Vadose Zone Flow Research Articles

Development of a conceptual groundwater model for a complex basement aquifer system: The case OF OSUN drainage basin in southwestern Nigeria

Sound knowledge of hydrogeological setting is required for effective groundwater resource management and protection. In the basement environment where groundwater occurrence is discontinuous and highly heterogeneous, hydrogeological conceptual model is required to understand the occurrence and flow pattern of groundwater. This study presents an integrated 2D and 3D GIS method for conceptual modeling of the hydrogeological setting of a basement complex aquifer system in Osun Drainage Basin, Southwestern Nigeria. The efficiency of conjunctive use of RockWorks, Suffer and ArcGIS for surface and subsurface 3D hydrogeological modeling of a basement aquifer system was evaluated.Three distinct aquifers (Weathered Layer Aquifer (WLA), Unconfined Fractured Aquifer (UFA) and Confined Fractured Aquifer (CFA)) were delineated. WLA is characterized by low hydraulic conductivity (K = 0.34–27.54md−1, mean = 4md−1), hence low yield potential. Transmissivity (T) range and mean (0.6–1368 m2d−1, 85 m2d−1) indicate restricted flow within WLA. UFA is hydraulically connected to the underlying multi-layered CFA in the contact zones. The occurrence of UFA is localized within meta-sedimentary environment and it acts as the recharge redistributing medium, connecting the deep seated aquifers with the highly porous surface flaggy rubbles. CFA is characterized by relatively high mean K (0.2–91md−1, mean = 11md−1) and relatively high T (1.6–7185 m2d-1, mean = 483 m2d−1). Comparative analyses of aquifers’ hydraulic properties and spatial pattern of occurrence affirm that regional groundwater flow only takes place within CFA. The variation of hydraulic properties along fault lines is attributed to alternating occurrence of bedrocks of different structural and physico-chemical characteristics. Three distinct groundwater flow systems were identified in the study basin: i) the radially concentric localized shallow groundwater flow systems that occur within the overburden, ii) the W-E oriented regional flow system that feeds the source of Ikogosi Warm Spring in the adjourning basin, and iii) the NNE-SSW trans-current flow system that traverses the study basin at the center. While the first flow system is controlled by surface topography and the hydraulic properties of the vadose zone, the second and third flow systems are structurally controlled. The efficiency and efficacy of the methodology presented in this study is sufficient to develop hydrogeological conceptual model for highly heterogeneous, discontinuous basement aquifer system. Therefore, the developed conceptual model could be transformed to a numerical model of the study basin.

Axisymmetric Modeling Using MODFLOW-USG.

Axisymmetric groundwater models are used for simulating radially symmetric conditions. Groundwater simulators built specifically to model axisymmetric conditions are most commonly used for simulating aquifer tests. Although some numerical models capable of simulating flow and solute transport that are developed in the cartesian coordinate system framework offer flexibility to simulate axisymmetric conditions, most of the numerical groundwater models, such as the MODFLOW family of codes, are based on structured grids in which axisymmetric flows cannot be directly simulated. Researchers in the past have provided methods to manipulate aquifer properties to mimic axisymmetric conditions. This study presents a methodology that takes advantage of the unstructured grids of MODFLOW-USG to simulate axisymmetric models within the MODFLOW framework. To develop axisymmetric models, the intercell interface area arrays of MODFLOW-USG were calculated to accurately represent coaxial cylindrical model cells. Three examples are presented to demonstrate the application of MODFLOW-USG for axisymmetric modeling: a pumping well with delayed yield effects, a vadose zone flow model simulating an infiltration basin, and a density-dependent saltwater intrusion problem for a circular island. Results were verified against analytical solutions and published numerical codes.

