Hydraulic Head Measurements Research Articles

Well-posed Bayesian geometric inverse problems arising in subsurface flow

In this paper, we consider the inverse problem of determining the permeability of the subsurface from hydraulic head measurements, within the framework of a steady Darcy model of groundwater flow. We study geometrically defined prior permeability fields, which admit layered, fault and channel structures, in order to mimic realistic subsurface features; within each layer we adopt either a constant or continuous function representation of the permeability. This prior model leads to a parameter identification problem for a finite number of unknown parameters determining the geometry, together with either a finite number of permeability values (in the constant case) or a finite number of fields (in the continuous function case). We adopt a Bayesian framework showing the existence and well-posedness of the posterior distribution. We also introduce novel Markov chain Monte Carlo (MCMC) methods, which exploit the different character of the geometric and permeability parameters, and build on recent advances in function space MCMC. These algorithms provide rigorous estimates of the permeability, as well as the uncertainty associated with it, and only require forward model evaluations. No adjoint solvers are required and hence the methodology is applicable to black-box forward models. We then use these methods to explore the posterior and to illustrate the methodology with numerical experiments.

Estimation of seawater–groundwater exchange rate: case study in a tidal flat with a large-scale seepage face (Laizhou Bay, China)

The exchange rate between seawater and groundwater in a tidal flat was investigated at Laizhou Bay, China, where there are large-scale seepage faces with horizontal extension of several hundred meters developed during low tides. Taking into account the effects of seepage face and density, a simple and efficient method for estimating seawater–groundwater exchange rate is proposed, based on field measurements of groundwater hydraulic head, temperature and salinity. First, the exchange rate at each well was obtained using the generalized Darcy’s law, then the results were interpolated and integrated along the whole transect. The total submarine groundwater discharge (SGD) and inflow were estimated to be 8.8 and 15.3 m3 d−1 m−1, respectively. The spatial distributions of SGD and inflow were different from those of sandy or gravel beaches possibly owing to the low-permeability sediment (silty sand with mud), very gentle slope, and the large-scale seepage faces. A freshwater discharge tube was identified near the low-tide line, as evidenced by significant increase in outflow and low salinity of groundwater observed there. The SGD from the seepage faces accounted for ∼21 % of the total SGD. The outflow rate that occurred from the seepage faces, and the ratio of the outflow from the seepage faces to the total outflow, decreased seaward significantly and monotonically.

Direct observations of evolving subglacial drainage beneath the Greenland Ice Sheet

Seasonal acceleration of the Greenland Ice Sheet is influenced by the dynamic response of the subglacial hydrologic system to variability in meltwater delivery to the bed via crevasses and moulins (vertical conduits connecting supraglacial water to the bed of the ice sheet). As the melt season progresses, the subglacial hydrologic system drains supraglacial meltwater more efficiently, decreasing basal water pressure and moderating the ice velocity response to surface melting. However, limited direct observations of subglacial water pressure mean that the spatiotemporal evolution of the subglacial hydrologic system remains poorly understood. Here we show that ice velocity is well correlated with moulin hydraulic head but is out of phase with that of nearby (0.3-2kilometres away) boreholes, indicating that moulins connect to an efficient, channelized component of the subglacial hydrologic system, which exerts the primary control on diurnal and multi-day changes in ice velocity. Our simultaneous measurements of moulin and borehole hydraulic head and ice velocity in the Paakitsoq region of western Greenland show that decreasing trends in ice velocity during the latter part of the melt season cannot be explained by changes in the ability of moulin-connected channels to convey supraglacial melt. Instead, these observations suggest that decreasing late-season ice velocity may be caused by changes in connectivity in unchannelized regions of the subglacial hydrologic system. Understanding this spatiotemporal variability in subglacial pressures is increasingly important because melt-season dynamics affect ice velocity beyond the conclusion of the melt season.

