Solute Transport Module Research Articles

Modelling the salt accumulation and leaching processes in arid agricultural areas with a new mass balance model

Soil salinization has become a widespread problem that seriously affects the sustainable development of agriculture. To deal with the salt accumulation and leaching problems in practical agricultural systems, a new soil water and solute transport model adopting the mass balance scheme is developed in this study. The soil water movement module is based on a previously published soil water mass balance model, named as UBMOD. The mobile-immobile assumption has been adopted to develop the soil solute transport module, which can describe the obvious anomalous transport phenomenon in practice. The advection, chemical reactions of solute, solute transfer between the mobile and immobile regions, and dispersion in the mobile region are considered in the solute transport module, while only chemical reactions and solute transfer between the mobile and immobile regions are considered in the immobile region. Three cases were designed to evaluate the performance of the model by comparing the simulation results with those obtained by experimental results, HYDRUS-1D, and practical observations. The results demonstrate that the developed model can solve the homogeneous and heterogeneous soil water movement and solute transport effectively, and it has looser restrictions on the discretization of space and time than the numerical solution of Richards’ equation and can keep the mass balance well, which make it more suitable for practical conditions. Moreover, the real-world application of salt accumulation and leaching problems in Hetao Irrigation District, Inner Mongolia, China shows that the RMSE of the simulated soil salt content with the immobile present region is about 20% better than that when ignoring the immobile present region. Therefore, it is necessary to take the significant anomalous transport into account when studying the salt accumulation and leaching processes in practical agricultural systems.

Management of soil salinity associated with irrigation of protected crops

Long-term evaluation of soil chemical changes in the soil is required to optimize irrigation of protected crops and to control associated environmental issues. In this study, the multi-component solute transport module UNSATCHEM of HYDRUS-1D was used to assess the effects of long-term (2018–2050) irrigation on salt build-up in the soil under unheated greenhouse conditions. Blended water (recycled water, artificially recharged groundwater and harvested rainwater) was used to irrigate tomato, cucumber, capsicum, and eggplant. Irrigation management included four leaching fractions (LF), i.e., accounting for 0, 15, 20, and 30% of applied excess water. The effects of four amounts of annual gypsum application, i.e. 0, 1.7, 2.6, and 3.4 t/ha, were also simulated.Model-simulated annual root water uptake by cucumber, tomato, capsicum, and eggplant was 303, 476, 642, and 649 mm, respectively, for an irrigation schedule based on outdoor temperature thresholds. Annual drainage was 4.1–6.1% of irrigation for these crops. Average salinity in the soil solution (ECsw) at the end of the simulation (year 2050) reached 6.5 dS/m for cucumber, 7.6 dS/m for tomato, 8.7 dS/m for capsicum, and 9.3 dS/m for eggplant. Soil salinity was building up at a nearly constant rate over 33 years. Exchangeable sodium percentage (ESP) at the end of the simulation was 30.8% for tomato, 27.1% for capsicum, 33.2% for eggplant, and 31.4% for cucumber. These results indicate that both salinity and sodicity will exceed critical threshold values and thus must be managed to maintain sustainability of irrigated horticulture. Modeling showed that higher irrigation (up to 15–20%) coupled with an annual addition of gypsum of 1.7 t/ha kept both salinity and sodicity (SAR, ESP) below critical thresholds while pH was reduced from 8.7 to 7.8, thus creating a favorable soil environment for long-term sustainable vegetable production under greenhouse conditions. The new gypsum application module implemented in the UNSATCHEM model has broad applicability in evaluating soil amendment scenarios to address sodicity hazards linked to long-term irrigation.

A mechanical-hydraulic-solute transport model for rough-walled rock fractures subjected to shear under constant normal stiffness conditions

The present study developed a two dimensional mechanical-hydraulic-solute transport model for rough-walled rock fractures under constant normal stiffness boundary conditions. First, the mechanical responses of a fracture during shear including the shear stress, the normal stress, the dilation and the sheared-off area of asperities at each shear step were calculated in a mechanical module. The surface roughness was characterized by two-order asperities, i.e., waviness and unevenness, and the degradation of asperities was estimated based on the principle of wear. The mechanical module was validated by comparison with laboratory experiments. The surface geometry and the dilation behavior were subsequently incorporated into a hydraulic module to estimate the hydraulic aperture and flow rate based on the cubic law. Finally, these data were input into a solute transport module to investigate the influences of the mechanical boundary conditions on the solute transport in the fracture and the matrix at different shear displacements and time. The model linked the complex mechanisms involved in the mechanical, hydraulic and solute transport processes for fractures subjected to shear, and revealed a controlling effect of normal stiffness on the transport behavior. The results show that the normal displacement, normal stress, cumulative sheared-off area and porosity of matrix change quickly in the initial stage of shear, and gradually reach some constant values when the fracture surface is sufficiently smoothed in the residual stage. In the entire shear process, the major asperities are substantially damaged, producing abundant gouge particles that contribute to the retardation of solute transport. The shear-induced dilation and the retardation induced by gouge particles from damaged asperities played competitive roles in solute transport that resulted in the nonlinear variations in the coefficient of retardation. The model established a solid platform for estimating the mechanical-hydraulic-solute transport processes in fractures subjected to shear, upon which more sophisticated modules such as pressure solution and clogging by particles may be developed to improve the understanding on the outstanding issues involved in the coupled processes.

