Variability Of Soil Water Content Research Articles

Analysis of spatial variability of soil water retention using the cumulative distribution function matching

The importance of modeling dependencies of spatial variability of soil water content on soil matric potential grows due to the proliferation of ensemble modeling, data assimilations, and other soil water modeling applications. The objective of this work was to investigate conversions between cumulative distribution functions of water contents at different matric potentials. In total, 80 samples were taken in the nodes of the grid to measure soil water retention using sand and sand–kaolin capillarimeters at absolute values of soil matric potential of 0.001, 0.003, 0.010, 0.020, and 0.050 MPa, and with the water vapor desorption method at 3, 21, 39, 82, and 142 MPa. The probabilities of distributions of both non-transformed and log-transformed soil water contents being normal appeared to be larger than 0.05 in most cases. Using the probit function to represent the observed variability allowed us to match cumulative probability distributions at different soil water potentials. Slopes of dependencies of probits on non-transformed and log-transformed water contents had one-parametric linear dependencies on the logarithms of the absolute value of soil matric potential in capillary and adsorptive potential ranges.

Simulation of plate sinkage in soil using discrete element modelling: Calibration of model parameters and experimental validation

Discrete element method (DEM) has presented promising advantages in simulation of soil-machine/tool interaction. To develop reliable DEM simulations, selection and parametrization of a suitable contact model is crucial. The plate sinkage test is widely applied to characterize the soil behavior during compaction. This study aimed at setting up a DEM model for plate sinkage test using the hysteretic spring-linear cohesion contact model to (i) find the key parameters of the model which need to be calibrated and (ii) develop a calibration method for DEM parameters of the model for varying soil water content and bulk density of a cohesive soil. It was tested whether confined compression tests in combination with direct shear tests could be used to calibrate DEM parameters of the contact model. Soil cohesion and internal coefficient of friction were directly obtained and used from experimental direct shear test whereas particle yield strength was estimated by matching the stress-sinkage relationship of experimental and DEM-simulated confined compression tests. A clay loam soil at two water contents (11 and 16 %) and two dry bulk densities (1000 and 1150 kg m−3) was used to test the calibration method. Simulations using the parameterization obtained from the combination of confined compression and direct shear tests resulted in remarkable underestimation of plate sinkage. A satisfactory agreement with measurements was only found when the simulated cohesion and internal coefficient of friction were several times higher than measured. The factors were indicated to be constant with changes in soil water content and bulk density. A strong correlation was found between the particle yield strength and precompression stress estimated from confined compression tests. Using the factors obtained for increasing the cohesion and internal coefficient of friction and estimation of particle yield strength from precompression stress, a viable method for calibration of plate sinkage test could be proposed.

Effects of vegetation on the distribution of soil water in gully edges in a semi-arid region

Gully bank is an important part of the eroded gully and is an ecologically fragile zone. Vegetation and soil water play critical roles for regulating the hydrological process of gully banks and preventing the soil collapse of gully edge in arid and semi-arid areas. However, the effects of different vegetation on the distribution of soil water in the gully edge are still unknown in semi-arid areas. In this study, the variation in soil water content (SWC) was studied at distances of 0.5 m (D0.5), 2.0 m (D2.0), and 3.5 m (D3.5) from the gully edge in a shrubland (SL), bare land after removal of shrubs (SBL), grassland (GL), and bare land after removal of grasses (GBL). The results showed that SWC reduced with proximity to the gully edge in SBL and GBL. Soil water storage in both SL and GL was higher at D0.5 than at D2.0 and D3.5. In both dry and wet seasons, SWC in SL was significantly lower than that in GL at all locations (P < 0.05). Compared with SBL, the SWC in SL in the wet season significantly reduced 15.42%, 27.56%, and 27.73% at D0.5, D2.0, and D3.5, respectively (P < 0.05). Our findings suggest that grasses should be considered as a suitable choice for revegetation in gully banks to prevent excessive soil water depletion by shrubs and reduce the risk of soil collapse in the gully edge. These findings provide insights into the effects of different vegetation types on soil hydrological processes in gully edges.