Estimation of heterogeneous aquifer parameters using centralized and decentralized fusion of hydraulic tomography data

Abstract. Characterization of spatial variability of hydraulic properties of groundwater systems at high resolution is essential to simulate flow and transport phenomena. This paper investigates two schemes to invert transient hydraulic head data resulting from multiple pumping tests for the purpose of estimating the spatial distributions of the hydraulic conductivity, K, and the specific storage, Ss, of an aquifer. The two methods are centralized fusion and decentralized fusion. The centralized fusion of transient data is achieved when data from all pumping tests are processed concurrently using a central inversion processor, whereas the decentralized fusion inverts data from each pumping test separately to obtain optimal local estimates of hydraulic parameters, which are consequently fused using the generalized Millman formula, an algorithm for merging multiple correlated or uncorrelated local estimates. For both data fusion schemes, the basic inversion processor employed is the ensemble Kalman filter, which is employed to assimilate the temporal moments of impulse response functions obtained from the transient hydraulic head measurements resulting from multiple pumping tests. Assimilating the temporal moments instead of the hydraulic head transient data themselves is shown to provide a significant improvement in computational efficiency. Additionally, different assimilation strategies to improve the estimation of Ss are investigated. Results show that estimation of the K and Ss distributions using temporal moment analysis is fairly good, and the centralized inversion scheme consistently outperforms the decentralized inversion scheme.

Estimating temporal changes in hydraulic head using InSAR data in the San Luis Valley, Colorado

The sustainability of the confined aquifer system in the San Luis Valley, Colorado is of utmost importance to the valley's agricultural economy. There is a dearth of hydraulic head measurements in the confined aquifer to which the current groundwater flow model can be calibrated. Here we investigate the extent to which spatially and temporally dense measurements of deformation from Interferometric Synthetic Aperture Radar (InSAR) data can be used to fill in spatial and temporal gaps in the head data set by calibrating the InSAR data with head at the monitoring well locations. We conduct this calibration at 11 wells where we expect sufficient deformation for reliable InSAR measurement, given the accepted level of uncertainty (∼1 cm). In the San Luis Valley, crop growth degrades the quality of the InSAR signal, which means that the high-quality deformation data may not be collocated with the wells. We use kriging to estimate the deformation directly at the well locations. We find that the calibration is valid at three well locations where the seasonal magnitude of the deformation is much larger than the uncertainty of the InSAR measurement. At these well locations, we predict head prior to and within the temporal sampling window of the head measurements. We find that 59% of the InSAR-predicted hydraulic head values agree with the measured values, within the uncertainty of the data. Given our success in extending the hydraulic head data temporally, the next step in our research is to use InSAR data to interpolate spatially between head measurements.

Highly parameterized model calibration with cloud computing: an example of regional flow model calibration in northeast Alberta, Canada

Expanding groundwater datasets collected by automated sensors, and improved groundwater databases, have caused a rapid increase in calibration data available for groundwater modeling projects. Improved methods of subsurface characterization have increased the need for model complexity to represent geological and hydrogeological interpretations. The larger calibration datasets and the need for meaningful predictive uncertainty analysis have both increased the degree of parameterization necessary during model calibration. Due to these competing demands, modern groundwater modeling efforts require a massive degree of parallelization in order to remain computationally tractable. A methodology for the calibration of highly parameterized, computationally expensive models using the Amazon EC2 cloud computing service is presented. The calibration of a regional-scale model of groundwater flow in Alberta, Canada, is provided as an example. The model covers a 30,865-km2 domain and includes 28 hydrostratigraphic units. Aquifer properties were calibrated to more than 1,500 static hydraulic head measurements and 10 years of measurements during industrial groundwater use. Three regionally extensive aquifers were parameterized (with spatially variable hydraulic conductivity fields), as was the aerial recharge boundary condition, leading to 450 adjustable parameters in total. The PEST-based model calibration was parallelized on up to 250 computing nodes located on Amazon’s EC2 servers.