Field Evidence of a Natural Capillary Barrier in a Gravel Alluvial Aquifer

Core Ideas A tracer test in a gravel aquifer detected a natural capillary barrier. Electrical resistivity data combined with well data found two separate vadose zone flow paths. Phosphorus transport is limited, as it needs to migrate through soil before gravel. Ozark streams commonly feature “composite” floodplains, in which the vadose zone consists of silt or silt loam soils (∼1 m thick) overlying gravel subsoil. Previous work has shown that preferential flow paths can exist within the gravel subsoil, which can conduct water and P at rates exceeding the sorption capacity of the gravel. At a site on Barren Fork Creek, a 1‐ by 1‐m infiltration plot was constructed and an infiltration experiment was performed using sequentially introduced solutes including P (the constituent of regulatory interest), Rhodamine‐WT (Rh‐WT, a visual tracer), and Cl− (an electrical tracer). The solute transport was measured with monitoring wells (MWs) placed 1 m from the plot boundary and 5 m down the groundwater flow gradient using an electrical resistivity imaging (ERI) array. The ERI method utilized differences between a pre‐infiltration background image and subsequent temporal images taken during the test to quantify changes induced by the tracers. The infiltration test maintained a steady‐state flow rate of 4.5 L min−1 for 84.75 h. Electrical resistivity imaging data showed significant changes in resistivity induced by the tracers within the soil vadose zone under the plot but no similar changes within the gravel, indicating that the interface was acting as a capillary barrier. Electrical resistivity images 5 m away from the plot showed tracer breakthrough into the gravel in areas not sampled by the MWs. Solute detection was limited in MWs, indicating that MWs could not adequately monitor movement below the capillary barrier because it controlled migration of solute to the heterogeneous phreatic zone.

Recharge and Nitrate Transport Through the Deep Vadose Zone of the Loess Plateau: A Regional‐Scale Model Investigation

AbstractThe Loess Plateau of China (LPC) is suffering from the impacts of overexploitation of groundwater, leading to a decline in the regional water table. This is compounded by relatively recent large‐scale transformation of land use, which may impact groundwater recharge and threaten future water quality from intensive nitrogen fertilizer application. Understanding the regional unsaturated water flow and nitrate travel time in the deep vadose zone is crucial for the sustainable management of the LPC groundwater system. We develop here a regional‐scale model that exploits an extensive database of soil properties derived from recent intensive soil profile sampling over the LPC, together with climate, vadose zone thickness, and land use data. The model solves the Richards equation and a conservative solute transport model using a multiple 1D column approach. A comparison between model simulations and observations at five sites reveals good model performance. Application over the LPC indicates that areas with dense irrigated agricultural land use contribute significantly to groundwater recharge. By using land use cover information from 1975 to 2008, simulations reveal that the transition from agriculture to forest and grassland accounts for a small reduction of the total annual recharge (6.1%). Modeled nitrate travel times through the deep unsaturated zone range between decades and centuries; however, recent loading of anthropogenic nitrate is expected to reach the aquifer in the near future under irrigated areas or where the vadose zone is relatively thin. The model simulations provide valuable information for assessing future vulnerability of groundwater resources over a regional scale.

Spatial and temporal dynamics of deep percolation, lag time and recharge in an irrigated semi-arid region

The time required for deep percolating water to reach the water table can be considerable in areas with a thick vadose zone. Sustainable groundwater management, therefore, has to consider the spatial and temporal dynamics of groundwater recharge. The key parameters that control the lag time have been widely examined in soil physics using small-scale lysimeters and modeling studies. However, only a small number of studies have analyzed how deep-percolation rates affect groundwater recharge dynamics over large spatial scales. This study examined how the parameters influencing lag time affect groundwater recharge in a semi-arid catchment under irrigation (in northeastern Iran) using a numerical modeling approach. Flow simulations were performed by the MODFLOW-NWT code with the Vadose-Zone Flow (UZF) Package. Calibration of the groundwater model was based on data from 48 observation wells. Flow simulations showed that lag times vary from 1 to more than 100 months. A sensitivity analysis demonstrated that during drought conditions, the lag time was highly sensitive to the rate of deep percolation. The study illustrated two critical points: (1) the importance of providing estimates of the lag time as a basis for sustainable groundwater management, and (2) lag time not only depends on factors such as soil hydraulic conductivity or vadose zone depth but also depends on the deep-percolation rates and the antecedent soil-moisture condition. Therefore, estimates of the lag time have to be associated with specific percolation rates, in addition to depth to groundwater and soil properties.

