Water Table Response Research Articles

Transient simulations of flow and transport in the Chalk unsaturated zone

Chalk is a fractured porous medium composed of matrix blocks bounded by interconnected fractures. Furthermore, there are a number of commonly observed, and apparently contradictory, phenomena. These include: a fast water table response, slow solute migration and very little solute dispersion. To the authors' knowledge, no unified physically-based, numerical model has been able to reconcile all of these points. In this paper, these issues were addressed through the development of a transient dual-permeability model of flow and transport. The behaviour of the model was explored through a hypothetical scenario. The one-dimensional model featured a fixed water table at 10 m depth. A recharge time-series, derived using the Penman-Grindley model in conjunction with daily precipitation and monthly potential evaporation data from the Kennet valley, Berkshire, was applied at the top of the model. A conservative solute (that could be tritium, chloride or nitrate) was applied to the recharge for 1 year. Subsequent years involved the input of clean water. The simulations suggested that although transient effects have been previously neglected, overlying soil and gravelly chalk layers are likely to cause significant attenuating effects on flow such that most flow occurs through the matrix. Fracture flow was found to be episodic and infrequent and represented between 17% and 30% of the annual recharge.

Impacts of logging disturbance on hillslope saturated hydraulic conductivity in a tropical forest in Peninsular Malaysia

Using a constant-head permeameter we found an abrupt natural decrease in saturated hydraulic conductivity ( K s) within the upper 1.0-m soil profile of an Orthoxic Tropudult at the Bukit Tarek Experimental Catchments research area in tropical, Peninsular Malaysia. The depth at which low K s could cause a perched water table in response to high-intensity rainfall, however, was too great to generate saturation overland flow on planar hillslopes in the study area. The effects of logging activity on K s at five subsurface depths (0.1, 0.25, 0.5, 0.75, and 1.0 m) on the non-roaded portion of the harvest area were examined at the three following sites, which differed in the degree of disturbance and recovery since timber harvesting: (1) selective logging conducted in the 1960s; (2) mechanized selective tree removal conducted 4 years ago; and (3) high-impact clear-cutting just prior to measurement. This recent logging greatly disturbed the soil surface (via compaction, topsoil/subsoil mixing, burning) and produced comparatively high variability in near-surface K s. Changes in K s at or below 0.25-m, however, were not detected with certainty at any sites. In terms of hillslope hydrologic response, the connectivity of zones of low K s in the harvest area with dense networks of skid trails and terraces was identified as one of the most important consequences of timber harvesting, although this phenomenon was not quantified. We estimate the recovery time for near-surface K s on the non-roaded hillslope to be less than 40 years.

Recharge, upflux and water table response for shallow water table conditions in southwest Florida

A disproportionate increase or decrease in water table in response to minor water input or drainage is observed in shallow water table conditions inside drainage lysimeters. This increase happens because the capillary fringe of the shallow water table reaches up to or near the surface (Wieringermeer effect). The correlations between water table level changes and rainfall, seepage irrigation, drip irrigation, and drainage were analysed. Correlations with rainfall, seepage irrigation, and drainage were high (R2 ranged from 0·46 to 0·97). Drip irrigation had low correlations due to the low rates of application (R2 ranged from 0·26 to 0·44). Conventional methods of calculating recharge, such as multiplying the specific yield with the water table fluctuations, cannot be used for Wieringermeer effect situations. A method using water balance data and soil moisture at different depths in the lysimeters was developed to estimate recharge and upflux. The recharge results were used to develop the apparent specific yield Sya, which could be used to calculate consequent recharge events from water table fluctuation data. Combining the water table fluctuation relationships developed with the Sya value will allow the prediction of recharge from rainfall and irrigation events without the need for soil moisture equipment. Copyright © 2005 John Wiley & Sons, Ltd.