Global sensitivity analysis and calibration of parameters for a physically-based agro-hydrological model

Efficient parameter identification is an important issue for mechanistic agro-hydrological models with a complex and nonlinear property. In this study, we presented an efficient global methodology of sensitivity analysis and parameter estimation for a physically-based agro-hydrological model (SWAP-EPIC). The LH-OAT based module and the modified-MGA based module were developed for parameter sensitivity analysis and inverse estimation, respectively. In addition, a new solute transport module with numerically stable schemes was developed for ensuring stability of SWAP-EPIC. This global method was tested and validated with a two-year dataset in a wheat growing field. Fourteen parameters out of the forty-nine total input parameters were identified as the sensitive parameters. These parameters were first inversely calibrated by using a numerical case, and then the inverse calibration was performed for the real field experimental case. Our research indicates that the proposed global method performs successfully to find and constrain the highly sensitive parameters efficiently that can facilitate application of the SWAP-EPIC model.

Unsaturated zone flow and solute transport modelling with MIKE SHE: model test and parameter sensitivity analysis using lysimeter data

This study presents the test of the widely used model MIKE SHE at the lysimeter in Wagna in Southern Styria (Austria). At the lysimeter comprehensive data of vegetation, i.e., cultivation types, and soil hydraulic properties are available to parameterise the model. The aim of the study is to test the water movement and solute transport simulation in the unsaturated zone and the evapotranspiration module by a comparison with measured recharge rates at the lysimeter bottom and measured soil water contents and soil tensions in different depths. For the unsaturated zone calculation, the Richards equation and the Van Genuchten–Mualem approach is used. With the parameterised model the solute transport module is tested against data of a tracer experiment (Bromide). The water content simulation in the different soil depths yielded good results. However, particularly in the gravel zone (below 70 cm) the strong recharge dynamics and at the same time also strong water content dynamics can hardly be reproduced by one individual soil physical parameter set. Two different sets were identified and the corresponding soil water retention functions are located at the opposite ends of the relatively large range obtained by the measurements in this zone. Also, the saturated hydraulic conductivity had to be increased by an order of magnitude to accurately represent the recharge dynamics. The test of the solute transport simulation showed generally very good results. However, the early rapid rise of the measured concentrations could not be reproduced with either parameter set. The discrepancies found in the study indicate a complex pore system in the gravel zone consisting of different flow paths (e.g., system of matrix flow and macro-pore flow) activated at different soil moisture conditions, which cannot be fully represented by the used approach. However, the parameter study herein highlights the main parameters to be altered in a calibration procedure at point scale. The paper includes also a step from point to regional scale. For comparison test calculations with data, which are usually available on regional scale, showed good results for the 5-year average balance (−7 % compared to measurements) but rather large deviations from the measurements at shorter time scales.

Calibration of WAVE in Irrigated Maize: Fallow vs. Cover Crops

Nitrate leaching decreases crop available N and increases water contamination. Replacing fallow by cover crops (CC) is an alternative to reduce nitrate contamination, because it reduces overall drainage and soil mineral N accumulation. A study of the soil N and nitrate leaching was conducted during 5 years in a semi-arid irrigated agricultural area of Central Spain. Three treatments were studied during the intercropping period of maize (Zea mays L.): barley (Hordeum vulgare L.), vetch (Vicia villosa L.), and fallow. Cover crops, sown in October, were killed by glyphosate application in March, allowing direct seeding of maize in April. All treatments were irrigated and fertilised following the same procedure. Soil water content was measured using capacity probes. Soil Nmin accumulation was determined along the soil profile before sowing and after harvesting maize. Soil analysis was conducted at six depths every 0.20 m in each plot in samples from 0 to 1.2-m depth.The mechanistic water balance model WAVE [1] was applied in order to calculate drainage and plant growth of the different treatments, and apply them to the N balance. We evaluated the water balance of this model using the daily soil water content measurements of this field trial. A new Matlab version of the model was evaluated as well. In this new version improvements were made in the solute transport module and crop module. In addition, this new version is more compatible with external modules for data processing, inverse calibration and uncertainty analysis than the previous Fortran version.The model showed that drainage during the irrigated period was minimized in all treatments, because irrigation water was adjusted to crop needs, leading to nitrate accumulation on the upper layers after maize harvest. Then, during the intercrop period, most of the nitrate leaching occurred. Cover crops usually led to a shorter drainage period, lower drainage water amount and lower nitrate leaching than the treatment with fallow. These effects resulted in larger nitrate accumulation in the upper layers of the soil after CC treatments.

