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Precipitation is the most important water resource in semi-arid regions of China. The redistribution of precipitation among atmospheric water, soil water and groundwater are related to the land surface afforested ecological system. The study took widely replanted Pinus sylvestris var. Mongolica (PSM) in Mu Us Sandy Land (MUSL) as a research object and monitored precipitation, soil moisture, sap flow, and deep soil recharge (DSR) to find out moisture distribution in shallow soil layers. Results showed that the restoration process of PSM in MUSL changed the distribution of precipitation, with part of it infiltrating downward as DSR and part of it being stored in the shallow soil. Consequently, evapotranspiration increased and DSR significantly decreased, resulting in up to 466.9 mm of precipitation returning to the atmosphere through evapotranspiration in 2016. Vegetation increased soil water storage (SWS) capacity, with maximum SWS in PSM plot and bare sandy land (BSL) being 260 mm and 197 mm per unit horizontal area, respectively in 2016. DSR decreased from 54% of precipitation in the BSL plot to 0.2% of precipitation in the PSM plot in 2016. A great portion of infiltrated water was stored in the PSM ecosystem, resulting in a time lag of infiltration to reach the deep soil layer, and the infiltration rate in the BSL plot was 11 times of that in the PSM plot. SWS decreased 16 mm and 7.6 mm per unit horizontal area over a one-year period (from March to October, non-freezing time) in 2017 and 2019, respectively. The PSM annual sap flow was maintained at a relatively constant level of 154 mm/yr. Through in-situ measurement and comparative analysis of the precipitation redistribution of the BSL plot and the PSM plot, we find that PSM can significantly reduce the shallow soil water storage and DSR. However, substantial reduction of shallow soil water storage and DSR is detrimental for the long-term development of PSM forest. Therefore, it is necessary to reduce PSM density to cut the water consumption by PSM per unit area, thus to augment the shallow SWS and DSR, which will be beneficial for the PSM to survive under extreme drought conditions in the future. This study helps us understand the role of precipitation-induced groundwater recharge in the process of vegetation restoration in semi-arid regions and explains the possible causes of PSM forest degradation.

Objective Vadose zone well (VZW) injection is an effective method of managed aquifer recharge (MAR). To improve VZW injection management, it is of great importance to accurately describe the unsaturated zone properties. Several analytical models have been developed for VZW injection based on the two-parameter constitutive model (Gardner model). As the three-parameter model (MB model) and four-parameter model (MN model) have been proposed, it is of interest to know whether the application of more flexible constitutive models is able to improve the analysis of unsaturated-saturated flow induced by VZW injection. Methods In this study, the MN model (which includes the Gardner model and MB model as subsets) was employed to establish a VZW injection model. The model was solved using COMSOL Multiphysics. The results are utilized to investigate the unsaturated-saturated flow induced by VZW injection for different values of the unsaturated zone properties to analyse the influences of the ground surface flux (GSF), and to compare the hydraulic responses based on different exponential constitutive models. Results The analysis demonstrates that the hydraulic response induced by VZW injection and the influence of ground surface flux are affected by the hydraulic conductivity and the water storage capacity of the unsaturated zone. The relative hydraulic conductivity exponent ωk affects the change in hydraulic conductivity of the unsaturated zone. The moisture retention exponent ωc affects the water storage capacity of the unsaturated zone. The approximation of ωk=ωc=ω will result in some errors in the calculation and prediction of the hydraulic response caused by VZW injection. When the absolute value difference of the pressure head threshold b1=ψa-ψk is small, its effect on the water head increment is small. In this case, it is reasonable to assume that b1=ψa-ψk=0. Conclusion This study can help scholars improve the understanding of the VZW injection process and has important practical significance for the design, implementation and management of injection schemes.