The Response of Playa and Sabkha Hydraulics and Mineralogy to Climate Forcing

Dry playa lakes and sabkhat often represent the terminus of large ground water flow systems and act as integrators of both upgradient (recharge) and downgradient discharge (evaporation). Ground water levels beneath playa/sabkha systems show a variety of surprising responses driven by large evaporation demands and chemical processes not typically encountered in more humid regions. When the water table is very close to the land surface, almost instantaneous rises can be observed with little observed change in either upgradient ground water recharge or potential evaporation. Conversely, when water tables are several meters below the playa surface, water table responses to interannual variability of recharge can be damped and lag significantly behind such changes. This review of the dynamics of shallow water tables in playa lakes and sabkhat discusses the pertinent hydraulic and solute processes and extracts a simple but comprehensive model based on soil physics for predicting the water table response to either upstream recharge changes or changes in potential evaporation at the playa/sabkha. Solutes and associated authigenic minerals are also shown to be important in discriminating both the causes and effects of water level fluctuations.

Borehole time domain reflectometry in layered sandstone: Impact of measurement technique on vadose zone process identification

The hydraulic behaviour of the vadose zone of a layered sandstone aquifer has been investigated using borehole-based Time Domain Reflectometry (TDR). Both a commercially available portable packer TDR system (TRIME-B3L Borehole Packer Probe) and specially designed borehole-emplaced TDR probes were used to monitor seasonal fluctuations in water content in the vadose zone of a layered sandstone over 1 year under natural rainfall loading. The data show that the vadose zone contains seasonal perched water tables that form when downward percolating water reaches layers of fine grained sandstone and siltstone and causes local saturation. The formation of perched water tables is likely to lead to lateral flow bypassing the less permeable, finer layers. This contrasts with behaviour inferred from previous studies of the same aquifer that used borehole radar and resistivity, which suggested its vadose zone behaviour was characterized by uniform downwards migration of wetting fronts. To investigate the impact of measurement technique on observed response, the TDR data reported here were used to produce simulated zero offset profile (ZOP) borehole radar responses. This simulation confirmed the limited ability of ZOP borehole radar to detect key vadose zone processes, because the phenomenon of critical refraction minimizes the sensitivity of the results to high water content layers. The ability of the resistivity method to detect perched water table responses is also limited, because of the relatively large sampling volume of the technique. The study illustrates that inappropriate technique selection results in hydrological process mis-identification, with serious consequences for the usefulness of data in hydrological modeling.

Analysis of interfacial error in saturated–unsaturated flow models

Interfacial error results from estimation of interblock conductivities related to the saturated–unsaturated interface. Both interfacial conductivity error ( IE K ) and interfacial pressure error ( IE h ) were analyzed under the arithmetic mean scheme while IE K was numerically investigated under the averaging schemes arithmetic, geometric and harmonic. IE K , dependent on the media pore size, is regularly less than zero while IE h , associated with the height of capillary fringe, may be greater than zero. An interfacial discretization technique was developed to add two complementary equations into the saturated–unsaturated model with respect to the interface. The proposed interfacial approach may eliminate interfacial error from the approximations of interblock conductivities. Underestimation of the water-table response to infiltration is related to the negative IE K . The water-table response error reaches −5.13% in our investigation, which is an accumulated result from IE K .

Influence of capillarity on a simple harmonic oscillating water table: Sand column experiments and modeling

Comprehensive measurements of the water table response to simple harmonic forcing at the base of a sand column are presented and discussed. In similar experiments, Nielsen and Perrochet (2000) observed that fluctuations in the total moisture were both damped and lagged relative to the water table fluctuations. As a result, the concept of a complex effective porosity was proposed as a convenient means to account for the damping and phase lag through its magnitude and argument, respectively. The complex effective porosity then enables simple analytical solutions for the water table (and total moisture) dynamics including hysteresis. In this paper, these previous experiments are extended to cover a wider range of oscillation frequencies and are conducted for three well‐sorted materials with median grain diameters of 0.082, 0.2, and 0.78 mm, respectively. In agreement with existing theory, the influence of the capillary fringe is shown to increase with the oscillation frequency. However, the complex effective porosity model corresponding to the classical Green and Ampt (1911) capillary tube approximations is shown to be inadequate when compared to the data. These limitations are overcome by the provision of an empirical, frequency‐dependent complex effective porosity model fit to the data. Using measured moisture retention parameters, numerical simulation of the data solving a nonhysteretic van Genuchten–Richards' equation type model is unable to replicate the observations. Existing results of a hysteretic numerical model are shown to be in good agreement with the extended database.

