Distribution Of Soil Water Content Research Articles

Estimation of surface soil moisture by combining a structural equation model and an artificial neural network (SEM-ANN)

Soil moisture is an important variable of the environment that directly affects hydrological, ecological, and climatic processes. However, owing to the influence of soil type, soil structure, topography, vegetation, and human activities, the distribution of soil water content is spatially heterogeneous. It is difficult to accurately monitor the distribution of soil moisture over large areas. To investigate the direct or indirect influence of various factors on soil moisture and obtain accurate soil moisture inversion results, we used structural equation models (SEMs) to determine the structural relationships between these factors and the degree of their influence on soil moisture. These models were subsequently transformed into the topology of artificial neural networks (ANN). Finally, a structural equation model coupled with an artificial neural network was constructed (SEM-ANN) for soil moisture inversion. The results showed the following: (1) The most important predictor of the spatial variability of soil moisture in the April was the temperature–vegetation dryness index, while land surface temperature was the most important predictor in the August; (2) After the ANN model was improved, the inversion accuracy of surface soil moisture by SEM-ANN model was improved, and the R2 of verification set was increased by 0.01 and 0.02 in April and August, respectively, and the relative analysis error was reduced by 0.5 % and 1.13 %. (3) There were no significant differences in soil moisture distribution trends between the April and August.

Apple–Soybean Mixed Stand Increased Fine Root Distribution and Soil Water Content with Reduced Soil Nitrate Nitrogen

In dryland agroecosystems, intercropping fruit trees with legumes is often an important option for maintaining and improving soil N quality and fertility. The relationships of fine root length density (FRLD), soil water content (SWC), and soil NO3−-N content (SNC) in agroforestry systems is essential for optimal orchard management. Our objective was to understand the temporal and spatial dynamics of FRLD, SWC, and SNC in an apple–soybean intercropping system and competition between species for ecological niches. We established an orchard with soybean and apple, including monoculture apple (MA), monoculture soybean (MS), and apple–soybean intercrop (AS) treatments. We collected data on the distribution of FRLD, SWC, SNC, and productivity under the MA, AS, and MS from 2020 to 2021. This study showed that AS had more FRLD compared to MA, and intercropping apple (IA) FRLD increased at 20–60 cm soil depth at 50 and 100 cm from the tree and decreased at 0–40 cm soil depth at 150 and 200 cm from the tree. Intercropping increased the SWC of the system and decreased the SNC, and the effect of intercropping was concentrated in the soybean cover area. The competition between apple and soybean reached its maximum at soybean seed filling stage, with competition occurring mainly at 100 cm from the tree and at 0–20 cm soil depth. Intercropping affected soybean growth and also reduced soybean yield with a land equivalent ratio = 1.45. By understanding the dynamics of subsoil resources in the apple–soybean intercropping system and interspecific competition, we can provide a theoretical basis for exploring the potential of sustainable intensification of agroforestry systems.

Relationship between soil water content and vegetation distribution in a small area before and after coal seam mining: a case study of coal mining subsidence area in Northwest China

Relationship between soil water content and vegetation distribution in a small area before and after coal seam mining: a case study of coal mining subsidence area in Northwest China

Comparing water use patterns of sand‐binding species in an alpine semi‐arid desert

AbstractThe processes of species obtaining water resources are crucial to evaluate the adaptability of plantations, which can affect the establishment and survival of restored ecosystems and functions of water‐limited ecosystems. However, there are still limitations in understanding water use patterns of different four plantations (Populus sylvestris, Salix cheliophe, Hippophae rhamnoides and Artemisia ordosica) in the Qinghai Lake basin. Stable isotopes of xylem water of four individual species were analysed at different dunes located at the eastern shore of Qinghai Lake in 2018 and 2019. The purpose of this study was to explore potential water sources used by different life form plants, to identify whether the soil moisture content and root distribution determined the plants water use patterns. Results from the Bayesian mixing model MixSIAR showed that all species mainly uses three levels of soil water, but they extracted soil water from different layers in different growing seasons among species. All species primarily depended on water from upper soil layers at the early growing stage, but P. sylvestris and S. cheliophe also absorbed much proportion middle and deep soil layers' water in dry year of 2019. All four species shift to use middle or deep soil layer water rather than shallow water in July with the most rainfall and soil water content (SWC). However, it was only used by P. sylvestris and H. rhamnoides in 2018. In all, seasonal water use pattern of different species was affected by SWC, soil texture and root distribution. Compared with moratorium plantations such as S. cheliophe, mixed afforestation of P. sylvestris and H. rhamnoides maximized of soil water sources absorbed by the plants. The study can shed light on plant–water relationships to facilitate the woody species for afforestation and desertification restoration management in the semi‐arid desert ecosystem.

