Saturated Groundwater Flow Model Research Articles

Controls on flood managed aquifer recharge through a heterogeneous vadose zone: hydrologic modeling at a site characterized with surface geophysics

Abstract. In water-stressed regions of the world, managed aquifer recharge (MAR), the process of intentionally recharging depleted aquifers, is an essential tool for combating groundwater depletion. Many groundwater-dependent regions, including the Central Valley in California, USA, are underlain by thick unsaturated zones (ca. 10 to 40 m thick), nested within complex valley-fill deposits that can hinder or facilitate recharge. Within the saturated zone, interconnected deposits of coarse-grained material (sands and gravel) can act as preferential recharge pathways, while fine-textured facies (silts and clays) accommodate the majority of the long-term increase in aquifer storage. However, this relationship is more complex within the vadose zone. Coarse facies can act as capillary barriers that restrict flow, and contrasts in matric potential can draw water from coarse-grained flow paths into fine-grained, low-permeability zones. To determine the impact of unsaturated-zone stratigraphic heterogeneity on MAR effectiveness, we simulate recharge at a Central Valley almond orchard surveyed with a towed transient electromagnetic system. First, we identified three outcomes of interest for MAR sites: infiltration rate at the surface, residence time of water in the root zone and saturated-zone recharge efficiency, which is defined as the increase in saturated-zone storage induced by MAR. Next, we developed a geostatistical approach for parameterizing a 3D variably saturated groundwater flow model using geophysical data. We use the resulting workflow to evaluate the three outcomes of interest and perform Monte Carlo simulations to quantify their uncertainty as a function of model input parameters and spatial uncertainty. Model results show that coarse-grained facies accommodate rapid infiltration rates and that contiguous blocks of fine-grained sediments within the root zone are >20 % likely to remain saturated longer than almond trees can tolerate. Simulations also reveal that capillary-driven flow draws recharge water into unsaturated, fine-grained sediments, limiting saturated-zone recharge efficiency. Two years after inundation, fine-grained facies within the vadose zone retain an average of 37 % of recharge water across all simulations, where it is inaccessible to either plants or pumping wells. Global sensitivity analyses demonstrate that each outcome of interest is most sensitive to parameters that describe the fine facies, implying that future work to reduce MAR uncertainty should focus on characterizing fine-grained sediments.

Fen ecohydrologic trajectories in response to groundwater drawdown with an edaphic feedback.

Fens are high conservation value ecosystems that depend on consistent discharge of groundwater that saturates the near surface for most of the growing season. Reduced groundwater inputs can result in losses of native diversity, decreases in rare-species abundance and increased invasion by non-native species. As such, fen ecosystems are known to be particularly susceptible to changes in groundwater conditions including reduction in water levels due to nearby groundwater pumping. However, research is lacking on whether floristic degradation is influenced by feedbacks between hydrology and soil properties. We present a model of an archetype hillslope fen that couples a hydrological niche model with a variably saturated groundwater flow model to predict changes in vegetation composition in response to different groundwater drawdown scenarios. The model explores a potential edaphic feedback through the use of an observed relationship between fen floristic quality and soil/peat water retention characteristics that is attenuated with separate edaphic and floristic memory terms representing lags in biophysical responses to dewatering. Model parameters were determined based on data collected from six fens in Wisconsin under various states of degradation. We observed different water retention characteristics between sites that were minimally impacted versus degraded that are likely due to peat decomposition, oxidation and compaction at the degraded sites. These characteristics were also correlated with floristic quality. The results reveal a complex response to drawdown where changes in peat hydraulic properties following dewatering lead to even drier conditions and further shifts away from typical fen species.

Solving hindered groundwater dynamics in restored tidal marshes by creek excavation and soil amendments: A model study