Spatio‐temporal variation of near‐surface soil water content in China from 1988 to 2016

AbstractThe near‐surface soil water content (SWC) can reflect the agricultural drought levels. In addition, understanding the historical trends of near‐surface SWC change in China is an important step in combating climate change. The monthly near‐surface SWC data set during 1988–2016 published by the European Space Agency Climate Change Initiative were used in this study to analyse the variations of SWC in whole China and its seven natural divisions. Results indicated South and Central China had higher SWC than others because of their larger precipitation amounts. Significant (p &lt; .05) decreasing trend in temporal SWC was found in Northeast China, which is related to the significant (p &lt; .05) decreasing trend in the aridity index. However, significant (p &lt; .01) increasing trends were found in Northwest China and Qing‐Tibet, which may be associated with the climate change (e.g., the ice and snow melting caused by increase of air temperature) and human activities (e.g., tillage and vegetation restoration). The monthly SWC in all divisions and whole China showed a main period scale (corresponding to peak values of wavelet variance) of about 225 months and 2 dry‐wet cycles. From 2016, the next 5–6 years are likely to be the wet years. The findings are of great significance to the management of soil water resources in China under global change.

Response of soil water content and temperature to rangeland desertification in an alpine region with seasonally frozen soil and plateau pika (Ochotona curzoniae) burrows

Variations in soil temperature and water content are important indicators of land desertification, and play a significant role in ecological environments in desert regions, thus could help in establishing effective measures to combat desertification. In this study, the changing characteristics of plateau pika (Ochotona curzoniae) burrows, soil water content, and temperature under three levels of desertification (slight, medium, and severe) were assessed. The number of pika burrows was investigated in quadrats with areas of 400 m2. The areas of pika burrow mounds were obtained by taking pictures vertically and measured by AutoCAD, 2007. Soil temperature and water content were monitored by soil temperature and moisture smart sensors installed at depths of 5, 10, 20, and 50 cm. Data were recorded at 10-min intervals by Hobo Data Loggers. The greatest number of pika burrows and the greatest area of pika burrow mounds were found in the medium desertified site, followed by the slightly desertified site, and no pika burrows were found in the severely desertified site. With increasing desertification degree, vegetation coverage and soil water content decreased. The onset dates of soil freeze and thaw in the severely desertified site were both earlier, and the duration of freezing days was shorter, than those in the slightly desertified site at each soil depth. The medium desertified site showed the earliest onset of soil freeze, latest onset of soil thaw, and longest duration of soil freezing days. The annual mean soil temperature was highest in the severely desertified site within 50-cm soil depth of the ground surface due to the loss of vegetation coverage and soil water content, and lowest in the medium desertified site within 20-cm soil depth due to the cooling effects of pika burrows. Desertification resulted in the loss of vegetation cover and reduced soil water content, leading to higher soil temperatures. However, the cooling effects of pika burrows could reduce soil temperatures. These findings provide insights into the impacts of rangeland desertification on soil environments, thereby helping in establishing effective measures to combat desertification.

Strength and Consolidation Characteristics for Cement Stabilized Cohesive Soil Considering Consistency Index

The undrained shear strength and consolidation characteristics for both remolded and stabilized soil are required in many geotechnical applications. In this paper a comprehensive laboratory work was carried out on cement-stabilized cohesive soil. The undrained shear strength of both untreated and treated soil at different consistency indices particularly at liquid and plastic limits has been determined based on direct and reverse extrusion tests. Furthermore, a series of oedometer test was performed in order to estimate the consolidation characteristics in terms of compression index, swelling index and yield stress of cement-stabilized soil in comparison to untreated soil. Three different soils available in Upper Egypt were extensively used under the influence of four different cement contents. Besides, the effect of curing period on soil strength and the variation of soil water content during treatment were also presented. The laboratory results were used to establish nonlinear regression formula for untreated soil combining the consistency index and undrained shear strength. The results indicated, through the variation of parameters, that the soil strength due to cement stabilization ranges between 152 and 625 times that for untreated soil at liquid limit. In addition, compression index for treatment soil decreased by about 92% to 37%.

