Soil Water Transport Research Articles

Coupling HYDRUS-1D with ArcGIS to estimate pesticide accumulation and leaching risk on a regional basis

HYDRUS-1D is a well-established reliable instrument to simulate water and pesticide transport in soils. It is, however, a point-specific model which is usually used for site-specific simulations. Aim of the investigation was the development of pesticide accumulation and leaching risk maps for regions combining HYDRUS-1D as a model for pesticide fate with regional data in a geographical information system (GIS). It was realized in form of a python tool in ArcGIS. Necessary high resolution local soil information, however, is very often not available. Therefore, worldwide interpolated 250-m-grid soil data (SoilGrids.org) were successfully incorporated to the system. The functionality of the system is shown by examples from Thailand, where example regions that differ in soil properties and climatic conditions were exposed in the model system to pesticides with different properties. A practical application of the system will be the identification of areas where measures to optimize pesticide use should be implemented with priority.

A New Scheme for Considering Soil Water‐Heat Transport Coupling Based on Community Land Model: Model Description and Preliminary Validation

AbstractLand surface models (LSMs) have developed significantly over the past few decades, with the result that most LSMs can generally reproduce the characteristics of the land surface. However, LSMs fail to reproduce some details of soil water and heat transport during seasonal transition periods because they neglect the effects of interactions between water movement and heat transfer in the soil. Such effects are critical for a complete understanding of water‐heat transport within a soil thermohydraulic regime. In this study, a fully coupled water‐heat transport scheme (FCS) is incorporated into the Community Land Model (version 4.5) to replaces its original isothermal scheme, which is more complete in theory. Observational data from five sites are used to validate the performance of the FCS. The simulation results at both single‐point and global scale show that the FCS improved the simulation of soil moisture and temperature. FCS better reproduced the characteristics of drier and colder surface layers in arid regions by considering the diffusion of soil water vapor, which is a nonnegligible process in soil, especially for soil surface layers, while its effects in cold regions are generally inverse. It also accounted for the sensible heat fluxes caused by liquid water flow, which can contribute to heat transfer in both surface and deep layers. The FCS affects the estimation of surface sensible heat (SH) and latent heat (LH) and provides the details of soil heat and water transportation, which benefits to understand the inner physical process of soil water‐heat migration.

Arbuscular Mycorrhiza Improves Substrate Hydraulic Conductivity in the Plant Available Moisture Range Under Root Growth Exclusion.

Arbuscular mycorrhizal fungi (AMF) proliferate in soils and are known to affect soil structure. Although their contribution to structure is extensively investigated, the consequences of those processes for soil water extractability and transport has, so far, gained surprisingly little attention. Therefore we asked, whether AMF can affect water retention and unsaturated hydraulic conductivity under exclusion of root ingrowth, in order to minimize plant driven effects. We carried out experiments with tomato inoculated with Rhizoglomus irregulare in a soil substrate with sand and vermiculite that created variation in colonization by mixed pots with wild type (WT) plants and mycorrhiza resistant (RMC) mutants. Sampling cores were introduced and used to assess substrate moisture retention dynamics and modeling of substrate water retention and hydraulic conductivity. AMF reduced the saturated water content and total porosity, but maintained air filled porosity in soil spheres that excluded root ingrowth. The water content between field capacity and the permanent wilting point (6–1500 kPa) was only reduced in mycorrhizal substrates that contained at least one RMC mutant. Plant available water contents correlated positively with soil protein contents. Soil protein contents were highest in pots that possessed the strongest hyphal colonization, but not significantly affected. Substrate conductivity increased up to 50% in colonized substrates in the physiologically important water potential range between 6 and 10 kPa. The improvements in hydraulic conductivity are restricted to substrates where at least one WT plant was available for the fungus, indicating a necessity of a functional symbiosis for this effect. We conclude that functional mycorrhiza alleviates the resistance to water movement through the substrate in substrate areas outside of the root zone.

