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... Notwithstanding, Vandenberg (1980) applied and improved the work of Toth (1962) by developing transient model in a unit basin and studied flow field distortion under a periodically changing water table. Additionally, Zhao et al. (2018) extended the work of Toth (1963) by deriving a transient solution under a periodically changing water table in a complex basin and examined the transient behavior of nested flow systems, which was an extended application of Vandenberg (1980) method. Nevertheless, the Tothian theories of groundwater flow in unit basin (Toth 1962) and complex basins (Toth 1963) in most cases assume that water table is always a subdued replica of topography (Haitjema and Mitchell-Bruker 2005). ...
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Decision-makers require correct and adequate information on groundwater flow systems in a basin in order to formulate sustainable water resources development strategies. However, the practicality and realism of groundwater flow system models depend on the validity, reliability and availability of quality data and information, and how they are used in model development and calibration. This goes hand in hand with how the underlying theories, tenets and assumptions are understood, interpreted and applied. The more uncertain and contentious the information is, the wider are the knowledge and theoretical gaps, and thus the less useful the model results are for decision-making. The understanding of water table types in groundwater basins has become one of the additional factors for an in-depth understanding and modeling of nested groundwater flow systems. The classification of water table types using a water table ratio provides that if the ratio is more than 1, this depicts a topography-controlled area and a ratio of less than 1 depicts a recharge-controlled terrain. Log transformation of the water table ratio proved the same interpretation. This paper therefore reviews the evolution of groundwater flow systems theory, the prevailing knowledge and theoretical gaps by specifically pinpointing the theoretical and conceptual contentions and additional factors which can possibly limit the application of groundwater flow theories in regional groundwater modeling studies. The implications of how the conceptual and theoretical contentions affect groundwater modeling for decision-making in groundwater development and management are also pinpointed in this paper.
... Therefore, no data of storativity, specific yield, or aquifer diffusivity can be obtained. These parameters are vital for the management of groundwater resources, the evaluation of surface water-groundwater interactions, and the protection of the ecological environment [22][23][24][25]. Solutions regarding transient behaviors should be considered in the future. ...
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Straightforward solutions have long been expected for the analysis of multiwell aquifer tests. In this paper, we derive series analytical solutions of steady-state groundwater flow in a rectangular-shaped aquifer with pumping/injection wells for both confined and unconfined conditions. Double Fourier Transform (DFT) technique is applied to deal with different combinations of impermeable and specified head boundaries on sides. The obtained solutions are compact and concise in mathematics and flexible in terms of well number, well locations, and pumping/injection rates. Hatoucaidang, a groundwater resource field in the Ordos Plateau, Northwestern China, is introduced as a field case study, where a multiwell aquifer test was conducted. One of the analytical solutions derived herein is used to estimate hydraulic conductivities by applying a direct calculation method and a least square estimation method regarding observed versus calculated drawdowns. By comparing with nearby single-well pumping tests, the reliability of the derived analytical solutions is proven. This study facilitates utilizing the multiwell aquifer test to analyze the general behavior of groundwater movement in aquifer systems.
... It is found that for flowing wells with different depths and different locations when α = 0.7, d eq * ranges from 0.57 to 0.82, which corresponds to the lower half of the flowing wells (Fig. 8b). Although the size of the zone with flowing wells is sensitive to water-table undulations (Wang et al. 2015b;Zhao et al. 2018), the range of d eq * is insensitive to water-table undulations. Under all water-table undulations, when the flowing well is close to the boundary of the zone with flowing ...
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Groundwater sampled at the outlets of uncased flowing wells in a thick unconfined aquifer, which has undergone mixing, has been found to have hydrochemistry similar to deep groundwater in discharge areas. To identify the hydrodynamic causes, transient models of groundwater flow and age in a three-dimensional homogeneous unit basin with flowing wells are constructed to obtain flow rates in wells and groundwater mean age around wells. Inflow of groundwater to the well in the deep part leads to mixing of groundwater from different sources, and the finally mixed groundwater is found to have the same age as groundwater in the aquifer at a specific depth, termed the equivalent position (EP). EP is always found in the lower half of the flowing well, indicating that a mixed sample at the outlet could represent deep groundwater. Outflow from the well to the unconfined aquifer in the shallow part results in aging of groundwater around the well. For fully penetrating flowing wells in confined aquifers, EP is found in the upper half of the aquifer. The different relative depths of EP to the screen interval in the two types of flowing wells are mainly due to the profiles of horizontal velocity in the inflow segment, which is basically uniform in a confined aquifer but increases from zero to a maximum value in unconfined aquifers. Thus, groundwater at the outlets of topography-controlled flowing wells is a window of the deep part of a basin, and existing long-screen wells could have the potential for groundwater sampling.
... By changing α, different degrees of water table undulation can be characterized (J. Z. Wang, Jiang, Wan, Wörman, et al., 2015;Zhao et al., 2018). Figure 2b shows that a decrease in α would lead to lower vertical hydraulic gradients between the basin bottom and the water table, which is similar to the effect of R/K in models with a specified-flux top boundary (Liang et al., 2013). ...
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The occurrence of flowing wells in basins has been found to be closely related to the discharge area with an upward hydraulic gradient. Unfortunately, previous studies on upward gradient induced wellbore flow with equaling total inflow (Qin) in the deep and total outflow (Qout) in the shallow could not explain the occurrence of flowing wells. By representing wells using the MNW2 Package imbedded in MODFLOW 2005, we obtain the exchange of groundwater between the aquifer and the well in the discharge area of 3D unit basins and identify three scenarios: Qin=Qout, Qin>Qout>0 and Qin>Qout=0. The relationship of Qin>Qout well explains why flowing wells only develop in a limited part of the discharge area. Sensitivity analysis shows that well location, water table undulation and basin length‐depth ratio do not change the profile of the ratio of cumulative flow rate in the flowing well to total inflow (Qv/Qin) versus the relative elevation in the inflow segment, zin*, but could significantly change the length of the inflow segment; well depth could change both the length of the inflow segment and the profile of Qv/Qin versus zin*. Based on numerical results in homogeneous and isotropic basins with different dimensions, the ratio of inflow in the lower half part of a flowing well to the total inflow is found to be at least 68% and could be close to 100%, indicating that water at the outlets of flowing wells with long open sections is mainly from the deep part of the well.
... The different behavior of Inner and Outer Lake is consistent with the findings of Zhao et al. (2016) who examined transient behavior of nested flow systems and found that water table fluctuations are greatest near the flow divides and are less variable in the main discharge areas. ...