Coupling model for assessing anti-seepage behavior of curtain under dam foundation

The seepage under a dam foundation is mainly controlled by the performance of the curtain. Its anti-seepage behavior may be weakened by the long term physic-chemical actions from groundwater. According to seepage hydraulics and geochemistry theory, a coupling model for assessing the behavior of the curtain under a dam foundation is set up, which consists of seepage module, solute transport module, geochemistry module and curtain erosion module, solved by FEM. A case study was carried out. The result shows that the curtain efficiency is weakened all the time. Aqueous calcium from the curtain is always in dissolution during the stress period for simulation, which leads to the increasing amount in groundwater reaching 846.35–865.312 g/m3. Within the domain, reaction extent differs in different parts of the curtain. The dissolution of Ca(OH)2 accounts to 877.884 g/m3 near the bottom and is much higher than that of the other parts. The erosion is much more serious near the bottom of the curtain than the other parts, which is the same to the upstream and downstream. Calcium dissolution is mainly controlled by hydraulic condition and dispersion, and it varies in a non-linear way within the domain.

Field evaluation of a multicomponent solute transport model in soils irrigated with saline waters

Soil salinization, sodification, and non-point source pollution are among the most important and widespread environmental problems in agricultural regions with scarce water resources. Models evaluating these environmental problems should therefore consider an integrated approach to avoid favoring one problem over the other. The HYDRUS-1D software package was used to simulate water movement and solute transport in two complex experiments carried out under field conditions in Alvalade and Mitra, Portugal. The experiments involved irrigating maize with synthetic saline irrigation waters blended with fresh irrigation waters and waters with different nitrogen concentrations. The major ion chemistry module of HYDRUS-1D was used to model water contents (RMSE ⩽ 0.04 cm3 cm−3), the overall salinity given by the electrical conductivity of the soil solution (ECsw) (RMSE ⩽ 2.35 dS m−1), the concentration of soluble cations Na+ (RMSE ⩽ 13.86 mmol(c) L−1), Ca2+ (RMSE ⩽ 5.66 mmol(c) L−1), Mg2+ (RMSE ⩽ 4.16 mmol(c) L−1), and SAR (RMSE ⩽ 6.27 (mmol(c) L−1)0.5) in different experimental plots. RMSE were always lower for the soil with coarse texture of Mitra. The standard HYDRUS solute transport module was used to model N–NH4+ (RMSE ⩽ 0.07 mmol(c) L−1) and N–NO3- (RMSE ⩽ 2.60 mmol(c) L−1) concentrations in the soil solution while either including or neglecting the effects of the osmotic stress on nutrient uptake. The model was able to successfully simulate root water and nutrient uptake reductions due to osmotic stress. Consequently, modeled fluxes of N–NH4+ and N–NO3- leached from the soil profiles increased due to the effects of the salinity stress on water and nutrient uptake. Possible causes of disagreements between the modeling and experimental data are discussed. HYDRUS-1D proved to be a powerful tool for analyzing solute concentrations related to overall soil salinity and nitrogen species.

A simplified numerical model of 3-D groundwater and solute transport at large scale area

A simplified numerical model of groundwater and solute transport is developed. At large scale area there exists a big spatial scale difference between horizontal and vertical length scales. In the resultant model, the seepage region is particularly divided into several virtual layers along the z direction and vertical 1-D columns covering x-y 2-D area according to stratum properties. The numerical algorithm is replacing the full 3-D water and mass balance analysis as the 2-D Galerkin finite element method in x- and y-directions and 1-D finite differential approach in the z direction. The reasonable method of giving minimum thickness is successfully used to handle transient change of water table, drying cells and problem of rewetting. The solution of the simplified model is preconditioned conjugate gradient and ORTHOMIN method. The validity of the developed 3-D groundwater model is tested with the typical pumping and backwater scenarios. Results of water balance of the computed example reveal the model computation reliability. Based on a representative 3-D pollution case, the solute transport module is tested against computing results using the MT3DMS. The capability and high efficiency to predict non-stationary situations of free groundwater surface and solute plume in regional scale problem is quantitatively investigated. It is shown that the proposed model is computationally effective.

