Calibration Of Groundwater Flow Models Research Articles

Improving calibration of groundwater flow models using headwater streamflow intermittence

AbstractNon‐perennial streams play a crucial role in ecological communities and the hydrological cycle. However, the key parameters and processes involved in stream intermittency remain poorly understood. While climatic conditions, geology and land use are well identified, the assessment and modelling of groundwater controls on streamflow intermittence remain a challenge. In this study, we explore new opportunities to calibrate process‐based 3D groundwater flow models designed to simulate hydrographic network dynamics in groundwater‐fed headwaters. Streamflow measurements and stream network maps are considered together to constrain the effective hydraulic properties of the aquifer in hydrogeological models. The simulations were then validated using visual observations of water presence/absence, provided by a national monitoring network in France (ONDE). We tested the methodology on two pilot unconfined shallow crystalline aquifer catchments, the Canut and Nançon catchments (Brittany, France). We found that both streamflow and stream network expansion/contraction dynamics are required to calibrate models that simultaneously estimate hydraulic conductivity and porosity with low uncertainties. The calibration allowed good prediction of stream intermittency, both in terms of flow and spatial extent. For the two catchments studied, Canut and Nançon, the hydraulic conductivity is close reaching 1.5 × 10−5 m/s and 4.5 × 10−5 m/s, respectively. However, they differ more in their storage capacity, with porosity estimated at 0.1% and 2.2%, respectively. Lower storage capacity leads to higher groundwater level fluctuations, shorter aquifer response times, an increase in the proportion of intermittent streams and a reduction in perennial flow. This new modelling framework for predicting headwater streamflow intermittence can be deployed to improve our understanding of groundwater controls in different geomorphological, geological and climatic contexts. It will benefit from advances in remote sensing and crowdsourcing approaches that generate new observational data products with high spatial and temporal resolution.

Groundwater Flow Model Calibration Using Variable Density Modeling for Coastal Aquifer Management

The paper investigates the mechanism of seawater intrusion and the performance of free and open-source codes for the simulation of variable density flow problems in coastal aquifers. For this purpose, the research focused on the Marathon Watershed, located in the northeastern tip of Attica, Greece. For the simulation of the groundwater system, MODFLOW, MT3DMS and SEAWAT codes were implemented, while sensitivity analysis and calibration processes were carried out with UCODE. Hydraulic head calibration was performed on the MODFLOW model, and TDS concentration was validated in the SEAWAT model. The calibrated parameters of the MODFLOW model were obtained for the variable density flow simulation with SEAWAT. The MODFLOW and SEAWAT hydraulic head outputs were analyzed and compared to one another. The outcome of this analysis is that SEAWAT produced slightly better results in terms of the hydraulic heads, concluding that parameter transferability can take place between the two models. For the purpose of the seawater intrusion assessment, the use of the SEAWAT code revealed that the aquifer is subjected to passive and passive–active seawater intrusion during wet and dry seasons, respectively. Finally, an irregular shape of a saltwater wedge is developed at a specific area associated with the hydraulic parameters of the aquifer.

High spatial resolution prediction of tritium (3H) in contemporary global precipitation

Tritium (3H) in Earth’s precipitation is vigilantly monitored since historical nuclear bomb tests because of radiological protection considerations and its invaluable role as a tracer of the global water cycle in quantifying surface, groundwater, and oceanic fluxes. For hydrological applications, accurate knowledge of 3H in contemporary local precipitation is prerequisite for dating of critical zone water and calibrating hydrogeologic transport and groundwater protection models. However, local tritium input in precipitation is hard to constrain due to few 3H observation sites. We present new high-spatial resolution global prediction maps of multi-year mean 3H in contemporary “post-bomb” (2008–2018) precipitation by using a robust regression model based on environmental and geospatial covariates. The model accurately predicted the mean annual 3H in precipitation, which allowed us to produce global 3H input maps for applications in hydrological and climate modelling. The spatial patterns revealed natural 3H in contemporary precipitation sufficient for practical hydrological applications (1–25 TU) but variable across continental regions and higher latitudes due to cumulative influences of cyclical neutron fluxes, stratospheric inputs, and distance from tropospheric moisture sources. The new 3H maps provide a foundational resource for improved calibration of groundwater flow models and critical zone vulnerability assessment and provides an operational baseline for quantifying the potential impact of future anthropogenic nuclear activities and hydroclimatic changes.