Article
Climate change can directly influence groundwater systems through modification of recharge. Affecting not only groundwater levels and flow dynamics, climate change can also modify the fragmentation and hierarchy of groundwater flow systems. In this study, the influence of climate change - impacted recharge on groundwater levels and on inter-connected groundwater flow patterns is evaluated. Special emphasis is placed on how flow system hierarchy may change, to examine possible consequences on groundwater-related shallow surface water bodies and on groundwater – surface water interaction. As a test site with no significant anthropogenic impacts, the Tihany Peninsula in Hungary was an ideal area for the study. We address the following issues: i) How might a groundwater system, including groundwater-surface water interaction, be modified by predicted climate change?, ii) Given the variable groundwater levels and flow patterns, how will the water levels and fluxes be impacted around surface water bodies?, and iii) How sensitive are groundwater-related wetlands to these changes, and will they be maintained or will they eventually disappear? In order to answer these questions, two-dimensional transient numerical simulations were performed based on site-specific measurements and climatic prediction at the Tihany Peninsula. Results show that future climate trends can cause dynamic evolution and dissipation of transient groundwater flow systems, and the characteristic flow system hierarchy can change from nested flow systems to a set of single flow cells. Preservation of associated groundwater-dependent ecosystems would be challenging under these conditions since long-term climate change could potentially have serious consequences, including wetland disappearance. Understanding these transient processes in two-dimensions can also help to set-up three-dimensional site-specific models.
... One limitation of the current study is that compressible groundwater flow, which could play an important role in deep basins (El-Rawy et al., 2016), has not been considered. Some previous studies on transient compressible flow in 2-D cross-sectional models have revealed the effect of specific storage on the pattern of water table-induced flow (Vandenberg, 1980;Zhao et al., 2016) in deep basins; however, incorporating a discussion on the effect of specific storage in 3-D basins is beyond the scope of the current study. ...
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Although it has been increasingly acknowledged that groundwater flow pattern is complicated in the three-dimensional (3-D) domain, two-dimensional (2-D) water table-induced flow models are still widely used to delineate basin-scale groundwater circulation. However, the validity of 2-D cross-sectional flow field induced by water table has been seldom examined. Here, we derive the analytical solution of 3-D water table-induced hydraulic head in a Tóthian basin and then examine the validity of 2-D cross-sectional models by comparing the flow fields of selected cross sections calculated by the 2-D cross-sectional model with those by the 3-D model, which represents the “true” cases. For cross sections in the recharge or discharge area of the 3-D basin, even if head difference is not significant, the 2-D cross-sectional models result in flow patterns absolutely different from the true ones. For the cross section following the principal direction of groundwater flow, although 2-D cross-sectional models would overestimate the penetrating depth of local flow systems and underestimate the recharge/discharge flux, the flow pattern from the cross-sectional model is similar to the true one and could be close enough to the true one by adjusting the decay exponent and anisotropy ratio of permeability. Consequently, to determine whether a 2-D cross-sectional model is applicable, a comparison of hydraulic head difference between 2-D and 3-D solutions is not enough. Instead, the similarity of flow pattern should be considered to determine whether a cross-sectional model is applicable. This study improves understanding of groundwater flow induced by more natural water table undulations in the 3-D domain and the limitations of 2-D models accounting for cross-sectional water table undulation only.
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The responses of the nested groundwater flow systems (NGFS) to rainfall fluctuations and the impacts of the NGFS on surface runoff are explored using a fully coupled variably saturated groundwatersurface water model in conjunction with spectral analysis. Numerical experiments are designed to investigate the fractal behaviors of hydraulic head and surface runoff in different scenarios. The results show that the inlets, outlets, and flow patterns of the sub-systems of NGFS can change with rainfall. The scaling-exponent, defined as the slope of the power spectra of the fluctuations of hydraulic head or runoff, is used to depict the fractal behaviors. The scaling-exponent of the hydraulic head is highly variable within the unsaturated zone and local flow systems but remains constant within the intermediate and regional systems. Whether the scaling-exponent of surface runoff exhibits a linear relation with the watershed size depends on the proportion of baseflow in the runoff.
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We present analytical and semi-analytical flow and transport models for 2D (x–z vertical plane) flow and particle transport in stratified rock of sedimentary type or vertically non-uniform bedrock aquifers, which are essentially phreatic. The main focus is on the flow induced by the areal discharge in watershed aquifers in hard-rock formations (e.g. granite, gneiss), the weathering profiles of which comprise two or more stratiform zones differing in hydraulic conductivity, k, which thus can be represented by a decay function of the vertical coordinate, i.e., the depth, k(z). Several models of depth-dependent hydraulic conductivity (stepwise, power-law, exponential) have been tested to show the sensitivity of groundwater flow paths and ages (transit or residence time distributions) to various geological and hydraulic simplifications related to rock heterogeneity. The mathematical setup handles the Dupuit–Forchheimer approximation the validity of which, as well as the suggested solutions, were verified via comparison with numerical results obtained using MODFLOW–PMPATH packages.
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Numerical models with spatially-varying head as top boundary conditions were used in previous studies to understand topography-driven groundwater flow. The head boundary conditions could cause artifacts of extremely large, but unrealistic recharge rates owing to unlimited supply of water. This study adopted a fully-coupled surface–subsurface hydrologic modeling approach to simulate transient topography-driven groundwater flow and also surface-water flow under homogeneous and isotropic settings. Two 100-year climate datasets and five hydraulic conductivities (K, 0.01 - 100 m/d) were tested in numerical experiments. In the base case with a wet climate (annual precipitation 1696 mm/y) and K of 1 m/d, groundwater head at two different locations close to both lateral boundaries fluctuates only within 5.1 m and 9.6 m, respectively, during the 100-year period. Despite the local water table fluctuations caused by the variability in the climatic record, large-scale groundwater flow systems can be assumed in dynamic equilibrium provided stationary climate. Long-term average exchange fluxes are spatially constant and limited by precipitation infiltration when surface water is absent, whereas they vary from positive to negative values (i.e., recharge to discharge) spatially when surface water is present. Sensitivity analysis suggests that wetter climate and smaller K lead to more inundation of the land surface, stronger hierarchical nesting of groundwater flow systems and more variable exchange fluxes. Overall, our first fully-coupled modeling of topography-driven groundwater flow implies that attention must be paid to causality between head and flow, and climatic record as boundary conditions may be more appropriate due to its relaxed manner.
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Although it has been reported that flowing artesian wells could be topographically-controlled, there is no quantitative research on artesian flow conditions in unconfined aquifers. In this study, the water table, which has a lower amplitude than the land surface, is damped from the topography and used as the boundary condition to obtain the analytical solution of hydraulic head of a unit basin with a single flow system. The term artesian head is defined to characterize the condition of flowing artesian wells. The zone with positive artesian head is called artesian zone while with negative artesian head is non-artesian zone. The maximum artesian head and the size of artesian zones are found to increase with the damping factor and the anisotropy ratio, and decrease with the ratio of basin width to depth and the depth-decay exponent of hydraulic conductivity. Moreover, the artesian head increases with depth nearby the valley and decreases with depth near by the divide, and the variation rates are influenced by the decay exponent and the anisotropy ratio. Finally, the distribution of flowing artesian wells and the artesian head measurements in different depths of a borehole in a small catchment in the Ordos Plateau, Northwestern China is used to illustrate the theoretical findings. The change in artesian head with depth was used to estimate the anisotropy ratio and the decay exponent. This study opens up a new door to analyze basin-scale groundwater flow. This article is protected by copyright. All rights reserved.