Macropores regulate CO2 behavior in an andisol soil

Macropores resulting from soil pedogenesis, biological activity, and agricultural practices play important roles in soil water, chemical and gas transport; however, seldom studies focus on the effect of soil macropores on CO2 behavior. In this study, a 150-day soil column incubation experiment was conducted to investigate the CO2 behavior in a homogeneous soil column and a soil column with an artificial macropore, which have the same total porosity. The results showed that the cumulative CO2 fluxes observed in the soil with a macropore (57.2 g m−2) were higher than those in the homogeneous soil (52.7 g m−2). The soil cumulative CO2 fluxes measured using column incubation can fit the kinetic model, and a higher carbon mineralization rate in the soil with the macropore was found. The results of the incubation experiment also suggest that macropore increased the gas diffusivities, and thus decreased the CO2 concentrations in the soil profile. This study proposed a simulation experiment and quantified the effect of macropore on soil CO2 behavior, which could help to understand the mechanism of CO2 emission from soil with macropores especially caused by agricultural practices.

Numerical investigation of water evaporation from Fontainebleau sand in an environmental chamber

The global warming and climate change have aroused increasing attention to the effect of intensive soil water evaporation on earth constructions. When investigating the hydro-mechanical behavior of soil during evaporation, it is of paramount importance to consider the coupled transport of heat and water in soil. In this study, four different evaporation tests of Fontainebleau sand conducted in an environmental chamber are investigated. A numerical approach combing a coupled hydro-thermal model and a soil-atmosphere interaction model is adopted, enabling the numerical estimation of soil temperature and soil water content variations. Comparison between the measurements and the simulation results shows the relevance of the proposed approach with consideration of suitable thermal and hydraulic boundary conditions and soil parameters. Further inspection on the variations of soil water content and soil temperature suggests the significant influence of water and heat flux boundary conditions at the soil-atmosphere interface. In this study, it is proposed to estimate the mode of soil temperature variations directly based on the observed actual evaporation curve. Moreover, the simulation results also reveal the necessity of accounting for the dry layer formed in sand during evaporation.

Physically Accurate Soil Freeze‐Thaw Processes in a Global Land Surface Scheme

AbstractThe model Soil‐Litter‐Iso (SLI) calculates coupled heat and water transport in soil. It was recently implemented into the Australian land surface model CABLE, which is the land component of the Australian Community Climate and Earth System Simulator (ACCESS). Here we extended SLI to include accurate freeze‐thaw processes in the soil and snow. SLI provides thence an implicit solution of the energy and water balances of soil and snow as a standalone model and within CABLE. The enhanced SLI was tested extensively against theoretical formulations, laboratory experiments, field data, and satellite retrievals. The model performed well for all experiments at wide‐ranging temporal and spatial scales. SLI melts snow faster at the end of the cold season compared to observations though because there is no subgrid variability within SLI given by the implicit, coupled solution of energy and water. Combined CABLE‐SLI shows very realistic dynamics and extent of permafrost on the Northern hemisphere. It illustrated, however, also the limits of possible comparisons between large‐scale land surface models and local permafrost observations. CABLE‐SLI exhibits the same patterns of snow depth and snow water equivalent on the Northern hemisphere compared to satellite‐derived observations but quantitative comparisons depend largely on the given meteorological input fields. Further extension of CABLE‐SLI with depth‐dependence of soil carbon will allow realistic projections of the development of permafrost and frozen carbon stocks in a changing climate.

Impact of Temporal Macropore Dynamics on Infiltration: Field Experiments and Model Simulations

