter. With the acceleration of global warming, if the process of vegetation degeneration continues and soil temperatures keep rising, the ecosystem is likely to undergo irreversible degradation.

Development of a FBG water content sensor adopting FDM method and its application in field drying-wetting monitoring test

Fused deposition modelling (FDM) has been widely used in the field of sensor production, however, sensor fabricated adopting FDM in monitoring water pressure is seldomly discussed. In this study, one Fiber Bragg Grating (FBG) water content sensor has been designed and fabricated in order to monitor the variation of water pressure at the field. It’s worth noting that the advanced material, Polylactic Acid (PLA), has been used in fabricating sensor with favorable stability. After the fabrication of FBG sensor, calibration test was conducted by controlling and measuring water content of soil sample in a sealed chamber through manipulating pressure volume controller. It has been validated that this pressure sensor adopting FDM fabrication process has quick response and high sensitivity. Afterwards, field application of this type of sensors have been used in measuring the variation of soil water content continuously, while reading from different depth of this sensor can be compared and analyzed to summarize the influence of nature rainfall condition about the ground. To validate and verify the reliability and authenticity of developed FBG sensors, the conventional sensors have been implanted at the same depth of FBG sensors and comparison have been conducted in the entire monitoring period. It has been verified that FBG sensors have good performance when embedded into the melted PLA material, while its application in measuring soil water pressure is more sensitive and accurate.

Mesoscale spatial variability of soil‐water content in an alpine meadow on the northern Tibetan Plateau

AbstractSoil water is an important limiting factor for restoring alpine meadows on the northern Tibetan Plateau. Field studies of soil‐water content (SWC), however, are rare due to the harsh environment, especially in a mesoscale alpine‐meadow ecosystem. The objective of this study was to assess the spatial variability of SWC and the temporal variation of the spatial variability in a typical alpine meadow using a geostatistical approach. SWC was measured using a neutron probe to a depth of 50 cm at 113 locations on 22 sampling occasions in a 33.5‐hm2 alpine meadow during the 2015 and 2016 growing seasons. Mean SWC in the study plot for the two growing seasons was 18.7, 14.0, 13.9, 14.3, and 14.8% for depths of 10, 20, 30, 40, and 50 cm, respectively, and SWC was significantly larger at 10 cm than at other depths. SWC was negatively correlated with its spatial variability, and the spatial variability was higher when SWC was lower. Thirty‐three sampling locations in this study plot met the requirement of accuracy of the central limit theorem. A Gaussian model was the best fit for SWC semivariance at depths of 10, 20, and 30 cm, and the spatial structural ratio was between 0.997 and 1, indicating a strong spatial dependence of SWC. The sill and range fluctuated temporally, and the nugget and spatial structural ratio did not generally vary with time. The sill was significantly positively correlated with SWC and was initially stable and then tend to increase with SWC. The nugget, range, and spatial structure ratio, however, were not correlated with SWC. These results contribute to our understanding of SWC spatial distribution and variation in alpine meadows and provide basic empirical SWC data for mesoscale model simulations, optimizing sampling strategies and managing meadows on the Tibetan Plateau.

Soil water dynamics, water productivity and radiation use efficiency of maize under multi-year conservation agriculture during contrasting rainfall events