Riparian Vadose Zone Preferential Flow: Review of Concepts, Limitations, and Perspectives

Core Ideas Riparian ecosystems are hotspots for macropore flow due to their biogeomorphology. Macropore prevalence can reduce riparian buffer pollution mitigation effectiveness. Characterization of macropore morphology and connectivity is needed in riparian zones. An effect of seasonal shallow water table on macropore flow is proposed. Integration of infiltration–soil moisture–macropore flow model into design tools is needed. The design and analysis of surface water pollution control practices such as vegetative filter strips and riparian buffers typically focus on surface runoff, with limited attention given to subsurface flow and transport. Field evidence suggests a prevalence of macropore flow (MF) in the riparian vadose zone (RVZ) due to abundant biological activity (e.g., fauna and roots) and steep hydraulic gradients created by the adjacent stream and the presence of a seasonally shallow water table (SWT). Because rapid leaching and subsurface transport of contaminants can be significant with MF, their prevalence in riparian buffers can negate the intended benefits of this widely adopted surface runoff pollution control practice. While theories exist for modeling preferential flow processes, experimental and modeling techniques are still lacking to characterize in situ RVZ macropore network morphologies at the soil profile and landscape scales. Importantly, the presence of a seasonal SWT can increase MF and transport processes neglected in current analyses. Additional research is needed to evaluate holistic modeling frameworks that can represent MF from measurable parameters at the riparian field scale. In this work, we review various MF theories and concepts suitable to RVZ conditions and identify current limitations and knowledge gaps. We emphasize the use of dual‐permeability approaches as a compromise between model complexity and parameter identifiability. We also identify the need for well‐controlled experimental studies using the latest monitoring technology and validation studies at the laboratory and field scales. Only then can decision‐support tools realistically predict the influence of preferential flow processes on the performance of riparian buffers as a surface water quality control practice.

ANOVA-based transformed probabilistic collocation method for Bayesian data-worth analysis

Abstract Bayesian theory provides a coherent framework in quantifying the data worth of measurements and estimating unknown parameters. Nevertheless, one common problem in Bayesian methods is the considerably high computational cost since a large number of model evaluations is required in the likelihood evaluation. To address this issue, a new surrogate modeling method, i.e., ANOVA (analysis of variance)-based transformed probabilistic collocation method (ATPCM), is developed in this work. To cope with the strong nonlinearity, the model responses are transformed to the arrival times, which are then approximated with a set of low-order ANOVA components. The validity of the proposed method is demonstrated by synthetic numerical cases involving water and heat transport in the vadose zone. It is shown that, the ATPCM is more efficient than the existing surrogate modeling methods (e.g., PCM, ANOVA-based PCM and TPCM). At a very low computational cost, the ATPCM-based Bayesian data-worth analysis provides a quantitative metric in comparing different monitoring plans, and helps to improve the parameter estimation. Although the flow and heat transport in vadose zone is considered in this work, the proposed method can be equally applied in any other hydrologic problems.

