Moving Water Table Research Articles

A hydroponic system for natural turf that can take extreme wear as an alternative to artificial grass surfaces

AbstractNatural grass areas in high‐traffic and stress locations are being converted into artificial grass by inconsistent moisture conditions in root‐zone and poor grass conditions. As an alternative solution to this method, natural grass was planted instead in a washed sand without organic matter and with a constantly moving water table and no overhead irrigation. Water consumption and surface quality after heavy machinery exposure was measured. A significantly increase in the potential for traffic and play was observed, along with water savings of up to 85% compared with normal overhead irrigation amounts for similar surfaces (golf course tees).

Quantitative Analysis of Concrete Lining Damage in Field Canals by Frost Heave Using a Water–Heat–Mechanical Coupling Model

Concrete linings of field canals are seriously damaged by frost heave in the north of China. However, the mechanism causing lining damage is too complex to be described quantitatively, and present models are insufficient in considering soil–lining interaction. Based on mechanical analysis and the direct shear test, the restrictive relationship between the freezing soil and lining is introduced as a key point to quantitatively simulate the process of lining damage. Heat convection, solar radiation, and a moving water table boundary are also incorporated in the present model to simulate boundary conditions with daily climate data. Finally, two cases in the Hetao irrigation district are simulated. The developed model agrees with experiment data and can reflect the failure characteristics of lining. The results show: (1) the contact interface between lining and freezing soil will slide early in the freezing season, (2) reducing the freezing strength or weakening the restrictive relationship between the lining and freezing soil can reduce the maximum stress of the lining, (3) new lining structures with shapes like parabola and catenary perform better than a round-bottom-triangular (RT) shape because of a gentler side slope, and (4) large amounts of migration water accumulate and freeze beneath north (shady) slopes and cause a high damage risk for the lining. This paper can provide a reference for the further study of efficient and economical engineering methods for solving the lining damage problem.

Numerical Investigation of Ground-Borne Vibration Mitigation by Infilled Trenches in a Poroelastic Half-Space Considering the Moving Water Table

Abstract The moving water table would vary soil layering and may significantly affect the isolation efficiency of wave barriers. A two-dimensional finite-element model is then developed using COMSO...

Research on the effect of the moving water table on passive isolation effectiveness using a multiple open trench barrier

Research on the effect of the moving water table on passive isolation effectiveness using a multiple open trench barrier

The influence of pore-fluid in the soil on ground vibrations from a tunnel embedded in a layered half-space

A computationally efficient semi-analytical solution for ground-borne vibrations from underground railways is proposed and used to investigate the influence of hydraulic boundary conditions at the scattering surfaces and the moving ground water table on ground vibrations. The arrangement of a dry soil layer with varying thickness resting on a saturated poroelastic half-space, which includes a circular tunnel subject to a harmonic load at the tunnel invert, creates the scenario of a moving water table for research purposes in this paper. The tunnel is modelled as a hollow cylinder, which is made of viscoelastic material and buried in the half-space below the ground water table. The wave field in the dry soil layer consists of up-going and down-going waves while the wave field in the tunnel wall consists of outgoing and regular cylindrical waves. The complete solution for the saturated half-space with a cylindrical hole is composed of down-going plane waves and outgoing cylindrical waves. By adopting traction-free boundary conditions on the ground surface and continuity conditions at the interfaces of the two soil layers and of the tunnel and the surrounding soil, a set of algebraic equations can be obtained and solved in the transformed domain. Numerical results show that the moving ground water table can cause an uncertainty of up to 20 dB for surface vibrations.

A semi-analytical solution for slug tests in an unconfined aquifer considering unsaturated flow

Summary A semi-analytical solution considering the vertical unsaturated flow is developed for groundwater flow in response to a slug test in an unconfined aquifer in Laplace space. The new solution incorporates the effects of partial penetrating, anisotropy, vertical unsaturated flow, and a moving water table boundary. Compared to the Kansas Geological Survey (KGS) model, the new solution can significantly improve the fittings of the modeled to the measured hydraulic heads at the late stage of slug tests in an unconfined aquifer, particularly when the slug well has a partially submerged screen and moisture drainage above the water table is significant. The radial hydraulic conductivities estimated with the new solution are comparable to those from the KGS, Bouwer and Rice, and Hvorslev methods. In addition, the new solution also can be used to examine the vertical conductivity, specific storage, specific yield, and the moisture retention parameters in an unconfined aquifer based on slug test data.