Increasing resource degradation (soil) and resource scarcity (water) in Indo Gangetic Planes (IGP) had popularized the concept of conservation agriculture (CA). CA practices can alter the soil hydraulic properties by improving soil physical environment and hence water productivity (WP). Since CA practices are highly site-specific in nature, its worthiness under different cropping systems, soil types and agro-climatic zones must be studied. In the context of above-mentioned scenario of crop management practices, Hydrus-2D model was used to evaluate most suitable CA-based practice for north western (NW) IGP to understand the process of water transport. The experiment had 3-tillage and crop establishment practices [ZT-zero tillage; PB-permanent raised bed and CT-conventional tillage] in main plot and 4-cropping systems [MWMb-maize-wheat-mungbean; MCS-maize-chickpea-Sesbania; MMuMb-maize-mustard-mungbean and MMS-maize-maize-Sesbania] in sub-plot. The validation of Hydrus-2D showed that nRMSE was observed between 0.19-0.25 and R2 value between 0.80-0.87 at P<0.01. This indicates model is performing efficiently. The R2 value (0.87) was observed highest and RMSE (0.03) and nRMSE (0.15) was observed least in the CA-based PB plots. CA-based plots had 23%–37% less evaporation than CT plots. Cumulative root water uptake (CRWU) under PB was 11% and 15% lower, whereas CRWU under ZT was 20% and 30% higher than CT plots in 2012 and 2013, respectively. Cumulative deep drainage (CDD) in 2013 in PB was higher by 3.64 and 5.88 cm than ZT and CT, respectively. With 10-cm higher rainfall during 2013, variability in the soil water content (SWC) was increased by 22% in CT and 2% in ZT plots. PB had 14–35% and 30–36% higher water productivity than ZT and CT plots, respectively. Radiation use efficiency (RUE) for dry-matter was observed higher under CA based plots. However, RUE for grain yield was observed similar among CA and CT plots during 2012, but in 2013, ZT outperformed than CT plots. The outcome of this study is, the higher rainfall during 2013 mainly contributed to more CDD. Highly variable SWC was observed in the CT plots. With lesser deep drainage in ZT, root water uptake (RWU) was higher and WP was lower, whereas PB plots had comparatively less-moist soil profile and per unit of RWU + evaporation, a proportionately higher dry-matter was produced. In a well-drained soil profile of PB plots frequent and light irrigation may be advocated, whereas in ZT with better profile storage and lesser variability of SWC during both the year irrigation may be required less frequently. Therefore, adoption of CA-based practices leads to higher crop productivity and resource use efficiency, because of favorable soil hydrological propertywith less fluctuation in soil water content in maize under diversified cropping systems of IGP of south Asia.

Stable water and fertilizer supply by negative pressure irrigation improve tomato production and soil bacterial communities

The objective of this study is intended to explore the effect of maintain high efficiency water and fertilizer supply for improving the tomato production and soil bacterial communities by negative pressure irrigation (NPI) in greenhouse. The pot culture experiments consisted of three irrigation treatments: conventional irrigation (CI), drip irrigation (DI) and NPI, which have been conducted in greenhouse for two consecutive years. The study found that NPI reduced water consumption remained stable with slight variations on dynamic soil water content, as well as increased the water use efficiencies compared with CI and DI, respectively. The NPI reduced water consumption by 20% and 18% compared to CI in 2016 and 2017, respectively. Under NPI treatment, the variations in soil water content were stabilized, within the range of 12.3–14.7% in the two years, which was smaller than that of CI and DI treatments of 8.6–15.6%. Moreover, the tomato yields, fruit quality, uptake quantitates of nutrition and soil rhizosphere microbial diversity were increased in the NPI treatment comparison with the CI and DI treatments. With NPI, the yield of tomato was significantly increased by 26% and 8, 41 and 17% compared to that under CI and DI treatments in 2016 and 2017, respectively. NPI was also considered to benefit plant growth by increasing the abundances of Bacillus, Streptomyces and Pseudolabrys at the genus level. Therefore, NPI should be utilized as an advanced technique for saving water and producing high yields in future sustainable agriculture.

Transpiration dynamics and water sources for selected indigenous trees under varying soil water content

The major route through which water from the earth’s surface re-enters the hydrologic cycle in forested ecosystems is via tree transpiration (T). It is therefore important to have detailed understanding of the quantity and source of water transpired by different tree species. The aims of this study were to i) assess the trends of T for selected tree species (camel thorn, sweet thorn, shepherd’s tree and buffalo thorn) across a range of soil water content conditions and ii) partition the total T of the selected tree species growing in arid environments dominated by open cast mining activities into soil water and groundwater. Tree T was measured using the compensation heat pulse velocity (CHPV) method, while soil water content was monitored using DFM capacitance probes. The soil water content within the upper 50 cm soil profile ranged from 11 mm during the dry season to 20 mm during the wet season. The deeper soil layer (50–120 cm) was generally wetter compared to the top layer with water content was up to >30 mm during the wet season. The measured tree T ranged from 0.2 mm day−1 on buffalo thorn during the dry season to 1.9 mm day−1 on sheperd’s tree in summer. It was also revealed that T of large (diameter at breast height =46 cm) camel thorn trees is not responsive to seasonal variations of soil water availability and remained constant at approximately 1.2 mm day−1. Diurnal patterns of T did not effect changes on the soil water depletions within the top 120 cm soil profile, which indicated that the trees sourced water beyond this zone. Signs of daytime redistribution were observed within the canopy areas of the investigated trees during very limited soil water conditions of the dry season. It was concluded that the water use of trees is inclined to the seasonal variations, which however is not the case in old trees. Close to 100% of the water transpired by trees in the study area is sourced below 1.2 m (vadose zone and water table). We recommended investigation of daytime redistribution among the indigenous tree species of the study area. We also recommended extension of tree water use studies to other species for comprehensive catchment tree water use calculations to inform water budgets.