INCREASE IN GROUNDWATER LEVEL DUE TO ARTIFICIAL RECHARGE

ABSTRACT Artificial recharge of the ground-water table from rectangular basins has been the subject of extensive study in the last few decades. We develop a numerical model, based on the Finite Difference Method with an explicit scheme, to predict the transient position of the water table in response to recharge from a rectangular basin. The rate of recharge is varied by applying a time decay constant in order to simulate realistic infiltration behaviour. The model is validated through comparison with an existing analytical solution and is then applied to estimate the mounding below a proposed recharge basin at the Indian Institute of Technology Kanpur.

Toward a conceptual model of floodplain water table response

Hydrological processes operating within floodplains in temperate midlatitudes have significant implications for water management by controlling pollutant transfer between the catchment and the fluvial system. However, there is a lack of relevant high‐resolution data from which the dynamics of floodplain hydrology during flood events can be inferred. A detailed analysis of water table fluctuations during flood events within a typical European lowland floodplain system (River Severn, United Kingdom) is presented. Data collected hourly along two 120‐meter‐long transects, each comprising four piezometers, plus one river stage sensor, are analyzed for the winter season 1998–1999 using correlation analysis, hysteresis curves, and water table maps. The objective is to develop a conceptual model that provides mechanistic understanding of floodplain water table response during flood events. River stage is shown to be the principal driver of water table fluctuations. Piezometers with similar water table response are identified; their consistent pattern of response in different flood events is attributed to sedimentary and morphological controls on the floodplain and adjoining hillslopes. Deviations from the general pattern are a function of low antecedent soil moisture, which is only a significant factor at the beginning of the winter season, when the floodplain is initially dry. Our conceptual model adopts a kinematic wave process whereby river stage change induces rapid responses of the water table over many tens of meters across the floodplain, associated with flux velocities several orders of magnitude higher than would be expected for Darcian flow. The occurrence of a groundwater ridge within the floodplain dams hillslope drainage and causes the water table to rise at the back of the floodplain. The disappearance of the groundwater ridge during the recession reestablishes hillslope flow into the floodplain, resulting in significant three‐dimensional hydraulic gradients directed both perpendicular and parallel to the channel axis.

Farm salinity appraisal with water reuse

The need for a better understanding of the interaction between irrigation practices and the elevation and quality of the water table is of paramount importance for developing irrigation management strategies to ameliorate the regional problems of elevated saline water tables in the San Joaquin Valley, California. An area of approximately 3000 ha which includes portions of the Diener Ranch and the adjacent University of California, Westside Research and Extension Center, located south of Five Points in the Westlands Water District on the west side of the San Joaquin Valley was chosen for extensive field measurements. Field work consisted of four main activities namely, field instrumentation, collection of records of field activities, periodic data collection, and analyses of field data. Field measurements of water table carried out during 1994 indicated that the water table elevation was sensitive to the irrigation practices. There was a general increase in the area with a water table close to the surface during the irrigation season, and a return to water table elevations similar to the starting conditions at the end of the season. During the study period, the surface water quality deteriorated more in areas irrigated with reuse water and persisted through the end of the season. Depth averaged electrical conductivity for the study area over 6.5 m decreased between December 1993 and December 1994. Vertical hydraulic gradients in the saturated zone, were found to be an order of magnitude larger than horizontal gradients. The direction of vertical gradients changed, with downward gradients following pre-irrigations and upward gradients later in the season, when crop water requirements increased. Based on the results of the field study, it can be concluded that the irrigation management practices have a direct effect on local water table response as well as on water quality. Therefore, irrigation practices that promote less deep percolation losses may be helpful in controlling the water table rise.