Optimized Pilot Point Emplacement Based Groundwater Flow Calibration Method for Heterogeneous Small-Scale Area

Groundwater flow modeling in a small-scale area requires practical techniques to obtain high accuracy results. The effectiveness of the model calibration is the most challenging for simulating the hydraulic head. In pursuit of this, we proposed an optimized groundwater flow calibration method based on the pilot point emplacement technique for a 3D small-scale area in this work. Subsequently, two emplacement structures were tested during the experimentation, the regular pilot point placement, and the middle head measurement down gradient (MHMDG) placement with two different densities. The parameter estimation (PEST) numerical code applying the kriging interpolation was used to estimate the hydraulic conductivity field by MODFLOW. Moreover, geological SGrid models were chosen for the conceptual model. Thirty-seven observation wells were used for experimental simulations to test the proposed method in a heterogeneous confined aquifer. The result shows that the small-scale modeling was complicated, and the studying area presented a significant heterogeneity in horizontal hydraulic conductivity. The middle head measurement down gradient (MHMDG) pilot point case with the larger density gave the best R-squared 0.901 and minimum residual error of 0.0053 m compared to 0.880 and 0.078 m, respectively, for the regular placement. The calibration accuracy depended on the frequency and the emplacement of the pilot point. Therefore, the initial value should be technically selected to minimize the computation burden. The proposed techniques help to improve the groundwater flow model calibration based on the pilot point methodology for groundwater resources management.

USING OF LOW COST MOISTURE SENSORS IN LABORATORY EXPERIMENTS

<jats:p>Soil moisture is one of crucial parameters in modeling and analysis of groundwater flow. Measurement of soil water dynamics improves the understanding of processes and is vital for good groundwater flow model calibration. Determining soil moisture change during time by laboratory methods, as an integral part of practical classes, requires significant material and financial resources. Nowadays, relatively cheap sensors for measuring soil moisture are available. They could be combined with affordable data loggers to speed up the measurement procedure and to reduce cost price. In this paper, the possibility of using these sensors for measuring soil moisture and their usage for the educational experimental exercises in a field of hydraulic engineering was investigated. The paper first presents the procedure of sensor calibration for four different sensors used in two types of soil, and then their usage in a simple hydraulic engineering experiment.</jats:p>

Groundwater Flow Model Calibration of a Coastal Multilayer Aquifer System Based on Statistical Sensitivity Analysis

Groundwater Flow Model Calibration of a Coastal Multilayer Aquifer System Based on Statistical Sensitivity Analysis

Groundwater flow velocities in karst aquifers; importance of spatial observation scale and hydraulic testing for contaminant transport prediction.

We review scale dependence of hydraulic conductivities and effective porosities for prediction of contaminant transport in four UK karst aquifers. Approaches for obtaining hydraulic parameters include core plug, slug, pumping and pulse tests, calibration of groundwater flow models and spring recession curves. Core plug and slug tests are unsuitable because they do not characterize a large enough volume to include a representative fracture network. Pumping test values match regional-scale hydraulic conductivities from flow modelling for the less intensively karstified aquifers: Magnesian Limestone, Jurassic Limestone and Cretaceous Chalks. Reliable bulk hydraulic conductivities were not available for the intensively karstified Carboniferous Limestone due to dominance of flow through pipe conduits in Mendips. Here, the only hydraulic conductivity value found from spring recession is one order of magnitude higher than that indicated by pumping tests. For all four carbonate aquifers, effective porosities assumed for transport modelling are two orders of magnitude higher than those found from tracer and hydrogeophysical tests. Thus, a combination of low hydraulic conductivities and assumed flowing porosities resulted in underestimated flow velocities. The UK karst aquifers are characterized by a range of hydraulic behaviours that fit those of karst aquifers worldwide. Indeed, underestimation of flow velocity due to inappropriate parameter selection is common to intensively karstified aquifers of southern France, north-western Germany and Italy. Similar issues arise for the Canadian Silurian carbonates where the use of high effective porosities (e.g. 5%) in transport models leads to underestimation of groundwater velocities. We recommend values in the range of 0.01-1% for such aquifers.