Core Ideas Dynamics of earthworm activity change macropore networks temporally. In the topsoil, macropore numbers were largest in autumn and smallest in summer. Temporal dynamics in macropore networks affected their hydrological impact. Modeling confirmed that temporal macropore dynamics affect infiltration patterns. Macropores greatly affect water and solute transport in soils. Most macropores are of biogenic origin; however, the resulting seasonal dynamics are often neglected. Our study aimed to examine temporal changes in biopore networks and the resulting infiltration patterns. We performed infiltration experiments with Brilliant Blue on pastureland in the Luxembourgian Attert catchment (spring, summer, and autumn 2015). We developed an image‐processing scheme to identify and quantify changes in biopores and infiltration patterns. Subsequently, we used image‐derived biopore metrics to parameterize the ecohydrological model echoRD (ecohydrological particle model based on representative domains), which includes explicit macropore flow and interaction with the soil matrix. We used the model simulations to check whether biopore dynamics affect infiltration. The observed infiltration patterns revealed variations in both biopore numbers and biopore–matrix interaction. The field‐observed biopore numbers varied over time, mainly in the topsoil, with the largest biopore numbers in spring and the smallest in summer. The number of hydrologically effective biopores in the topsoil seems to determine the number and thereby the fraction of effective biopores in the subsoil. In summer, a strong biopore–matrix interaction was observed. In spring, the dominant process was rapid drainage, whereas in summer and autumn, most of the irrigated water was stored in the examined profiles. The model successfully simulated infiltration patterns for spring, summer, and autumn using temporally different macropore setups. Using a static macropore parameterization the model output deviated from the observed infiltration patterns, which emphasizes the need to consider macropores and their temporal dynamics in soil hydrological modeling.

Evaluating Hydraulic Properties of Biochar‐Amended Soil Aggregates by High‐Performance Pore‐Scale Simulations

Core Ideas SR‐mCT was used to characterize soil pore structures. Graphics processing unit–enhanced LBM was used to simulate the water flow. Effects of biochar amendment on soil hydraulic properties were evaluated. Biochar amendment can change the soil pore architecture and improve soil physical and hydraulic properties. In this study, two clayey soils were selected and treated with or without biochar amendment (four treatments). Synchrotron‐based X‐ray micro‐computed tomography (SR‐mCT) was used to characterize pore structures of the selected aggregates from the treatments. Based on the high‐resolution soil pore structure information, the three‐dimensional lattice Boltzmann (LB) method was used to simulate water flow in pores. The high‐resolution soil pore structures were also used to define the internal boundary conditions in the LB method that was accelerated by graphics processing unit parallel computing. Results showed that biochar amendment increased soil permeability by at least one order of magnitude, reduced tortuosity by 20 to 30%, and resulted in more uniformly distributed pore water velocities, which was attributed to the change in soil pore geometry. This study is the first physically based modeling effort to assess the effect of biochar amendment on hydraulic properties of soil aggregates. The SR‐mCT method and modeling approach developed in the research play an important role in understanding the mechanisms of water and nutrient transport in soil at the pore scale.

Quantification of wetting front movement under the influence of surface topography

Soil surface topography affects fundamental hydrologic processes, such as infiltration and soil water percolation. Topographic variations potentially alter both the magnitude and directions of unsaturated flow. The objective of this study is to evaluate the effects of surface topography on wetting front moving patterns under different rainfall and soil conditions through combined experimental and numerical modelling studies. Specifically, laboratory-scale infiltration and unsaturated flow experiments and HYDRUS-2D modelling were conducted for different topographic surfaces, rainfall intensities, and soil types. The simulated and observed wetting front distributions were compared and evaluated. Two different stages were observed: topography-dominated two-dimensional flow and uniform one-dimensional flow. A uniformly distributed wetting front was eventually achieved although soil surfaces had dissimilar topographic characteristics. However, the timing or duration to reach such a uniform flat wetting front varied, mainly depending on surface topography, rainfall characteristics, and soil hydraulic properties. The findings from this study are important to better understand the mechanism of topography-controlled unsaturated flow, wetting front movement, and overland flow generation, and to further improve modelling of soil water flow and transport processes under such complex conditions across different scales.

Preferential flow in the vadose zone and interface dynamics: Impact of microbial exudates