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Two analytical solutions using segregation variable method to calculate the hydraulic head under steady and unsteady flow conditions based on Tóth’s classical model were developed. The impacts of anisotropy ratio, hydraulic conductivity ( K ), and specific yield ( μ s ) on the flow patterns were analyzed. It was found that the area of the equal velocity region increases and the penetrating depth of the flow system decreases at steady state with anisotropy ratio increases, which is defined as ε = K x / K z . In addition, stagnant zones can be found in the flow field where the streamlines have opposite directions. These stagnant zones move toward the surface as the horizontal hydraulic conductivity increases. The results of the study on transient flow indicate that a relative increase in hydraulic conductivity produces a positive impact on hydraulic head and a relative enhancement in specific yield produces a negative effect on hydraulic head at early times.
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The distribution of groundwater fluxes in aquifers is strongly influenced by topography, and organized between hillslope and regional scales. The objective of this study is to provide new insights regarding the compartmentalization of aquifers at the regional scale and the partitioning of recharge between shallow/local and deep/regional groundwater transfers. A finite-difference flow model was implemented, and the flow structure was analyzed as a function of recharge (from 20 to 500 mm/yr), at the regional-scale (1400 km2), in three dimensions, and accounting for variable groundwater discharge zones; aspects which are usually not considered simultaneously in previous studies. The model allows visualizing 3-D circulations, as those provided by Tothian models in 2-D, and shows local and regional transfers, with 3-D effects. The probability density function of transit times clearly shows two different parts, interpreted using a two-compartment model, and related to regional groundwater transfers and local groundwater transfers. The role of recharge on the size and nature of the flow regimes, including groundwater pathways, transit time distributions, and volumes associated to the two compartments, have been investigated. Results show that topography control on the water table and groundwater compartmentalization varies with the recharge rate applied. When recharge decreases, the absolute value of flow associated to the regional compartment decreases, whereas its relative value increases. The volume associated to the regional compartment is calculated from the exponential part of the two-compartment model, and is nearly insensitive to the total recharge fluctuations.
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Spatial and temporal variability of weather and climatic forcings induce a dynamic response in hydrologic systems. Regional groundwater systems and stream hyporheic zones are examples of hydrologic systems driven by forcings varying at several time scales, such as daily, seasonal, interannual, decadal and longer. Hydrologic systems are characterized by flow paths and residence time distributions. Residence times vary in space, with positions further along flow paths exhibiting older ages. If the hydrologic flow system is in steady state the flow paths do not change in time and water present at a given point has a stable residence time distribution. But hydrologic flow paths and residence times can change dynamically with weather and climate temporal variability. Traditionally, this dynamic response is ignored and modeled and observed residence times are evaluated as if the flow was in steady state. A finite element scheme is used to model the transient flow and transport of an ideal tracer into a Thothian-like domain, and to illustrate the effect of dynamically changing systems on residence-time estimation.
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The existence of stagnation points in nested flow systems is relevant to a range of geologic processes. There has been no analytical study on the characteristics and locations of stagnation points in nested flow systems. We derived analytical solutions for hydraulic head and stream function in basins with isotropic and depth-decaying hydraulic conductivity. The solutions of hydraulic head and stream function are used to identify the positions of stagnation points and discuss the dynamics of groundwater around the stagnation points. Three types of stagnation points are identified by analytical and graphical means. For stagnation points on the basin bottom below the valley, only two regional flow systems converge from opposite directions. For stagnation points on the basin bottom below the regional high, only two regional flow systems part toward opposite directions. In contrast, for stagnation points under counterdirectional local flow systems, flow systems converging from and parting toward opposite directions coexist, and these stagnation points move deeper as the water table configuration becomes more rugged and the decay exponent of hydraulic conductivity increases. Moreover, the dividing streamlines around stagnation points under counterdirectional local flow systems are used to divide the local, intermediate, and regional flow systems accurately, from which the penetration depths of local and intermediate flow systems are precisely determined. A clear understanding of the location of stagnation points is critical for characterizing the pattern of hierarchically nested flow systems and has potential implication in studying solute and mineral concentration distributions in drainage basins.
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Most stochastic models of solute transport assume flow to be at steady state. However, even locally uniform gradients tend to show seasonal fluctuations in magnitude and direction. Such transients affect the prediction of flow and plume migration and spread. The question is how and to what extent. We address this question by developing low-order approximations for autocovariances and cross covariances of velocity, head, and log hydraulic conductivity under quasi-steady state flow, then using a first-order Lagrangian approach to examine their effect on advective transport. Our results show that whereas periodic temporal fluctuations in the magnitude of the mean velocity may either enhance or reduce dispersion, similar fluctuations in its direction always cause longitudinal dispersion to decrease and transverse dispersion to increase. .
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On the basis of the parallel pattern of the water divides and the valleys in parts of central Alberta and the inferred difference in permeability between the Paskapoo and Edmonton formations, basins are considered to be separate units of flow in the groundwater regime. For the cross-sectional distribution of the fluid potential in a basin of homogeneous lithology an equation is found that relates the fluid potential to the acceleration of gravity, topographic gradient of the valley flank, horizontal distance between water divide and valley bottom, elevation of the water table at the valley bottom above the horizontal impermeable boundary, elevation above the hori- zontal impermeable boundary, and horizontal distance from the valley bottom. The validity of the assumption that groundwater runoff is discharged mainly at the valley bottoms is disputed. A boundary between areas of recharge and discharge is proved mathematically. A theory is ad- vanced to explain the systematic deviation of the theoretical value of the fluid potential from the observed values. Local anomalies of the piezometric surface are accounted for by the presence of lenticular bodies of relatively high permeability. Mathematical formulas are used to express the relation between those anomalies and the permeability ratios and the size and shape of the lenses. A schematic cross section of possible potential distribution and flow pattern across a watershed is presented. Introduction. An attempt is made to under- stand groundwater movement in small drainage basins of known physiographic and hydrogeologic characteristics. The method of reasoning em- ployed is inductive; on the basis of field observa- tions a mathematical model has been set up to account both for the general features of the flow systems involved and for the apparent anomalies in the general flo w pattern. The area in which the field observations were carried out is in.central Alberta, Canada (Figure 1). The relatively well defined morphologic and geologic characteristics of the area offer a com- paratively clear-cut basis for mathematical reasoning.