Perched Water Table Responses to Forest Clearing in Northern Idaho

Perched Water Table Responses to Forest Clearing in Northern Idaho

Perched Water Table Responses to Forest Clearing in Northern Idaho

Many forested soils of northern Idaho containing fragipans and seasonal perched water tables (PWTs) have been cleared for agricultural use. However, little information exists on the direct effect of canopy removal on PWTs in these soils. We compared PWTs in soils where forest canopy had been removed with those in adjacent soils with intact canopy. Study sites were selected such that both cleared and forested treatments occurred within the same soil map unit. Perched water table levels in shallow wells were monitored weekly or biweekly from November to June or July for 3 yr. Results indicate that canopy removal substantially affects both average height and duration of seasonal PWTs. Average PWT levels were 6 to 107% higher under cleared treatments, with greatest increases observed when seasonal precipitation was close to long‐term averages. Seasonal PWTs developed 2 to 8 wk sooner under cleared treatments compared with forested treatments. Additionally, it took as much as four months before PWTs in the forested treatments reached an equivalent height as those in the cleared treatments. At one study site, the average volume of perched water in the cleared treatment was 7.5 cm greater than that in the forested treatment over the period of episaturation. Results suggest that land‐use interpretations based on duration and proximity of a seasonal PWT to the soil surface may need to be adjusted when vegetation cover is altered. It may also be appropriate to distinguish between cleared and forested phases of fragipan‐containing soil series when developing hydrologic interpretations.

Hydrological Dynamics of the Panola Mountain Research Watershed, Georgia

AbstractRelations between stormflow characteristics and soil‐moisture content, water table elevation in wells along several transects perpendicular to the stream, and precipitation were evaluated for the Panola Mountain Research Watershed, Georgia, from water years 1986 through 2001 to refine the conceptualization of streamflow generation, including the relations between ground water dynamics and streamflow. For 759 rainstorms during 16 water years, stormflow water yield correlates best with maximum soil‐moisture content and with maximum water table elevation; the relations are linear above thresholds for the entire period and vary by season. For moderate to large rainstorms, relations among wetness parameters and stormflow water yield are better (less scatter) and more consistent when the watershed is seasonally wet than when the watershed is dry. During rainstorms, soil‐moisture content (minimum or maximum) is highly correlated among soil depths and with water table elevations. The soil‐moisture measurements were made at one site at the base of a hillslope, and the values at any depth are likely to vary spatially and with respect to timing, other landscape positions, and soil types. Stormflow water yield is linearly related to soil‐moisture content at the deepest location (70 cm) above 37% volumetric moisture content, which occurs ∼20% of the time. Maximum water table elevation is linearly related to maximum soil‐moisture content above a threshold, and the soil‐moisture threshold increases with upslope position; the soil‐moisture threshold for flow to occur at a trench hillslope site is 41%, which occurs <0.5% of the time. The general relations among streamflow, soil moisture, and water table response are attributed to variable source areas, particularly the ground water contribution from a riparian zone aquifer ≤5 m thick that expands as the watershed becomes wetter.

Hillslope‐storage Boussinesq model for subsurface flow and variable source areas along complex hillslopes: 2. Intercomparison with a three‐dimensional Richards equation model

The Boussinesq equation for subsurface flow in an idealized sloping aquifer of unit width has recently been extended to hillslopes of arbitrary geometry by incorporating the hillslope width function w(x) into the governing equation, where x is the flow distance along the length of the hillslope [Troch et al., 2003]. Introduction of a source/sink term N allows simulation of storm‐interstorm sequences in addition to drainage processes, while a function Sc(x) representing the maximum subsurface water storage can be used to account for surface saturation response in variable source areas activated by the saturation excess mechanism of runoff generation. The model can thus simulate subsurface flow and storage dynamics for nonidealized (more realistic) hillslope configurations. In this paper we assess the behavior of this relatively simple, one‐dimensional model in a series of intercomparison tests with a fully three‐dimensional Richards equation model. Special attention is given to the discretization and setup of the boundary and initial conditions for seven representative hillslopes of uniform, convergent, and divergent plan shape. Drainage and recharge experiments are conducted on these hillslopes for both gentle (5%) and steep (30%) bedrock slope angles. The treatment and influence of the drainable porosity parameter are also considered, and for the uniform (idealized) hillslope case the impact of the unsaturated zone is examined by running simulations for different capillary fringe heights. In general terms, the intercomparison results show that the hillslope‐storage Boussinesq model is able to capture the broad shapes of the storage and outflow profiles for all of the hillslope configurations. In specific terms, agreement with the Richards equation results varies according to the scenario being simulated. The best matches in outflow hydrographs were obtained for the drainage experiments, suggesting a greater influence of the unsaturated zone under recharge conditions due to transmission of water throughout the hillslope. In the spatiotemporal water table response a better match was observed for convergent than divergent hillslopes, and the bedrock slope angle was not found to greatly influence the quality of the agreement between the two models. On the basis of the intercomparison experiments we make some suggestions for further development and testing of the hillslope‐storage model.