Validation of finite water‐content vadose zone dynamics method using column experiments with a moving water table and applied surface flux

AbstractData from laboratory experiments on a 143 cm tall and 14.5 cm diameter column, packed with Wedron sand with varied constant upper boundary fluxes and water table velocities for both falling and rising water tables are used to validate a finite water‐content vadose zone simulation methodology. The one‐dimensional finite water‐content Talbot and Ogden (2008) (T‐O) infiltration and redistribution method was improved to simulate groundwater table dynamic effects and compared against the numerical solution of the Richards equation using Hydrus‐1D. Both numerical solutions agreed satisfactorily with time series measurements of water content. Results showed similar performance for both methods, with the T‐O method on average having higher Nash‐Sutcliffe efficiencies and smaller absolute biases. Hydrus‐1D was more accurate in predicting deponding times in the case of a falling water table, while Hydrus‐1D and the T‐O method had similar errors in predicted ponding times in the case of a rising water table in six of nine tests. The improved T‐O method was able to predict general features of vadose zone moisture dynamics with moving water table and surface infiltration using an explicit, mass‐conservative formulation. The advantage of an explicit formulation is that it is numerically simple, using forward Euler solution methodology, and is guaranteed to converge and to conserve mass. These properties make the improved T‐O method presented in this paper a robust and computationally efficient alternative to the numerical solution of the Richards equation in hydrological modeling applications involving groundwater table dynamic effects on vadose zone soil moistures.

Design of a resistively loaded modulated scatterer for ground penetrating radar measurements

ABSTRACTA broadband resistively loaded modulated scatterer (RLMS) is developed for ground penetrating radar experiments and optimized to reflect signals that are high in strength and have a flat frequency response with a linear phase variation. The linear phase makes the scattering compact in time and easily interpreted in the time‐domain. A novel method of measuring the modulation with a standard network analyzer is presented, and two example measurements are presented. The RLMS is used to measure the two‐way pattern of a pair of Resistive‐Vee dipole antennas, and to measure the response of a scatterer in soil with a moving water table. The RLMS performs well in these experiments. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:1557–1561, 2014

Application of level set method for groundwater flow with moving boundary

A potential of level set methods for the numerical modeling of moving boundaries and interfaces in groundwater flow problems is shown by the application for a particular model of the flow with moving water table (the phreatic surface). The main ingredients of the level set methods like the advection of level set function, the computation of signed distance function and the extrapolation of missing data are explained for a class of possible applications. The numerical solution of the groundwater flow problem is proposed using the flux-based level set method and the immersed interface method that are based on a finite volume discretization on any fixed (unstructured) grid. Presented examples show good properties of the method concerning a small mass balance error and a good grid convergence even for an example with merging boundaries. An example with seepage boundary is computed using the locally adapted grids by refining and coarsening the mesh according to the position of moving water table.

Can atmospheric pressure and water table fluctuations be neglected in soil vapor extraction?

Solutions to soil vapor extraction (SVE) are indispensable to characterize the unsaturated zone and to optimize SVE. Most of the existing solutions neglect the fluctuations of atmospheric pressure and water table. This study presents a new semi-analytical solution for SVE by considering the atmospheric pressure and water table fluctuations. Comparisons between the new and previous solutions indicate that the water table effect is negligible but the atmospheric pressure effect is non-negligible for the interpretation of gas pressure in a non-coastal site where the daily water table fluctuation is in centimeters scale; both the water table and atmospheric pressure fluctuations need to be considered in a coastal site where the daily water table fluctuation is in tens of centimeters scale. Tidal-induced downward moving water table increases the depth-averaged radius of influence, which is insensitive to atmospheric pressure fluctuation. Less vertical gas permeability leads to greater atmospheric pressure and water table effect.

Modeling slug tests in unconfined aquifers taking into account water table kinematics, wellbore skin and inertial effects

Two models for slug tests conducted in unconfined aquifers are developed by (a) extending the unconfined KGS solution to oscillatory responses, yielding a model referred to herein as the unified model, and (b) replacing the constant head condition with the linearized kinematic condition at the water table. The models can be used to analyze the full range of responses from highly oscillatory to overdamped. The second model, refered to as the moving water table (MWT) model, is only applicable when effects of well bore skin are negligible. The models are validated by comparison with published solutions, and by application to a published case study of field tests conducted in wells without skin in an unconfined aquifer at the MSEA site in Nebraska. In this regard (a) the MWT model essentially yields the same results as the confined KGS model, except very close to the water table, and (b) the unified model yields slightly smaller aquifer K-values relative to the MWT model at all positions in the well. All model solutions yield comparable results when fitted to published field data obtained in an unconfined fluvial aquifer at the MSEA site in Nebraska. The unified model is fitted to field data collected in wells known to exhibit positive skin effects at the Boise Hydrogeophysical Research Site (BHRS) in Boise, Idaho. It is shown to yield hydraulic conductivity estimates of comparable magnitude to those obtained with the KGS model for overdamped responses, and the Springer–Gelhar model for oscillatory responses. Sensitivity of the MWT model to specific yield, S y , and hydraulic anisotropy, κ is evaluated and the results, when plotted in log–log space and with consideration of log-scale time derivatives of the response, indicate that these two parameters should be estimable from slug test data, though challenges still remain.