Objective The conceptual model of the single-well push test is a hot topic in groundwater hydrogeology. Methods In this study, a new mathematical model was developed for radial solute transport in an aquifer near injection wells. The heterogeneity of the aquifer was considered, and the MIM (Mobile-Immobile) convective diffusion model was used to describe the solute transport process in the aquifer. The skin effect and mixing effect are also included in this conceptual model. The semi-analytical solution was derived by using the Laplace transform and Stehfest numerical inverse transform methods. The influence of effective porosity and radial dispersion of the skin zone and the radius of the wellbore on the solute breakthrough curves (BTCs) of a fixed observation point and solute concentration distribution curves at given times were investigated. Results Results show that wellbore mixing and skin effects have significant impacts on BTCs, solute radial transport processes and the influence area. The larger the radius of the wellbore is, the more obvious the wellbore mixing effect is. For the skin zone, a larger porosity leads to a smaller velocity of solute migration. The larger the radial dispersion is, the steeper the solute concentration curve of the observation point is, indicating that the solute concentration changes at a faster rate and can reach a stable value earlier. Conclusion Compared with previous studies, this model can better describe the solute radial dispersion process near the injection wells.

Objective Groundwater numerical models often have uncertainties due to the complexity of the hydrogeological conditions and the economic and time constraints in collecting a sufficiently large dataset as inputs for conducting modelling exercises. In the past 50 years, stochastic methods have been one of the main methods of uncertainty analysis. The interval uncertainty is different from the stochastic uncertainty, and it considers the hydrogeological parameters as the intervals (ranges) without considering their stochastic properties. Methods From the perspective of interval uncertainty, a numerical simulation method based on first-order perturbation expansion was proposed for simulating unsteady two-dimensional confined flow with known hydrogeological parameters as intervals in this paper.The proposed method is implemented based on GFModel, a three-dimensional (3D) numerical simulation platform for groundwater flow and pollutant migration. Results The analysis shows that the relative error can be controlled within 10% when the parameter change rate is less than 0.1. The computational efficiency of the proposed method is obviously higher than that of the continuous sampling method with equal spacing. Conclusion This method allows the interval of the head or water budget to be calculated without the requirement for detailed statistical information (which is usually unavailable in advance) if the intervals of hydrogeological parameters are known.It provides a theoretical basis for decisions on the use and protection of groundwater resources.

Study region: China Study focus: This study focuses on the continental-scale spatiotemporal calibration of the Blaney–Criddle equation (BC) for different climate zones in China. Daily meteorological data from 82 weather stations in China were collected for the periods of 1970–2004 and 2005–2014, respectively, to calibrate and validate the BC model. The coefficients of the BC model were spatially and temporally calibrated in different climate types via a linear regression method. New hydrological insights for the region: (1) Performance of the quarterly calibrated BC model was the most satisfactory. Statistical indices were in the range of 0.449–0.797 mm d−1 for the mean absolute error, 0.567–0.969 mm d−1 for the root mean square error and 0.82–0.953 for the index of agreement. The accuracy ranges increased from 42.95–96.56 % to 87.18–98.74 %; (2) Correction coefficient b and a for every quarter was mapped. Value of b showed a bell-shaped consist with temperature, while a showed an opposite trend. The quarterly calibrated BC model provided satisfactory results in the temperate monsoon and temperate continental climate zones; and (3) results indicated maximum temperature was the most influential factor for reference evapotranspiration (ET0), followed by minimum temperature. This study provides a more accurate BC model to calculate ET0 for different climate types in China and a reference for agricultural decision-makers in irrigation areas that lack sufficient meteorological data.