Grouted-in Pressure Transducers: Implications for Hydrogeological Testing and Monitoring in Bedrock

Grouted-in pressure transducers are commonly deployed at open pit and underground mines to monitor hydraulic heads in bedrock near the mine working and to provide information on the progress of dewatering/depressurization efforts. Grout mixtures typically have a lower hydraulic conductivity than permeable features that act as conduits for groundwater flow in bedrock. Therefore, a question often arises as to how grout properties affect measurements of hydraulic head when compared to installations with pressure transducers embedded in sand pack or installed in conventional standpipe piezometers. A series of numerical simulations have been completed to illustrate the hypothetical response of a grouted-in pressure transducers and the response of an “ideal” instrument that is not affected by grout properties. For the range of parameters tested, these simulations indicated that the grouted-in transducers resulted in a “lag” in pressure response of a few hours following initiation of the hydraulic stress compared to the response of the sand packed transducer. Thus, care needs to be exercised when estimating hydrogeological properties from tests that rely on a network of grouted-in transducers. The numerical simulations also suggested that approximately 1 day after the initiation of hydraulic stress, the readings recorded by a grouted-in transducer were essentially the same as readings that would be obtained from an “ideal” instrument. Longer-term monitoring of hydraulic heads behind the pit walls using such instruments thus appears to be a viable alternative at mine sites.

Three-dimensional geostatistical inversion of synthetic tomographic pumping and heat-tracer tests in a nested-cell setup

A main purpose of groundwater inverse modeling lies in estimating the hydraulic conductivity field of an aquifer. Traditionally, hydraulic head measurements, possibly obtained in tomographic setups, are used as data. Because the groundwater flow equation is diffusive, many pumping and observation wells would be necessary to obtain a high resolution of hydraulic conductivity, which is typically not possible. We suggest performing heat tracer tests using the same already installed pumping wells and thermometers in observation planes to amend the hydraulic head data set by the arrival times of the heat signals. For each tomographic combinations of wells, we recommend installing an outer pair of pumping wells, generating artificial ambient flow, and an inner well pair in which the tests are performed. We jointly invert heads and thermal arrival times in 3-D by the quasi-linear geostatistical approach using an efficiently parallelized code running on a mid-range cluster. In the present study, we evaluate the value of heat tracer versus head data in a synthetic test case, where the estimated fields can be compared to the synthetic truth. Because the sensitivity patterns of the thermal arrival times differ from those of head measurements, the resolved variance in the estimated field is 6 to 10 times higher in the joint inversion in comparison to inverting head data only. Also, in contrast to head measurements, reversing the flow field and repeating the heat-tracer test improves the estimate in terms of reducing the estimation variance of the estimate. Based on the synthetic test case, we recommend performing the tests in four principal directions, requiring in total eight pumping wells and four intersecting observation planes for heads and temperature in each direction.

Is high-resolution inverse characterization of heterogeneous river bed hydraulic conductivities needed and possible?

Abstract. River–aquifer exchange fluxes influence local and regional water balances and affect groundwater and river water quality and quantity. Unfortunately, river–aquifer exchange fluxes tend to be strongly spatially variable, and it is an open research question to which degree river bed heterogeneity has to be represented in a model in order to achieve reliable estimates of river–aquifer exchange fluxes. This research question is addressed in this paper with the help of synthetic simulation experiments, which mimic the Limmat aquifer in Zurich (Switzerland), where river–aquifer exchange fluxes and groundwater management activities play an important role. The solution of the unsaturated–saturated subsurface hydrological flow problem including river–aquifer interaction is calculated for ten different synthetic realities where the strongly heterogeneous river bed hydraulic conductivities (L) are perfectly known. Hydraulic head data (100 in the default scenario) are sampled from the synthetic realities. In subsequent data assimilation experiments, where L is unknown now, the hydraulic head data are used as conditioning information, with the help of the ensemble Kalman filter (EnKF). For each of the ten synthetic realities, four different ensembles of L are tested in the experiments with EnKF; one ensemble estimates high-resolution L fields with different L values for each element, and the other three ensembles estimate effective L values for 5, 3 or 2 zones. The calibration of higher-resolution L fields (i.e. fully heterogeneous or 5 zones) gives better results than the calibration of L for only 3 or 2 zones in terms of reproduction of states, stream–aquifer exchange fluxes and parameters. Effective L for a limited number of zones cannot always reproduce the true states and fluxes well and results in biased estimates of net exchange fluxes between aquifer and stream. Also in case only 10 head data are used for conditioning, the high-resolution characterization of L fields with EnKF is still feasible. For less heterogeneous river bed hydraulic conductivities, a high-resolution characterization of L is less important. When uncertainties in the hydraulic parameters of the aquifer are also regarded in the assimilation, the errors in state and flux predictions increase, but the ensemble with a high spatial resolution for L still outperforms the ensembles with effective L values. We conclude that for strongly heterogeneous river beds the commonly applied simplified representation of the streambed, with spatially homogeneous parameters or constant parameters for a few zones, might yield significant biases in the characterization of the water balance. For strongly heterogeneous river beds, we suggest adopting a stochastic field approach to model the spatially heterogeneous river beds geostatistically. The paper illustrates that EnKF is able to calibrate such heterogeneous streambeds on the basis of hydraulic head measurements, outperforming zonation approaches.