Methane oxidation and diffusivity in mollisols under an urban forest in Argentina

Methane (CH4) oxidation in aerated soils is regulated mainly by physical factors which limit CH4 diffusion into the soil. Using the static chamber technique, we measured CH4 flux at the soil-atmosphere interface (SAI) and estimated its diffusivity (DsCH4) in a plantation of mixed deciduous species during a year. Methane diffusivity (DsCH4) was determined through two in situ techniques (Fick's Law -FL- and SF6 methods) and a particular soil parameter model (SP method). Mean CH4 oxidation was 15.61 ± 6.39 ng CH4/m2/s and varied significantly among seasons, coinciding with variation in soil water content (W%) (R = −0.8697, p = 5 × 10−4). CH4 diffusivity presented mean values of 0.0151 ± 0.0085 cm2/s and 0.0173 ± 0.0070 cm2/s for SP and FL methods respectively, and 0.0222 ± 0.0093 cm2/s for SF6 method, with a statistically significant difference from the other two (p < 0.05). For the three methods, temporal variation in CH4 diffusion exhibited the same response as CH4 oxidation and a good negative linear correlation was found between W% and DsCH4 (p < 0.01). Results suggest that Fick's Law could be used for a first estimation of CH4 oxidation and that, in this temperate region, SP is the most appropriate method to obtain a first approximate measure of DsCH4 (R = 0.83, p = 0.001).

The behavior of wetting front on slopes with different slope morphologies during rainfall

The aim of this research is to investigate the influence of slope morphology on the wetting front arrival during rainfall. Temporal variations in soil water content at different times and depths are measured in a slope gully and in a non-gullied slope. The arrival time of wetting front (Ts) and the time when soil shear strength reaches its lowest level during rainfall (Te) at different depths and at different elevations in the slopes and in rainfall events with different intensities are collected in the study and are used to represent the hydrological behavior in a slope. Ts and Te values noticeably decrease with depth in the slope gully lower elevation during moderate intensity rainfall, whereas these values increase with depth in a non-gullied slope. Rainfall characteristics considerably affect the behavior of Ts and Te values in the slope gully. Ts values vary slightly along depths during extremely high intensity rainfall compared with that of moderate rainfall intensity. Slope morphology and rainfall characteristics considerably affect the hydrological behavior in a slope. Subsurface lateral flow in a slope gully during rainfall is more conspicuous than in a non-gullied slope due to its topographic features. In a landslide prone area, soil instability in a slope gully may occur faster than in a non-gullied slope during severe rainfall activity.

Underestimation of denitrification rates from field application of the &amp;lt;sup&amp;gt;15&amp;lt;/sup&amp;gt;N gas flux method and its correction by gas diffusion modelling