Frequent dust storms on the Mongolian Plateau have adversely affected the ecological environmental quality of East Asia. Studying the dynamic changes in vegetation coverage is one of the important means of evaluating ecological environmental quality in the region. In this study, we used Landsat remote sensing images from 2000 to 2019 on the Mongolian Plateau to extract yearly Normalized Difference Vegetation Index (NDVI) data during the growing season. We used partial correlation analysis and the Hurst index to analyze the spatiotemporal characteristics of the NDVI before and after the establishment of nature reserves and their influencing factors on the GEE cloud platform. The results showed that (1) the proportion of the region with an upwards trend of NDVI increased from 52.21% during 2000–2009 to 67.93% during 2010–2019, indicating a clear improvement in vegetation due to increased precipitation; (2) the increase in precipitation and positive human activities drove the increase in the NDVI in the study region from 2000 to 2019; and (3) the overall trend of the NDVI in the future is expected to be stable with a slight decrease, and restoration potential is greater for water bodies and grasslands. Therefore, it is imperative to strengthen positive human activities to safeguard vegetation. These findings furnish scientific evidence for environmental management and the development of ecological engineering initiatives on the Mongolian Plateau.

AbstractAbrupt changes in aperture (sudden expansion and contraction) are commonly seen in naturally occurred or artificial single fractures. The relevant research mainly focuses on the changes in fluid properties caused by the sudden expansion of the aperture in smooth parallel fractures. To investigate the effects of roughness on the nonlinear flow properties in a single rough fracture with abruptly aperture change (SF-AC), the flow characteristics of the fractures under different Reynolds numbers Re (50~2000) are simulated by the turbulence k-ε steady-state modulus with the Naiver-Stokes equation. The results show that, in a rough SF-AC, the growth of the eddy and the flow path deflection of the mainstream zone are more obvious than those in a smooth SF-AC, and the discrepancies between the rough and smooth SF-ACs become even more obvious when the relative roughness and/or Re values become greater. The increase of the fracture roughness leads to the generation of more local eddies on the rough SF-ACs and enhances the flow path deflection in the sudden expansion fracture. The number of eddies increases with Re, and the size of eddy area increases linearly with Re at first. When Re reaches a value of 300-500, the growth rate of the eddy size slows down and then stabilizes. Groundwater flow in a rough SF-AC follows a clearly visible nonlinear (or non-Darcy) flow law other than the linear Darcy’s law. The Forchheimer equation fits the hydraulic gradient-velocity (J-v) better than the linear Darcy’s law. The corresponding critical Re value at which the nonlinear flow starts to dominate in a rough SF-AC is around 300~500.

Understanding solute transport through macroscopic interfaces is essential to understand the effects of geological heterogeneity on contaminant transport in porous media. Studies of solute transport in compartmental porous media have noted the asymmetry of breakthroughs (BTCs) in solute movement across material interfaces, indicating the presence of discontinuous concentration that makes solute transport directionally dependent. Transition interfaces are more common in nature than sharp interfaces. To understand solute transport across transition interfaces, well-controlled laboratory experiments were performed. A numerical model was also built to understand mass accumulation and concentration discontinuity through transition as well as sharp interfaces. We conclude that directionally dependent asymmetry of BTCs was found with both sharp and transition interfaces. The asymmetry of BTCs was more pronounced at a transition interface than at a sharp interface. The mobile and immobile (MIM) model can better capture the directionally dependent transport of solutes through a sharp/transition interface than the advection-dispersion equation (ADE). The mobile water partition coefficient (β) and mass transfer coefficient (ω) in MIM were lager in the direction from fine sand to coarse sand (F-C). The time difference between tracer replace and tracer input is greater in the presence of an interface, especially transition interfaces. Even at small Reynolds numbers (1 × 10−4 to 0.116), solute transport across a discontinuous interface under reversible flow directions is most likely dominated by convective dispersion rather than an assumed diffusion process.