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1] Water levels in aquifers typically vary in response to time-varying rates of recharge, suggesting the possibility of inferring time-varying recharge rates on the basis of long-term water level records. Presumably, in the southwestern United States (Arizona, Nevada, New Mexico, southern California, and southern Utah), rates of mountain front recharge to alluvial aquifers depend on variations in precipitation rates due to known climate cycles such as the El Niño-Southern Oscillation index and the Pacific Decadal Oscillation. This investigation examined the inverse application of a one-dimensional analytical model for periodic flow described by Lloyd R. Townley in 1995 to estimate periodic recharge variations on the basis of variations in long-term water level records using southwest aquifers as the case study. Time-varying water level records at various locations along the flow line were obtained by simulation of forward models of synthetic basins with applied sinusoidal recharge of either a single period or composite of multiple periods of length similar to known climate cycles. Periodic water level components, reconstructed using singular spectrum analysis (SSA), were used to calibrate the analytical model to estimate each recharge component. The results demonstrated that periodic recharge estimates were most accurate in basins with nearly uniform transmissivity and the accuracy of the recharge estimates depends on monitoring well location. A case study of the San Pedro Basin, Arizona, is presented as an example of calibrating the analytical model to real data.
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Flow-system analysis is based on the concept of hierarchical groundwater flow systems. The topography of the water table, which is strongly related to the topography of the land surface, is a major factor in the hierarchical nesting of gravity-driven groundwater flow, resulting in flow systems of different orders of magnitude in lateral extent and depth of penetration. The concept of flow systems is extremely useful in the analysis of spatial and temporal scales and their mutual relationships. Basic equations on the laboratory scale are extended to larger, regional scales. Making use of Fourier analysis further develops Tóth's original idea of topography-driven flow systems. In this way, the different spatial scales of the water table are separated in a natural way, leading to a simple expression for the penetration depth of a flow system. This decomposition leads also to the relationship between spatial and temporal scales. Analogous to flow systems, water bodies with different water quality may be called 'transport systems.' Field studies, numerical micro-scale modeling over macro-scale domains, and stochastic dispersion theory indicate that between systems with steady transport, the interfaces are relatively thin. The interfaces are much thinner than the relatively large mixing zones predicted by the conventional engineering approach to macrodispersion, in which relatively large, time-independent macrodispersion lengths are applied. A relatively simple alternative engineering approach is presented. For macrodispersion of propagating solute plumes, the alternative dispersion term gives the same results as the conventional engineering approach and gives correct results for steady-state transport.
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On the basis of the parallel pattern of the water divides and the valleys in parts of central Alberta and the inferred difference in permeability between the Paskapoo and Edmonton formations, basins are considered to be separate units of flow in the groundwater regime. For the cross-sectional distribution of the fluid potential in a basin of homogeneous lithology an equation is found that relates the fluid potential to the acceleration of gravity, topographic gradient of the valley flank, horizontal distance between water divide and valley bottom, elevation of the water table at the valley bottom above the horizontal impermeable boundary, elevation above the horizontal impermeable boundary, and horizontal distance from the valley bottom. The validity of the assumption that groundwater runoff is discharged mainly at the valley bottoms is disputed. A boundary between areas of recharge and discharge is proved mathematically. A theory is advanced to explain the systematic deviation of the theoretical value of the fluid potential from the observed values. Local anomalies of the piezometric surface are accounted for by the presence of lenticular bodies of relatively high permeability. Mathematical formulas are used to express the relation between those anomalies and the permeability ratios and the size and shape of the lenses. A schematic cross section of possible potential distribution and flow pattern across a watershed is presented. Introduction. An attempt is made to understand groundwater movement in small drainage basins of known physiographic and hydrogeologic characteristics. The method of reasoning employed is inductive; on the basis of field observations a mathematical model has been set up to
Book
This textbook provides an introduction to the study of hydrogeology, and maintains the process oriented approach of the earlier edition. The introduction is followed by chapters on: the origin of porosity and permeability; groundwater movement; equations of flow, boundary conditions and flow nets; groundwater in the basin hydrologic cycle; hydraulic testing; groundwater resources; stress, strain and pore fluids; heat transport in groundwater flow; solute transport; aqueous geochemistry; chemical reactions; colloids and microorganisms; mass transport equations; mass transport in natural groundwater systems and groundwater flow; contaminant hydrology; modelling of dissolved contaminant transport; multiphase fluid systems; remediation; and in situ destruction and risk assesment.
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Based on historical analysis of regional geography and geology with the confirmed evidences from environmental isotopes and succeeding change of continental salinized salt water, it is found that the groundwater flow systems in the Quaternary of Hebei Plain have undergone three major evolution stages since the Last Glacial Maximum. In the low sea level stage of 18-15 ka BP, groundwater recharge from abound rainfall almost fully replaced the original water in the Quaternary aquifers, and the regional flow system developed. In the period of sea level sharply rising stage of 15-12 ka BP, the flow driven force weakened as the result of topographic potential differences decreased; the early intermediate flow systems penetrating to the third aquifer developed and the regional flow system tended to be stagnant. From the stage of 2.5 ka BP to the present, the present fluvial morphology formed, the topographic potential differences between mountain and sea decreased continually, and the potential differences between high river beds and low river beds or depressions became the dominated flow driven force, so that the late intermediate flow systems penetrating to the first and second aquifers developed. Along with sea level rising, the penetrating depth of later flow systems being less than the former, thus, the later partly cut and superimposed on the former. At present, it is a temporal and spatial assemblage of groundwater flow systems of different evolution stages in the Quaternary of Hebei Plain. Such a scene might be seen in other costal basin, even in inland basin if the erosion weakened with time. The continental salinized saline water originated in around 12 ka BP, resulting in saline water and soil salinization by upward flow, is the key negative factor in water resources management and in ecology and environment protection of Hebei Plain.
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This book recognises groundwater flow as a fundamental geologic agent, and presents a wide-ranging and illustrated overview of its history, principles, scientific consequences and practical utilization. The author, one of the founding fathers of modern hydrogeology, highlights key interrelationships between seemingly disparate processes and systems by tracing them to a common root cause - gravity-driven groundwater flow. Numerous examples demonstrate practical applications in a diverse range of subjects, including land-use planning, environment protection, wetland ecology, agriculture, forestry, geotechnical engineering, nuclear-waste disposal, mineral and petroleum exploration, and geothermal heat flow. The book contains numerous user-friendly features for a multidisciplinary readership, including full explanations of the relevant mathematics, emphasis on the physical meaning of the equations, and an extensive glossary. It is a key reference for researchers, consultants and advanced students of hydrogeology and reservoir engineering. © J. Tóth 2009 and Cambridge University Press, 2009. All rights reserved.
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The interactions between groundwater and surface water are complex. To understand these interactions in relation to climate, landform, geology, and biotic factors, a sound hydrogeoecological framework is needed. All these aspects are synthesized and exemplified in this overview. In addition, the mechanisms of interactions between groundwater and surface water (GW–SW) as they affect recharge–discharge processes are comprehensively outlined, and the ecological significance and the human impacts of such interactions are emphasized. Surface-water and groundwater ecosystems are viewed as linked components of a hydrologic continuum leading to related sustainability issues. This overview concludes with a discussion of research needs and challenges facing this evolving field. The biogeochemical processes within the upper few centimeters of sediments beneath nearly all surface-water bodies (hyporheic zone) have a profound effect on the chemistry of the water interchange, and here is where most of the recent research has been focusing. However, to advance conceptual and other modeling of GW–SW systems, a broader perspective of such interactions across and between surface-water bodies is needed, including multidimensional analyses, interface hydraulic characterization and spatial variability, site-to-region regionalization approaches, as well as cross-disciplinary collaborations.