In the hydrological cycle, the infiltration process is a critical component in the distribution of water into the soil and in the groundwater system. The nonlinear dynamics of the soil infiltration process yield preferential flow which affects the water distribution in soil. Preferential flow is influenced by the interactions between water, soil, plants, and microorganisms. Although the relationship among the plant roots, their rhizodeposits and water transport in soil has been the subject of extensive study, the effect of microbial exudates has been studied in only a few cases. Here the authors investigated the influence of two artificial microbial exudates–catechol and riboflavin–on the infiltration process, particularly unstable fingered flow, one form of preferential flow. Flow experiments investigating the effects of types and concentrations of microbial exudates on unstable fingered flow were conducted in a two-dimensional tank that was filled with ASTM C778 graded silica sand. The light transmission method (LTM) which is based on capturing the light intensity transmitted through a sand-water system and then converting it into degree of water saturation was used to visualize and characterize the flow of water in porous media as well as to image and measure the spatial and temporal distribution of water in porous media. Flow patterns, vertical and horizontal profiles of the degree of water saturation of the fingers, as well as measurements of the fingers dimension (width), number, and velocity were determined using the light transmission method. Interfacial experiments exploring the influence of microbial exudates on the wettability behavior of water were performed by measuring the contact angle and the interfacial tension of the (solid)-gas-microbial exudate solution systems. Unstable wetting front generating fingered flow was observed in all infiltration experiments. The experimental results showed that the microbial exudate addition affected the infiltration process, as the measurements of the degree of saturation profiles and widths of the fingers differed from those of the control NaCl solution. These differences may be due to an improved water holding capacity in the presence of the microbial exudates. The lowest catechol solution concentration (10 μM) produced the largest finger width (9.69 cm) among the tested catechol solution concentrations and all the other solutions including the control solution (7.24 cm). Moreover, the wettability of the medium for the catechol solution increased with an increase in concentration. The highest riboflavin solution concentration (1000 μM) generated the highest finger width (7.75 cm) among the tested riboflavin solution concentrations. However, the wettability of the medium for the riboflavin solution decreased with an increase in concentration. Our study demonstrated that the microbial exudates which are biochemical compounds produced and released by microbes in the environment are capable of influencing the soil infiltration process. The results of this study also demonstrated that the influence of the contact angle expressed as (cosθ)1/2 should be integrated in the scaling of the finger dimension, i.e., finger width, when the Miller and Miller (1956) scaling theory is applied for the hydrodynamic scaling in porous media.

Mathematical Models of Water and Solute Transport in Soil

Improved understanding of water flow and solute transport through the unsaturated zone is important for the sustainable management of soils. As soils are complex and heterogeneous systems, quantification of the transport processes is difficult. More knowledge on the relationship between solute transport process, soil structure, hydrologic initial and boundary conditions, and observation scale is needed here.Modeling unsaturated flow and transport with mathematical or numerical methods is an important tool for predicting the infiltration and redistribution of soil water and the transport of solutes in the unsaturated zone. Flow and transport models are commonly used to support the decision making process in agricultural management, environmental impact assessment, toxic waste control, remediation design, and subsurface cleanup monitoring. The movement of contaminants through porou media describs by the combination of advection, diffusion-dispersion and chemical retardation. The most common model that describes solute transport by convection and dispersion is the convection-dispersion equation (CDE). This equation describes the change in concentration at any point along the flow path as a function of time. This paper is mainly dedicated to a discussion of basic processes for modelling of water flow and contaminant transport in saturated and unsaturated soils. After a brief description of the classical approach for simulating water flow and solute transport in porous media, issues related to water and solute trasport equation in soil.

Assessing the Potential Exposure of Groundwater to Pesticides: A Model Comparison

Core Ideas Numerical models were compared for transport of four pesticides in nine pedoclimatic scenarios. Conceptual uncertainty in soil physical representation of pesticide transport was assessed. HYDRUS (2D/3D) can be used as an alternative model for pesticide assessment studies. Many pesticide‐leaching studies rely on the use of numerical models that account for various physicochemical and biological processes at a range of temporal and spatial scales. In leaching assessments for the registration of pesticides in the European Union and the United States, one‐dimensional models are used that describe the vertical transport of soil water and pesticides to groundwater. One‐dimensional models are most representative for spray and broadcast applications of pesticides, but they may not be adequate for spatially nonuniform applications such as microdrip irrigation. We tested whether an advanced numerical model capable of one‐, two‐, and three‐dimensional flow and pesticide transport simulation [HYDRUS (2D/3D)] provides results consistent with one‐dimensional numerical models commonly used for pesticide registration (PEARL, PELMO). Model comparison was used to assess conceptual uncertainty in soil physical representation of pesticide transport among numerical models across a range of applications. Simulations of soil water flow and pesticide and heat transport were conducted for four different hypothetical pesticides and nine European regulatory standard pedoclimatic scenarios (FOCUS). The results show that the predicted annual amount of pesticide mass leached below the depth of 1 m, for all substances, locations, and simulation years, was on the same order of magnitude among the three models. Additionally, the simulated 80th percentile of annual pesticide concentrations at the 1‐m depth was not statistically different among models. HYDRUS (2D/3D) can, therefore, be used as an alternative model for pesticide assessment studies and provides a conceptual framework consistent with PEARL and PELMO but capable of two‐ and three‐dimensional applications as well.