Abstract. Common methods for measuring soil denitrification in situ include monitoring the accumulation of 15N-labelled N2 and N2O evolved from 15N-labelled soil nitrate pool in closed chambers that are placed on the soil surface. Gas diffusion is considered to be the main transport process in the soil. Because accumulation of gases within the chamber decreases concentration gradients between soil and the chamber over time, the surface efflux of gases decreases as well, and gas production rates are underestimated if calculated from chamber concentrations without consideration of this mechanism. Moreover, concentration gradients to the non-labelled subsoil exist, inevitably causing downward diffusion of 15N-labelled denitrification products. A numerical 3-D model for simulating gas diffusion in soil was used in order to determine the significance of this source of error. Results show that subsoil diffusion of 15N-labelled N2 and N2O – and thus potential underestimation of denitrification derived from chamber fluxes – increases with chamber deployment time as well as with increasing soil gas diffusivity. Simulations based on the range of typical soil gas diffusivities of unsaturated soils showed that the fraction of N2 and N2O evolved from 15N-labelled NO3- that is not emitted at the soil surface during 1 h chamber closing is always significant, with values up to &gt;50 % of total production. This is due to accumulation in the pore space of the 15N-labelled soil and diffusive flux to the unlabelled subsoil. Empirical coefficients to calculate denitrification from surface fluxes were derived by modelling multiple scenarios with varying soil water content. Modelling several theoretical experimental set-ups showed that the fraction of produced gases that are retained in soil can be lowered by lowering the depth of 15N labelling and/or increasing the length of the confining cylinder. Field experiments with arable silt loam soil for measuring denitrification with the 15N gas flux method were conducted to obtain direct evidence for the incomplete surface emission of gaseous denitrification products. We compared surface fluxes of 15N2 and 15N2O from 15N-labelled micro-plots confined by cylinders using the closed-chamber method with cylinders open or closed at the bottom, finding 37 % higher surface fluxes with the bottom closed. Modelling fluxes of this experiment confirmed this effect, however with a higher increase in surface flux of 89 %. From our model and experimental results we conclude that field surface fluxes of 15N-labelled N2 and N2O severely underestimate denitrification rates if calculated from chamber accumulation only. The extent of this underestimation increases with closure time. Underestimation also occurs during laboratory incubations in closed systems due to pore space accumulation of 15N-labelled N2 and N2O. Due to this bias in past denitrification measurements, denitrification in soils might be more relevant than assumed to date. Corrected denitrification rates can be obtained by estimating subsurface flux and storage with our model. The observed deviation between experimental and modelled subsurface flux revealed the need for refined model evaluation, which must include assessment of the spatial variability in diffusivity and production and the spatial dimension of the chamber.

Unraveling location-specific and time-dependent interactions between soil water content and environmental factors in cropped sandy soils using Sentinel-1 and moisture probes

Advances in sensor technologies provide the potential for real-time monitoring of soil moisture status. Little work has been conducted to study the spatial and temporal variations of soil water content at the field scale and the interactions between soil water content and environmental controlling factors in cropped fields during the growing season. In this study, we assessed the spatial and temporal variations of surface soil water content (0–0.1 m) using seven in-situ soil moisture probes and Sentinel-1 satellite data over the 2016 and 2017 cropping seasons in irrigated fields of the Central Sands Plains, Wisconsin, USA. An empirical multiple linear regression model was developed between the soil water content with Sentinel-1 backscatter data. The model was evaluated with leave-one-group-out cross-validation, which showed an R2 of 0.60, mean error of –0.00 m3 m−3, root mean square error of 0.02 m3 m−3, and Lin’s concordance correlation coefficient of 0.75. Soil volumetric water content (VWC) and soil water deficit (SWD) were mapped at 30-m resolution every 6–12 days across one field (DU field at the Wallendal farm). Using two statistical metrics (“first metric” and “last metric”) and a bootstrapping approach, the location-specific and time-dependent interactions between estimated soil water content and environmental controlling factors across the DU field during the cropping season were evaluated, and it was found that such interactions were affected by initial soil water content and plant stress status. The robustness of the model needs to be improved in the future by collecting more VWC measurements in different soil and crop conditions. The method has the potential to provide a baseline soil moisture map and can be coupled with mechanistic models and subsoil VWC measurements for improved irrigation management and water use efficiency under a changing climate.

