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"Soil water content"
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Effects of irrigation amount and frequency on soil water and salt dynamics and water use efficiency of mulched drip-irrigated cotton in Southern Xinjiang
【Objective】Cotton is a major economic crop in saline-alkali regions of southern Xinjiang, where soil salinity and water availability affect crop growth and yield. Optimizing irrigation is essential for improving water use efficiency (WUE) and maintaining stable production of cotton in these regions. This paper experimentally studies suitable drip-irrigation strategies for cotton cultivated in saline-alkali soil with plastic film mulch.【Method】A field experiment was conducted from April to August 2018 to evaluate the influence of plant row spacing and irrigation methods on cotton growth. There were four irrigation-amount treatments by irrigating 45, 37.5, 30 mm and 22.5 mm of water in each irrigation; the irrigation interval was 10 and 7 days. In the experiment, we measured soil water and salt content in the root zone, growth and WUE of the cotton. 【Result】During the first 70 days after sowing, water content in the 0-40 cm soil layer decreased with decreasing irrigation amount. At the seedling stage, soil moisture difference between wide and narrow plant row spacing was negligible, while by the bud stage, water content difference in soil below 40 cm between different row spacings became increasingly pronounced. Soil salt content in the top 0-40 cm soil layer showed minimal variation among treatments during the seedling stage, but significant differences emerged after flowering began. Salt content in the 60-100 cm soil layer steadily declined in the first 80 days post-sowing and then stabilized thereafter. Irrigation 45 mm every 10 days during the bud stage followed by 30 mm every 7 days during the flowering stage achieved the highest WUE of 1.42 kg/m3, with a seed cotton yield of (6 916.2 ± 338.6) kg/hm2. Irrigating 45 mm every 10 days during the bud stage and 37.5 mm every 7 days during the flowering stage yielded the highest seed cotton production of (7 703.9 ± 641.9) kg/hm2, with a WUE of 1.37 kg/m3. 【Conclusion】For cotton cultivated with drip irrigation under plastic film mulch in saline-alkali soils, the optimal irrigation strategy is irrigating 45 mm every 10 days during the bud stage followed by 37.5 mm every 7 days during the flowering stage. This can effectively balance soil moisture and salinity dynamics, while maximizing water use efficiency and seed cotton yield.
Journal Article
Effect evaluation of grass on shallow stability of unsaturated volcanic soil slope in seasonal cold region
This study investigates effects of grass on hydro-thermal-mechanical properties and shallow stability of unsaturated soil slope. A field measurement consisted of two cut slopes, namely bare slope and grassed slope, was carried out. Field measurement results show that grass has effects on reducing the volumetric water content and increasing matric suction. There is a greater gap in matric suction between grassed soil slope and bare soil slope when the snow appears. Besides, the soil temperature in grassed slope is lower than bare slope in warm seasons. However, contrasted result of the soil temperature is recorded in winter. The triaxial tests were conducted for studying mechanical properties of grassed saturated soil with different grass ages. Experimental results reveal that there is an increase in peak shear strength and effective cohesion of grassed soil as the grass age increases, whereas a slightly higher effective angle of internal friction is observed. Furthermore, the volumetric strain decreased with the rise in grass root volume ratio. The approach of coupled nonisothermal-seepage numerical analysis for unsaturated soil slope considering impacts of grass is suggested. It is followed by slope stability analysis considering enhanced shear strength of soil due to grass roots. The good agreement regarding soil temperature and volumetric water content between simulation and field measurement indicates that the proposed approach is feasible to consider the influences of grass on the hydro-thermal behaviors of unsaturated soil slope. Furthermore, the higher values of factor of safety (FOS) of grassed slope present that the grass is effective in resisting the shallow landslide-prone area.
Journal Article
Recent Advances in Dielectric Properties-Based Soil Water Content Measurements
Dielectric properties are crucial in understanding the behavior of water within soil, particularly the soil water content (SWC), as they measure a material’s ability to store an electric charge and are influenced by water and other minerals in the soil. However, a comprehensive review paper is needed that synthesizes the latest developments in this field, identifies the key challenges and limitations, and outlines future research directions. In addition, various factors, such as soil salinity, temperature, texture, probing space, installation gap, density, clay content, sampling volume, and environmental factors, influence the measurement of the dielectric permittivity of the soil. Therefore, this review aims to address the research gap by critically analyzing the current state-of-the-art dielectric properties-based methods for SWC measurements. The motivation for this review is the increasing importance of precise SWC data for various applications such as agriculture, environmental monitoring, and hydrological studies. We examine time domain reflectometry (TDR), frequency domain reflectometry (FDR), ground-penetrating radar (GPR), remote sensing (RS), and capacitance, which are accurate and cost-effective, enabling real-time water resource management and soil health understanding through measuring the travel time of electromagnetic waves in soil and the reflection coefficient of these waves. SWC can be estimated using various approaches, such as TDR, FDR, GPR, and microwave-based techniques. These methods are made possible by increasing the dielectric permittivity and loss factor with SWC. The available dielectric properties are further synthesized on the basis of mathematical models relating apparent permittivity to water content, providing an updated understanding of their development, applications, and monitoring. It also analyzes recent mathematical calibration models, applications, algorithms, challenges, and trends in dielectric permittivity methods for estimating SWC. By consolidating recent advances and highlighting the remaining challenges, this review article aims to guide researchers and practitioners toward more effective strategies for SWC measurements.