Analysis of subsurface velocity using CMP gathers picked up by unshielded GPR system: results from an experimental NAPL contaminated test site

Adequate knowledge of velocity is required for accurate data imaging and depth conversion, as well as for quantifying the distribution of soil water content. Without complementary borehole information in the form of dielectric permittivity and/or porosity logs along the profile, it is currently impossible to reliably estimate the high-frequency electromagnetic velocity distribution in the probed subsurface region. Here, we present a new method for calculating the precise subsurface velocity structure from ground penetrating radar (GPR) reflection data that does not require boreholes or log data. This study investigates the ability of the pulse_EKKO PRO GPR system to predict a vertical profile for the possible velocity estimation of a layered and contaminated geophysical test site in Hangzhou, China. All data were acquired and saved on the GPR system in various files (projects) before analysis using GPR software to obtain approximated velocity modelling using common midpoint (CMP) gathers. Using the velocity spectrum analysis, a vertical profile of the interval velocities can be derived from each CMP gather. The findings of this study indicate that the proposed method is effective and sustainable. Furthermore, owing to the efficacy of the method in terms of field effort and computational complexity, it can easily be expanded to 3D GPR velocity exploration, increasing its importance in comparison to standard offset-based techniques for estimating velocity using GPR.

A Spatial Distribution Empirical Model of Surface Soil Water Content and Soil Workability on an Unplanted Sugarcane Farm Area Using Sentinel-1A Data towards Precision Agriculture Applications

Obtaining soil water content and soil workability data using remote sensing technology with passive sensors has some limitations due to cloud cover, cloud shadow, haze and smoke. This study proposes a method for computing soil water content and soil workability over large areas, faster and in near real-time based on Sentinel-1A (SAR) data. Sample data collected from sugarcane plantations in the Kediri and Sidoarjo districts in East Java, Indonesia, were used to develop a mathematical model of the proposed method using multi-polynomial regression. The performance indicators of the model (RMSE, MAPE and accuracy) were calculated with the results of RMSE = 0.213 and 0.250, MAPE = 16.39% and 18.79%, and accuracy = 83.6% and 81.2% for the training and testing models, respectively. The distribution of soil water content and soil workability can be computed and visualized using a spatial map. The future contribution of this work is to develop a decision support system for the selection of appropriate machinery for sugarcane field operations based on the principles of precision agriculture.

The Seasonal and Diurnal Variation Characteristics of Soil Moisture at Different Depths from Observational Sites over the Tibetan Plateau

Using observational data of soil moisture from the third Tibetan Plateau Experiment for atmospheric science (TIPEX III), the seasonal and diurnal variations characteristics of soil moisture at different depths of 5–160 cm from seven stations were analyzed, with emphasis on the comparative analysis of the differences of soil moisture between different sites and the differences of the synergistic relationship between soil moisture and temperature. The soil moisture was wet in the southeast and dry in the northwest. The studied sites were Lhari, Biru, Nyainrong, Amdo, Nagqu, Baingoin and Seng-ge Kambab in descending order, according to the soil moisture. The seasonal variation of soil moisture at the different sites showed a significant three-peak structure, which was more obvious in the shallow layer than in the deep layer. The first peak occurred from March to May, which was mainly due to the soil thawing in spring. The other two peaks corresponded to the two rainy seasons in the plateau. Soil moisture was the greatest during this rainy period. The diurnal variations of soil moisture and temperature in Amdo, Nagqu, Nyainrong and Baingoin showed a significant positive correlation in the four seasons. The soil moisture and temperature in Lhari and Biru were significantly positively correlated in winter and spring but negatively correlated in summer and autumn. The profiles of the soil moisture with depth varied greatly at different stations in different seasons. The distribution of soil water content at each observational site did not increase or decrease with depth but showed a certain high aquifer, which might be related to the types of the underlying surface and physical properties of soil. During the summer monsoon period, soil moisture in the shallow layer of 5–10 cm was higher at all observational sites. The spatial distribution of soil moisture in the plateau was more heterogeneous than that in the plain area, and only in the central part of the Tibetan Plateau, the soil moisture varied greatly from site to site. This also indicated that it was unreasonable to only use the soil moisture of several stations to represent the overall soil moisture of the region. The results provided a multi-angle observational basis for the validation of satellite data and parameterization of the numerical model of soil moisture over the Tibetan Plateau.