Challenges of Groundwater Flow Model Calibration Using MODFLOW in Ethiopia: With Particular Emphasis to the Upper Awash River Basin

In this work, most important problems related to model calibration have been assessed using MODFLOW. Particular emphasis is given to the Upper Awash river basin where many boreholes have been drilled for municipal and industrial uses compared with other regions in Ethiopia. Static Water Level (SWL) records from water supply wells drilled for about 32 years in the Upper Awash basin is considered to illustrate the commonly used groundwater flow model calibration procedures and associated problems. The assumptions made in the modeling procedures to use SWL data collected over many years from water supply boreholes to calibrate steady state models is too much of an assumption. Alternatives on steady and pseudo transient model calibration approaches in data scarce areas based on logical assumptions and reasonable representation of groundwater systems has been suggested. Hence, numerical groundwater flow models may play the expected key role for the sustainable groundwater resource management of the country, which is solving practical ground-water related problems.

Sensitivity analysis of two-dimensional steady-state aquifer flow equations. Implications for groundwater flow model calibration and validation

This paper presents the analytical properties of the sensitivity of the two-dimensional, steady-state groundwater flow equation to the flow parameters and to the boundary conditions, based on the perturbation approach. These analytical properties are used to provide guidelines for model design, model calibration and monitoring network design. The sensitivity patterns are shown to depend on the nature of both the perturbed parameter and the variable investigated. Indeed, the sensitivity of the hydraulic head to the hydraulic conductivity extends mainly in the flow direction, while the sensitivity to the recharge spreads radially. Besides, the sensitivity of the flow longitudinal velocity to the hydraulic conductivity propagates in both the longitudinal and transverse directions, whereas the sensitivity of the flow transverse velocity propagates in the diagonal directions to the flow. The analytical results are confirmed by application examples on idealized and real-world simulations. These analytical findings allow some general rules to be established for model design, model calibration and monitoring network design. In particular, the optimal location of measurement points depends on the nature of the variable of interest. Measurement network design thus proves to be problem-dependent. Moreover, adequate monitoring well network design may allow to discriminate between the possible sources of error.

On the value of lithofacies data for improving groundwater flow model accuracy in a three‐dimensional laboratory‐scale synthetic aquifer

Improvement of the prediction accuracy of groundwater flow models has been receiving substantial attention from many researchers through the development of enhanced characterizations of the structure of subsurface lithofacies and of the distribution of hydraulic conductivity. In this study, we investigated how incorporating increasing amounts of lithofacies data into the construction of a conceptual model of aquifer heterogeneity helps to reduce prediction error and uncertainty in groundwater flow models. An approach based on both laboratory experiments and numerical simulations was tested using data from an intermediate‐scale synthetic heterogeneous aquifer. The heterogeneous aquifer consisted of five lithofacies, corresponding to five test sands. Three pumping tests were conducted and provided experimental data to perform groundwater flow model calibration and validation. The pumping tests were also simulated numerically in order to provide a series of error‐free synthetic hydraulic data sets. On the basis of Markov chains models of transition probabilities, a total of 901 random realizations of the heterogeneous distribution of lithofacies were created using varying amounts of conditioning lithofacies data sampled along randomly placed hypothetical boreholes. For each realization and for two other simplified lithofacies models, parameter estimation was performed to estimate the hydraulic conductivity of the lithofacies using the experimental and synthetic hydraulic data from the three pumping tests. The results generally showed that the use of more lithofacies data in the construction of the lithofacies realizations led to an improvement in groundwater flow model prediction accuracy. When using the error‐free synthetic hydraulic data, the calibration‐prediction error and uncertainty decreased drastically when the mean borehole spacing was on the order of twice the horizontal correlation length or less. When the experimental hydraulic data were used, this drastic improvement in the calibration‐prediction error was somewhat obscured and, in some cases, exhibited a local minimum. This local minimum, although beyond practical limits, corresponded to an optimal number of boreholes. Finally, the effect of incorporating more lithofacies data for the construction of lithofacies realizations was found to have a similar impact on the quality of model calibration and on the quality of predictive simulations conducted using the calibrated model.