Where does water go when it rains? Moving beyond the variable source area concept of rainfall‐runoff response

Where does water go when it rains? What flow path does it take to the stream? How long does it reside in the catchment? These questions were articulated by John Hewlett within the context of his variable source area (VSA) concept almost 40 years ago (Hewlett and Hibbert, 1967). Today, we still grapple with these often vexing questions—now using new tools and approaches, but, as then, still searching for answers. Rapid progress is being made on the rainfallrunoff modelling front in catchment hydrology vis-a-vis parameter estimation techniques, model uncertainty analysis, examination of parameter identifiability in our models, downward approaches to hydrologic prediction, etc. (Beven 2001; Sivapalan, 2003). However, I wonder if we have somewhat neglected updating our understanding of the rainfall-runoff process and how this informs our needed model structures and response to these three basic questions central to our conceptualization of how catchments work? One could argue that our sharpening perception of water source, flowpath and age in upland headwater catchments is radically different to what the framers of the VSA theory thought a half century ago (i.e. Hewlett in the USA, Cappus in France and Tsukamoto in Japan). Our best models still rely on mechanistic notions underlying the VSA, including saturation excess overland flow and subsurface stormflow (I will avoid mentioning how our operational models often are based on another whole older generation of streamflow generation concepts related exclusively to Horton!). The VSA concept has been distilled into our widely used research model structures by collapsing the process complexity into simple mathematical assumptions of things like the decline in saturated hydraulic conductivity with depth, steady-state catchment water table response, topographically defined water flowpaths and linear wetting and drying from the valley bottom upwards to the ridge (depending upon storm size, intensity and antecedent wetness conditions). Much discussion is now devoted in the modelling literature towards the balance between practical simplifications of the VSA details and justifiable model complexity. This commentary takes a critical look at our process underpinning by discussing new field evidence of where water goes when it rains that directly challenges the status quo. New model structures informed by this new process understanding are then discussed in the context of how data

The role of bedrock topography on subsurface storm flow

We conducted a detailed study of subsurface flow and water table response coupled with digital terrain analysis (DTA) of surface and subsurface features at the hillslope scale in Panola Mountain Research Watershed (PMRW), Georgia. Subsurface storm flow contributions of macropore and matrix flow in different sections along an artificial trench face were highly variable in terms of timing, peak flow, recession characteristics, and total flow volume. The trench flow characteristics showed linkages with the spatial tensiometer response defining water table development upslope. DTA of the ground surface did not capture the observed spatial patterns of trench flow or tensiometric response. However, bedrock surface topographic indices significantly improved the estimation of spatial variation of flow at the trench. Point‐scale tensiometric data were also more highly correlated with the bedrock surface‐based indices. These relationships were further assessed for temporal changes throughout a rainstorm. Linkages between the bedrock indices and the trench flow and spatial water table responses improved during the wetter periods of the rainstorm, when the hillslope became more hydrologically connected. Our results clearly demonstrate that in developing a conceptual framework for understanding the mechanisms of runoff generation, local bedrock topography may be highly significant at the hillslope scale in some catchments where the bedrock surface acts as a relatively impermeable boundary.

Evaluation of a computer model to simulate water table response to subirrigation

The objective of this work was to evaluate the water flow computer model, WATABLE, using experimental field observations on water table management plots from a site located near Hastings, FL, USA. The experimental field had scale drainage systems with provisions for subirrigation with buried microirrigation and conventional seepage irrigation systems. Potato (Solanum tuberosum L.) growing seasons from years 1996 and 1997 were used to simulate the hydrology of the area. Water table levels, precipitation, irrigation and runoff volumes were continuously monitored. The model simulated the water movement from a buried microirrigation line source and the response of the water table to irrigation, precipitation, evapotranspiration, and deep percolation. The model was calibrated and verified by comparing simulated results with experimental field observations. The model performed very well in simulating seasonal runoff, irrigation volumes, and water table levels during crop growth. The two-dimensional model can be used to investigate different irrigation strategies involving water table management control. Applications of the model include optimization of the water table depth for each growth stage, and duration, frequency, and rate of irrigation.