From point to field scale - indirect monitoring of soil moisture variations at the DWD test site in Falkenberg

Information regarding the spatial distribution of soil water content is key in many disciplines and applications including soil and atmospheric sciences, hydrology, and agricultural engineering. Thus, within the past decades various experimental methods and strategies have been developed to map spatial variations in soil moisture distribution and to monitor temporal changes. Our study examines the combination of electrical resistivity mapping and point observations of soil moisture to infer the spatial and the temporal variability of soil moisture. Over a period of around two years, we performed field measurements on six days to collect repeated electrical resistivity mapping data for a nine-hectare test site south-east of Berlin, Germany. Permanently installed TDR probes, temporary TDR measurements within permanently installed tubes, and gravimetric measurements using soil samples provided soil moisture data at various selected points. In addition, soil analysis and classification results are available for 132 regularly distributed positions up to depths of 1.2 m. We compare and link three-dimensional resistivity models obtained via data inversion to soil composition and soil moisture as provided by our point data. Both the soil samples and the resistivity models indicate a two-layer medium characterized by a sandy top layer with varying thickness and a loamy bottom soil. For all six field campaigns, we observe similar resistivity patterns reflecting the temporally stable influence of soil texture. While the overall patterns are stable, the range of resistivity values changes with soil moisture. Finally, to estimate spatial models of soil moisture, we link our soil moisture and resistivity data using empirical petrophysical models relying on a second order polynomial function. We observe a mean prediction error for soil moisture of +/- 0.034 m3 • m−3 using all observation points while we notice that point-specific models further reduce the error. Thus, we conclude that our experimental and data analysis strategies represent a reliable approach to establish site-specific models and to estimate three-dimensional moisture distribution including temporal variations.

An approximate solution to one-dimensional upward infiltration in soils for a rapid estimation of soil hydraulic properties

• A concise soil water content profile expression was obtained by an analytical method. • An approximate solution was derived for one-dimensional upward infiltration. • A method with a standard ring was proposed to determine BC model parameters. Soil hydraulic properties quantify soil pore structure and are essential input parameters for simulating soil hydrological processes in the vadose zone. Although methods have been proposed to estimate soil hydraulic properties, efficient and applicable ones are still scarce. In this study, we derived an approximate solution for a one-dimensional upward infiltration process with initially uniform soil water content distribution. Founded on our solution, a nonlinearly constrained optimization method was developed to simultaneously estimate the parameters K s and n , h d of the Brooks-Corey model by measuring the upward infiltration process in a 5-cm high soil column. Numerical experiments simulated with Hydrus-1D were performed on six theoretical soils to evaluate the accuracy of the new method. The results indicate that the approximate solution can accurately simulate the upward infiltration process at the low initial soil water content range, with a relative error ( Re ) of 2.85% to 5.47% for soil water content distribution in space and time. For the cumulative infiltration curve, the Re ranges from 0.2% to 1.01%. The estimated parameters K s and n , h d of the Brooks-Corey model are close to the actual parameter values, with a Re of 0.59% to 4.68% for n , 0.00% to 7.91% for h d , and 0.50% to 8.83% for K s , respectively. It is concluded that the new method provides a simple and valuable tool for the accurate estimation of soil hydraulic properties.