Defining tillage need for edible bean production under no-tillage: Classical and time series analyses

Mechanical amelioration techniques may be used in long-term no-till compacted soils to improve soil structure and increase crop yield. The objectives of this study were to investigate the effect of ploughing and chiseling on changes in soil physical properties, quantify the number of days that the soil moisture is outside the critical limits of the least limiting water range, compare crop performance, and define the best tillage method using classical and applied statistical analysis. Treatments consisted of continuous no-tillage for 5 years (NT), disc plough (Plo) and chisel plough (Chi) both on soil previously under no-tillage for 5 years. High degree-of-compactness and low least limiting water range on the medium-term no-tillage Acrisol justified the need for evaluating inverting and non-inverting tillage. Soil penetration resistance, dry bulk density, soil temperature, and volumetric moisture content was measured at different times and soil layers during bean growth and development, and correlated with black bean roots distribution and yield. Soil moisture content and temperature data were subjected to time-series analysis. Low penetration resistance was observed in the 0.03 – 0.05 m soil layer, which was greater in NT than in Plo or Chi soil, but always below 1.5 MPa. During the cropping season, spatial variations in penetration resistance was lesser in the deep soil layers compared to the surface layers, following the trend on lower oscillations in soil moisture. Soil bulk density at 0.075 m depth decreased due to soil disruption: 1.72, 1.65 and 1.52 Mg m−3 for NT, Chi and Plo, respectively, and increased with soil depth. The number of days the soil was in good moisture conditions had the order Plo ≈ Chi > NT. Although no-till soil is near the critical/restrictive condition to crops and soil tillage improves soil quality, tilling does not guarantee increased crop yield. The degree-of-compactness and the least limiting water range do not capture soil physical quality for bean growth and yield. Soil tillage influenced the autocorrelation and cross correlation of soil water content in all soil layers. The estimation of soil water content using state-time analysis was affected by soil management, with the best performance from Chi soil. Significant temporal relations were observed between soil water content and other soil-atmospheric variables, with rainfall being the highest controlling factor. The time series analysis can be combined with classical and spatial statistics, for assessing soil quality under different management based on local environmental properties and climatic conditions.

Characteristics of soil water and salt associated with Tamarix ramosissima communities during normal and dry periods in a semi-arid saline environment

Tamarisk (Tamarix ramosissima) plants are widely distributed in semi-arid regions with saline soils. In these environments, the survival and sustainability of the Tamarisk plants are critically related to the availability and distribution of soil water and salt. In this study, we analyzed the soil water content (SWC) and soil electrical conductivity (EC) to the depth of 60 cm in natural Tamarisk communities, located in a semi-arid saline region of northwestern China, in July 2016 (a normal period) and July 2017 (a dry period). The results showed that SWC varied from 1.91 to 7.82% and soil EC ranged from 9.65 to 25.38 mS/cm in the 0–60 cm soil layer. Despite the deficiency of precipitation in the study period of 2017, the SWC in all plots was higher at the 20–60 cm depth than that of the normal study period in 2016. Compared to 2016, the soil was also less saline in 2017, representatively by lower soil EC values at the 0–40 cm depth. Additionally, the SWC decreased with Tamarisk density, indicating that Tamarisk plants increase water consumption. The SWC and soil EC both revealed “island” distribution, with highest level found inside or underneath the Tamarisk plant canopies. Both the SWC and soil EC presented a strong or moderate spatial dependency, and the spatial dependency in soil EC were significantly affected by community composition. Correlations (Pearson’s r) showed that there were positive correlations between SWC and soil EC. The results above indicate that Tamarisk communities can ameliorate the semi-arid high saline soil environment, and that the growth and distribution of Tamarisk plants influence the spatiotemporal variability of SWC and soil EC. In addition, the results also demonstrate that the possible appropriate density in the process of constructing Tamarisk plants should be considered in order to achieve the optimal water condition. The study may provide a scientific basis for the ecological restoration and halophytes establishment in the semi-arid saline region of northwestern China and other areas with similar ecologies.

Belowground soil water response in the afforestation-cropland interface under semi-arid conditions

Agroforestry is an effective measure to control soil erosion and maintain or increase productivity in semi-arid areas. However, the belowground soil water responses in the afforestation-cropland interface (ACI) is not well known. This study analyzed the variability of soil water storage (SWS) and deficit in three ACIs taking into account the distance between the forest and cropland. The variations of soil water content up to 4 m depth and at five distances from the interface (−5 m (in forest), 0 m (interface), 1 m, 3 m and 5 m (in cropland)) were estimated in three artificial forests (Salix matsudana, Sophora japonica, and Populus cathayana) and their adjacent croplands (maize). The results showed that soil water at the interfaces was significantly affected by forest. This effect was effective up to 160–170 cm of soil depth, and the ACI of S. matsudana had the greatest impact on the farmland soil water. There was no significant relative soil water deficit between 1 and 3 m length, and the ACI of P. cathayana showed the lowest changes in the lateral direction. The SWS of S. japonica in the ACI was clearly higher than the other two artificial forests (P < 0.05). Our findings indicated that S. japonica was the most suitable forest species for agriculture sustainability in the study area. The tree species and the distance between cropland and forest should be considered during the establishment of agroforestry systems. This study provided insights for water conservation and effective management of ACIs in semi-arid areas.