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Ground-water flow modeling is an important tool fre-quently used in studies of ground-water systems. Reviewers and users of these studies have a need to evaluate the accuracy or reasonableness of the ground-water flow model. This report provides some guidelines and discussion on how to evaluate complex ground-water flow models used in the investigation of ground-water systems. A consistent thread throughout these guidelines is that the objectives of the study must be specified to allow the adequacy of the model to be evaluated.
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Theoretically, three types of flow systems may occur in a small basin: local, intermediate, and regional. The local systems are separated by subvertical boundaries, and the systems of different order are separated by subhorizontal boundaries. The higher the topographic relief, the greater is the importance of the local systems. The flow lines of large unconfined flow systems do not cross major topographic features. Stagnant bodies of groundwater occur at points where flow systems meet or branch. Recharge and discharge areas alternate; thus only part of the basin will contribute to the baseflow of its main stream. Motion of groundwater is sluggish or nil under extended flat areas, with little chance of the water being freshened. Water level fluctuations decrease with depth, and only a small percentage of the total volume of the groundwater in the basin participates in the hydrologic cycle.
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Analytical models of groundwater flow with a spatially varying elevation of a top boundary are widely used. However, a vast majority of previous analytical studies truncated the irregularly shaped top section with little to no analyses of the shortcomings of the approximate solutions for the resulting rectangles or parallelepipeds. We present an analytical approach based on a perturbation technique that treats complete domains. It is especially accurate near the top boundary, where fluid circulation is most pronounced and higher accuracy is typically needed, such as in regional or hyporheic systems flow. The approach is illustrated by analyzing flow for a Tóthian unit basin.
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The relationship between base flow recession characteristics in steep watersheds and geomorphologic and soil parameters is investigated. The formulation for the groundwater outflow was obtained by means of a hydraulic approach applied to a simple conceptual model for a hillslope. Long-term flow data of 19 representative basins in the Allegheny Mountain section of the Appalachian Plateaus were analyzed on the basis of this formulation. Results showed that the reaction factor, which is a time scale of base flow recession, is dependent on the mean land slope, the drainage density, and the ratio (K/f) of the hydraulic conductivity and the drainable porosity. On account mainly of the nonuniform distribution of the physical characteristics within a basin, the reaction factor for a given watershed is somewhat variable with time, but the adoption of a constant value is useful to represent average conditions for a recession period. Analysis of the (K/f) dependency showed that macropores and other structural features may greatly affect the watershed base flow. Evaporation from groundwater appears to constitute only a minor portion of overall basin evaporation.
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[1] A new analytical solution of the flow equation has been developed to estimate the time to reach a near-equilibrium state in mixed aquifers, i.e., having unconfined and confined portions, following a large hydraulic perturbation. Near-equilibrium is defined as the time for an initial aquifer perturbation to dissipate by an average 95% across the aquifer. The new solution has been obtained by solving the flow system of a simplified conceptual model of a mixed aquifer using Laplace transforms. The conceptual model is based on two assumptions: (1) the groundwater flow can be reduced to a horizontal 1-D problem and (2) the transmissivity, a function of the saturated thickness, is assumed constant on the unconfined portion. This new solution depends on the storativity of the unconfined portion, the lengths of the unconfined and confined portions and the transmissivity, assumed to be constant and equal in both portions of the mixed aquifer. This solution was then tested and validated against a numerical flow model, where the variations of the saturated thickness and therefore variations of the transmissivity were either ignored, or properly modeled. The agreement between the results from the new solution and those from the numerical model is good, validating the use of this new solution to estimate the time to reach near-equilibrium in mixed aquifers. This solution for mixed aquifers, as well as the solutions for a fully confined or fully unconfined aquifer, has been used to estimate the time to reach near-equilibrium in 13 large aquifers in the world. For those different aquifers, the time to reach near-equilibrium ranges between 0.7 kyr to 2.4 × 107 kyr. These results suggest that the present hydraulic heads in these aquifers are typically a mixture of responses induced from current and past hydrologic conditions and thus climate conditions. For some aquifers, the modern hydraulic heads may in fact depend upon hydrologic conditions resulting from several past climate cycles.
Article
A numerical model is used to examine groundwater flow in vertical section near surface water bodies, such as lakes, wetlands, ponds, rivers, canals, and drainage and irrigation channels. Solutions are generated partly by superposition to achieve computational efficiency. A large number of flow regimes are identified, with their characteristics controlled by regional water table gradients, recharge to the aquifer, water body length, aquifer anisotropy, and the hydraulic resistance of the bottom sediments. Different flow regimes are distinguished by the presence and nature of groundwater mounds or depressions near the edges of a surface water body and by corresponding stagnation points. Ranges of values for dimensionless flow parameters over which particular regimes occur are determined for six representative geometries and presented in the form of transition diagrams. Increasing anisotropy or sediment resistance and decreasing the length of a water body relative to aquifer thickness are shown to have similar effects on flow geometry, the main effect being an increasing tendency for stagnation points to form in the interior of the aquifer. Flow-through behavior becomes more prevalent with decreasing anisotropy and sediment resistance and increasing water body length.
Article
The classic work of Tóth in 1963 on 2D, steady state, isotropic, basinal, flow modelling is revisited with a stagnation point and critical streamline analysis. It is found that narrow channels of groundwater flow may exist which have previously been undiscovered. Although these channels do not change the overall concept of dividing basinal flow into regional, intermediate and local flow zones, they do show that small portions of recharge areas in regional zones connect with predominantly intermediate discharge zones and also intermediate recharges connect with predominantly local discharge zones. Depending on basin depth and surface potential parameters, the size and capacity of these channels is determined by the existence of paired stagnation points relative to the basin vertical centreline and the degree of asymmetry of stream functions. These channels are likely to occur in other basinal models and will affect the assessment of discharge water temperature, age and chemical composition.
Article
Tidal motions of the water table height inside a sloping beach are investigated via field measurements and theoretical considerations. Only the movements forced by the tide are considered, so a beach with negligible wave activity was chosen for the field measurements. The data show that even in the absence of precipitation the time averaged inland water table stands considerably above the mean sea level. Also the water table at a fixed point inside the beach is far from sinusoidal even though its variation is forced by an essentially sinusoidal tide. This latter effect is due to the boundary condition along the sloping beach face which acts as a highly nonlinear filter. The observed behavior of the water table is explained in terms of perturbation extensions to the classical "deep aquifer solution." One extension deals with the nonlinearity in the interior, the other with the boundary condition at the sloping beach face.