An Assessment of Pedotransfer Function Performance for the Estimation of Spatial Variability of Key Soil Hydraulic Properties

Core Ideas PTFs have been widely applied in environmental and hydrological watershed studies. PTFs can simulate the spatial variability of hydraulic functions. The simulation of hydraulic maps using PTF data should be performed with care. The reliable applications of soil water dynamics and its spatial variability are very important in hydrological and environmental studies. However, determination of some hydraulic properties requires time and sometimes high costs. Pedotransfer functions (PTFs) have been applied as an alternative for the prediction of soil hydraulic properties by working with some easily measurable physical properties. This study examined the performance of the Splintex and Rosetta PTFs for estimating the spatial variability of some soil hydraulic properties such as available water capacity (AWC), field capacity (θfc), permanent wilting point (θpwp), specific water capacity [C(θ)], conductivity [K(θ)], and diffusivity [D(θ)] in a sandy soil from southeastern Brazil. Based on the log‐likelihood, Akaike, and Bayesian information criteria, semivariogram models were fitted and used to krige the assessed variable maps. The results showed the feasibility of using these PTFs to estimate the spatial variability in soil hydraulic properties and functions, which would greatly benefit vadose zone flow and transport modeling. Geostatistical analyses presented similarities in kriged maps of both observed K(θ) and C(θ) data with kriged maps by Rosetta estimates, as well as the kriged map of the observed AWC data with the kriged map by Splintex estimates sometimes revealing the same semivariogram data pattern. Predicted maps of θfc, θpwp, and D(θ) were, however, different from kriged maps of the observed data, showing that this approach should be used carefully. The use of the range parameter from the semivariogram models was efficient to determine the optimal sample density for the studied area.

Forward modeling to investigate inversion artifacts resulting from time-lapse electrical resistivity tomography during rainfall simulations

Time-lapse electrical resistivity tomography (ERT) is commonly used as a minimally invasive tool to study infiltration processes. In 2014, we conducted field studies coupling variable intensity rainfall simulation with high-resolution ERT to study the real-time partitioning of rainfall into surface and subsurface response. The significant contrast in resistivity in the subsurface from large changes in subsurface moisture resulted in artifacts during the inversion process of the time-lapse ERT data collected using a dipole-dipole electrode array. These artifacts, which are not representative of real subsurface moisture dynamics, have been shown to arise during time-lapse inversion of ERT data and may be subject to misinterpretation. Forward modeling of the infiltration process post field experiments using a two-layer system (saprolite overlain by a soil layer) was used to generate synthetic datasets. The synthetic data were used to investigate the influence of both changes in volumetric moisture content and electrode configuration on the development of the artifacts identified in the field datasets. For the dipole-dipole array, we found that a decrease in the resistivity of the bottom layer by 67% resulted in a 50% reduction in artifact development. Artifacts for the seven additional array configurations tested, ranged from a 19% increase in artifact development (using an extended dipole-dipole array) to as much as a 96% decrease in artifact development (using a wenner-alpha array), compared to that of the dipole-dipole array. Moreover, these arrays varied in their ability to accurately delineate the infiltration front. Model results showed that the modified pole-dipole array was able to accurately image the infiltration zone and presented fewer artifacts for our experiments. In this study, we identify an optimal array type for imaging rainfall-infiltration dynamics that reduces artifacts. The influence of moisture contrast between the infiltrating water and the bulk subsurface material was characterized and shown to be a major factor in contributing to artifact development. Through forward modeling, this study highlights the importance of considering array type and subsurface moisture conditions when using time-lapse resistivity to obtain reliable estimates of vadose zone flow processes during rainfall-infiltration events.

Image analysis method for the measurement of water saturation in a two-dimensional experimental flow tank

A novel, non-invasive imaging technique is proposed that determines 2D maps of water content in unsaturated porous media. This method directly relates digitally measured intensities to the water content of the porous medium. This method requires the classical image analysis steps, i.e., normalization, filtering, background subtraction, scaling and calibration. The main advantages of this approach are that no calibration experiment is needed, because calibration curve relating water content and reflected light intensities is established during the main monitoring phase of each experiment and that no tracer or dye is injected into the flow tank. The procedure enables effective processing of a large number of photographs and thus produces 2D water content maps at high temporal resolution. A drainage/imbibition experiment in a 2D flow tank with inner dimensions of 40cm×14cm×6cm (L×W×D) is carried out to validate the methodology. The accuracy of the proposed approach is assessed using a statistical framework to perform an error analysis and numerical simulations with a state-of-the-art computational code that solves the Richards’ equation. Comparison of the cumulative mass leaving and entering the flow tank and water content maps produced by the photographic measurement technique and the numerical simulations demonstrate the efficiency and high accuracy of the proposed method for investigating vadose zone flow processes. Finally, the photometric procedure has been developed expressly for its extension to heterogeneous media. Other processes may be investigated through different laboratory experiments which will serve as benchmark for numerical codes validation.