Groundwater fluxes in tidal marshes largely control key ecosystem functions and services, such as vegetation growth, soil carbon sequestration, and nutrient cycling. In tidal marshes restored on formerly embanked agricultural land, groundwater fluxes are often limited as compared to nearby natural marshes, as a result of historical agricultural soil compaction. To improve the functioning of restored tidal marshes, knowledge is needed on how much certain design options can optimize soil-groundwater interactions in future restoration projects. Based on measured data on soil properties and tidally induced groundwater dynamics, we calibrated and evaluated a 2D vertical model of a creek-marsh cross-section, accounting for both saturated and unsaturated groundwater flow and solute transport in a variably saturated groundwater flow model. We found that model simulations of common restoration practices such as soil amendments (increasing the depth of porous soil on top of the compact layer) and creek excavation (increasing the creek density) increase the soil aeration depth and time, the drainage depth and the solute flux, and decrease the residence time of solutes in the porewater. Our simulations indicate that increasing the depth to the compact layer from 20 cm to 40 cm, or increasing the creek density from 1 creek to 2 creeks along a 50 m marsh transect (while maintaining the total creek cross-sectional area), in both cases more than doubles the volume of water processed by the marsh soil. We discuss that this may stimulate nutrient cycling. As such, our study demonstrates that groundwater modelling can support the design of marsh restoration measures aiming to optimize groundwater fluxes and related ecosystem services.

Analyzing variation of the water table level with three-dimensional numerical simulations to assess reclamation techniques for an acidic tailings impoundment

Tailings with sulphide minerals exposed to oxygen and water can oxidize and produce acid mine drainage (AMD). This study evaluated the impact of natural conditions and of a thin cover deposition on the water table level, with respect to selecting the reclamation technique to control AMD on the elevated portion of an abandoned tailings impoundment. The reactive tailings were partly covered with alkaline tailings transported as a pulp from a nearby mine. A three-dimensional numerical variably saturated groundwater flow model of the tailings impoundment and its surrounding area was built. The simulation results reproduced well the field observations before, during, and after the deposition of a thin layer of tailings. The calibrated model was then used to forecast the evolution of the water table position for the next 100 years under various site conditions, including a period of 5 years with dry summers. The results show that the water table levels are on average several metres below the interface between the reactive tailings and the cover, thus failing to meet the target criterion to control the production of AMD determined from previous column tests. The results are analyzed and discussed in terms of the site configuration and atmospheric recharge.

The topographic control on land surface energy fluxes: A statistical approach to bias correction

Subsurface hydrodynamics are an important component of the hydrological cycle and a key factor in the partitioning of land surface water and energy fluxes. Because of computational reasons they are often neglected, or strongly simplified, in numerical weather prediction and climate models. Particularly in regions where the water table is shallow, soil moisture acts as a link between groundwater, land and atmosphere. Because of its long-term memory, groundwater represents a buffer for the effects of climate variability. To dynamically model this system we study the outputs of a variably saturated groundwater flow model (ParFlow) coupled with a surface model (CLM) and propose an empirical approach for the statistical correction of the bias of the energy fluxes in a highly-parametrized simulation. Corrections are based on a comparison of the simple scheme with the fully-coupled subsurface-land-atmosphere simulations. This simple dynamical scheme computes the potential latent heat flux in case of near saturation, while the fully-coupled simulations approximate the reality. Our statistical model examines the evapotranspiration surplus, i.e., the difference in latent heat flux between the two runs, and aims to correct the bias in the latent heat flux of the simple scheme, based on the characteristics of each point of the domain. In particular, we focus on the ability of topography-related indices, such as the topographic wetness index and the depth-to-water index, to provide information on the availability of water for evapotranspiration. Our results confirm that topographic indices are good predictors for moisture availability. While small-scale structures cannot be accurately reproduced by our model, large-scale biases of latent heat flux over the domain are effectively removed. Moreover, the bias corrected fluxes show a better agreement with the fluxes of the full modelling system than commonly used free-drainage simulations. Thus, the proposed approach can be useful in approximating the effect of groundwater on land surface water and energy fluxes in, e.g., regional climate models.

Solute dynamics across the stream‐to‐riparian continuum under different flood waves

AbstractTo enhance the understanding of solute dynamics within the stream‐to‐riparian continuum during flood event‐driven water fluctuation (i.e., flood wave), a variable saturated groundwater flow and solute transport model were developed and calibrated against in situ measurements of the Inbuk stream, Korea, where seasonal flooding prevails. The solute dynamics were further investigated for flood waves (varying by amplitude [A], duration [T], roundness [r], and skewness [tp]) that were parameterised by real‐time stream stage fluctuations. We found that the solute transferred faster and farther in the riparian zone, especially within the phreatic zone, above which in the variable saturated zone the concentration required a significantly longer time, particularly at higher altitudes, to return to the initial state. By comparison, solute transferred shallowly in the streambed where the solute plume exhibited an exponential growth trend from the centre to the bank. The dynamic changes of solute flux and mass along the stream–aquifer interface and stream concentration were linked to the shape of flood wave. As the flood wave became higher (A↗), wider (T↗), rounder (r↘), and less skewed (tp↗), the maximum solute storage in aquifer increased. Maximum stream concentration (Cstrˍmax) not only presented a positive linear relationship with A or tp but also showed a negative logarithmic trend with increasing T or r. The sensitivity of Cstr_max to A was approximately two times that of tp, and between these values, the r was slightly more sensitive than T. Cstrˍmax linearly increased as hydraulic conductivity increased and logarithmically increased as longitudinal dispersivity increased. The former relationship was more sensitive than the latter.