Nitrogen nutrition of poplar trees

Many forest ecosystems have evolved at sites with growth-limiting nitrogen (N) availability, low N input from external sources and high ecosystem internal cycling of N. By contrast, many poplar species are frequent constituents of floodplain forests where they are exposed to a significant ecosystem external supply of N, mainly nitrate, in the moving water table. Therefore, nitrate is much more important for N nutrition of these poplar species than for many other tree species. We summarise current knowledge of nitrate uptake and its regulation by tree internal signals, as well as acquisition of ammonium and organic N from the soil. Unlike herbaceous plants, N nutrition of trees is sustained by seasonal, tree internal cycling. Recent advances in the understanding of seasonal storage and mobilisation in poplar bark and regulation of these processes by temperature and daylength are addressed. To explore consequences of global climate change on N nutrition of poplar trees, responses of N uptake and metabolism to increased atmospheric CO(2) and O(3) concentrations, increased air and soil temperatures, drought and salt stress are highlighted.

Simulated gravitational response to hydraulic testing of unconfined aquifers

The gravitational response to hydraulic testing of shallow unconfined aquifers (<50 m) has been simulated to assess whether gravity surveying can be used as a practical tool to detect groundwater extraction. A general expression is derived to calculate the response based on the gravitational attraction of the drawdown cone. The expression can accommodate any radially symmetric drawdown solution that assumes instantaneous release of groundwater above the moving water table as exemplified by Neuman's solution. Based on simulations, hydraulic testing of shallow unconfined aquifers with typical properties produces gravitational responses on the order of several tens of microGals (1 μGal=1×10 −8 m/s 2) at the pumping well. The response at the 1 μGal level can extend hundreds of meters away from the pumping well. At a distance of 1 m, survey resolutions between 0.002 to 0.15 and 0.03 to 2 μGal are required to delineate incremental drawdown at the 0.02- and 0.3-m levels, respectively, below the point of measurement.

Improved water table dynamics in MODFLOW

The standard formulation of a block-centered finite-difference model, such as MODFLOW, uses the center of the cell as the location of a cell node. Simulations of a dynamic water table can be improved if the node of a cell containing the water table is located at the water table rather than at the center of the cell. The LPF package of MOD-FLOW-2000 was changed to position a cell's node at the water table in convertible cells with a water table. Improved accuracy in the upper regions of an unconfined aquifer is demonstrated for pumping from a partially penetrating well. The change introduces a nonlinearity into the solution of the flow equations that results in slightly slower convergence of the flow solution, 7% slower in the presented demonstration. Accuracy of simulations is improved where vertical flow is dominated by a moving water table, but not when a large water table gradient dominates over the water table movement.

A Leaky Aquifer Model For Hard Rock Aquifers

A comparison is made between the unsteady parallel fracture model using the double porosity approach and the anisotropic aquifer-water table aquitard system. The presence of a moving water table in the weathered zone (unconfined block), influenced by recharge from rainfall or other sources and leakage to the fracture zone, is discussed. The effect of fracture skin on the block-fracture interface is studied for both double-porosity and aquifer-water table aquitard systems. Typical type curves are presented. It is found that the effect of both fracture skin and water table is to lead to a flat drawdown behaviour for a large time. The applicability of the present model for parameters estimation is illustrated on a field pumping test of 7 days duration.

Unsaturated and Saturated Flow in Response to Pumping of an Unconfined Aquifer: Numerical Investigation of Delayed Drainage

AbstractIn this study, numerical methods are used to examine the capillary fringe hypothesis proposed by Nwankwor et al. (1992) to explain variations in the storage property of an unconfined aquifer during pumping. Based on field observations, the hypothesis suggested that the response of an unconfmed aquifer to the stress imposed by pumping is largely controlled by the magnitude of vertical hydraulic gradients developed above the moving water table. After validation with a 1‐D hysteretic laboratory experiment and a 2‐D axisymmetric variably saturated semianalytical model, the numerical solution was used to simulate the pumping test of Nwankwor et al. (1992). Numerical parameters used for the simulation were chosen from experiments independent of the pumping test. There was good agreement between numerical results and field data at different depths and different radial distances from the pumping well. Numerically obtained variations in the magnitude of vertical gradients above the water table and the resulting variations in values of apparent specific yield also showed similar trends with the field data. The good agreement between the numerical and field results provides convincing evidence that the field observations and the resulting capillary fringe hypothesis concerning time variations of specific yield, are indeed consistent with the physical principles of variably saturated flow. The high storativity values frequently obtained by fitting early‐time data to type curves were shown to correspond to the period of increasing hydraulic gradients, and may not be related to intrinsic properties of the aquifer, such as compressibility. In agreement with the “volume‐balance method” of Nwankwor et al. (1984), it was also shown that the results confirm that the ultimate specific yield of an unconfmed aquifer is more related to the drainable porosity than the low values often obtained from type‐curve analysis.