A New Zonation Algorithm with Parameter Estimation Using Hydraulic Head and Subsidence Observations

Parameter estimation codes such as UCODE_2005 are becoming well-known tools in groundwater modeling investigations. These programs estimate important parameter values such as transmissivity (T) and aquifer storage values (Sa ) from known observations of hydraulic head, flow, or other physical quantities. One drawback inherent in these codes is that the parameter zones must be specified by the user. However, such knowledge is often unknown even if a detailed hydrogeological description is available. To overcome this deficiency, we present a discrete adjoint algorithm for identifying suitable zonations from hydraulic head and subsidence measurements, which are highly sensitive to both elastic (Sske) and inelastic (Sskv) skeletal specific storage coefficients. With the advent of interferometric synthetic aperture radar (InSAR), distributed spatial and temporal subsidence measurements can be obtained. A synthetic conceptual model containing seven transmissivity zones, one aquifer storage zone and three interbed zones for elastic and inelastic storage coefficients were developed to simulate drawdown and subsidence in an aquifer interbedded with clay that exhibits delayed drainage. Simulated delayed land subsidence and groundwater head data are assumed to be the observed measurements, to which the discrete adjoint algorithm is called to create approximate spatial zonations of T, Sske , and Sskv . UCODE-2005 is then used to obtain the final optimal parameter values. Calibration results indicate that the estimated zonations calculated from the discrete adjoint algorithm closely approximate the true parameter zonations. This automation algorithm reduces the bias established by the initial distribution of zones and provides a robust parameter zonation distribution.

Applications of Diverse Data Combinations in Subsurface Characterization using D-optimality Based Pilot Point Methods (DBM)

Many cases of strategically designed groundwater remediation have lack of information of hydraulic conductivity or permeability, which can render remediation methods inefficient. Many studies have been carried out to minimize this shortcoming by determining detailed hydraulic information either through direct or indirect measurements. One popular method for hydraulic characterization is the pilot point method (PPM), where the hydraulic property is estimated at a small number of strategically selected points using secondary measurements such as hydraulic head or tracer concentration. This paper adopted a D-optimality based pilot point method (DBM) developed previously for hydraulic head measurements and extended it to include both hydraulic head and tracer measurements. Based on different combinations of trials, our analysis showed that DBM performs well when hydraulic head is used for pilot point selection and both hydraulic head and tracer measurements are used for determining the conductivity values.

Estimation of the Height of Complete Groundwater Drainage Above Mined Longwall Panels

The height of complete groundwater drainage above subsided longwall panels (referred to as H) at underground mines is determined using a data base of hydraulic head measurements made with multiple devices down the depth profile at each of a number of sites worldwide. H is shown to be relatively independent of most parameters except the geometry of the mined void and the overburden thickness. An empirical equation linking H to these parameters is developed using hydraulic head data, and confirmed using an independent data base of ground movement. H is shown to be the same as the height of the zone of major ground movement above a panel. H for special cases (above longwall chain pillars, above pillar extraction panels, and underneath significant water bodies) is invariably smaller than H above center panel for ordinary cases. A new caving model, from a groundwater perspective, is proposed for continuously sheared longwall panels at ordinary locations. It removes complexity and reduces the uncertainty in estimating H. The derived equation for H applies to a variety of strata types.