Impact of groundwater flow across tectonic aquifer compartments in a Miocene sandstone aquifer: three-dimensional hydrogeological modeling of the Kasserine aquifer system in central Tunisia and northeastern Algeria

The Kasserine Aquifer System (KAS) is a transboundary aquifer, located in an arid region in central Tunisia and extending into northeastern Algeria. The system consists of four compartments: Oum Ali-Thelepte, Feriana-Skhirat, and the Plateau and the Plaine of Kasserine. The challenge of this study was to evaluate the influence of regional faults on groundwater flow in the different compartments of the KAS and to estimate the regional impact of current and future groundwater use. A three-dimensional saturated regional groundwater flow model for the steady state and transient conditions (1980–2015) was created and calibrated. This work was achieved using numerical flow modelling, coupled with geological modelling, using FEFLOW and GeoModeller software. The significance of regional faults as potential barriers or conduits to groundwater flow in the different aquifer compartments was evaluated by considering the different recharge rates. Two connectivity hypotheses were proposed at each major fault, and the general hydraulic relationship of units that are juxtaposed by each fault were considered. This study contributes rigorous estimates for the diffuse and concentrated recharge in the arid study region, and evaluates the groundwater behavior that shows a gradual decline in the water table over time, using a regional model. Different predicted outcomes for the KAS based on variable potential groundwater extraction scenarios for the period 2015–2050 have been developed. The results of numerical simulation provide useful information regarding the behavior of the KAS aquifers, and contribute significant knowledge to guide sustainable practice for present and future groundwater management.

Improved Inverse Modeling by Separating Model Structural and Observational Errors

A practical formal likelihood function (L) is developed to separate model structure errors and observation errors by the separation of correlated and uncorrelated model residuals. L overcomes the time-consuming problem of likelihood functions proposed by previous studies, and combines the Mean Square Error (MSE) and first-order Auto-Regression (AR(1)) models. For comparison of the effect of different error models, MSE, AR(1), and L are used as efficiency criteria to calibrate the three-dimensional variably saturated ground-water flow model (MF2K-VSF) based on the soil tank seepages of rainfall–runoff experiments. Results of L are nearly the same as those of AR(1) due to negligible observational errors. Although all calibrated models well mimic the seepage discharges, MF2K-VSF with MSE cannot capture the groundwater level and soil suction processes because of the considerable autocorrelation of model residuals owing to model inadequacies (e.g., neglect of the soil moisture hysteresis), which obviously violates the statistical assumption of MSE. By contrast, L accounts for the model structural errors and thus enhances the reliability of hydrological simulations.

High resolution modelling of soil moisture patterns with TerrSysMP: A comparison with sensor network data

The prediction of the spatial and temporal variability of land surface states and fluxes with land surface models at high spatial resolution is still a challenge. This study compares simulation results using TerrSysMP including a 3D variably saturated groundwater flow model (ParFlow) coupled to the Community Land Model (CLM) of a 38ha managed grassland head-water catchment in the Eifel (Germany), with soil water content (SWC) measurements from a wireless sensor network, actual evapotranspiration recorded by lysimeters and eddy covariance stations and discharge observations. TerrSysMP was discretized with a 10×10m lateral resolution, variable vertical resolution (0.025–0.575m), and the following parameterization strategies of the subsurface soil hydraulic parameters: (i) completely homogeneous, (ii) homogeneous parameters for different soil horizons, (iii) different parameters for each soil unit and soil horizon and (iv) heterogeneous stochastic realizations. Hydraulic conductivity and Mualem-Van Genuchten parameters in these simulations were sampled from probability density functions, constructed from either (i) soil texture measurements and Rosetta pedotransfer functions (ROS), or (ii) estimated soil hydraulic parameters by 1D inverse modelling using shuffle complex evolution (SCE).The results indicate that the spatial variability of SWC at the scale of a small headwater catchment is dominated by topography and spatially heterogeneous soil hydraulic parameters. The spatial variability of the soil water content thereby increases as a function of heterogeneity of soil hydraulic parameters. For lower levels of complexity, spatial variability of the SWC was underrepresented in particular for the ROS-simulations. Whereas all model simulations were able to reproduce the seasonal evapotranspiration variability, the poor discharge simulations with high model bias are likely related to short-term ET dynamics and the lack of information about bedrock characteristics and an on-site drainage system in the uncalibrated model. In general, simulation performance was better for the SCE setups. The SCE-simulations had a higher inverse air entry parameter resulting in SWC dynamics in better correspondence with data than the ROS simulations during dry periods. This illustrates that small scale measurements of soil hydraulic parameters cannot be transferred to the larger scale and that interpolated 1D inverse parameter estimates result in an acceptable performance for the catchment.