Evaluation of a pumping test of the Snake River Plain aquifer using axial-flow numerical modeling

The Snake River Plain aquifer in southeast Idaho is hosted in a thick sequence of layered basalts and interbedded sediments. The degree to which the layering impedes vertical flow has not been well understood, yet is a feature that may exert a substantial control on the movement of contaminants. An axial-flow numerical model, RADFLOW, was calibrated to pumping test data collected by a straddle-packer system deployed at 23 depth intervals in four observation wells to evaluate conceptual models and estimate properties of the Snake River Plain aquifer at the Idaho National Engineering and Environmental Laboratory. A delayed water-table response observed in intervals beneath a sediment interbed was best reproduced with a three-layer simulation. The results demonstrate the hydraulic significance of this interbed as a semi-confining layer. Vertical hydraulic conductivity of the sediment interbed was estimated to be about three orders of magnitude less than vertical hydraulic conductivity of the lower basalt and upper basalt units. The numerical model was capable of representing aquifer conceptual models that could not be represented with any single analytical technique. The model proved to be a useful tool for evaluating alternative conceptual models and estimating aquifer properties in this application.

Modeling groundwater-surface water interactions including effects of morphogenetic depressions in the Chernobyl exclusion zone

Morphogenetic depressions or "dishes" in the Chernobyl exclusion zone play an important role in the transport of water and solutes (in particular the radionuclides 137Cs and 90Sr), functioning as accumulation basins and facilitating their transfer between the surface and subsurface via return flow (under conditions of high soil water saturation) and infiltration. From a digital elevation model (DEM) of the 112-km2 study area, 583 dishes (covering about 10% of the area) are identified and classified into four geometric types, ranging in size from 2,500 to 22,500 m2, and a with a maximum depth of 2 m. The collective influence of these depressions on the hydrology of the study basin is investigated with a coupled model of three-dimensional saturated and unsaturated subsurface flow and one-dimensional (along the rill or channel direction s) hill-slope and stream overland flow. Special attention is given to the handling of dishes, applying a "lake boundary-following" procedure in the topographic analysis, a level pool routing algorithm to simulate the storage and retardation effects of these reservoirs, and a higher hydraulic conductivity in the topmost 3 m of soil relative to non-dish cells in accordance with field observations. Modeling the interactions between the surface and subsurface hydrologic regimes requires careful consideration of the distinction between potential and actual atmospheric fluxes and their conversion to ponding, overland flow, and infiltration, and this coupling is described in some detail. Further consideration is given to the treatment of snow accumulation, snowmelt, and soil freezing and thawing processes, handled via linear and step function variations over the winter months in atmospheric boundary conditions and in upper soil hydraulic conductivities. A 1-year simulation of the entire watershed is used to analyze the water table response and, at the surface, the ponding heads and the infiltration/exfiltration fluxes. Saturation patterns and return flow and seepage fluxes are similar between the dishes and the main channel networks, with, as expected, preferential infiltration in the dishes.

Groundwater dynamics along forest-marsh transects in a southeastern salt marsh, USA: Description, interpretation and challenges for numerical modeling

Long-term and spatially dense time series of total head measured alongthree forest-marsh piezometer transects across a finger marsh basin,together with bimonthly groundwater salinity measurements, reveal dynamicfeatures that present challenges to the interpretation and modeling of thisshallow water table aquifer. These include:1. Rapid response of forest water table to rain events.2. Daytime lowering (drawdown) of the water table in the forest and highmarsh due to evapotranspiration and its recovery due to rain, seepage ortidal inundation.3. Upward gradients in head along the western margin of the marsh butdownward gradients in the eastern, more seaward, high marsh and forest.4. Rapid head responses to the tide in parts of the high marsh andforest that are not actually inundated.5. Asymmetrical distribution of salinity with higher values in theeastern marsh and forest than in the west.6. Sharp salinity gradients across the tidal creek that bisects the basin. We discuss modeling issues related to these features because acomprehensive understanding of them may have a bearing on patterns ofbotanical zonation and primary production, the transport of nutrients andcontaminants, and the response of this system to sea level rise.