Modeling water table contribution to crop evapotranspiration

A model was developed to account for the time-dependent contribution of the water table to crop evapotranspiration. The same numerical approximation used to solve the water flow in the unsaturated zone was also modified for saturated conditions. For unsaturated flow, the hydraulic conductivity changes with water content and the specific water capacity has finite values. For saturated flow, hydraulic conductivity is constant, and the specific water capacity is zero. The proposed approach considers saturated flow as a special case of unsaturated flow with a constant saturated water content and very small but not zero specific water capacities. Thus flow can be simulated in either unsaturated or saturated zones. The contribution of upward flow to crop evapotranspiration was evaluated during lysimeter experiments in the greenhouse. Spring wheat was planted on asilty clay loam and a fine sandy loam with either no water table or constant water table depths at 50, 100 or 150 cm. Irrigation was applied whenever soil water was depleted below about 50% plant available water. Model predictions of water content and cumulative upward flux as a function of time, for the different water table depths and soils, agreed closely with measured values. The contribution of the water table to evapotranspiration (ET) was found to be 90, 41 and 7% for 50, 100, and 150 cm water table depths respectively for the silty clay loam. Corresponding computed values were 89, 45 and 6%. For the fine sandy loam measured contribution of the water table to ET was 92, 31, and 9% for 50, 100 and 150 cm water tables respectively. Corresponding computed values were 99, 29, and 11 %. It was not practical to simulate the saturated-unsaturated (moving water table) predictions of the model under greenhouse conditions because of the height of the lysimeters needed. Therefore the model was also used to simulate field irrigation management options under several bottom boundary conditions where the water table contributions were significant to crop water use. Results from a one-year simulation were consistent with data for sugarcance grown under similar conditions in the Cauca Valley of Colombia.

PEATLANDS AND MARSHES IN THE WETLAND REGIONS OF CANADA

AbstractPeatland development in Canada is influenced by climatic conditions, specifically a north–south temperature and an east–west precipitation gradient, resulting in specific and characteristic kinds of wetlands. The wetland classes (bogs, fens, marshes) are broad ecological units, each composed of distinctive groups of vegetation growing in appropriate habitats. Bogs usually develop in nutrient-poor areas where there is little movement of groundwater, whereas fens occur in more mineral-rich environments having a slowly moving water table. Marshes develop in areas that are periodically inundated by oxygen-rich water. Included within the peatlands are water environments ranging from small puddles to pools, each with a water quality that is influenced by the surrounding peatland.

Green and Ampt analyses for rising water table and intermittent surface flux conditions

Equations developed for a Green and Ampt type analysis for moving water table conditions in an initially saturated homogeneous profile of porous material and used previously in a falling water table study are now applied to a falling‐rising water table sequence in a sand. The ability of the developed equations to be used in a system involving a directional change in water table movement relates to the inclusion in the equations of a term describing the initial saturated depth applicable to each new stage of the analysis. The predictive capability of the Green and Ampt approach for such a system is tested by comparing the results with those obtained from a computer‐based numerical analysis. The relevant subroutine in the computer program required certain restructuring to accommodate the rising water table component. An interpolative hysteresis model is used to describe the accompanying wetting process with the material in question wetting up along primary wetting scanning curves. In comparison with the complexity of the real system during rewetting, the simplicity of the Green and Ampt approach is very marked. Notwithstanding this, it is found that reasonable correspondence occurs, although care is necessary in specifying an appropriate value, applicable to the wetting process, for the characteristic pressure head at the sharp front. The Green and Ampt approach is also extended to include a falling water table system where an applied surface flux of constant magnitude is discontinued during the gravity drainage process. The analysis can be readily adapted to such intermittent inflow and, again, comparisons with a numerical simulation indicate that satisfactory accuracy is achievable.

Solar pond ground heat loss to a moving water table

An analytical solution is presented that calculates the heat loss from the bottom of a solar pond (or any heated object) to a soil that contains a moving water table. The water table is treated as a fluid slab moving as a slug flow in one dimension. Edge effects and horizontal heat conduction are ignored. Both steady-state and time-dependent solutions are presented. Results are presented in terms of an effectiveness ratio—the actual heat flux divided by the steady-state heat flux resulting from a constant temperature heat sink at the depth of the water table. The only water-table parameter that strongly affects the effectiveness is the fluid capacity rate. Thus, for any potential solar pond site, a measurement of the mass flow rate of the water table combined with knowledge of the soil thermal properties will allow a good estimation of the ground heat loss expected over the lifetime of the pond.