Analysing the mechanisms of soil water and vapour transport in the desert vadose zone of the extremely arid region of northern China

The transport of water and vapour in the desert vadose zone plays a critical role in the overall water and energy balances of near-surface environments in arid regions. However, field measurements in extremely dry environments face many difficulties and challenges, so few studies have examined water and vapour transport processes in the desert vadose zone. The main objective of this study is to analyse the mechanisms of soil water and vapour transport in the desert vadose zone (depth of ∼350 cm) by using measured and modelled data in an extremely arid environment. The field experiments are implemented in an area of the Gobi desert in northwestern China to measure the soil properties, daily soil moisture and temperature, daily water-table depth and temperature, and daily meteorological records from DOYs (Days of Year) 114–212 in 2014 (growing season). The Hydrus-1D model, which simulates the coupled transport of water, vapour and heat in the vadose zone, is employed to simulate the layered soil moisture and temperature regimes and analyse the transport processes of soil water and vapour. The measured results show that the soil water and temperatures near the land surface have visible daily fluctuations across the entire soil profile. Thermal vapour movement is the most important component of the total water flux and the soil temperature gradient is the major driving factor that affects vapour transport in the desert vadose zone. The most active water and heat exchange occurs in the upper soil layer (depths of 0–25 cm). The matric potential change from the precipitation mainly re-draws the spatio-temporal distribution of the isothermal liquid water in the soil near the land surface. The matric potential has little effect on the isothermal vapour and thermal liquid water flux. These findings offer new insights into the liquid water and vapour movement processes in the extremely arid environment.

Accounting for sub-resolution pores in models of water and solute transport in soils based on computed tomography images: Are we there yet?

In the last decade, X-ray computed tomography (CT) has become widely used to characterize the geometry and topology of the pore space of soils and natural porous media. Regardless of the resolution of CT images, a fundamental problem associated with their use, for example as a starting point in simulation efforts, is that sub-resolution pores are not detected. Over the last few years, a particular type of modeling method, known as “Grey” or “Partial Bounce Back” Lattice-Boltzmann (LB), has been adopted by increasing numbers of researchers to try to account for sub-resolution pores in the modeling of water and solute transport in natural porous media. In this short paper, we assess the extent to which Grey LB methods indeed offer a workable solution to the problem at hand. We conclude that, in spite of significant computational advances, a major experimental hurdle related to the evaluation of the penetrability of sub-resolution pores, is blocking the way ahead. This hurdle will need to be cleared before Grey LB can become a credible option in the microscale modeling of soils and sediments. A necessarily interdisciplinary effort, involving both modelers and experimentalists, is needed to clear the path forward.

Assessment the effect of homogenized soil on soil hydraulic properties and soil water transport

Soil hydraulic properties play a crucial role in simulating water flow and contaminant transport. Soil hydraulic properties are commonly measured using homogenized soil samples. However, soil structure has a significant effect on the soil ability to retain and to conduct water, particularly in aggregated soils. In order to determine the effect of soil homogenization on soil hydraulic properties and soil water transport, undisturbed soil samples were carefully collected. Five different soil structures were identified: Angular-blocky, Crumble, Angular-blocky (different soil texture), Granular, and subangular-blocky. The soil hydraulic properties were determined for undisturbed and homogenized soil samples for each soil structure. The soil hydraulic properties were used to model soil water transport using HYDRUS-1D.The homogenized soil samples showed a significant increase in wide pores (wCP) and a decrease in narrow pores (nCP). The wCP increased by 95.6, 141.2, 391.6, 3.9, 261.3%, and nCP decreased by 69.5, 10.5, 33.8, 72.7, and 39.3% for homogenized soil samples compared to undisturbed soil samples. The soil water retention curves exhibited a significant decrease in water holding capacity for homogenized soil samples compared with the undisturbed soil samples. The homogenized soil samples showed also a decrease in soil hydraulic conductivity. The simulated results showed that water movement and distribution were affected by soil homogenizing. Moreover, soil homogenizing affected soil hydraulic properties and soil water transport. However, field studies are being needed to find the effect of these differences on water, chemical, and pollutant transport under several scenarios.