Journal Article
Assessment of the Temperature Effects in SMAP Satellite Soil Moisture Products in Oklahoma
Soil moisture is a notably important component in various studies in water sciences, including hydrology, agriculture, and water management. To achieve extensive or global spatial coverage, satellites focusing on soil moisture observation have been launched, and many satellite products, such as SMAP and SMOS soil moisture products, have been provided. Most of these satellite observations are based on the dielectric properties of wet soil, and most soil moisture retrieval algorithms are calibrated or evaluated using in situ soil moisture. While the in situ data observed by dielectric sensors, which are the most widely used, are reported to include errors caused by the so-called “temperature effects” of these sensors, the temperature dependency of bulk soil dielectric constant has rarely been discussed on satellite data. Since both in situ dielectric measurements and satellite observations are based on the same physical variable, the dielectric constant and the dielectrically measured in situ soil moisture data are also used as ground truth, it is necessary to assess the impact of temperature effects on satellite products. In this work, we attempted to identify the existence of the temperature effects and evaluate the consequences of removing these effects on in situ and satellite soil moisture and the relationships between the brightness temperature at the soil surface and the brightness temperature provided by satellite observation. To achieve the goals of this study, we analyzed the temperature effects on surface soil moisture data provided by a SMAP mission in Oklahoma, the United States. The results show that temperature effects exist in SMAP soil moisture products in Oklahoma, and the removal of these effects will potentially improve the accuracy of these products.
Journal Article
Drought differentially affects autotrophic and heterotrophic soil respiration rates and their temperature sensitivity
Climate change predictions indicate that extreme drought is likely to become more frequent in the future. In this study, the impact of drought on soil respiration (Rs) and its autotrophic (Ra) and heterotrophic components (Rh) were studied in a cultivated Black Chernozemic soil in central Alberta, Canada. The mean Rs was 24.4% lower in the drought relative to the plots with ambient precipitation (P < 0.001), with a larger decrease in Ra (26.8%) relative to Rh (21.0%), and a higher (P < 0.05) contribution of Rh to Rs under drought (52.8%) than under the ambient condition (47.7%). Both Rs and its Ra and Rh components had an exponential relationship with soil temperature and a quadratic relationship with soil water content. Drought caused a greater decrease in the tipping point of soil water content for Rh (a 39.6% reduction) than for Ra (a 15.1% reduction) relative to the ambient precipitation treatment. In addition, drought resulted in a greater increase in the temperature sensitivity (Q10 values) of Ra (a 45.0% increase) than that of Rh (a 14.1% increase) relative to the ambient precipitation treatment. The results suggest that drought amplified the water limitation effect on CO2 emission, especially that from microbial respiration, and resulted in a tighter relationship between temperature and root or autotrophic respiration, based on this 1-year study. We conclude that it is important to assess the impact of drought on soil respiration components rather than the total soil respiration, and such differential effects of drought on soil respiration components should be incorporated into global carbon circulation models.
Journal Article
Temporal Variation and Hysteresis of Soil Respiration and Sap Flow of Pinus densiflora in a Cool Temperate Forest, Japan
Soil respiration (Rs) consists of autotrophic (Ra) and heterotrophic (Rh) respiration, and the metabolic responses of Ra and Rh are supposedly affected by environmental factors. Our hypothesis was that the contribution of Ra and Rh would be affected by seasons. To characterize seasonal patterns of Rs, sap flow (as an indicator of photosynthetic activity), and environmental factors, we continuously measured temporal variation in Rs using an automated opening and closing chamber system in a cool temperate forest. Rs had counterclockwise hysteresis (Rs: spring < autumn) with soil temperature at a depth of 5 cm. Daily maximum Rs had a significant positive relationship with daily maximum sap flow over three seasons, and daily maximum sap flow was lower in autumn than in spring (Ra: spring > autumn). The amount of leaf litterfall increased significantly from August to October; the high Rs in autumn would be due to an increase in Rh (Rh: spring < autumn). These results suggest that Ra contributes more in spring than in autumn, and the contribution of Rh is high from summer through autumn.