The ability of plants to absorb unsaturated atmospheric water vapor is a controversial topic. To study how vegetation in arid areas survives under limited water resources, this study uses Tamarisk in the Ulan Buh Desert of China as an example. The in-situ observation of a newly designed Lysimeter and sap flow meter system were used to monitor the precipitation infiltration and the utilization efficiency of Tamarisk of atmospheric vapor. The results show that the annual precipitation of 84 mm in arid areas could still result in deep soil recharge (DSR) with a recharge rate of 5 mm/year. Furthermore, DSR is detectable even in the winter, and the 5-year average DSR was 5.77% of the annual precipitation. It appears that the small precipitation events are critically important for the survival of Tamarisk. When the atmospheric relative humidity reaches 70%, Tamarisk leaves can absorb the unsaturated atmospheric vapor, which accounts for 13.2% of the annual precipitation amount. To adapt to the arid environment, Tamarisk can harvest its water supply from several sources including atmospheric vapor and micro-precipitation events (whose precipitation is below the measurement limit of 0.2 mm of the precipitation gauge) and can still permit a certain amount of recharge to replenish the deep soil moisture. Such an ecohydrological dynamic is of great significance to desert vegetation.

Abstract Accurate estimation of the temporal and spatial root water uptake patterns in root zone is needed for an improved understanding of water and chemical transport dynamics in vadose zone. Rooting system is of great importance to describe the plant root water uptake directly. In this study, the diffusive root growth model was coupled with the Environmental Policy Integrated Climate (EPIC) crop growth model through changing the boundary condition, and a new semianalytical solution of the modified root diffusive model was derived considering the dynamic root length density distribution simulated by the EPIC crop growth model. To test the modified root growth model, the field‐measured data of maize (Zea mays L.) and tomato (Solanum lycopersicum L.) root length density distribution were used for parameter optimization. A MATLAB program was developed by coupling the modified root diffusive model with the genetic algorithm to facilitate the parameters optimization. Results showed that the simulated root length density distribution at different times was in a good agreement with the observed values. The RMSE and bias values ranged from 0.22 to 0.25 cm cm–3 and from −3.0 to 24.5%, respectively. The modified diffusive root growth model can therefore be used to simulate the two‐dimensional root growth during the crop growing period.

BackgroundChina’s so-called Three North Shelterbelt Program (3NSP) has produced a vast area of lined forest reconstruction in the semi-arid regions. This study uses the lined rain-fed Pinus sylvestris var. mongolica (PSM) sand-fixing forest in the eastern part of Mu Us Sandy Land in Northwestern China as an example to investigate the ecohydrological process in this region. Rain gauges, newly designed lysimeters and soil moisture sensors are used to monitor precipitation, deep soil recharge (DSR) and soil water content, where DSR specifically refers to recharge that can reach a depth more than 200 cm and eventually replenish the underneath groundwater reservoir.ResultsThis study shows that there are two obvious moisture recharge processes in an annual base for the PSM forest soil: a snowmelt-related recharge process in the spring and a precipitation-related recharge process in the summer. The recharge depth of the first process can reach 180 cm without DSR occurring (in 2018). The second process results in noticeable DSR in 2018. Specifically, the DSR values over 2016–2018 are 1, 0.2, and 1.2 mm, respectively. To reach the recharge depths of 20, 40, 80, 120, 160, and 200 cm, the required precipitation intensities have to be 2.6, 3.2, 3.4, 8.2, 8.2, and 13.2 mm/d, respectively. The annual evapotranspiration in the PSM forest is 466.94 mm in 2016, 324.60 mm in 2017, and 183.85 mm in 2018.ConclusionThis study concludes that under the current precipitation conditions (including both dry- and wet-years such as 2016–2018), water consumption of PSM somewhat equals to the precipitation amount, and PSM has evolved over years to regulate its evapotranspiration in response to annual precipitation fluctuations in Mu Us Sandy Land of China.