Influence of ionized saline water on soil water and salt transport in saline soils

AbstractThe utilization of saline water plays an important role in agricultural production in saline areas to overcome freshwater shortages. Ionized saline water might alleviate the adverse effects of salinity on soil and plants. In this study, field experiments were conducted to study the effect of ionized saline water on the soil water content and salinity distribution in a sandy soil under a surface emitter. Passing groundwater through an ionization system installed on a feeding pipeline resulted in producing ionized saline water. Three emitter discharges (1.0, 1.8, 3.1 L h−1) and two water types (non‐ionized and ionized saline water) were considered in the experiments. As a result of using ionized saline water, soil water content and salt leaching increased. Ionized treatment increased the salinity desalination rate by 31, 33 and 26% for surface soil layer in comparison with non‐ionized treatments of 1.0, 1.8 and 3.1 L h−1, respectively. It was found that using ionized saline water could decrease the surface wetting radius by 6–9% and increase the wetting depth by 11–17%. A discharge rate of 1.8 L h−1 was recommended as the optimal discharge rate for ionized saline water under drip irrigation. The results indicated that saline water could be improved through ionized treatment for agricultural irrigation in saline soils in an arid region.

Effects of Underground Coal Mining on Soil Spatial Water Content Distribution and Plant Growth Type in Northwest China.

The impact of coal mining subsidence on surface ecology involves the influence of several ecological elements such as water, soil, and vegetation, which is systematic and complex. Given the unclear understanding of the synergistic change patterns of the water–soil–vegetation ecological elements in the influence of coal mining in the west, this paper investigates the impact of coal mining on the surface ecology, especially the distribution of soil water content (SWC). In 2020, this study collected 3000 soil samples from 60 sampling points (at depth of 0–10 m) and tested the SWC. All samples come from three different temporal and spatial areas of coal mining subsidence in the desert mining area of Northwest China where soil types are mainly aridisols. At the same time, the interactions among deep SWC and surface soil physical and chemical properties, surface SWC and soil fertility, and pH were analyzed. The spatial variability of soil moisture is reflected by kriging interpolation, and SWC values at different depths are predicted as a basis for monitoring the environmental impact of different coal mining subsidence years. The research has shown that the ground subsidence leads to a decrease in SWC value and changes in surface soil pH, physical and chemical properties, and covering vegetation, which have occurred from the beginning of coal mining. The impact of coal mining on the SWC of the unsaturated zone is mainly at the depth of 0–6 m, where SWC is not directly related to the nutrient content of the surface soil. The overall settlement of the ground will stir up simultaneous decline in the quality of deep SWC and topsoil. The findings of this investigation suggest that changes in the soil structure caused by coal mining subsidence are the key factor in SWC loss. Timely monitoring and repairing 0–6 m ground fissures, as well as selecting shrubs on the surface is the best choice for the restoration of the ecological environment and prevention of soil erosion in this area.

Spatial distribution of soil water, plant roots, and water use pattern under different drip fertigation regimes in an apple-soybean intercropping system on the Loess Plateau, China

The irrational utilization of water resources restricts the sustainable development of tree-crop intercropping in the Loess region. Drip fertigation was applied for two years in an intercropping system to save water resources, and improve water use efficiency (WUE) and crop yield. The spatial distribution of soil water, plant roots, and water use pattern were investigated in a drip fertigated intercropping system. A two-factor randomized block design was adopted. Four irrigation levels: 60% (W1), 70% (W2), 80% (W3), and 90% (W4) of field capacity (Fc), together with three nitrogen fertilization treatments [59.40 (F1), 92.00 (F2), and 124.32 kg ha–1 (F3)], were set up with a blank control (CK), which was rain-fed with base fertilizer applied before sowing. The soil water content (SWC) distribution had a “U” shape in the horizontal direction, and the vertical water competition area moved downward as the intercropping year increased. The soybean root length density (RLD) increased as the distance from the tree row increased, but apple showed the reverse trend. The overlap between apple and soybean roots generally decreased as the distance from the tree row increased, but the vertical overlap of roots extended from 0–40 cm (2018) to 0–60 cm (2019). The interspecific competition increased with the increase in intercropping years, resulting in a downward shift of the competition area. Water consumption in 2018 and 2019 increased as the irrigation level rose. The SWC, RLD, soybean yield, and WUE of W3F2 were the highest among the treatments. Excessive irrigation can lead to vigorous vegetative growth, but reduced yields. The negative effects of intense interspecific competition were relieved under drip fertigation. The results suggested that drip irrigation at 80% Fc combined with 92 kg ha–1 N fertilization can improve the yield and WUE of the apple-soybean system on the Loess Plateau.