Article
The North China Plain (NCP) is one of the global hotspots of groundwater depletion. Currently, our understanding is limited on spatiotemporal variability in depletion and approaches toward more sustainable groundwater development in this region. This study was intended to simulate spatiotemporal variability in groundwater depletion across the entire NCP and explore approaches to reduce future depletion. Simulated predevelopment groundwater recharge (∼ 13 km yr) primarily discharged as base ow to rivers and evapotranspiration. Initial groundwater storage was estimated to be 1500 km of drainable storage in shallow aquifers and 40 km of compressive storage in deep aquifers. Simulated groundwater depletion from 1960s to 2008 averaged ∼4 km /yr. Cumulative depletion was 50 km (∼20% of pumpage) in the piedmont district, 103 km (∼20%) in the central plain, and 5 km (12%) in the coastal plain. However, depletion varied with time : ∼2.5 km /yr in the 1970s, ∼4.0 in the 1980s, ∼2.0 in 1990-1996; ∼7.0 in 1997-2001, and ∼4.0 in 2002-2008. Recharge also varied spatially, averaging ∼120 mm/yr and concentrated in the piedmont district (200-350 mm/yr) while lower in the central and coastal plains (50-100 mm/yr). Simulation of several alternatives, including managed aquifer recharge, increased water use efficiency, brackish water use, and interbasin water transfer, indicated that the combination of these strategies could be used to recover groundwater storage by 50 km over a 15-year period. This study provides valuable insights for developing more sustainable groundwater management options for the NCP ; the methods are useful for managing other depleted aquifers.
Article
Natural systems are driven by dynamic forcings that change in time as well as space, behavior that is inherited by the system flow field and results in time-varying age distributions (ADs). This work presents a review of the mathematical tools and solution approaches used to model ADs in dynamic time-varying flow systems. A simple conceptual, numerical model is then used to explore the role of flow dynamics in ADs for topography-driven flow systems. This model is an analog for regional groundwater systems and hyporheic zones. This model demonstrates that relatively small fluctuations in the forcing, even though importantly affecting the flow in the system, can have minimal effects in ADs. However, as the intensity of fluctuation increases, still within the bounds observed in natural systems, ADs in shallow parts of the system become highly sensitive to dynamic flow conditions, leading to considerable changes in the moments and modality of the distributions with time. In particular, transient flow can lead to emergence of new modes in the AD, which would not be present under steady flow conditions. The discrepancy observed between ADs under steady and transient flow conditions is explained by enhancement of mixing due to temporal variations in the flow field. ADs in deeper parts of the system are characterized by multimodality and tend to be more stable over time even for large forcing fluctuations.
Article
Numerical simulation of variably saturated porous media indicates that groundwater recharge is variable in time and space, depending on the thickness of the unsaturated zone through which infiltrating water must move. The resulting complex, transient groundwater flow systems have significant impact on contiguous surface water. In very permeable media, small, local, closed groundwater flow systems can develop and dissipate within a few weeks to several months after major recharge. These have a direct effect on contiguous surface water by alternately causing seepage to and seepage from the surface water. The transient nature of these flow systems indicates that reversals of the direction of groundwater flow may be common. In less permeable media the same complex flow systems may occur, but the time for development and dissipation is much greater. For example, it is conceivable that small, local flow systems may exist for many months or years as a result of major recharge. Therefore directions of flow in such systems are more stable, and the effect on contiguous surface water also is more stable. The findings of this study indicate that wells and groundwater quality sampling sites need to be carefully located to define accurately water table configuration, groundwater recharge, direction of seepage through the beds of surface water bodies, and complex geochemical processes related to changing directions of groundwater flow.
Article
Three zones of different generations of formation-fluid flow systems were identified from analyses of the potentiometric surface, hypsographic distribution of fresh water heads, pressure-depth relations, water table evaluations, and dynamic pressure increments observed in five extensive water-bearing units in a 52,840 km**2 geologically mature area in northern Alberta. In each one, fluids move in gravity-induced flow systems maintained by cross-formational energy transfer and subject to past or present boundary conditions. The force fields and associated flow systems in the basal zone of Middle Devonian aquifers were generated by and adjusted to the topography of the Pliocene continental surface.
Article
Regional impact studies of the effects of future climate change are necessary because projected changes in meteorological variables vary regionally, and different hydrological systems can react in various ways to the same changes. In this study the effects of climate change on groundwater recharge, storage, and discharge to streams are compared in two geologically and climatologically different regions in Denmark. Outputs are used for the periods 1961 to 1990 and 2071 to 2100 from a regional climate model representing the Intergovernmental Panel on Climate Change (IPCC) scenarios A2 and B2. A physically based, distributed hydrological model simulates changes in groundwater head, recharge, and discharge. Precipitation, temperature, and reference evapotranspiration increased for both the A2 and B2 scenarios. This results in a significant increase in mean annual net precipitation, but with decreased values in the summer months. The magnitude of the hydrological response to the simulated climate change is highly dependent on the geological setting of the model area. In the Jylland area, characterized by sandy top soils and large interconnected aquifers, groundwater recharge increased significantly, resulting in higher groundwater levels and increasing groundwater-river interaction. On Sjaelland, where the topsoil is dominated by low-permeability soils and the aquifers are protected by thick clay layers of regional extent, only minor changes in groundwater levels are predicted. The primary effect in this area is the change in stream discharge, caused by changes in drain flow and overland flow, with up to 50% increase in winter and 50% decrease in summer. This study shows the added value of studying different climate scenarios and hydrological systems, so that the simulated effects can be compared both qualitatively and quantitatively.
Article
Several researchers have observed seasonal reversals in the direction of groundwater flow around lakes. If these reversals are prolonged and are accompanied by the formation of a stagnation point, they may have a significant effect on a lake's water and nutrient budgets. The formation of a stagnation point at a flow-through lake (i.e., a lake that receives groundwater through part of the lake basin and recharges the groundwater system over the rest of the lake basin) is accomplished by the formation of a groundwater mound on the downgradient side of the lake. In this paper the seasonal formation of a stagnation point at Snake Lake, Wisconsin, is investigated with the aid of two-dimensional transient computer models applied in cross section and areally. The analysis demonstrates the potential for the seasonal formation of a stagnation point at a flow-through lake and provides some insight into the transient development of the stagnation point.
Article
The natural basin yield of a ground water basin can be calculated from a quantitative analysis of the flow net obtained from a digital computer solution to a numerical mathematical model of the basin. The natural basin yield is an unique property of the basin and can be considered as a measure of the ground water recharge to the basin and a conservative estimate of the safe yield. Rates of ground water recharge and discharge vary from place to place throughout the areal extent of a basin; a quantitative analysis can be used to determine the positions of concentration. Quantitative interpretation of ground water flow nets can play an important role in the calculation of basin-wide water balances due to the inter- relationships between ground water recharge and infiltration at one end of the flow system and ground water discharge, evapotranspiration, and stream baseflow at the other.