Weighted recalibration of the Rosetta pedotransfer model with improved estimates of hydraulic parameter distributions and summary statistics (Rosetta3)

Pedotransfer functions (PTFs) have been widely used to predict soil hydraulic parameters in favor of expensive laboratory or field measurements. Rosetta (Schaap et al., 2001, denoted as Rosetta1) is one of many PTFs and is based on artificial neural network (ANN) analysis coupled with the bootstrap re-sampling method which allows the estimation of van Genuchten water retention parameters (van Genuchten, 1980, abbreviated here as VG), saturated hydraulic conductivity (Ks), and their uncertainties. In this study, we present an improved set of hierarchical pedotransfer functions (Rosetta3) that unify the water retention and Ks submodels into one. Parameter uncertainty of the fit of the VG curve to the original retention data is used in the ANN calibration procedure to reduce bias of parameters predicted by the new PTF. One thousand bootstrap replicas were used to calibrate the new models compared to 60 or 100 in Rosetta1, thus allowing the uni-variate and bi-variate probability distributions of predicted parameters to be quantified in greater detail. We determined the optimal weights for VG parameters and Ks, the optimal number of hidden nodes in ANN, and the number of bootstrap replicas required for statistically stable estimates. Results show that matric potential-dependent bias was reduced significantly while root mean square error (RMSE) for water content were reduced modestly; RMSE for Ks was increased by 0.9% (H3w) to 3.3% (H5w) in the new models on log scale of Ks compared with the Rosetta1 model. It was found that estimated distributions of parameters were mildly non-Gaussian and could instead be described rather well with heavy-tailed α-stable distributions. On the other hand, arithmetic means had only a small estimation bias for most textures when compared with the mean-like “shift” parameter of the α-stable distributions. Arithmetic means and (co-)variances are therefore still recommended as summary statistics of the estimated distributions. However, it may be necessary to parameterize the distributions in different ways if the new estimates are used in stochastic analyses of vadose zone flow and transport. Rosetta1 and Posetta3 were implemented in the python programming language, and the source code as well as additional documentation is available at: http://www.cals.arizona.edu/research/rosettav3.html.

Time‐Lapse Electrical Imaging of Water Infiltration in the Context of Soil Aquifer Treatment

Core Ideas We investigated geoelectrical monitoring of water infiltration within soil aquifer treatment. Time‐lapse ERT is able to track water infiltration under very high recurrent fluxes. Individual apparent resistivity captures temporal dynamics of water infiltration. Spatiotemporal ERT monitoring results used for hydraulic subsurface zonation. In this study, we investigated the ability of online geoelectrical monitoring to characterize water dynamics in the context of soil aquifer treatment (SAT). In spring 2014, we conducted several infiltration experiments at the Shafdan SAT facility in Israel, where treated wastewater is recurrently infiltrated through large ponds into the sandy coastal plain aquifer. Temporally dense electrical resistivity tomography (ERT) measurements were collected to investigate vadose zone flow processes and the influence of heterogeneous sedimentation on the infiltration pattern. Measurements of water content as well as cross‐hole ground‐penetrating radar (GPR) profiles and lithological analyses from boreholes were used to support the ERT results. The observed electrical responses reflect the high temporal dynamics in subsurface water content associated with the applied infiltration–drying cycles. Individual apparent resistivity monitoring data captured general temporal dynamics, providing an easy and cost‐effective means for infiltration monitoring. Time‐lapse ERT results reveal different temporal dynamics in regions of the subsurface as a result of spatially varying hydraulic properties. We propose to use, based on a simple quantitative parametric description, the imaged temporal behavior of electrical conductivity to discriminate zones with different hydraulic properties, which represents a new approach for aquifer zonation based on the observed process dynamics. Our results demonstrate that time‐lapse ERT is a powerful tool for the monitoring of SAT water infiltration with high spatiotemporal resolution. The provided information may be used for assessing and optimizing applied infiltration–drying cycles and, thus, for more effective SAT management.