Nitrogen and phosphorus are commonly recognized as causing eutrophication in aquatic systems, and their transport in subsurface environments has also aroused great public attention. This research presented four natural clay minerals (NCMs) evaluated for their effectiveness of NH4+ and PO43- adsorption from wastewater. All the NCMs were fully characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), BET analysis, and adsorption kinetics and isotherms to better understand the adsorption mechanism-property relationship. The results show that the adsorption efficiency of the four NCMs for phosphate was better than that for ammonia nitrogen. The removal rate of phosphate was higher than 65%, generally in the range of 80%-90%, while the removal rate of ammonia nitrogen was less than 50%. The adsorption kinetic behavior followed the pseudo-second-order kinetic model. The ammonia nitrogen adsorption isotherm was in good agreement with the Freundlich isotherm equilibrium model, and the phosphate adsorption isotherm matched the Langmuir model. Among all the NCMs studied, bentonite (7.13 mg/g) and kaolinite (5.37 mg/g) showed higher adsorption capacities for ammonia nitrogen, while zeolite (0.21 mg/g) and attapulgite (0.17 mg/g) showed higher adsorption capacities for phosphate. This study provides crucial baseline knowledge for the adsorption of nitrogen and phosphate by different kinds of NCMs.

Reactive solute transport in a stratified multi-layer system (such as a fracture-matrix or an aquifer-aquitard system) is a classical problem and serves as a benchmark for testing new ideas and theories in subsurface hydrology. This system resembles a “sandwich” model that includes a much permeable layer (such as a fracture or an aquifer) bounded by two much less permeable layers (such as rock matrixes or aquitards). Transport in such a system usually involves fully coupled processes at the interfaces of different layers. The system becomes asymmetric when the less permeable layers have different transport properties. The scale dependency of dispersion in a fully coupled stratified system has never been examined before. This study constructs new transport models and associated solutions to describe reactive solute transport in such a stratified system considering scale-dependent as well as scale-independent dispersion schemes. We will analyze the spatiotemporal concentration distributions based on the newly developed solutions. Breakthrough curves (BTCs) obtained from the models are tested against the experimental data to evaluate the effect of scale dependency of dispersion, and the analysis shows that dispersivity appears to be scale-independent in the experiments of concern. Meanwhile, this study reveals the possible existence of a dynamic diffusion phenomenon in the less permeable layers as the diffusion coefficients in those layers appear to decline with the advective time in the permeable layer. This may be attributed to the increasingly tortuous pathways for the diffusion to take place in those layers when advective time increases. However, theoretical understanding of such a dynamic diffusion phenomenon is still not available and needs further investigation. Keywords: Stratified layers, Reactive transport, Time-dependent, Dispersion, Diffusion

Heavy metals such as Cu(II) are widespread in the environment, and the impact of heavy metals on the environment of soils depends on the ability of soils to immobilize these pollutants. It is necessary to investigate the mechanism of interaction between heavy metal and soil from a soil remediation perspective. In this study, a series of experiments were conducted to investigate the adsorption and desorption behavior of Cu(II) in silty clay. Several impact factors such as pH, organic matter, temperature, and coexisted ions Zn(II) were considered. It was found that the adsorption process reached equilibrium after 4 hours of the experiment, and the data can be fitted well by the Elovich model and the double-constant model for the kinetic sorption process. The isothermal adsorption results showed that the adsorption rate reached a peak value when the initial concentration was about 20 mg L−1. The decrease of H+ can increase the adsorption activity of Cu(II) and reduce the ability of the desorption of Cu(II) ions. The adsorption capacity of Cu(II) is less than the desorption capacity under the condition of strong acidity and low concentration of Cu(II). In addition, the adsorption capacity of the native soil on Cu(II) was larger than that of the soil with the removal of organic matter, while the opposite was true for the desorption capacity on Cu(II). The maximum adsorption of Cu(II) occurred at 35°C for this study, and the binding energy increased as the temperature increased. Thermodynamic analysis revealed that the adsorption process of Cu(II) was spontaneous and endothermic. The Freundlich, Langmuir, Temkin, and Henry adsorption models were used for analyzing the adsorption isotherm of Cu(II), and it was found that the Freundlich model agreed the best with the experimental data compared with other three models. The results of the competitive adsorption experiments indicated that the competitive capacity of Cu(II) was greater than that of Zn(II) in low-permeability media such as silty clay, and the existence of binary metals can weaken the adsorption force between the single metal and the soil surface.