Hydraulic and thermal response of groundwater–surface water exchange to flooding in an experimental aquifer

The effects of flooding on interactions between streams and shallow aquifers are poorly studied due to the unpredictable nature of flooding events. This study examines groundwater–surface water interactions in a constructed experimental meander over several simulated flood events using hydrologic and thermal monitoring. Detailed steady-state hydraulic head measurements were taken of the stream surface and in 59 piezometer nests during baseflow, bankfull flow, and overbank flooding conditions, allowing for three-dimensional hydrologic characterization. Additionally, a lithium tracer test was conducted, and pressure transducers and thermistors were deployed to capture transient behavior. Results demonstrate extensive coupling between the stream and adjacent alluvial aquifer under all discharge conditions. Water table elevation responds rapidly to changes in stream stage and re-equilibrates to order of magnitude increases in discharge within about 1 h. Though flooding elevates the water table, steady-state hydraulic gradients within the meander are independent of stream stage. This results from the flow boundary condition imposed by the stream, which deepens in response to flooding but has a slope that is essentially independent of discharge. In addition to the stream boundary, flow within the meander is also controlled by loss of stream water to the subsurface, which directs hydraulic gradients towards the base of the meander. Finally, the temperature distribution within the meander during normal and bankfull flow mimics the water table distribution and results from advected warm stream water being progressively cooled by vertical conduction in the direction of groundwater flow. These findings suggest that areas of high reaction rates within meanders (hot spots) will be sensitive to whether a stream reach is gaining or losing water to the subsurface. Further, the location of these hot spots of is likely independent of stream stage, though other controls on reaction rates may be affected.

A Simple Model to Assess Nitrogen and Phosphorus Contamination in Ungauged Surface Drainage Networks: Application to the Massaciuccoli Lake Catchment, Italy

Modeling is a common practice to evaluate factors affecting water quality in environmental systems impaired by point and nonpoint losses of N and P. Nevertheless, in situations with inadequate information, such as ungauged basins, a balance between model complexity and data availability is necessary. In this paper, we applied a simplified analytical model to an artificially drained floodplain in central-western Italy to evaluate the importance of different nutrient sources and in-stream retention processes and to identify critical source areas. We first considered only a set of chemical concentrations in water measured from February through May 2008 and from November 2008 through February 2009. We then broadened available data to include water discharge and hydraulic-head measurements to construct a hydrogeological model using MODFLOW-2000 and to evaluate the reliability of the simplified method. The simplified model provided acceptable estimates of discharge (ranging from 0.03-0.75 m s) and diffuse nutrient inputs from water table discharge and in-stream retention phenomena. Estimates of PO-P and total P retention (ranging from 1.0 to 0.6 μg m s and from 1.18 to 0.95 μg m s for PO-P and total P, respectively) were consistent with the range of variability in literature data. In contrast, the higher temporal variability of nitrate concentrations decreased model accuracy, suggesting the need for more intensive monitoring. The model also separated the dynamics of different reaches of the drainage network and identified zones considered critical source areas and buffer zones where pollutant transport is reduced.

Fully coupled hydrogeophysical inversion of a laboratory salt tracer experiment monitored by electrical resistivity tomography

We perform a salt tracer experiment, monitored by time‐lapse electrical resistivity tomography, in a quasi‐two‐dimensional sandbox with the aim of determining the hydraulic conductivity distribution in the domain. We use sodium chloride as a tracer, together with cochineal red for visual monitoring. The time series of observed resistance for each electrode configuration is characterized by its temporal moments. We invert the mean arrival time of electrical potential perturbations and a few steady state hydraulic head measurements using the fully coupled hydrogeophysical approach recently introduced by Pollock and Cirpka (2010). This is the first application of the approach to experimental data. The results obtained show a reasonable agreement between the estimated hydraulic conductivity field and the pattern of the actual sandbox filling. Using this estimation, a transient simulation is performed to compute the propagation of the salt tracer plume through the sandbox. The latter is compared to pictures taken during the experiment. These results show an even better agreement, indicating that the lenses of different sand types are not entirely homogeneous and some unexpected preferential flow paths are present. We conclude that temporal moments of potential perturbations obtained during salt tracer tests provide a good basis for inferring the hydraulic conductivity distribution by fully coupled hydrogeophysical inversion.