A simplified modelling approach for pesticide transport in a tile-drained field: The PESTDRAIN model

The paper presents a simplified model called PESTDRAIN. It simulates pesticide transport in a subsurface tile-drained field. It computes surface runoff and tile-drainage flow rates, along with the associated pesticide concentrations, with a variable event-driven time step. PESTDRAIN consists of three coupled modules: SIDRA, SIRUP and SILASOL. SIDRA and SIRUP are the water flow simulation modules in the saturated and unsaturated zones, respectively. SIDRA follows a simplified physically based approach while SIRUP follows a conceptual capacitive approach. SILASOL is the solute transport module for both the saturated and unsaturated zones and is based on transfer functions. It includes simple representations of adsorption and degradation of pesticides. PESTDRAIN was tested on field data sets collected for three drainage seasons at the La Jaillière experimental site in north-western France, for the wheat herbicides isoproturon (IPU) and diflufenican (DFF). After model calibration, relative errors for drainage and surface runoff flows over the season were 14% and 4%, respectively, and the Nash–Sutcliffe efficiency coefficient ( N eff ) value for drainage discharge was 0.58. A fair reproduction of a high temporal resolution IPU concentration data set in drainage discharge was also obtained ( N eff = 0.28 ). For the validation data sets, PESTDRAIN was able to simulate accurately drainage discharge with Nash–Sutcliffe efficiency coefficients of 0.57 and 0.69. The global N eff was 0.44 for all flow-weighted average weekly concentrations in drainage. Relative errors for the pesticide losses were 2.5% and 35% (IPU), and 60% (DFF). For surface runoff the results were not as accurate, but they remained correct in terms of time location and order of magnitude. Although further validation is necessary with more field data, PESTDRAIN appears as a promising tool for agricultural water management.

Testing the PEARL model in the Netherlands and Sweden

The Plant Protection Product Directive (91/414/EEC) stresses the need of validated models to calculate predicted environmental concentrations. The use of models has become an unavoidable step before pesticide registration. The main topic of the research presented here is the validation of the PEARL model for two well-instrumented sites located in the Netherlands and Sweden. First the water transport module was calibrated, and then the solute transport module, keeping unchanged the water transport parameters. The Dutch site is characterised by a sandy soil. PEARL predictions were very satisfactory for both soil moisture content and groundwater level. Predictions for the tracer, bentazone and ethoprophos concentrations in the soil profile were also successful. The Swedish site is characterised by a structured clay soil. PEARL was successful in predicting soil moisture profiles and the draining water. PEARL performed well in predicting the soil concentration of bentazone at different depths, however, it did not predict accurately solute concentration in the drainage water. A sensitivity analysis of the PEARL model performed on the Swedish site showed the greater importance of the pesticide degradation properties over the soil hydraulic properties.

Colloid‐Facilitated Solute Transport in Variably Saturated Porous Media: Numerical Model and Experimental Verification

Strongly sorbing chemicals (e.g., heavy metals, radionuclides, pharmaceuticals, and explosives) in porous media are associated predominantly with the solid phase, which is commonly assumed to be stationary. However, recent field‐ and laboratory‐scale observations have shown that in the presence of mobile colloidal particles (e.g., microbes, humic substances, clays, and metal oxides), colloids can act as pollutant carriers and thus provide a rapid transport pathway for strongly sorbing contaminants. To address this problem, we developed a one‐dimensional numerical model based on the HYDRUS‐1D software package that incorporates mechanisms associated with colloid and colloid‐facilitated solute transport in variably saturated porous media. The model accounts for transient variably saturated water flow, and for both colloid and solute movement due to advection, diffusion, and dispersion, as well as for solute movement facilitated by colloid transport. The colloid transport module additionally considers the processes of attachment/detachment to/from the solid phase and/or the air–water interface, straining, and/or size exclusion. Various blocking and depth dependent functions can be used to modify the attachment and straining coefficients. The solute transport module uses the concept of two‐site sorption to describe nonequilibrium adsorption–desorption reactions to the solid phase. The module further assumes that contaminants can be sorbed onto surfaces of both deposited and mobile colloids, fully accounting for the dynamics of colloid movement between different phases. Application of the model is demonstrated using selected experimental data from published saturated column experiments, conducted to investigate the transport of Cd in the presence of Bacillus subtilis spores in alluvial gravel aquifer media. Numerical results simulating bacteria transport, as well as the bacteria‐facilitated Cd transport, are compared with experimental results. A sensitivity analysis of the model to various parameters is also presented.