OpenSimRoot: widening the scope and application of root architectural models.

Summary OpenSimRoot is an open‐source, functional–structural plant model and mathematical description of root growth and function. We describe OpenSimRoot and its functionality to broaden the benefits of root modeling to the plant science community.OpenSimRoot is an extended version of simroot, established to simulate root system architecture, nutrient acquisition and plant growth. OpenSimRoot has a plugin, modular infrastructure, coupling single plant and crop stands to soil nutrient and water transport models. It estimates the value of root traits for water and nutrient acquisition in environments and plant species.The flexible OpenSimRoot design allows upscaling from root anatomy to plant community to estimate the following: resource costs of developmental and anatomical traits; trait synergisms; and (interspecies) root competition. OpenSimRoot can model three‐dimensional images from magnetic resonance imaging (MRI) and X‐ray computed tomography (CT) of roots in soil. New modules include: soil water‐dependent water uptake and xylem flow; tiller formation; evapotranspiration; simultaneous simulation of mobile solutes; mesh refinement; and root growth plasticity.OpenSimRoot integrates plant phenotypic data with environmental metadata to support experimental designs and to gain a mechanistic understanding at system scales.

Modeling Coupled Water and Heat Transport in the Root Zone of Winter Wheat under Non-Isothermal Conditions

Temperature is an integral part of soil quality in terms of moisture content; coupling between water and heat can render a soil fertile, and plays a role in water conservation. Although it is widely recognized that both water and heat transport are fundamental factors in the quantification of soil mass and energy balance, their computation is still limited in most models or practical applications in the root zone under non-isothermal conditions. This research was conducted to: (a) implement a fully coupled mathematical model that contains the full coupled process of soil water and heat transport with plants focused on the influence of temperature gradient on soil water redistribution and on the influence of change in soil water movement on soil heat flux transport; (b) verify the mathematical model with detailed field monitoring data; and (c) analyze the accuracy of the model. Results show the high accuracy of the model in predicting the actual changes in soil water content and temperature as a function of time and soil depth. Moreover, the model can accurately reflect changes in soil moisture and heat transfer in different periods. With only a few empirical parameters, the proposed model will serve as guide in the field of surface irrigation.

Geometric and fractal analysis of dynamic cracking patterns subjected to wetting-drying cycles

Desiccation cracks detrimentally affect the hydraulic and mechanical properties of soils. Soil cracking dynamic subjected to wetting-drying (W-D) cycles aids in the understanding of soil shrinking-swelling mechanism and the regularity of water movement in unsaturated soils with variable solid phase. In the present paper, we experimentally investigate the geometric and fractal features of dynamic cracking patterns with the soils undergoing four W-D cycles. Crack patterns were recorded by a digital camera mounted above the soil specimen. Digital image processing and morphological algorithms were employed to yield the geometric and fractal indexes that depict crack patterns (incl. crack ratio, length, mean width, similarity degree, box-counting fractal and multifractal dimensions). The analysis of dynamic crack patterns suggest an inconsistency exhibited in the crack morphology towards the drying (DR) process and backwards the wetting (WT) process, which could be possibly attributed to the different suction stress between soil particles during moisture desorption and sorption, closely analogous to the hysteresis phenomenon of soil-water characteristic curve. The gradient of crack propagating and dissolving related to water content reaches different peak values during an individual W-D cycle. High repeatability is found across the stabilized crack patterns in the four drying cycles and the cracking degree reaches equilibrium with the increasing number of W-D cycles. The crack patterns in the subsequent drying processes are significantly dominated by the first cracking. The similarity degree between the first and respectively the second, third, fourth crack patterns decreases with the increasing W-D cycles, suggesting the soil cracked in the previously cracking vicinity, and saturated wetting/submerge hardly healed the failure zones that caused the repeatable cracking. Attempt of fractal and multifractal methods applied to the crack networks corroborates the applicability of the scaling method to the crack patterns. The analysis indicates that soil cracks progress in hierarchical networks in the first drying cycle, while the gradational crack segments nearly simultaneously emerged or vanished in the subsequent drying or wetting cycles. The study reveals the discrepancy of crack morphology between the crack formation and dissolution during W-D cycles, and demonstrates the repeatability degree and hierarchy of cracks across W-D cycles, which would aid in quantifying physical processes in cracked soil shrinking/swelling and modelling of water transport in unsaturated soils of dynamic structure.