Solute transport in low-permeability media such as clay has not been studied carefully up to present, and we are often unclear what the proper governing law is for describing the transport process in such media. In this study, we composed and analyzed the breakthrough curve (BTC) data and the development of leaching in one-dimensional solute transport experiments in low-permeability homogeneous and saturated media at small scale, to identify key parameters controlling the transport process. Sodium chloride (NaCl) was chosen to be the tracer. A number of tracer tests were conducted to inspect the transport process under different conditions. The observed velocity-time behavior for different columns indicated the decline of soil permeability when switching from tracer introducing to tracer flushing. The modeling approaches considered were the Advection-Dispersion Equation (ADE), Two-Region Model (TRM), Continuous Time Random Walk (CTRW), and Fractional Advection-Dispersion Equation (FADE). It was found that all the models can fit the transport process very well; however, ADE and TRM were somewhat unable to characterize the transport behavior in leaching. The CTRW and FADE models were better in capturing the full evaluation of tracer-breakthrough curve and late-time tailing in leaching.

Abstract Soft magnetic materials (SMMs) are indispensable for electromechanical energy conversion in high-efficiency applications, but they are exposed to increasing mechanical loading conditions in electric motors due to higher rotational speeds. Enhancing the yield strength of SMMs is essential to prevent the degradation in magnetic performance and failure from plastic deformation, yet most SMMs have yield strengths far below one gigapascal. Here, we present a multicomponent nanostructuring strategy that doubles the yield strength of SMMs while maintaining ductility. We introduce morphologically anisotropic nanoprecipitates through dislocation-driven precipitation induced by preceding deformation during heat treatment in an iron–nickel–cobalt–tantalum material. With all dimensions of the precipitates below the magnetic domain wall width, we achieve a high precipitate number density with a large specific surface area, small interprecipitate spacing, and high lattice mismatch, which impede dislocation glide and strengthen the material. Both the matrix and precipitates are ferromagnetic, yielding a high magnetic moment. This nanostructuring approach offers a pathway to two-gigapascal-strong ductile SMMs with moderately increased coercivity that can be tolerated in exchange for significantly improved mechanical performance for sustainable electrification.

Abstract Aerosol hygroscopic behavior plays a central role in determining climate effects and environmental influence of atmospheric particulates. Water-soluble organic acids (WSOAs) constitute a significant fraction of organic aerosols. These organic acids have a complex impact on aerosol hygroscopicity due to their physical and chemical interactions with atmospheric inorganic salts. The mixing of WSOAs with inorganic salts exerts a multiple influence on the hygroscopic growth and phase behaviors of aerosol particles, largely depending on the composition ratio, acid properties, particle size and interactions between particle components. The WSOAs play a critical role in determining water uptake characteristics of aerosol particles, especially in the low and moderate RH ranges. The previous studies reveal the occurrence of aerosol chemistry related to chloride/nitrate/ammonium depletions in aerosol droplets containing WSOAs and inorganic salts. The potential influence of WSOAs on the atmospheric recycling of HCl/HNO3/NH3 due to the chloride/nitrate/ammonium depletion may contribute to the atmospheric budget of reactive gases. A fundamental understanding for the hygroscopic behavior and aerosol chemistry of inorganic–WSOA systems is essential for the accurate parameterization of aerosol behaviors in atmospheric models. However, there is still lack of a comprehensive understanding of the hygroscopicity and related aerosol chemistry of internally mixed inorganic–WSOA systems. The present review comprehensively summarizes the impacts of WSOAs on hygroscopicity and phase changes of atmospherically relevant inorganic salts in aerosol particles especially under subsaturated conditions, and overviews the recent advances on aerosol chemistry related to the hygroscopic process for the internally mixed inorganic–WSOA aerosols.