Electrokinetic spontaneous polarization in porous media: petrophysics and numerical modelling

The behaviour of streaming potential is directly related to movement of groundwater. The responses for typical subsurface flows are modelled to investigate possibilities of spontaneous polarization (SP) when performing quantitative data interpretation. The physical properties of geomaterials related to streaming potential are described. A magnitude range of streaming current coefficient is assessed for geomaterials and found to be from −10−10 to −10−8A/Pam, depending on water salinity and rock composition. The electrical sources of SP caused by groundwater flow in saturated media are theoretically described. It is shown that SP is completely defined by three types of electrical sources situated (1) on boundaries where water enters or quits porous media, (2) in areas with transient regime, and (3) on the boundaries of rocks with different properties (hydraulic conductivity, streaming current coefficients and electrical conductivity). A 2D-computer program based on the method of finite difference was created to provide numerical successive modelling of both groundwater flow and SP. Using Sill's [Geophysics 48 (1983) 76] approach, first the water heads are calculated. Then, electrical sources of SP are obtained on the basis of the calculated heads and coefficient of streaming current. Finally, the SP is obtained on the basis of calculated electrical sources and subsurface electrical conductivity. Numerical examples illustrating SP responses of infiltration, of barrage with different electrical conductivity, and of well pumping are discussed. Field data obtained at the site containing the dam between superficial artificial reservoir and the river are interpreted on the basis of numerical modelling. The discussed method can be mainly used for additional calibration of groundwater flow models.

A Controlled Experiment in Ground Water Flow Model Calibration

AbstractNonlinear regression was introduced to ground water modeling in the 1970s, but has been used very little to calibrate numerical models of complicated ground water systems. Apparently, nonlinear regression is thought by many to be incapable of addressing such complex problems. With what we believe to be the most complicated synthetic test case used for such a study, this work investigates using nonlinear regression in ground water model calibration. Results of the study fall into two categories. First, the study demonstrates how systematic use of a well designed nonlinear regression method can indicate the importance of different types of data and can lead to successive improvement of models and their parameterizations. Our method differs from previous methods presented in the ground water literature in that (1) weighting is more closely related to expected data errors than is usually the case; (2) defined diagnostic statistics allow for more effective evaluation of the available data, the model, and their interaction; and (3) prior information is used more cautiously. Second, our results challenge some commonly held beliefs about model calibration. For the test case considered, we show that (1) field measured values of hydraulic conductivity are not as directly applicable to models as their use in some geostatistical methods imply; (2) a unique model does not necessarily need to be identified to obtain accurate predictions; and (3) in the absence of obvious model bias, model error was normally distributed. The complexity of the test case involved implies that the methods used and conclusions drawn are likely to be powerful in practice.

Calibration of groundwater flow models using Monte Carlo simulations and geostatistics : Hoeksma, R J; Clapp, R B Proc Conference on Calibration and Reliability in Groundwater Modelling, The Hague, 3–6 September 1990 P33–42. Publ Wallingford: IAHS Press, 1990 (IAHS Publication No. 195)

Groundwater flow models require specification of several input parameter fields which are inferred from limited data. In this paper the hydraulic conductivity and recharge rate are estimated for an unconfined aquifer under steady flow conditions. Usually point observations of conductivity and head are available for the estimation of the distributed conductivity field and the recharge rate. Use of numerical flow models require that these fields be prescribed as average values over finite elements. The geostatistical solution to this problem uses linear estimation to obtain the distributed conductivity field and the recharge rate. Monte Carlo simulations are used here to compute the covariances associated with the head data. Results obtained for both artificial and real data show that the head data is effective in improving the estimates that would result using conductivity data alone. The use of Monte Carlo simulations results in a method which can be used under a wider variety of modeling conditions than previous applications of the geostatistical approach. 11 refs., 2 figs.