Characterising deep vadose zone water movement and solute transport under typical irrigated cropland in the North China Plain

AbstractUnderstanding the dynamics and mechanisms of soil water movement and solute transport is essential for accurately estimating recharge rates and evaluating the impacts of agricultural activities on groundwater resources. In a thick vadose zone (0–15 m) under irrigated cropland in the piedmont region of the North China Plain, soil water content, matric potential, and solute concentrations were measured. Based on these data, the dynamics of soil water and solutes were analysed to investigate the mechanisms of soil water and solute transport. The study showed that the 0–15‐m vadose zone can be divided into three layers: an infiltration and evaporation layer (0–2 m), an unsteady infiltration layer (2–6 m), and a quasi‐steady infiltration layer (6–15 m). The chloride, nitrate, and sulphate concentrations all showed greater variations in the upper soil layer (0–1 m) compared to values in the deep vadose zone (below 2 m). The average concentrations of these three anions in the deep vadose zone varied insignificantly with depth and approached values of 125, 242, and 116 mg/L. The accumulated chloride, sulphate, and nitrate were 2,179 ± 113, 1,760 ± 383, and 4,074 ± 421 kg/ha, respectively. The soil water potential and solute concentrations indicated that uniform flow and preferential flow both occurred in the deep vadose zone, and uniform flow was the dominant mechanism of soil water movement in this study. The piston‐like flow velocity of solute transport was 1.14 m per year, and the average value of calculated leached nitrate nitrogen was 107 kg/ha∙year below the root zone. The results can be used to better understand recharge processes and improve groundwater resources management.

Evaluating the effects of irrigation water salinity on water movement, crop yield and water use efficiency by means of a coupled hydrologic/crop growth model

Numerical simulation is an efficient approach for investigating the salt and water movement through soil profile and predicting of crop response to soil water deficit and salinity. In this study a coupled model that describes the soil water and solute transport by means of HYDRUS-1D model and crop growth process by means of growth module of EPIC was used. The model was calibrated and validated against field data, collected during two growing seasons in field plots of winter wheat irrigated with four levels of irrigation amount and water salinity of 5dsm−1 at the Fengqiu State Key Agro-Ecological Experimental Station, in North China Plain. The model was also used to evaluate the salinity stress on evapotranspiration (ET), grain yield and water use efficiency (WUE) and long-term use of saline water on grain yield and salt accumulation. Visual inspection and the obtained statistical parameters values showed that good agreement between measured and simulated data of soil water content, salt concentration, ET and grain yield. Evapotranspiration values of winter wheat were reduced under salinity stress conditions, mainly by reducing crop transpiration. The grain yields were reduced due to salinity stress, but the change trend of WUE was associated with precipitation amount. Increasing per saline water irrigation amount is an effective way to reduce the WUE decline rate under the low rainfall growing season as like that in simulation year. The average of ten years grain yield confirmed that a yield potential exceeding 86% could be maintained by saline water with 5dsm−1 when per irrigation volume more than 0.8 E, E denotes evaporation from an uncovered, 20cm diameter pan positioned 0–5cm above the crop canopy. There has a slight salt build-up after ten years simulation, and the quantity of salt accumulation decreased with the increase of irrigation volume. Thus, more attention should be paid to the sustainability of irrigated agriculture with low irrigation volume when using saline water irrigation. On the whole, field observations combined with the coupled model could be used to evaluate different agricultural managements on grain yield and soil salinity.