Numerical modeling of subsurface release and fate of benzene and toluene in coastal aquifers subjected to tides

A numerical study was undertaken to investigate subsurface release and fate of benzene and toluene in a tidally influenced beach. The simulations were conducted by using a numerical model BIOMARUN, which coupled a multi-Monod kinetic model BIOB to a density-dependent variably saturated groundwater flow model MARUN. The fate and transport of the contaminant plume were characterized by computing its centroid trajectory, spreading area and percentage of biodegradation in beach saturated and unsaturated zone, respectively. Key factors likely affecting this process were investigated, including tide amplitude, capillarity and hydraulic conductivity. It was found that aerobic biodegradation was the major fate of the contaminant plume in the beach. Tidal action twisted the centroid of the contaminant plume during its migration in the beach, which increased the residence time of the plume in the beach. High tidal range significantly altered the spatial distribution of the contaminant biodegradation in the beach. In contrast, the capillary fringe had impacts on the percentage of benzene biodegraded in the saturated and unsaturated zone of the beach. The increase in capillary fringe enhanced the percentage of the contaminant biodegraded in the unsaturated zone, up to 40%, which is comparable to that in the saturated zone. Hydraulic conductivity seemed to have large impacts on the biodegradation rate of the contaminant in the beach. Higher hydraulic conductivity induced faster contaminant biodegradation in the beach.

Biodegradation of subsurface oil in a tidally influenced sand beach: Impact of hydraulics and interaction with pore water chemistry

AbstractThe aerobic biodegradation of oil in tidally influenced beaches was investigated numerically in this work using realistic beach and tide conditions. A numerical model BIOMARUN, coupling a multiple‐Monod kinetic model BIOB to a density‐dependent variably saturated groundwater flow model 2‐D MARUN, was used to simulate the biodegradation of low‐solubility hydrocarbon and transport processes of associated solute species (i.e., oxygen and nitrogen) in a tidally influenced beach environment. It was found that different limiting factors affect different portions of the beach. In the upper intertidal zone, where the inland incoming nutrient concentration was large (1.2 mg N/L), oil biodegradation occurred deeper in the beach (i.e., 0.3 m below the surface). In the midintertidal zone, a reversal was noted where the biodegradation was fast at shallow locations (i.e., 0.1 m below the surface), and it was due to the decrease of oxygen with depth due to consumption, which made oxygen the limiting factor for biodegradation. Oxygen concentration in the midintertidal zone exhibited two peaks as a function of time. One peak was associated with the high tide, when dissolved oxygen laden seawater filled the beach and a second oxygen peak was observed during low tides, and it was due to pore oxygen replenishment from the atmosphere. The effect of the capillary fringe (CF) height was investigated, and it was found that there is an optimal CF for the maximum biodegradation of oil in the beach. Too large a CF (i.e., very fine material) would attenuate oxygen replenishment (either from seawater or the atmosphere), while too small a CF (i.e., very coarse material) would reduce the interaction between microorganisms and oil in the upper intertidal zone due to rapid reduction in the soil moisture at low tide.