High quality InSAR data linked to seasonal change in hydraulic head for an agricultural area in the San Luis Valley, Colorado

In the San Luis Valley (SLV), Colorado legislation passed in 2004 requires that hydraulic head levels in the confined aquifer system stay within the range experienced in the years 1978–2000. While some measurements of hydraulic head exist, greater spatial and temporal sampling would be very valuable in understanding the behavior of the system. Interferometric synthetic aperture radar (InSAR) data provide fine spatial resolution measurements of Earth surface deformation, which can be related to hydraulic head change in the confined aquifer system. However, change in cm‐scale crop structure with time leads to signal decorrelation, resulting in low quality data. Here we apply small baseline subset (SBAS) analysis to InSAR data collected from 1992 to 2001. We are able to show high levels of correlation, denoting high quality data, in areas between the center pivot irrigation circles, where the lack of water results in little surface vegetation. At three well locations we see a seasonal variation in the InSAR data that mimics the hydraulic head data. We use measured values of the elastic skeletal storage coefficient to estimate hydraulic head from the InSAR data. In general the magnitude of estimated and measured head agree to within the calculated error. However, the errors are unacceptably large due to both errors in the InSAR data and uncertainty in the measured value of the elastic skeletal storage coefficient. We conclude that InSAR is capturing the seasonal head variation, but that further research is required to obtain accurate hydraulic head estimates from the InSAR deformation measurements.

Joint inversion of steady-state hydrologic and self-potential data for 3D hydraulic conductivity distribution at the Boise Hydrogeophysical Research Site

► We combine sedimentological and hydro-geophysical information to characterize a real aquifer. ► We characterize the distribution of materials at the site in the spirit of multiple- (overlapping) continua concepts. ► A Multiple Indicator Kriging is adopted to estimate the probability of occurrence of each geo-material. ► Self potential and hydraulic head are embedded jointly within a three-dimensional inverse groundwater model. ► We obtained a very detailed reconstruction of the heterogeneity of the site better than previous studies. We combine sedimentological, hydraulic and geophysical information to characterize the 3D distribution of transport properties of an heterogeneous aquifer. We focus on the joint inversion of hydraulic head and self-potential measurements collected during an extensive experimental campaign performed at the Boise Hydrogeophysical Research Site (BHRS), Boise, Idaho, and involving a series of dipole tests. While hydraulic head data obtained from piezometric readings in open wells represent a depth-averaged value, self-potential signals provide an estimate of the water table location. The aquifer is conceptualized as a multiple-continuum, where the volumetric fraction of a geo-material within a cell of the numerical flow model is calculated by Multiple Indicator Kriging. The latter is implemented on the basis of available sedimentological information. The functional format of the indicator variograms and associated parameters are estimated on the basis of formal model identification criteria. Self-potential and hydraulic head data have been embedded jointly within a three-dimensional inverse model of groundwater flow at the site. Each identified geo-material (category) is assumed to be characterized by a constant hydraulic conductivity. The latter constitute the set of model parameters. The hydraulic conductivity associated with a numerical block is then calculated as a weighted average of the conductivities of the geo-materials which are collocated in the block by means of Multiple Indicator Kriging. Model parameters are estimated by a Maximum Likelihood fit between measured and modeled state variables, resulting in a spatially heterogeneous distribution of hydraulic conductivity. The latter is effectively constrained on the sedimentological data and conditioned on both self-potential and borehole hydraulic head readings. Minimization of the Maximum Likelihood objective function allows estimating the relative weight of measurement errors associated with self-potential and borehole-based head data. The procedure adopted allowed a reconstruction of the heterogeneity of the site with a level of details, which was not obtained in previous studies and with relatively modest computational efforts. Further validation against dipole tests which were not used in the inversion procedure supports the robustness of the results.