Spatiotemporal variations in deep soil moisture and its response to land-use shifts in the Wind–Water Erosion Crisscross Region in the Critical Zone of the Loess Plateau (2011–2015), China

Soil water content (SWC) with a deep profile is crucial to land use management, soil and water conservation, and ecological restoration worldwide, including the wind–water erosion crisscross region in the Critical Zone of the Chinese Loess Plateau. To ascertain the spatiotemporal variation of deep SWC and its response to land-use shifts in the region, the SWC was monitored at four representative land uses (farmland, shrubland, natural grassland, and planted grassland) to a depth of 21 m. Monitoring occurred between 2011 and 2015 in the Liudaogou watershed of the Chinese Loess Plateau. The Hydrus-1D model was then applied to simulate the SWC dynamics for the different land-use types and scenarios of land-use shifts. The SWC exhibited a high spatial variability in the vertical direction both within a given soil profile and among the different land uses. In the 0–2 m layer, the soil texture and root biomass controlled the spatial-temporal variability of the SWC. The temporal variability was also controlled by meteorological factors. In the 2–21 m layer, spatial variability was controlled by the soil texture, while soil texture and root biomass influenced the temporal variability of the SWC. The Hydrus-1D model accurately simulated spatiotemporal variations of SWC in the 0–21 m layer and was more effective on layers minimally influenced by weather. Land-use shifts from farmland with shallow-rooted plants to those with deep-rooted plants intensified soil water consumption in the 3.5–5 m layer. For best-management practices, appropriate plant species should be considered based on climatic conditions and SWC regimes. A better understanding of this information is useful for plant species selection, soil water resource utilization, and land use policymaking in the wind–water erosion crisscross region of the Critical Zone and other similar regions around the world.

Laboratory characterization of sandy soil water content during drying process using electrical resistivity/resistance method (ERM)

Drought-induced evaporation can reduce soil water content and significantly alter soil hydro-mechanical behavior. Understanding the temporal and spatial distribution characteristics of soil water content during evaporation is of great significance for evaluating the encountered geotechnical and geo-environmental problems in arid or semi-arid regions. In this study, an electrical resistivity/resistance method (ERM) with a high spatial resolution of centimeter-level was developed for a small-scale laboratory test and applied to quantitatively characterize the evaporation-induced water content variations along a depth gradient. A total of 8 groups of initially saturated sandy soil columns (84 mm in diameter and 290 mm in height) were prepared, and eight pairs of mini electrodes (3.5 mm in diameter) were installed in each soil sample with a vertical distance of 30 mm. The soil columns were subjected to continuous drying. The changes in soil electrical resistance at different depths were monitored by the electrode couples. The gravimetric water contents at different depths were also measured at the end of drying. It is found that soil water content decreases exponentially with increasing electrical resistance. Based on the obtained data, a calibration relationship between soil gravimetric water content and corrected electrical resistance was well established with consideration of temperature effect. This relationship was validated successfully by the experimental results, indicating the feasibility of the developed ERM to characterize the soil water content dynamics during the drying process. Besides, the drying process with the movement of the evaporation front was discussed. The results of this study demonstrate the good performance of ERM in the estimation of temporal and spatial variations of soil water content and its potential application in arid or semi-arid regions with frequent droughts.

Factors determining soil water heterogeneity on the Chinese Loess Plateau as based on an empirical mode decomposition method