Numerical modeling of water flow and salt transport in bare saline soil subjected to evaporation

Summary A numerical study, based on a density-dependent variably saturated groundwater flow model MARUN, was conducted to investigate subsurface flow and salt transport in bare saline aquifers subjected to evaporation, which was simulated using the bulk aerodynamic formulation. As evaporation was assumed to depend on the pore moisture, the evaporation flux evolved gradually causing a gradual increase in the pore salinity. This is in contrast to prior studies where the high salinity was imposed instantaneously on the ground surface. Key factors likely affecting subsurface hydrodynamics were investigated, including saturated hydraulic conductivity, capillary drive, relative humidity in the air, and surrounding groundwater replenishment. The simulations showed two temporal regimes where the first consists of rapid evaporation for a duration of hours followed by slow evaporation, until evaporation ceases. In the absence of surrounding groundwater replenishment, evaporation-induced density gradient generated an upward water flow initially, and then the flow decreased at which time a high density salt “finger” formed and propagated downwards. Capillary properties and atmospheric condition had significant impacts on subsurface moisture distribution and salt migration in response to the evaporation. The results also suggested that the presence of subsurface water replenishment to the evaporation zone tended to produce a steady evaporation rate at the ground surface.

Evapotranspiration of bush encroachments on a temperate mire meadow – A nonlinear function of landscape composition and groundwater flow

As widely recognized, bush encroachment on mire meadows induces species and habitat biodiversity. However, it is unclear if expansion of either a coherent, continuous forest or randomly distributed groups of trees, so-called tree isles, in formerly open landscapes of mire meadows influences differently the water balance of the system due to changing vegetation structure, landscape, and thus evapotranspiration. In this paper we use a quasi-3D unsaturated–saturated groundwater flow model to reveal the feedback between the actual evapotranspiration and diurnal phreatic groundwater level dynamics. A schematized numerical experiment was setup to elucidate the ecohydrological functioning of mire meadows in the Biebrza Valley (NE Poland), which have been influenced by bush encroachment due to abandonment of mowing. Model calibration and validation was based on field collected data on groundwater dynamics showing the feedback between the evapotranspiration and diurnal groundwater level fluctuations. The model was applied for different landscape compositions of encroachments as well as different stages of shrubby vegetation expansion in a formerly open meadow. It is concluded that for coupled unsaturated–saturated flow models of mires, analysis of diurnal groundwater level fluctuations combined with evapotranspiration quantification is an efficient yet simple method for model calibration and validation. Results of the modelling experiment indicated that the tree-isle-type of bush encroachment entails higher losses of water from the system due to evapotranspiration than coherent forest expansion. For a hypothetical summer drought and an encroachment cover of 50% the total evapotranspiration is shown to be 13% higher for the tree-isle-type encroachment than for expansion of a coherent forest. Consequently, it is concluded that conservation of mires requires continuous control of encroachment not only because of potential loss of biodiversity, but more importantly to limit significant loss of water due to increased evapotranspiration.

A Scale-Consistent Terrestrial Systems Modeling Platform Based on COSMO, CLM, and ParFlow

AbstractA highly modular and scale-consistent Terrestrial Systems Modeling Platform (TerrSysMP) is presented. The modeling platform consists of an atmospheric model (Consortium for Small-Scale Modeling; COSMO), a land surface model (the NCAR Community Land Model, version 3.5; CLM3.5), and a 3D variably saturated groundwater flow model (ParFlow). An external coupler (Ocean Atmosphere Sea Ice Soil, version 3.0; OASIS3) with multiple executable approaches is employed to couple the three independently developed component models, which intrinsically allows for a separation of temporal–spatial modeling scales and the coupling frequencies between the component models.Idealized TerrSysMP simulations are presented, which focus on the interaction of key hydrologic processes, like runoff production (excess rainfall and saturation) at different hydrological modeling scales and the drawdown of the water table through groundwater pumping, with processes in the atmospheric boundary layer. The results show a strong linkage between integrated surface–groundwater dynamics, biogeophysical processes, and boundary layer evolution. The use of the mosaic approach for the hydrological component model (to resolve subgrid-scale topography) impacts simulated runoff production, soil moisture redistribution, and boundary layer evolution, which demonstrates the importance of hydrological modeling scales and thus the advantages of the coupling approach used in this study.Real data simulations were carried out with TerrSysMP over the Rur catchment in Germany. The inclusion of the integrated surface–groundwater flow model results in systematic patterns in the root zone soil moisture, which influence exchange flux distributions and the ensuing atmospheric boundary layer development. In a first comparison to observations, the 3D model compared to the 1D model shows slightly improved predictions of surface fluxes and a strong sensitivity to the initial soil moisture content.