Soil moisture heterogeneity and sensor deployment in uniformly managed field with unitextural soil

AbstractThis research investigated the spatial and temporal distribution of volumetric soil water content (VWC) and total soil water across 64 sampling locations (8 × 8 grid) in a production‐scale field, reported as a unitextural soil unit by SSURGO. Consequently, the required number and placement of soil moisture monitoring locations were calculated to represent soil water status with varying degrees of variability. Each soil layer in the root zone was subject to 5–9% of mean spatial VWC variability and maximum variability as high as 21% during the 2014 and 2015 growing seasons. Total soil water showed spatial behavior, which was as high as 13%, with mean spatial variability of 5%. In 2014, the field minimum and maximum temporal CVs in VWC were 9 and 27%, respectively, which increased to 14 and 33% in 2015, respectively. Representative soil moisture and its variability were accounted for using 1–45 monitoring locations, depending on observed variability, acceptable measurement error, and confidence interval (CI). When the commonly used marginal error of 2% was considered, the numbers of sensor deployment locations were 8 (90% CI) and 11 (95% CI), which reduced to 5 (90% CI) and 4 (95% CI) sensors with 3% marginal error. We demonstrate that although SSURGO data can be valuable to make preliminary assessments, relying on it to spatially characterize soil units and their properties for effective within‐soil unit management, especially for variable rate water and nutrient applications, can create significant challenges.

Effects of tillage and seasonal variation of rainfall on soil water content and root growth distribution of winter wheat under rainfed conditions of the Loess Plateau, China

Drought is the leading cause of low and unstable wheat yield in dryland. The field experiment was performed from 2009 to 2017 in the eastern part of Loess Plateau, China, to study the effect of tillage (deep ploughing, DP and no-tillage, NT) and seasonal variation in precipitation on soil moisture and root traits and grain yield of wheat. In dry seasons, soil water content, root length surface density (RLSD), and root diameter peaked at 60 cm depth, but in the normal/wet seasons, peaked at 40 cm depth. The reduction of RLSD at anthesis by drought was 44% under NT and 29% with DP, while the reduction in above-ground dry weight was 67% under NT and 56% under DP. More soil water was depleted from the deep soil layer (80-100 cm depth) than that in the shallow layer (20-60 cm depth) from jointing in the dry seasons, and from anthesis in the normal/wet seasons. The higher amount of effective in-seasonal rainfall in the dry seasons increased RLSD at anthesis by 44% with NT and 21% with DP and increased grain yield by 24.1% under NT and 14.2% under DP. The average grain yield in the dry seasons was 28–41% lower than in the normal/wet seasons. The average water use efficiency (WUE) in the dry seasons was 42–54% lower than that in the normal/wet seasons. WUE with DP was 32% higher than that with NT in the dry seasons, while it was 15% higher than that with NT in the normal/wet seasons. The average contribution rates of soil water content at sowing to grain yield under DP and NT were 23–25% in the dry seasons, and 52–53% in the normal/wet seasons. Thus, deep ploughing in summer fallow should be adopted for high water storage and yield stability.

Does biochar affect soil wettability and flow pattern?