Temporal Variations of Water Productivity in Irrigated Corn: An Analysis of Factors Influencing Yield and Water Use across Central Nebraska.

Water Productivity (WP) of a crop defines the relationship between the economic or physical yield of the crop and its water use. With this concept it is possible to identify disproportionate water use or water-limited yield gaps and thereby support improvements in agricultural water management. However, too often important qualitative and quantitative environmental factors are not part of a WP analysis and therefore neglect the aspect of maintaining a sustainable agricultural system. In this study, we examine both the physical and economic WP in perspective with temporally changing environmental conditions. The physical WP analysis was performed by comparing simulated maximum attainable corn yields per unit of water using the crop model Hybrid-Maize with observed data from 2005 through 2013 from 108 farm plots in the Central Platte and the Tri Basin Natural Resource Districts of Nebraska. In order to expand the WP analysis on external factors influencing yields, a second model, Maize-N, was used to estimate optimal nitrogen (N)–fertilizer rate for specific fields in the study area. Finally, a vadose zone flow and transport model, HYDRUS-1D for simulating vertical nutrient transport in the soil, was used to estimate locations of nitrogen pulses in the soil profile. The comparison of simulated and observed data revealed that WP was not on an optimal level, mainly due to large amounts of irrigation used in the study area. The further analysis illustrated year-to-year variations of WP during the nine consecutive years, as well as the need to improve fertilizer management to favor WP and environmental quality. In addition, we addressed the negative influence of groundwater depletion on the economic WP through increasing pumping costs. In summary, this study demonstrated that involving temporal variations of WP as well as associated environmental and economic issues can represent a bigger picture of WP that can help to create incentives to sustainably improve agricultural production.

Development challenges in karst regions: sustainable land use planning in the karst of Slovenia

The comprehensive knowledge of karst surface, caves, waters and biodiversity were examined with special emphasis on planning for environmental management and land use in karst regions using examples of projects performed by the staff from the Karst Research Institute ZRC SAZU. Based on the knowledge of karst, karst phenomena, karst waters and karst biodiversity, karstological monitoring was developed throughout the construction of expressways and planning for railways on karst terrain. Newly revealed karst phenomena were properly preserved. Regular climatic and biological monitoring were developed and implemented, as well as karstology consultation with an emphasis on the impact of the use of caves as natural assets. To understand karst, its evolution dynamics and processes, models were developed with which the study of the risk posed by dam site construction in karst areas was possible. Karst aquifers and their protection were studied using tracer tests and different methods of the transfer of contaminants through the vadose zone and underground flow connections. In assessing the vulnerability and contamination risk of groundwater, a comprehensive method was elaborated. Microorganisms and fauna of dripping water in caves were used as monitors of cave ecosystem health. By applying high-quality karstological research results from individual spheres of karstology and interdisciplinary studies, a foundation for sustainable planning of life in karst regions that will consider the natural and cultural characteristics and vulnerability of karst landscapes has been established.