Groundwater flow and salt transport in a subterranean estuary driven by intensified wave conditions

A numerical study, based on a density-dependent variably saturated groundwater flow model, was conducted to investigate flow and salt transport in a nearshore aquifer under intensified wave conditions caused by offshore storms. Temporally varying onshore hydraulic gradients due to wave setup were determined as the seaward boundary condition for the simulated aquifer. The results showed a rapid increase in influxes across the aquifer-ocean interface in response to the wave event followed by a more gradual increase in effluxes. The upper saline plume first widened horizontally as the wave setup point moved landward. It then expanded vertically with recirculating seawater pushed downward by the wave-induced hydraulic gradient. The time for the salt distribution to return to the prestorm condition was up to a hundred days and correlated strongly with the time for seawater to recirculate through the aquifer. The pathways of recirculating seawater and fresh groundwater were largely modified by the wave event. These pathways crossed through the same spatial locations at similar times, indicating significant salt-freshwater mixing. The flow and salt transport dynamics were more responsive to wave events of longer duration and higher intensity, especially in more permeable aquifers with lower fresh groundwater discharge. Despite their larger response, aquifers with higher permeability and beach slope recovered more rapidly postevent. The rapid recovery of the flows compared with the salinity distribution should be considered in field data interpretation. Due to their long-lasting impact, wave events may significantly influence the geochemical conditions and the fate of chemicals in a subterranean estuary. Key Points Intensified waves perturb flow and transport in a subterranean estuary Lengthy period (months) for salinity distribution to recover from a wave event Exchange fluxes and flows respond and recover rapidly following a wave event

Improved predictions of saturated and unsaturated zone drawdowns in a heterogeneous unconfined aquifer via transient hydraulic tomography: Laboratory sandbox experiments

► Uniform parameters lead to biased drawdown predictions in the (un)saturated zones. ► Accounting for aquifer heterogeneity yields significant improvements in predictions. ► Considering unsaturated zone (UZ) heterogeneity produced a minor improvement. ► UZ heterogeneity plays a minor role in forming the S shape drawdown curve. ► Hydraulic tomography can be used to characterize unconfined aquifers. Interpretation of pumping tests in unconfined aquifers has largely been based on analytical solutions that disregard aquifer heterogeneity. In this study, we investigate whether the prediction of drawdown responses in a heterogeneous unconfined aquifer and the unsaturated zone above it with a variably saturated groundwater flow model can be improved by including information on hydraulic conductivity ( K ) and specific storage ( S s ) from transient hydraulic tomography (THT). We also investigate whether these predictions are affected by the use of unsaturated flow parameters estimated through laboratory hanging column experiments or calibration of in situ drainage curves. To investigate these issues, we designed and conducted laboratory sandbox experiments to characterize the saturated and unsaturated properties of a heterogeneous unconfined aquifer. Specifically, we conducted pumping tests under fully saturated conditions and interpreted the drawdown responses by treating the medium to be either homogeneous or heterogeneous. We then conducted another pumping test and allowed the water table to drop, similar to a pumping test in an unconfined aquifer. Simulations conducted using a variably saturated flow model revealed: (1) homogeneous parameters in the saturated and unsaturated zones have a difficult time predicting the responses of the heterogeneous unconfined aquifer; (2) heterogeneous saturated hydraulic parameter distributions obtained via THT yielded significantly improved drawdown predictions in the saturated zone of the unconfined aquifer; and (3) considering heterogeneity of unsaturated zone parameters produced a minor improvement in predictions in the unsaturated zone, but not the saturated zone. These results seem to support the finding by Mao et al. (2011) that spatial variability in the unsaturated zone plays a minor role in the formation of the S-shape drawdown-time curve observed during pumping in an unconfined aquifer.

Integrated numerical model for vegetated surface and saturated subsurface flow interaction

The construction of an integrated numerical model is presented in this paper to deal with the interactions between vegetated surface and saturated subsurface flows. A numerical model is built by integrating the previously developed quasi-three-dimensional (Q3D) vegetated surface flow model with a two-dimensional (2D) saturated groundwater flow model. The vegetated surface flow model is constructed by coupling the explicit finite volume solution of 2D shallow water equations (SWEs) with the implicit finite difference solution of Navier-Stokes equations (NSEs) for vertical velocity distribution. The subsurface model is based on the explicit finite volume solution of 2D saturated groundwater flow equations (SGFEs). The ground and vegetated surface water interaction is achieved by introducing source-sink terms into the continuity equations. Two solutions are tightly coupled in a single code. The integrated model is applied to four test cases, and the results are satisfactory.