Despite the known benefits of biochar application to soils, little is known about its effect on soil wettability and the consequences on soil water movement across the soil profile. Focusing on the latter, three types of commercial biochar derived from wheat, corn, and rice straw produced under low temperature (350–450 ℃) were mixed with a wettable sandy soil at rates of 0%, 2%, 5%, and 10% w/w. Soil water repellency (SWR) of the biochars and biochar–soil mixtures was determined by water drop penetration time (WDPT) test and sessile-drop contact angle (CA) measurement. The WDPT results (< 5 s) showed complete wettability for the biochar–soil mixtures, whereas CA results indicated some water repellency. The mean initial CA of the three biochars varied between 105.3° and 113.9°, with ∼ 30° for the pure sand, and between 56.8° and 75.7° for the biochar–soil mixtures. SWR increased with biochar-application rate, whereas biochar particle size (< 1 mm and > 1 mm) had no significant effects on SWR. A 2-D flow chamber experiment with point source water application at the surface was used to continuously monitor the flow pattern and soil water content (SWC) distribution across the soil profile during infiltration and redistribution periods. Biochar–soil mixtures (2% and 5% w/w) with the three biochar types were studied. Flow chamber results showed substantial differences in plumes shape and internal SWC distribution in the biochar–soil mixtures compared to the untreated sand during the wetting and drainage. The plumes' shape and nonmonotonic spatial SWC distribution suggested unstable flow in the biochar–soil mixtures. The internal fingers developed in the biochar treated sand indicate that SWR induced by biochar addition seemed to be physically induced via the blending of the biochar and soil particles, rather than chemically induced, via coating of the soil particles with amphiphilic molecules. Under field conditions, the primary and inner finger-like plumes may eventually form preferential flow paths that will affect the spatial distribution of water and fertilizer in soil profiles and their availability to plant roots.

High-resolution velocity estimation from surface-based common-offset GPR reflection data

SUMMARY Surface-based common-offset ground-penetrating radar (GPR) reflection profiling is a popular geophysical exploration technique for obtaining high-resolution images of the shallow subsurface in a cost-effective manner. One drawback of this technique is that, without complementary borehole information in form of dielectric permittivity and/or porosity logs along the profile, it is currently not possible to obtain reliable estimates of the high-frequency electromagnetic velocity distribution of the probed subsurface region. This is problematic because adequate knowledge of the velocity is needed for accurate imaging and depth conversion of the data, as well as for quantifying the distribution of soil water content. To overcome this issue, we have developed a novel methodology for estimating the detailed subsurface velocity structure from common-offset GPR reflection measurements, which does not require additional conditioning information. The proposed approach combines two key components: diffraction analysis is used to infer the smooth, large-scale component of the velocity distribution, whereas the superimposed small-scale fluctuations are inferred via inversion of the reflected wavefield. We test and validate our method on two synthetic data sets having increasing degrees of complexity and realism before applying it to a field example from the Boise Hydrogeophysical Research Site, where independent control data in the form of neutron–neutron porosity logs are available for validation. The results obtained demonstrate the viability and robustness of the proposed approach. Further, due to its efficiency, both in terms of field effort and computational cost, the method can be readily extended to 3-D, which further enhances its attractiveness compared to multi-offset-based GPR velocity estimation techniques.

The Dominant Control of Relief on Soil Water Content Distribution During Wet‐Dry Transitions in Headwaters

AbstractThe redistribution of hillslope soil water during and after rainstorms is affected by soil properties and topography. Therefore, understanding how soil‐terrain attributes affect the soil volumetric water content (VWC) distribution under various catchment storages is a prerequisite for accurate hydrological modeling. Herein, the relationships between soil‐terrain attributes and soil VWC were examined in a steep (average slope = 60%), forested, zero‐order catchment. Detailed topography, soil properties, and runoff, and high frequency (6‐min) soil moisture data observed from July 2016 to November 2017 were employed along with resampled VWC data in three time‐steps (daily, hourly, and 6‐min intervals) for correlation analysis between soil‐terrain attributes and measured VWC. The results showed that daily aggregated data overlooked the highly heterogeneous flow conditions under dry‐wet transitions, leading to a constantly high correlation between VWC and the topographic wetness index (TWI). In comparison, the 6‐min data captured short‐lived flow processes, highlighting that, in reality, VWC‐TWI correlations fluctuated frequently as a function of catchment storages and precipitation characteristics, with four wetting patterns identified. Under wet‐dry transitions, the VWC‐TWI correlations increased significantly (from 0.3 to 0.6) as the hillslope gradually drained of lateral subsurface water. Soil water behavior consists of diurnal oscillations superimposed on a declining trend, which demonstrates the existence of unsaturated flow and which contributed to the increase in the strength of the correlations. Finally, through correlation analysis of all drying (n = 4) and wetting (n = 45) periods, we found that soil‐terrain attributes and VWC correlations can be used to identify the dominant soil water processes in various catchment storages.