Finite analytic method based on mixed-form Richards’ equation for simulating water flow in vadose zone

Summary In this paper, we develop a finite analytic method (FAMM), which combines flexibility of numerical methods and advantages of analytical solutions, to solve the mixed-form Richards’ equation. This new approach minimizes mass balance errors and truncation errors associated with most numerical approaches. We use numerical experiments to demonstrate that FAMM can obtain more accurate numerical solutions and control the global mass balance better than modified Picard finite difference method (MPFD) as compared with analytical solutions. In addition, FAMM is superior to the finite analytic method based on head-based Richards’ equation (FAMH). Besides, FAMM solutions are compared to analytical solutions for wetting and drying processes in Brindabella Silty Clay Loam and Yolo Light Clay soils. Finally, we demonstrate that FAMM yields comparable results with those from MPFD and Hydrus-1D for simulating infiltration into other different soils under wet and dry conditions. These numerical experiments further confirm the fact that as long as a hydraulic constitutive model captures general behaviors of other models, it can be used to yield flow fields comparable to those based on other models.

An explicit approach to capture diffusive effects in finite water-content method for solving vadose zone flow

Summary Vadose zone flow problems are usually solved from the Richards equation. Solution to the Richards equation is generally challenging because the hydraulic conductivity and diffusivity in the equation are strongly non-linear functions of water content. The finite water-content method was proposed as an alternative general solution method of the vadose zone flow problem for infiltration, falling slugs, and vadose zone response to water table dynamics based on discretizing the water content domain into numerous bins instead of the traditional spatial discretization. In this study, we develop an improved approach to the original finite water-content method (referred to as TO method hereinafter) that better simulates diffusive effects but retains the robustness of the TO method. The approach treats advection and diffusion separately and considers diffusion on a bin by bin basis. After discretizing into water content bins, we treat the conductivity and diffusivity in individual bins as water content dependent constant evaluated at given water content corresponding to each bin. For each bin, we can solve the flow equations analytically since the hydraulic conductivity and diffusivity can be treated as a constant. We then develop solutions for each bin to determine the diffusive water amounts at each time step. The water amount ahead of the convective front for each bin is redistributed among water content bins to account for diffusive effects. The application of developed solution is straightforward only involving algebraic manipulations at each time step. The method can mainly improve water content profiles, but has no significant difference for the total infiltration rate and cumulative infiltration compared to the TO method. Although the method separately deals with advection and diffusion, it can account for the coupling effects of advection and diffusion reasonably well.

Quantitative evaluation of specific vulnerability to nitrate for groundwater resource protection based on process-based simulation model

It has been proved that groundwater vulnerability assessment is an effective tool for groundwater protection. Nowadays, quantitative assessment methods for specific vulnerability are scarce due to limited cognition of complicated contaminant fate and transport processes in the groundwater system. In this paper, process-based simulation model for specific vulnerability to nitrate using 1D flow and solute transport model in the unsaturated vadose zone is presented for groundwater resource protection. For this case study in Jilin City of northeast China, rate constants of denitrification and nitrification as well as adsorption constants of ammonium and nitrate in the vadose zone were acquired by laboratory experiments. The transfer time at the groundwater table t50 was taken as the specific vulnerability indicator. Finally, overall vulnerability was assessed by establishing the relationship between groundwater net recharge, layer thickness and t50. The results suggested that the most vulnerable regions of Jilin City were mainly distributed in the floodplain of Songhua River and Mangniu River. The least vulnerable areas mostly appear in the second terrace and back of the first terrace. The overall area of low, relatively low and moderate vulnerability accounted for 76% of the study area, suggesting the relatively low possibility of suffering nitrate contamination. In addition, the sensitivity analysis showed that the most sensitive factors of specific vulnerability in the vadose zone included the groundwater net recharge rate, physical properties of soil medium and rate constants of nitrate denitrification. By validating the suitability of the process-based simulation model for specific vulnerability and comparing with index-based method by a group of integrated indicators, more realistic and accurate specific vulnerability mapping could be acquired by the process-based simulation model acquiring. In addition, the advantages, disadvantages, constraint conditions and applying prospects of the quantitative approach for specific vulnerability assessment were discussed.