Simulation of groundwater flow within observation boreholes for confined aquifers

Observation wells around a pumping well are the primary means of monitoring the change of hydraulic head in an aquifer. Pipe flow in open observation boreholes will affect the measurement of hydraulic head in the observation wells and alter the flow patterns around the wells, but the influences of non-Darcian pipe flow in observation wells on the accuracy of hydraulic head measurements are not fully recognized. This paper presents a coupled seepage-pipe flow theory which integrates the aquifer and well as a continuous medium for assessing the influences. The objective of this study is to examine the pipe-flow effects in observation wells of varied diameters under several pumping scenarios including different penetration lengths of the pumping well. A numerical model based on the Galerkin finite element method and the finite difference method was developed for the study. Numerical simulations were conducted for a homogeneous and isotropic confined aquifer in which a pumping well penetrated a different thickness of the aquifer and a number of full penetrating observation wells were placed to obtain drawdowns. The simulation results indicated that vertical pipe flow within the observation wells was significant and the flow was non-linear. The pipe flow resulted in a conduit effect that disturbed the distribution of the hydraulic head within the observation wells. After comparing with the results of the Hantush (1961) integral by using the case study, which neglected the wellbore effect on the groundwater flow system, the authors found that the Hantush integral may be only conditionally applicable for estimating the drawdown within the observation wells when well diameter is greater than 0.20 m and radial distance is greater than the thickness of the aquifer. Our study suggests that use of hydraulic heads from observation wells for the evaluation of groundwater flow systems must be cautious before the pipe-flow effect can be assumed to be negligible.

Ensemble smoother assimilation of hydraulic head and return flow data to estimate hydraulic conductivity distribution

Numerical groundwater models, frequently used to enhance understanding of the hydrologic and chemical processes in local or regional aquifers, are often hindered by an incomplete representation of the parameters which characterize these processes. In this study, we present the use of a data assimilation algorithm that incorporates all past model results and data measurements, an ensemble smoother (ES) to provide enhanced estimates of aquifer hydraulic conductivity (K) through assimilation of hydraulic head (H) and groundwater return flow volume (RFV) measurements into groundwater model simulation results. On the basis of the Kalman filter methodology, residuals between forecasted model results and measurements, together with covariances between model results at measurement locations and nonmeasurement locations, are used to correct model results. Parameter estimation is achieved by incorporating model parameters into the algorithm, thus allowing the correlation between H, RFV, and K to correct the K fields. The applicability of the ES is demonstrated using a synthetic two‐dimensional transient groundwater modeling simulation. Sensitivity analyses are carried out to show the performance of the ES in regard to measurement error, number of measurements, number of assimilation times, correlation length of the K fields, and the number of stream gage locations. Results show that the departure of the K fields from a reference K field is greatly reduced through data assimilation and demonstrate that the ES scheme is a promising alternative to other inverse modeling techniques because of low computational burden and the ability to run the algorithm entirely independent of the groundwater model simulation.

Incorporating a Watershed-Based Summary Field Exercise into an Introductory Hydrogeology Course

We have developed and implemented a summary field exercise for an introductory hydrogeology course without a laboratory section. This exercise builds on lectures and problem sets that use pre-existing field data. During one day in April, students measure hydraulic heads, stream and spring flow, and stream-bed seepage within the rural watershed of a third-order perennial stream in western Kentucky. Students calculate net specific discharge at various scales, map groundwater flow in the watershed, and calculate vertical hydraulic gradients at the mouth of the watershed, where the stream enters a reservoir (Kentucky Lake). Distinctive features of the exercise include hydraulic head measurements in large-diameter domestic wells and in piezometers installed in the reservoir embayment. Kentucky Lake is raised ~ 2 m shortly before the field trip, thus providing an analog of bank storage. Former students who responded to a questionnaire indicated that the exercise was worthwhile. The exercise was based at a biological field station but could be completed at any field site where long-term hydrologic monitoring is in place or could be initiated.