Time-lapse gravity and levelling surveys reveal mass loss and ongoing subsidence in the urban subrosion-prone area of Bad Frankenhausen, Germany

Abstract. We present results of sophisticated, high-precision time-lapse gravity monitoring that was conducted over 4 years in Bad Frankenhausen (Germany). To our knowledge, this is the first successful attempt to monitor subrosion-induced mass changes in urban areas with repeated gravimetry. The method provides an approach to estimate the mass of dissolved rocks in the subsurface. Subrosion, i.e. leaching and transfer of soluble rocks, occurs worldwide. Mainly in urban areas, any resulting ground subsidence can cause severe damage, especially if catastrophic events, i.e. collapse sinkholes, occur. Monitoring strategies typically make use of established geodetic methods, such as levelling, and therefore focus on the associated deformation processes. In this study, we combine levelling and highly precise time-lapse gravity observations. Our investigation area is the urban area of Bad Frankenhausen in central Germany, which is prone to subrosion, as many subsidence and sinkhole features on the surface reveal. The city and the surrounding areas are underlain by soluble Permian deposits, which are continuously dissolved by meteoric water and groundwater in a strongly fractured environment. Between 2014 and 2018, a total of 17 high-precision time-lapse gravimetry and 18 levelling campaigns were carried out in quarterly intervals within a local monitoring network. This network covers historical sinkhole areas but also areas that are considered to be stable. Our results reveal ongoing subsidence of up to 30.4 mm a−1 locally, with distinct spatiotemporal variations. Furthermore, we observe a significant time-variable gravity decrease on the order of 8 µGal over 4 years at several measurement points. In the processing workflow, after the application of all required corrections and least squares adjustment to our gravity observations, a significant effect of varying soil water content on the adjusted gravity differences was figured out. Therefore, we place special focus on the correlation of these observations and the correction of the adjusted gravity differences for soil water variations using the Global Land Data Assimilation System (GLDAS) Noah model to separate these effects from subrosion-induced gravity changes. Our investigations demonstrate the feasibility of high-precision time-lapse gravity monitoring in urban areas for sinkhole investigations. Although the observed rates of gravity decrease of 1–2 µGal a−1 are small, we suggest that it is significantly associated with subterranean mass loss due to subrosion processes. We discuss limitations and implications of our approach, as well as give a first quantitative estimation of mass transfer at different depths and for different densities of dissolved rocks.

Tracing the soil water response to autumn rainfall in different land uses at multi-day timescale in a subtropical zone

Identifying the soil water response processes to rainfall events can provide scientific guidance to sustain and enhance ecosystem services of the terrestrial ecosystem. This study presents the results of soil water content and isotopic compositions response to a typical rainfall in the subtropical zone in east China. The response of soil water (including soil water content, hydrogen and oxygen isotope ratios) to rainfall event at three soil layers (0–10, 10–20, 20–30 cm) in two croplands, two orchards (tea and peach), and two forests were analyzed. The results showed that soil water recharge processes were varied in different land uses. The variation in the isotopic composition of soil water showed a hysteresis pattern, and this pattern was affected by land use types. The extents of isotopes depletion in topsoil (0–10 cm) were along the sequence of forest (9.90%) < orchard (10.86%) < cropland (13.70%), in contrast to the reduction of soil water content (17.46% in cropland, 32.06% in forest, and 41.78% in orchard). Little depletion in isotopes indicated limited evaporation effect on soil water, and the corresponding large reduction in water content suggested large infiltration or water recharge into deep soil, these implying a good water buffer for forest soils compared with crop soils. Soil texture and antecedent soil water were the main factors influencing the variations of both soil water content and isotopic compositions. The identification of different soil water changing processes may be important for water management and plant-available water evaluation.

Soil Salinity Resistance Effect on Evaporation

Soil salinity is an increasing threat to agriculture and is a major factor in reducing plant productivity in arid and semi-arid regions. Knowledge of the transport process of mass and energy in subsurface soil is vital for understanding the environmental and economic impact of agricultural practices in these areas. A finite difference model, WASH_1D, was applied to evaluate various transport mechanisms associated with temporal variations in soil water content and soil salinity in 5 cm sandy soil columns. Experiment was conducted in a laboratory with fixed temperature and relative humidity. Soil columns were initially saturated with distilled water or 3000 ppm NaCl solution. The accuracy of the model was evaluated using measured soil water content and soil salinity at seven depths 0.2, 0.6, 1.2, 1.7, 2.4, 3.4 and 4.4 cm at the end of the experiment. Simulated cumulative evaporation was compared with the data measured with an electronic balance. Results showed that the liquid flux of distilled water treatment was sharply declined sooner than NaCl solution treatment which was associated with decreasing in moisture diffusivity. The contribution of the water vapor flux to the total moisture flux was 0.01 and 0.2% for distilled water and solution treatments, respectively. The RMSE values between simulated and measured cumulative evaporation lines were 0.77 and 1.08 (g) for the distilled water and solution treatments, respectively. It was observed that the solution treatment significantly reduced total cumulative evaporation up to 11% as compared to the distilled water treatment. Simulated results demonstrated that the model predicted soil water content and salinity with adequate accuracy.