Journal Article
Estimation of the Soil Water Content Using the Early Time Signal of Ground-Penetrating Radar in Heterogeneous Soil
Ground-penetrating radar (GPR) is an important tool for measuring soil water content (SWC) at the field scale. The amplitude analysis of the early time signal (ETS) of GPR may provide a rapid way to estimate SWC. By assuming a homogeneous medium, various studies have been conducted on the relationship between the amplitude of ETS and the topsoil layer’s electromagnetic parameters (dielectric permittivity and conductivity) through numerical simulations, laboratory experiments, and field experiments. Soil is a typical inhomogeneous medium, and soil cultivation is a factor affecting its heterogeneity. In this context, we discuss the ability of the amplitude of ETS to estimate the water content of heterogeneous soil. First, we establish a multi-scale stochastic medium model with the inhomogeneous distribution of dielectric permittivity and conductivity and simulate the GPR response by the finite-difference time-domain (FDTD) method to observe the influence of medium heterogeneity on the GPR response. The heterogeneity of the soil models is evaluated by a geostatistical analysis described by two parameters, correlation length and variability. Then, we analyze the relationship between variability and the average envelope amplitude (AEA) of ETS. A strong soil heterogeneity increases the error of the AEA method in estimating SWC. Finally, the AEA method is used to estimate the SWC of two adjacent fields with different heterogeneities, which were caused by different cultivation methods. The results of the numerical simulation and field experiment indicate that the soil heterogeneity can have an impact on the estimation of SWC using EST, with an error lower than 3% within a depth range of 1/2 λ to λ (wavelength). This suggests that the EST of GPR can be applied to soil layers with relatively large lateral changes in water content.
Journal Article
Spatiotemporal Variations in Near-Surface Soil Water Content across Agroecological Regions of Mainland India: 1979–2022 (44 Years)
This study was undertaken to address how near-surface soil water content (SWC) patterns have varied across diverse agroecological regions (AERs) of mainland India from 1979 to 2022 (44 years) and how these variations relate to environmental factors. Grid-wise trend analysis using the Mann–Kendall (MK) trend test and Sen’s slope was conducted to determine the trends and their magnitudes. Additionally, we used Spearman’s rank correlation (ρ) to explore the relationships of ESA CCI’s near-surface SWC data with key environmental variables, including rainfall, temperature, actual evapotranspiration, and the normalized difference vegetation index (NDVI). The results revealed significant variations in SWC patterns and trends across different AERs and months. The MK trend test indicated that 17.96% of the area exhibited a significantly increasing trend (p < 0.1), while7.6% showed a significantly decreasing trend, with an average annual Sen’s slope of 0.9 × 10−4 m3 m−3 year−1 for mainland India. Areas with the highest decreasing trends were AER-16 (warm per-humid with brown and red hill soils), AER-15 (hot subhumid to humid with alluvium-derived soils), and AER-17 (warm per-humid with red and lateritic soils). In contrast, increasing trends were the most prominent in AER-5 (hot semi-arid with medium and deep black soils), AER-6 (hot semi-arid with shallow and medium black soils), and AER-19 (hot humid per-humid with red, lateritic, and alluvium-derived soils). Significant increasing trends were more prevalent during monsoon and post-monsoon months while decreasing trends were noted in pre-monsoon months. Correlation analysis showed strong positive correlations of SWC with rainfall (ρ = 0.70), actual evapotranspiration (ρ = 0.74), and NDVI (ρ = 0.65), but weak or negative correlations with temperature (ρ = 0.12). This study provides valuable insights for policymakers to delineate areas based on soil moisture availability patterns across seasons, aiding in agricultural and water resource planning under changing climatic conditions.
Journal Article
Potential of catchment-wide soil water content prediction using electromagnetic induction in a forest ecosystem
Mapping of soil water content (SWC) by electromagnetic induction (EMI) is an established method to obtain field-scale SWC information. However, the relationship between SWC and the apparent electrical conductivity (EC
a
) measured with EMI is complex and affected by several confounding factors at the catchment scale such as variable porosity (
ϕ
) and pore water electrical conductivity (
σ
w
). In this study, we investigated these confounding factors using a time-lapse EMI data set obtained in a forest ecosystem with soils of low EC
a
and catchment-wide SWC data provided by a wireless soil moisture sensor network. To assess the impact of variable
ϕ
on the accuracy of SWC estimates, we compared three different models to relate SWC and EC
a
: (i) a linear regression model and two nonlinear models based on Archie’s equation with (ii) constant
ϕ
and (iii) variable
ϕ
. The linear model reached a prediction accuracy of RMSE = 5.83 vol%, while the Archie models increased the accuracy to RMSE = 4.55 vol% (constant
ϕ
) and RMSE = 4.20 vol% (variable
ϕ
). Although we found strong spatial similarities between SWC and EC
a
maps, the temporal trends in SWC and EC
a
were inconsistent. This was attributed to temporal variations in
σ
w
due to seasonal changes in ion concentrations of the soil pore water. To support this hypothesis,
σ
w
was calculated from the measured EC
a
and the known soil saturation from SoilNet. The resulting
σ
w
maps showed highly structured and consistent patterns. We thus conclude that in addition to variation in SWC and
ϕ
, spatiotemporal variations of
σ
w
affected the EC
a
measured with EMI. These potentially confounding factors in the interpretation of EMI measurements in terms of SWC have not been sufficiently recognized in the literature so far, and the results presented in this study indicate a range of limitations for the use of EMI to monitor spatiotemporal changes in SWC at test sites with low EC
a
.
Journal Article