Long-term study on the seasonal water uptake of Platycladus orientalis in the Beijing mountain area, northern China

Understanding how extreme drought and precipitation influence the water uptake patterns of vegetation is essential to assessing the responses of vegetation to climate change and evaluating regional water security. We introduced stable hydrogen and oxygen isotopes to determine the seasonal pattern of water uptake by the evergreen coniferous species Platycladus orientalis from June 2011 to June 2014 in the semiarid rocky areas of North China with shallow soil. Temporal variations in the soil water content (SWC), transpiration and root distribution were examined to analyze the key role of groundwater in plant water sources during the extreme drought. Platycladus orientalis showed great dependence on the surface soil water (0–20 cm) and a water uptake proportion greater than 50% in all rainy seasons (June–September) was observed. Our study also revealed that P.orientalis preferred to use upper soil moisture (0–40 cm) in the dry seasons as long as the supply met the demand, and it would turn to deep soil water and groundwater under dramatic reduction in the surface soil water content. During prolonged extreme drought, groundwater use increased from 25.3% in the early period to 61.3% in the late period. These findings suggest that P. orientalis water uptake has great plasticity and that its water uptake depth varies with a decline in soil water. In addition, a long extreme drought period will lead to a preference for and an increase in groundwater use, which means that P. orientalis can adapt well to extreme climates.

Effect of biocrusts on profile distribution of soil water content and salinity at different stages of evaporation

In this study, the effect of biocrusts on daily variations of evaporation in soils of an arid rangeland was investigated during 142 days. Temporal and spatial variations of soil water and solute content were analyzed in three times of 17, 50 and 103 days, and four depths of 10, 20, 30 cm and 40 cm in six columns (20 cm diameter × 50 cm height) containing biocrusts (43 cm saline soil + 5 cm biocrusts) and four columns filled with bare soil (48 cm saline soil) respectively. Temporal variation of evaporation rate was supposed as a 3-stages process. Results showed that stage 1 and stage 2 are not distinguishable. We further found that, compared to bare soil, the biocrusts retarded water evaporation from soil surface (24 vs. 30 mm and 34 vs. 44 mm after 17 and 50 days respectively), and reduced solute accumulation in top soil, especially in the stage 2 of evaporation process (5 vs. 8 dS/m and 13 vs. 20 dS/m after 17 and 50 days respectively). The effect of biocrusts on the evaporation rate at the stage 3 of evaporation was not significant (9 vs. 9 mm after 103 days). Reduction of evaporation by biocrusts resulted in less soil salinity within entire soil profile. The effects of biocrusts on evaporation reduction are attributed to i) the biocrusts create mats on the soil surface, and swelling of mats blocks the soil pores in wet condition, and ii) the large pores of biocrusts disrupt the hydraulic connectivity between topsoil and subsoil, which minimizes the capillary rise. Since major part of water is evaporated during stage 2, and is controlled by water flow within the soil and at the surface, so conservation of the BSCs would be important for preserving hydrological functions advantage such as reducing evaporation and enhancing water retention in the salt affected and dry soils.

An attempt to find a suitable place for soil moisture sensor in a drip irrigation system

Determination of an appropriate location for monitoring soil water content (SWC) is a key factor in efficient use of water in precision agriculture, however, the main challenge is the dynamic movement of water and root development in the soil profile. The objective of this study was investigating how SWC distribution in a loam soil profile at two growth stages of maize may impact the suitable location for SWC monitoring in a drip-tape irrigation system. A new concept, Average Moisture Representative Surface (AMRS) was proposed to determine the surface of the soil profile, which represent the average soil moisture of the wetted volume. SWC samples were taken during two irrigation intervals (48~52 days after planting (DAP) and 68~73 DAP) and root growth pattern was studied through root length density (RLD) at 50 and 100 DAP. The results revealed that a non-uniform wetting pattern after irrigation limits the appropriate locations for SWC monitoring to point measurements and with time, SWC depletion resulted in enlarging AMRS. At the end of growing season, an increase of root growth around the drippers increased the variation of root water uptake in different soil layers, and thus optimal place for soil sensors was limited to the upper layers, where the maximum root water uptake occurred. Overall, it is recommended to install soil sensors, such as tensiometers and TDRs at a horizontal distance of 5~20 cm from the crop and a depth of 10~20 cm from the soil surface while drip-tape is aligned close to the maize row.