Spatio‐temporal heterogeneity in soil water stable isotopic composition and its ecohydrologic implications in semiarid ecosystems

AbstractSpatio‐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.

Spatial distribution of water-active soil layer along the south-north transect in the Loess Plateau of China

Soil water is an important composition of water recycle in the soil-plant-atmosphere continuum. However, intense water exchange between soil-plant and soil-atmosphere interfaces only occurs in a certain layer of the soil profile. For deep insight into water active layer (WAL, defined as the soil layer with a coefficient of variation in soil water content >10% in a given time domain) in the Loess Plateau of China, we measured soil water content (SWC) in the 0.0–5.0 m soil profile from 86 sampling sites along an approximately 860-km long south-north transect during the period 2013–2016. Moreover, a dataset contained four climatic factors (mean annual precipitation, mean annual evaporation, annual mean temperature and mean annual dryness index) and five local factors (altitude, slope gradient, land use, clay content and soil organic carbon) of each sampling site was obtained. In this study, three WAL indices (WALT (the thickness of WAL), WAL-CV (the mean coefficient of variation in SWC within WAL) and WALSWC (the mean SWC within WAL)) were used to evaluate the characteristics of WAL. The results showed that with increasing latitude, WAL-T and WAL-CV increased firstly and then decreased. WAL-SWC showed an opposite distribution pattern along the south-north transect compared with WAL-T and WAL-CV. Average WAL-T of the transect was 2.0 m, suggesting intense soil water exchange in the 0.0–2.0 m soil layer in the study area. Soil water exchange was deeper and more intense in the middle region than in the southern and northern regions, with the values of WAL-CV and WAL-T being 27.3% and 4.3 m in the middle region, respectively. Both climatic (10.1%) and local (4.9%) factors influenced the indices of WAL, with climatic factors having a more dominant effect. Compared with multiple linear regressions, pedotransfer functions (PTFs) from artificial neural network can better estimate the WAL indices. PTFs developed by artificial neural network respectively explained 86%, 81% and 64% of the total variations in WAL-T, WAL-SWC and WAL-CV. Knowledge of WAL is crucial for understanding the regional water budget and evaluating the stable soil water reserve, regional water characteristics and eco-hydrological processes in the Loess Plateau of China.

Interactions of soil moisture and plant community properties in meadows restored from abandoned farmlands on the Sanjiang Plain, China

Soil moisture is a major driving force of plant community succession in restored meadows. Existing studies mainly focus on diversity-productivity relationships. However, studies which determine the effects of soil moisture on the plant community properties in restored meadows are lacking. In this study, we conducted a chronosequence analysis of the interactions between soil water content variation and plant community properties in meadows following passive restoration (3-, 5-, 9-, 14-, 17-, 21-year restoration) of abandoned farmlands on the Sanjiang Plain, China. Results showed that the plant community was characterized by ruderal plants in the initial year of succession, and then by perennial plants such as Calamagrostis angustifolia and Carex spp. in older restored meadows. Similarity of restored community to target site increased across succession time whereas species diversity gradually decreased. Plant height, coverage and biomass increased with restoration time, with plant density being the exception. The community height, coverage and root/shoot ratio were positively related to the water content in the surface soil layer (0–10 cm). Conversely, plant density was significantly and negatively related with soil moisture at 0–10 cm soil depth. Plant diversity (Shannon index, Richness index and evenness) was closely correlated to soil water content at the soil depth of 0–10 cm. Our findings indicate that vegetation of cultivated meadows could be effectively restored by passive restoration. Change of plant species diversity is an especially important response to hydrological recovery in restored meadows on the Sanjiang Plain.