Snowmelt infiltration and storage within a karstic environment, Vers Chez le Brandt, Switzerland

Summary Even though karstic aquifers are important freshwater resources and frequently occur in mountainous areas, recharge processes related to snowmelt have received little attention thus far. Given the context of climate change, where alterations to seasonal snow patterns are anticipated, and the often-strong coupling between recharge and discharge in karst aquifers, this research area is of great importance. Therefore, we investigated how snowmelt water transits through the vadose and phreatic zone of a karst aquifer. This was accomplished by evaluating the relationships between meteorological data, soil–water content, vadose zone flow in a cave 53 m below ground and aquifer discharge. Time series data indicate that the quantity and duration of meltwater input at the soil surface influences flow and storage within the soil and epikarst. Prolonged periods of snowmelt promote perched storage in surficial soils and encourage surficial, lateral flow to preferential flow paths. Thus, in karstic watersheds overlain by crystalline loess, a typical pedologic and lithologic pairing in central Europe and parts of North America, soils can serve as the dominant mechanism impeding infiltration and promoting shallow lateral flow. Further, hydrograph analysis of vadose zone flow and aquifer discharge, suggests that storage associated with shallow soils is the dominant source of discharge at time scales of up to several weeks after melt events, while phreatic storage becomes import during prolonged periods without input. Soils can moderate karst aquifer dynamics and play a more governing role on karst aquifer storage and discharge than previously credited. Overall, this signifies that a fundamental understanding of soil structure and distribution is critical when assessing recharge to karstic aquifers, particularly in cold regions.

A new general 1‐D vadose zone flow solution method

AbstractWe have developed an alternative to the one‐dimensional partial differential equation (PDE) attributed to Richards (1931) that describes unsaturated porous media flow in homogeneous soil layers. Our solution is a set of three ordinary differential equations (ODEs) derived from unsaturated flux and mass conservation principles. We used a hodograph transformation, the Method of Lines, and a finite water‐content discretization to produce ODEs that accurately simulate infiltration, falling slugs, and groundwater table dynamic effects on vadose zone fluxes. This formulation, which we refer to as “finite water‐content”, simulates sharp fronts and is guaranteed to conserve mass using a finite‐volume solution. Our ODE solution method is explicitly integrable, does not require iterations and therefore has no convergence limits and is computationally efficient. The method accepts boundary fluxes including arbitrary precipitation, bare soil evaporation, and evapotranspiration. The method can simulate heterogeneous soils using layers. Results are presented in terms of fluxes and water content profiles. Comparing our method against analytical solutions, laboratory data, and the Hydrus‐1D solver, we find that predictive performance of our finite water‐content ODE method is comparable to or in some cases exceeds that of the solution of Richards' equation, with or without a shallow water table. The presented ODE method is transformative in that it offers accuracy comparable to the Richards (1931) PDE numerical solution, without the numerical complexity, in a form that is robust, continuous, and suitable for use in large watershed and land‐atmosphere simulation models, including regional‐scale models of coupled climate and hydrology.

Evaluating the Impact of Nonisothermal Flow on Vadose Zone Processes in Presence of a Water Table

Although the coupled theory of heat and mass transfer movement in the unsaturated zone has been developed for a long time, the interaction between water and heat is considered mainly in the shallow vadose zone, but not commonly in deeper soil profiles. The goal of this article was to evaluate the role of soil temperature and water vapor in the water flow in the whole vadose zone in the presence of a shallow water table by using two models with and without considering these effects. Simulated water contents and recharges were compared with observation data collected at the Hanwang Hydrology and Water Resources Experimental Station in China. The atmospheric data observed at this station were used to drive the two models during a 1-year period. Results show that the nonisothermal model better represented observed data than the isothermal model. However, the isothermal model underestimated the soil water content during the dry season and overestimated it during the rainy season. Meanwhile, the isothermal model significantly overestimated the annual cumulative net recharge to groundwater and underestimated the annual cumulative evaporation from the surface layer.