Article
A flow pattern is characterized by aquifer features and the number, type, and distribution of stagnation points (locations where the discharge is zero). This article identifies a condition for transition of flow patterns in two-dimensional groundwater flow obeying Darcy’s law by examining changes in stagnation points, using the Taylor series expansion of the discharge vector. It is found that the three standard types of stagnation points (minimums, maximums, and saddle points) are completely characterized by the first-order term containing the discharge gradient tensor. However, when the determinant of the tensor becomes zero, stagnation points of other types characterized by higher-order terms come into existence. In this article, we call these zero-determinant stagnation points as critical stagnation points; they may emerge suddenly, split to a set of new stagnation points, or disappear from the flow, resulting in transitions of flow patterns. Examples of both transient and steady flows are used to illustrate the usefulness and significance of critical stagnation points.
Article
Since the 1960s, most of the studies on groundwater flow systems by analytical and numerical modelling have been based on given-head upper boundaries. The disadvantage of the given-head approach is that the recharge into and discharge from a basin vary with changes in hydraulic conductivity and/or basin geometry. Consequently, flow patterns simulated with given-head boundaries but with different hydraulic conductivities and/or basin geometry may not reflect the effects of these variables. We conducted, therefore, numerical simulations of groundwater flow in theoretical drainage basins using flux as the upper boundary and realistically positioned fluid-potential sinks while changing the infiltration intensity, hydraulic conductivities, and geometric configuration of the basin. The simulated results demonstrate that these variables are dominant factors controlling the flow pattern in a laterally closed drainage basin. The ratio of infiltration intensity to hydraulic conductivity (Ric) has been shown to be an integrated pattern-parameter in a basin with a given geometric configuration and possible fluid-potential-sink distribution. Successively, the changes in flow patterns induced by stepwise reductions in Ric are identical, regardless of whether the reductions are due to a decrease in infiltration intensity or an increase in hydraulic conductivity. The calculated examples show five sequential flow patterns containing (i) only local, (ii) local–intermediate, (iii) local–intermediate–regional, (iv) local– regional, and (v) just regional flow systems. The Ric was found to determine also whether a particular sink is active or not as a site of discharge. Flux upper boundary is preferable for numerical simulation when discussing the flow patterns affected by a change of infiltration, the hydraulic conductivity, or the geometry of a basin.
Article
Climate change will have a significant impact on the hydrologic cycle, creating changes in freshwater resources, land cover and land-atmosphere feedbacks. Recent studies have investigated the response of groundwater to climate change but do not account for energy feedbacks across the complete hydrologic cycle. Although land-surface models have begun to include an operational groundwater-type component, they do not include physically based lateral surface and subsurface flow and allow only for vertical transport processes. Here we use a variably saturated groundwater flow model with integrated overland flow and land-surface model processes to examine the interplay between water and energy flows in a changing climate for the southern Great Plains, USA, an important agricultural region that is susceptible to drought. We compare three scenario simulations with modified atmospheric forcing in terms of temperature and precipitation with a simulation of present-day climate. We find that groundwater depth, which results from lateral water flow at the surface and subsurface, determines the relative susceptibility of regions to changes in temperature and precipitation. This groundwater control is critical to understand processes of recharge and drought in a changing climate.
Article
Based on the theory of gravity-driven groundwater flow systems, we have developed a complex Flow System Sand-Box Model (FSM). It enables the visual observations of the development and characteristics and temporal evolution of complex Tóthian flow systems in the laboratory. The configuration of the regional, intermediate and local flow systems can be controlled and observed; hydraulic head, flow direction and travel time can be measured; and the scale and shape of the sub-flow systems as well as the path lines and flow lines can be observed directly. The experiments demonstrate the Tóthian flow systems in a small basin with multiple sources and sinks. Greater local topographic (water table) undulation will lead to larger local flow systems. Greater regional and less local topographic undulation will enhance the development of intermediate and regional flow systems. In homogeneous media, increasing fluid-potential differences between source and sink increase the spatial scale of the generated flow systems. The FSM is a useful teaching aid and experimental device to study and develop an intuitive insight into gravity-driven groundwater flow systems. It helps to visualize and understand the hydraulic properties and controlling factors of Tóthian flow systems and may be used to study problems related to the chemical and temperature characteristics of the flow systems as well. Copyright © 2010 John Wiley & Sons, Ltd.
Article
Surface water and ground water watersheds commonly do not coincide. This condition is particularly relevant to understanding biogeochemical processes in small watersheds, where detailed accounting of water and solute fluxes commonly are done. Ground water watersheds are not as easily defined as surface watersheds because (1) they are not observable from land surface; (2) ground water flow systems of different magnitude can be superimposed on one another; and (3) ground water divides may move in response to dynamic recharge and discharge conditions. Field studies of relatively permeable terrain in Wisconsin, Minnesota, and Nebraska indicate that lakes and wetlands in small watersheds located near the lower end of extensive ground water flow systems receive ground water inflow from shallow flow systems that extend far beyond their surface watershed, and they may also receive ground water inflow from deeper regional flow systems that pass at depth beneath local flow systems. Field studies of mountainous terrain that have low-permeability deposits in New Hampshire and Costa Rica also indicate that surface water bodies receive ground water inflow from sources beyond their local surface watersheds. Field studies of lakes and wetlands in North Dakota, Nebraska, and Germany indicate that ground water divides move in response to changing climate conditions, resulting in a variable source of ground water inflow to those surface water bodies.
Article
Accurate estimation of groundwater recharge is extremely important for proper management of groundwater systems. Many different approaches exist for estimating recharge. This paper presents a review of methods that are based on groundwater-level data. The water-table fluctuation method may be the most widely used technique for estimating recharge; it requires knowledge of specific yield and changes in water levels over time. Advantages of this approach include its simplicity and an insensitivity to the mechanism by which water moves through the unsaturated zone. Uncertainty in estimates generated by this method relate to the limited accuracy with which specific yield can be determined and to the extent to which assumptions inherent in the method are valid. Other methods that use water levels (mostly based on the Darcy equation) are also described. The theory underlying the methods is explained. Examples from the literature are used to illustrate applications of the different methods.
Article
Surface-water bodies are integral parts of groundwater flow systems. Groundwater interacts with surface water in nearly all landscapes, ranging from small streams, lakes, and wetlands in headwater areas to major river valleys and seacoasts. Although it generally is assumed that topographically high areas are groundwater recharge areas and topographically low areas are groundwater discharge areas, this is true primarily for regional flow systems. The superposition of local flow systems associated with surface-water bodies on this regional framework results in complex interactions between groundwater and surface water in all landscapes, regardless of regional topographic position. Hydrologic processes associated with the surface-water bodies themselves, such as seasonally high surface-water levels and evaporation and transpiration of groundwater from around the perimeter of surface-water bodies, are a major cause of the complex and seasonally dynamic groundwater flow fields associated with surface water. These processes have been documented at research sites in glacial, dune, coastal, mantled karst, and riverine terrains.