Soil water is a critical resource, and as such is the focus of considerable physical research. Characterization of the distribution and spatial variability of soil water content (SWC) offers important agronomic and environmental information. Estimation of non-stationary and non-linear SWC distribution at different scales is a research challenge. Based on this context, we performed a case study on the Chinese Loess Plateau, with objectives of investigating spatial variability of SWC and soil properties (i.e., soil particle composition, organic matter and bulk density), and determining multi-scale correlations between SWC and soil properties. A total of 86 in situ sampling sites were selected and 516 soil samples (0-60 cm depth with an interval of 10 cm) were collected in May and June of 2019 along the Yangling-Wugong-Qianxian transect, with a length of 25.5 km, in a typical wheat-corn rotation region of the Chinese Loess Plateau. Classical statistics and empirical mode decomposition (EMD) method were applied to evaluate characteristics of the overall and scale-specific spatial variation of SWC, and to explore scale-specific correlations between SWC and soil properties. Results showed that the spatial variability of SWC along the Yangling-Wugong-Qianxian transect was medium to weak, with a variability coefficient range of 0.06-0.18, and it was gradually decreased as scale increased. We categorized the overall SWC for each soil layer under an intrinsic mode function (IMF) number based on the scale of occurrence, and found that the component IMF1 exhibited the largest contribution rates of 36.45%-56.70%. Additionally, by using EMD method, we categorized the general variation of SWC under different numbers of IMFs according to occurrence scale, and the results showed that the calculated scales among SWC for each soil layer increased in correspondence with higher IMF numbers. Approximately 78.00% of the total variance of SWC was extracted in IMF1 and IMF2. Generally, soil texture was the dominant control on SWC, and the influence of the three types of soil properties (soil particle composition, organic matter and bulk density) was more prominent at larger scales along the sampling transect. The influential factors of soil water spatial distribution can be identified and ranked on the basis of the decomposed signal from the current approach, thereby providing critical information for other researchers and natural resource managers.

An experimental application of electrical resistivity/resistance method (ERM) to characterize the evaporation process of sandy soil

Because of global climate change, drought occurs in increasing intensity and frequency across the world. Drought-induced evaporation can reduce soil water content and cause progressive desiccation cracking, which is responsible for various geotechnical and geo-environmental problems. This study aims to develop an ERM with a high spatial resolution and study the ERM performance in the characterization of soil water content dynamics from a quantitative point of view in small-scale evaporation tests, in order to support the future study of desiccation cracking in field investigations. An environmental chamber filled with initially saturated sand was designed for the evaporation test. As the chamber was subjected to drying, the evolutions of soil electrical resistance and water content at different depths were monitored continuously by the installed sixty electrodes and six time domain reflectometers (TDR) sensors, respectively. Experimental results indicate the good correlation between the measured soil electrical resistance and the water content. As evaporation continues, soil electrical resistance increases exponentially with decreasing water content. The variations of soil electrical resistance present an evidentially delayed effect along with the depth. A calibration relationship between the recorded soil electrical resistance by ERM and the water content measured by the oven drying method was established. Afterwards, the variations of soil water content at different depths were estimated based on the developed calibration relationship and compared with the TDR results. Besides, a numerical approach combining a soil-atmosphere interaction model and a coupled hydro-thermal model was employed to study the evaporation-induced variations of soil water content. The estimated soil water contents by ERM were further compared with the results obtained using the numerical method. The evaporation process with the movement of the evaporation front in the studied soil sample was also discussed in depth. This study presents that the developed ERM is effective to record soil moisture dynamics, especially in the near-surface zone, in small-scale evaporation tests. It also provides insights on the potential of ERM in field applications to characterize soil hydraulic responses to drought climate.

Regional variation in soil water and vegetation characteristics in the Chinese Loess Plateau

Abstract Soil water is the limiting factor that determines ecosystem development in arid and semi-arid regions. Thus, determining the relationships between soil water and vegetation can inform the restoration of vegetation and water resource management. However, these relationships have not been fully investigated at the regional scale. In this study, field observations were conducted in two typical vegetation zones of the Loess Plateau of China: forest-steppe and semi-arid steppe zones. Variation in soil water content (SWC) and vegetation characteristics (plant density and species richness) in addition to their correlations were analyzed in grasslands and forestlands (occupied by black locust) in each zone. SWC, plant density, and species richness in the forest-steppe zone were significantly higher than those in the semi-arid steppe zone (p

Growth phenology and water potential patterns of Periploca angustifolia in relation to water availability