Hydroecological model predictions indicate wetter and more diverse soil water regimes and vegetation types following floodplain restoration

Transitions between aquatic and terrestrial ecosystems represent zones where soil moisture is a dominant factor influencing vegetation composition. Niche models based on hydrological and vegetation observations can be powerful tools for guiding management of these zones, especially when they are linked with physically based hydrological models. Floodplain restoration represents a unique opportunity to utilize such a predictive vegetation tool when a site's hydrology is altered to create a wetter environment. A variably saturated groundwater flow model was developed and used to simulate the soil moisture regime across a floodplain in Wisconsin where post‐settlement alluvium was removed with the intent of increasing regionally threatened wetland plant species. Hydrological niche models based on simultaneous observations of vegetation composition and surface effective saturation were used to predict probability of presence for two plant species (Carex vulpinoidea (fox sedge) and Elymus canadensis(Canada wildrye)) and wetland indicator score (a composite indicator of relative frequency of species in five habitat categories) based on simulated surface effective saturation. The vegetation predictions following restoration are more wetland‐species dominant overall. However, zones of the study site where a confining layer is present that decouples groundwater from the near‐surface soil zone tend to be drier following restoration due to restricted upward groundwater flow and less soil water storage above the confining layer. As reflected by an increase in the interquartile range in the predicted wetland indicator score, this restoration technique may increase the site‐scale spatial diversity of plant community types while simultaneously accomplishing the goal of increasing wetland plant species occurrence.

A Quasi-Three-Dimensional Variably Saturated Groundwater Flow Model for Climate Modeling

Abstract In this study, a quasi-three-dimensional, variably saturated groundwater flow model was developed by approximately dividing the three-dimensional soil water and groundwater flow into an unsaturated vertical soil water flow and a horizontal groundwater flow to simulate the interactions among soil water, groundwater, and vegetation. The developed model consists of a one-dimensional unsaturated soil water flow model with the water table as the moving boundary using an adaptive grid structure for a vertical soil column formed based on discrete grid cells in a horizontal domain, a two-dimensional groundwater flow model for the horizontal domain, and an interface model connecting the two components for the horizontal grid cells in the domain. Synthetic experiments by the model were conducted to test the sensitivities of the model parameters of river elevation, ground surface hydraulic conductivity, and surface flux, and the results from the experiments showed the robustness of the proposed model under different conditions. Comparison of the simulation by the model and that by a full three-dimensional scheme showed its feasibility and efficiency. A case of stream water conveyance in the lower reaches of the Tarim River was then applied to validate the developed model for simulation of the water table elevations at the Yingsu section. Finally, a numerical experiment by the model for the Tarim River basin was conducted to discuss the groundwater latent flow for large-scale high-relief topography with stream water conveyance. The results show that the model can simulate the water table reasonably well.

Assessment of nitrogen contamination of groundwater in paddy and upland fields

This study aims to assess the nitrogen contamination of groundwater in paddy and upland fields. A reactive chemical transport model PHREEQC and a variable saturated groundwater flow and transport model FEMWATER were used to evaluate the vertical transport of nitrogen compound in various soil types of paddy and upland. The shallow groundwater quality monitoring data of 2003, 2006, 2009 in the Choushui river alluvial fan, the major agriculture production area in Taiwan, were applied to support the validity of the numerical simulation findings. Results from PHREEQC and FEMWATER simulations showed that the organic-rich impermeable plow sole layer underneath the muddy layer of rice paddy can effectively reduce NO3 − and N2 to NH4 + and retard the movement of NH4 +. However, in the upland field which has no plow sole layer, the NH4 + can move easily to the shallow aquifer and contaminate the groundwater. The spatiotemporal distribution of NO3 −–N and NH4 +–N in the Choushui river alluvial fan revealed that high nitrate–N contamination areas were located mainly in the upland field of the proximal fan, where the granular unconfined aquifer was vulnerable to surface contaminants. Moreover, the unconfined nature of the aquifer allows the oxidization of NH4 + to NO3 − and accelerates the plume movement. High ammonium–N concentration areas were mostly dispersed in the distal-fan area where upland planting and aquacultural farming were prevailed. The high NH4 +–N found in the northern Choushui river alluvial fan was attributed to the alternative planting of rice and upland crops, and the plow sole layer was broken to maintain the quick drainage upland crop needs.