Article
In the analysis of aquifer behavior, it is common to distinguish between steady-state and dynamic or unsteady behavior. A special case of the latter is periodic behavior, in which an aquifer responds to forcing that varies sinusoidally in time. This paper presents an efficient method for obtaining analytical solutions for periodic aquifer flow, based on the use of complex algebra. It also provides a number of solutions which are simpler in form than those that are currently available. The solutions provide valuable insights into the spatial variations of amplitudes of head fluctuations and of the phase lags between periodic forcing and aquifer response. The classical problem of the response of a one-dimensional aquifer to tidally fluctuating boundary levels is re-examined. A solution is presented for the response of a one-dimensional aquifer to seasonal or diurnal variations in net recharge; interesting results include the amplification of head fluctuations beyond the expected maximum, and an explanation for the 3-month and 6-h lags observed in annual and daily time series, respectively. Other examples include the effects of mixed boundary conditions, and the behavior of radially symmetric flow systems, either with flow inwards towards a pumping well, or flow outwards, as on a circular island. The amplitude and phase of fluxes through boundaries are also examined.
Article
A case is made for the use of simple models of groundwater flow emphasizing the salient regional characteristics of groundwater basins and for relating them to basin-wide features of groundwater flow. As an example a simple formula for the “natural basin yield” as defined by Freeze is calculated from a model which was introduced by Tóth. The Tóth model is then extended to take seasonal fluctuations of the water table into account; the type of response of the instantaneous head distribution in the basin due to elastic storage in the aquifer may range from severe distortions of Tóth's steady-state distribution to virtually no distortion. The type of response depends on two ratios which can be easily calculated from the basin parameters.
Article
Diurnal fluctuations of hydrological variables (e.g., shallow groundwater level or streamflow rate) are comparatively rarely investigated in the hydrologic literature although these short-term fluctuations may incorporate useful information for the characterization of hydro-ecological systems. The fluctuations can be induced by several factors like (a) alternating processes of freezing and thawing; (b) early afternoon rainfall events in the tropics; (c) changes in streambed hydraulic conductivity triggered by temperature variations, and; (d) diurnal cycle of water uptake by the vegetation. In temperate climates, one of the most important diurnal fluctuation-inducing factors is the water consumption of vegetation, therefore a detailed overview is provided on the history of such research. Beside a systematic categorization of the relevant historical studies, models that calculate groundwater evapotranspiration from diurnal fluctuations of groundwater level and/or streamflow rate have been reviewed. Compared to traditional evapotranspiration estimation methods these approaches may excel in that they generally employ a small number of parameters and/or variables to measure, are typically simple to use, and yet can yield results even on a short time-scale (i.e., hours). While, e.g., temperature-based methods of evapotranspiration are simple too, they cannot be applied or become inaccurate over shorter time periods. Similarly, traditional approaches (such as eddy-correlation or Bowen-ratio based) are accurate for shorter time steps but they require a number of measurable atmospheric input variables.
Article
Observations of periodic components of measured heads have long been used to estimate aquifer diffusivities. The estimations are often made using well-known solutions of linear differential equations for the propagation of sinusoidal boundary fluctuations through homogeneous one-dimensional aquifers. Recent field data has indicated several instances where the homogeneous aquifer solutions give inconsistent estimates of aquifer diffusivity from measurements of tidal lag and attenuation. This paper presents new algebraic solutions for tidal propagation in spatially heterogeneous one-dimensional aquifers. By building on existing solutions for homogeneous aquifers, comprehensive solutions are presented for composite aquifers comprising of arbitrary (finite) numbers of contiguous homogeneous sub-aquifers and subject to sinusoidal linear boundary conditions. Both Cartesian and radial coordinate systems are considered. Properties of the solutions, including rapid phase shifting and attenuation effects, are discussed and their practical relevance noted. Consequent modal dispersive effects on tidal waveforms are also examined via tidal constituent analysis. It is demonstrated that, for multi-constituent tidal forcings, measured peak heights of head oscillations can seem to increase, and phase lags seem to decrease, with distance from the forcing boundary unless constituents are separated and considered in isolation.
Article
The relationship between groundwater recharge and discharge is one of the most important aspects in the protection of ecologically valuable areas. Knowledge of groundwater systems is therefore a pre-requisite for up-to-date integrated land and water management. A methodology is presented for assessing the relative importance of different recharge–discharge systems, with respect to ecological status or development, including mapping of regional groundwater systems, and recharge and discharge areas. This methodology is applied to a land-use planning project in the Grote-Nete basin, Belgium. Discharge regions are delineated on the basis of their spatial discharge contiguity, position in the landscape and alkalinity of the plants habitat. The simulated discharge areas are verified by field mapping of phreatophytic vegetation. Particle tracking is used to delineate the recharge area associated with each discharge area, and to characterize each recharge–discharge groundwater system. Three groundwater flow and two vegetation parameters are used in a cluster analysis to obtain four different clusters of groundwater discharge systems. It is shown that the discharge clusters are significantly different in discharge intensity and alkalinity. The effects on the groundwater system due to anthropogenic impacts on the land-use are studied by simulation of the present, pre-development, and future situation. The results indicate the sensitivity and impact of the changes on the recharge and discharge areas, and groundwater discharge fluxes. The impact of the changes for the different areas for both the pre-development and the future situation appears to differ from large decrease to large increase in total groundwater discharge. Of additional ecological importance is the fact that some areas show an opposite behaviour regarding the changes in groundwater discharge area and fluxes. The delicate shifts in the groundwater systems, which cause the changes in the recharge and discharge, clearly show the need for hydrological modelling. The synergy of hydrological modelling and vegetation mapping proves advantageous and reveals some of the ecological differences in the catchment.
Article
The hydrologic and solute budgets of a lake can be strongly influenced by transient groundwater flow. Several shallow interdunal lakes in southwest Spain are in close hydraulic connection with the shallow ground water. Two permanent lakes and one intermittent lake have chloride concentrations that differ by almost an order of magnitude. A two-dimensional solute-transport model, modified to simulate transient groundwater-lake interaction, suggests that the rising water table during the wet season leads to local flow reversals toward the lakes. Response of the individual lakes, however, varies depending on the lake's position in the regional flow system. The most dilute lake is a flow-through lake during the entire year; the through flow is driven by regional groundwater flow. The other permanent lake, which has a higher solute concentration, undergoes seasonal groundwater flow reversals at its downgradient end, resulting in complex seepage patterns and higher solute concentrations in the ground water near the lake. The solute concentration of the intermittent lake is influenced more strongly by the seasonal wetting and drying cycle than by the regional flow system. Although evaporation is the major process affecting the concentration of conservative solutes in the lakes, geochemical and biochemical reactions influence the concentration of nonconservative solutes. Probable reactions in the lakes include biological uptake of solutes and calcite precipitation; probable reactions as lake water seeps into the aquifer are sulfate reduction and calcite dissolution. Seepage reversals can result in water composition that appears inconsistent with predictions based on head measurements because, under transient flow conditions, the flow direction at any instant may not satisfactorily depict the source of the water. Understanding the dynamic nature of groundwater-lake interaction aids in the interpretation of hydrologic and chemical relations between the lakes and the ground water.
Article
Typescript. Thesis (Ph. D.)--University of Massachusetts at Amherst, 1984. Includes bibliographical references (leaves 164-169).