AbstractPeriploca angustifoliaLabill., is a multipurpose shrub also used to rehabilitate drylands. It is essential to understand its phenological patterns under arid conditions. In order to understand how plants manage extreme drought, this study investigates the relationship between phenological traits of P. angustifolia Labill., water potential and climatic conditions. During two successive growing seasons (Sep 2009–Augst 2011) phenological patterns, soil water content and water potentials were measured monthly for plants of Periploca angustifoliaLabill growing under rain‐fed conditions (Annual rainfall = 168 mm) at the pastoretum of the Arid Regions Institute in southern Tunisia. The Ψmd decreased progressively and concomitantly with increasing seasonal drought, reaching the lowest values in late summer (down to –3 MPa). Water potentials of P. angustifoliaLabill were affected by spatial and temporal variations in soil water content. The increased diurnal amplitude values (∆ψ &gt; −1.3 MPa) explained the especially high biological activity of this species during the dry season. Our results clearly demonstrate that P. angustifolia Labill is a drought‐tolerant species reaching low water potentials during the driest months of summer. These characteristics make this shrub as promising for rehabilitating degraded arid ecosystems.

Production limits analysis of rain-fed maize on the basis of spatial variability of soil factors in North China

In precision agriculture, suitable soil moisture and nutrient levels are important for crop production. Therefore, studying the spatial variability in soil moisture and nutrient, and their impact on crop yield is essential. In this study, spatial variability in soil mechanical composition, bulk density, water content and nutrient content in north China along with its impact on crop yield is investigated. Results show that soil mechanical composition and bulk density respectively exhibited moderate and weak spatial variation at the 0–80 cm soil depth. Soil moisture and NH4+-N contents at the 0–80 cm soil depth showed moderate spatial variation, while soil NO3−-N content showed a moderate spatial variation from the seedling stage to the filling stage and a strong spatial variation from the milk-riping stage to the maturing stage. The maximum attainable yield was achieved under the optimum soil moisture, NH4+-N, or NO3−-N contents for the whole growth period and each growth stage. At high soil water condition, both soil NH4+-N and NO3−-N contents were found to be significantly different between the high and low yield areas, while at low soil water condition, only soil NO3−-N content affected the crop yield significantly. At either low or high soil NO3−-N level, soil NH4+-N content in high-producing areas was significantly lower than that in low-producing areas. Excess fertilization was found in the experimental area. This study can provide a scientific basis for developing appropriate crop irrigation and fertilization management practices to obtain high yield.

Measuring vertical soil water content profiles by combining horizontal borehole and dispersive surface ground penetrating radar data

ABSTRACTTo investigate transient dynamics of soil water redistribution during infiltration, we conducted horizontal borehole and surface ground penetrating radar measurements during a 4‐day infiltration experiment at the rhizontron facility in Selhausen, Germany. Zero‐offset ground penetrating radar profiling in horizontal boreholes was used to obtain soil water content information at specific depths (0.2, 0.4, 0.6, 0.8 and 1.2 m). However, horizontal borehole ground penetrating radar measurements do not provide accurate soil water content estimates of the top soil (0–0.1 m depth) because of interference between direct and critically refracted waves. Therefore, surface ground penetrating radar data were additionally acquired to estimate soil water content of the top soil. Due to the generation of electromagnetic waveguides in the top soil caused by infiltration, a strong dispersion in the ground penetrating radar data was observed in 500 MHz surface ground penetrating radar data. A dispersion inversion was thus performed with these surface ground penetrating radar data to obtain soil water content information for the top 0.1 m of the soil. By combining the complementary borehole and surface ground penetrating radar data, vertical soil water content profiles were obtained, which were used to investigate vertical soil water redistribution. Reasonable consistency was found between the ground penetrating radar results and independent soil water content data derived from time domain reflectometry measurements. Because of the improved spatial representativeness of the ground penetrating radar measurements, the soil water content profiles obtained by ground penetrating radar better matched the known water storage changes during the infiltration experiment. It was concluded that the combined use of borehole and surface ground penetrating radar data convincingly revealed spatiotemporal soil water content variation during infiltration. In addition, this setup allowed a better quantification of water storage, which is a prerequisite for future applications, where, for example, the soil hydraulic properties will be estimated from ground penetrating radar data.

Temporal stability of soil water content on slope during the rainy season in gully regulation watershed

To increase reserve farmland resources, ensure food security, and to improve the ecological environment, large-scale gully regulation projects in Yan'an area of the Loess Plateau have been undertaken in China since 2013. Understanding the spatial and temporal distribution characteristics of slope soil moisture during the rainy season is, therefore, important for vegetation selection used in restoration measures in this area. The temporal stability of soil water content in the upper 100 cm soil depth in a gully regulation watershed was examined using six measurements taken during the rainy season (May–November 2016). Temporal stability analysis of the soil water content was undertaken using Spearman’s rank correlation coefficient and the relative difference method. Results showed that soil water content in the 0–50 cm and 0–100 cm soil depths demonstrated moderate temporal and spatial variability, and soil water content variability gradually decreased with increasing soil depth. Soil water content in both depths had strong temporal stability, having a positive correlation with soil depth. Furthermore, temporal stability may decrease with increasing soil water content (p < 0.05). The number and position of representative locations were not constant, which varies with the soil depth and the estimation method. Five methods (mean relative difference, minimum relative difference standard equilibrium, temporal stability index, mean absolute deviation and root mean square error) can be used to determine the representative location of temporal stability. However, the prediction accuracy of the soil water content at representative locations obtained using the different methods for mean slope water content differed. Comparatively, the mean absolute deviation method had a higher accuracy, followed by the minimum relative difference standard equilibrium method. Results indicate that the prediction accuracy of five methods increased with soil depth.

Increasing leaf δ13C values of woody plants in response to water stress induced by tunnel excavation in a karst trough valley: Implication for improving water-use efficiency

Plant species growing in karst shallow-soil areas always experience water deficit especially in seasonally dry. Moreover, such water stress is aggravated by tunnel excavation in karst areas. However, the effects of tunnel excavation on karst ecosystems remain largely unknown. This study aimed at investigating whether there were variations in soil water contents and shifts of water use strategies of plants between rainy and dry seasons in a karst trough valley affected by tunnel excavation, and comparing their differences with those in a tunnel-free karst trough valley in Southwest China. Monthly soil water contents at two soil layers of the upper 0–20 cm and the lower 20–40 cm were measured from January 2017 to December 2018. Foliage of woody plants were sampled in the dry season of December 2017 and March 2018, and in the rainy season of June and September 2018, respectively, and leaf δ13C values were analyzed. The soil water contents at both valleys showed significant seasonal variations, and the soil water contents of two soil layers in the tunneling affected valley were significantly lower than that of the tunnel-free valley at both seasons. Plant water use strategy changed from profligate water-use pattern in the rainy season to conservative water-use pattern in the dry season. Moreover, increased leaf δ13C values at both seasons in the tunneling affected valley suggests that the water stress resulted from the tunnel excavation has significantly impacted the physiological process of plants in karst areas. These results highlight that improving water use efficiency (WUE) is a common water use strategy to overcome water limitation for native plants in karst areas during unfavorable drought conditions.

Soil properties of apple orchards on China's Loess Plateau

There are many apple orchards on the Loess Plateau because of their economic value and aggressive expansion is planned. However, little is known about their ecological impact in deep soil water, soil organic carbon (SOC) and soil aggregation. An accurate evaluation of the soil properties of apple orchards is crucial to ensure the establishment of sustainable ecosystems. We, therefore, measured the soil water content variation in deep layers (200–800 cm), SOC content and density (0–800 cm) and, soil aggregate stability (0–40 cm) in apple (Malus pumila) orchards, black locust (Robinia pesudoacacia) and korshinsk peashrub (Caragana korshinskii). We found that (1) there was generally less soil water in deep soil under apple orchards (13.29%), black locust (12.4%) and korshinsk peashrub (13.46%) than under arable land (18.35%) (p < 0.05). This implies that plantations caused intense reductions in soil water compared with arable land, leading to severe soil desiccation. (2) Apple orchards (1.85 to 5.49 g kg−1) had significantly (p < 0.05) lower SOC density (SOCD) than ecological plantations (2.15 to 8.95 g kg−1). It suggests that apple orchards have a low value for SOC sequestration because their clean cultivation management increases the risk of SOC loss by soil erosion. (3) soil aggregate stability (mean weight diameter, MWD) in apple orchards (0.26–0.63 mm) was significantly (p < 0.05) lower than under black locust (0.63–2.97 mm) and korshinsk peashrub (0.72–2.13 mm) plantations in the 0–40 cm layers, which means that apple orchards have low anti-erodibility. Our results suggest apple orchards and ecological plantations both consumed large amounts of deep soil water, but the ecological benefits (e.g., SOC sequestration, soil and water conservation) delivered by ecological plantations are much higher than those of apple orchards. In the interest of sustainable development in the region, apple cultivation should be undertaken with caution, especially in semiarid regions.