Conceptual model uncertainty in groundwater modeling: Combining generalized likelihood uncertainty estimation and Bayesian model averaging

in Water Resources Research (2008), 44

Uncertainty assessments in groundwater modeling applications typically attribute all sources of uncertainty to errors in parameters and inputs, neglecting what may be the primary source of uncertainty ... [more ▼]

Uncertainty assessments in groundwater modeling applications typically attribute all sources of uncertainty to errors in parameters and inputs, neglecting what may be the primary source of uncertainty, namely, errors in the conceptualization of the system.Confining the set of plausible system representations to a single model leads to underdispersive and prone-to-bias predictions. In this work, we present a general and flexible approach that combines generalized likelihood uncertainty estimation (GLUE) and Bayesian model averaging (BMA) to assess uncertainty in model predictions that arise from errors in model structure, inputs, and parameters. In a prior analysis, a set of plausible models is selected, and the joint prior input and parameter space is sampled to form potential simulators of the system. For each model, the likelihood measures of acceptable simulators, assigned to thembased on their ability to reproduce observed systembehavior, are integrated over the joint input and parameter space to obtain the integrated model likelihood. The latter is used to weight the predictions of the respective model in the BMA ensemble predictions. For illustrative purposes, we applied the methodology to a three-dimensional hypothetical setup. Results showed that predictions of groundwater budget terms varied considerably among competing models; despite this, a set of 16 head observations used for conditioning did not allow differentiating between the models. BMA provided average predictions that were more conservative than individual predictions obtained for individual models. Conceptual model uncertainty contributed up to 30% of the total uncertainty. The results clearly indicate the need to consider alternative conceptualizations to account for model uncertainty. [less ▲]

Hydrogeological study of Somes-Szamos transboundary alluvial aquifer

in Technical/Scientific Issues of Transboudary Water Management: Experience from around the world (2008)

Bayesian data fusion applied to water table spatial mapping

in Water Resources Research (2008), 44

Water table elevations are usually sampled in space using piezometric measurements that are unfortunately expensive to obtain and are thus scarce over space. Most of the time, piezometric data are ... [more ▼]

Water table elevations are usually sampled in space using piezometric measurements that are unfortunately expensive to obtain and are thus scarce over space. Most of the time, piezometric data are sparsely distributed over large areas, thus providing limited direct information about the level of the corresponding water table. As a consequence, there is a real need for approaches that are able at the same time to (1) provide spatial predictions at unsampled locations and (2) enable the user to account for all potentially available secondary information sources that are in some way related to water table elevations. In this paper, a recently developed Bayesian data fusion (BDF) framework is applied to the problem of water table spatial mapping. After a brief presentation of the underlying theory, specific assumptions are made and discussed to account for a digital elevation model and for the geometry of a corresponding river network. On the basis of a data set for the Dijle basin in the north part of Belgium, the suggested model is then implemented and results are compared to those of standard techniques such as ordinary kriging and cokriging. Respective accuracies and precisions of these estimators are finally evaluated using a ‘‘leave-one-out’’ cross-validation procedure. Although the BDF methodology was illustrated here for the integration of only two secondary information sources (namely, a digital elevation model and the geometry of a river network), the method can be applied for incorporating an arbitrary number of secondary information sources, thus opening new avenues for the important topic of data integration in a spatial mapping context. [less ▲]

Carte hydrogéologique, Bra - Lierneux 55/3-4, 1/25.000 : [notice explicative]

Book published by Ministère de la Région wallonne, Direction générale des ressources naturelles et de l'environnement - Edition provisoire : août 2007 (2007)

The effect of heterogeneity on the agreement between analytical and numerical solutions describing the recession flow in unconfined hillslope aquifers

in Proceedings of the Int. Congress on Development, Environment and Natural Resources: Multi-level and multi-scale Sustainability (2007, July)

Analytical approximations or linearized versions of the Boussinesq equation, describing recession flow in unconfined hillslope aquifers have been applied extensively in the past. One of the major ... [more ▼]

Analytical approximations or linearized versions of the Boussinesq equation, describing recession flow in unconfined hillslope aquifers have been applied extensively in the past. One of the major assumptions in the conceptual formulation is isotropic homogeneity of the aquifer. Notwithstanding the effect of aquifer heterogeneity on recession flow has been pointed out [1], only recent studies are increasingly focusing on the quantification of this effect [2,3]. In the present research, the relevance of heterogeneity is being studied by analyzing the agreement between analytical approximations and the numerical solution of the receding flow in a layered unconfined aquifer with different domain configurations (horizontal, inclined and concave impervious bedrock). For the application of the analytical approximations use is made of the equivalent hydraulic conductivity concept. [less ▲]

Intermediate report on the development of the Geer hydrological model (surface and subsurface water) for climatic change scenario on that sub-catchment

Report (2007)

In the framework of Workpackage BASIN R3 Meuse, the Hydrology Group of the University of Liège (Belgium) is developing a hydrological model of the Geer sub-catchment, in order to assess climate change ... [more ▼]

In the framework of Workpackage BASIN R3 Meuse, the Hydrology Group of the University of Liège (Belgium) is developing a hydrological model of the Geer sub-catchment, in order to assess climate change impacts on groundwater reserves. This report describes the calculation code used, the general hypotheses chosen to develop the model, the conceptual model, the climate change scenarios and the final objectives of the study. [less ▲]

Essais de traçages et modélisation des zones de prévention des sites de captage de Bassenge (Wonck et Roclenge)

Report (2007)

Comparative analysis between analytical approximations and numerical solutions describing recession flow in unconfined hillslope aquifers

in Hydrogeology Journal (2007), 15(6), 1077-1091

Recession flow of aquifers from a hillslope can be described by the non-linear Boussinesq equation. Under strong assumptions and for specific conceptual formulations, different authors derived analytical ... [more ▼]

Recession flow of aquifers from a hillslope can be described by the non-linear Boussinesq equation. Under strong assumptions and for specific conceptual formulations, different authors derived analytical approximations or linearized versions to this partial differential equation. A comparative analysis between some analytical approximations of the Boussinesq equation and the numerical solution of the recession flow of an unconfined homogeneous aquifer (horizontal, inclined and concave aquifer floor) was carried out. The objective was to define the range where the analytical solutions approximate the numerical solution. The latter was considered in this study as the reference method, because it requires fewer assumptions. From the considered analytical approximations, exponential decay relationships were found to be mainly valid for fine domain materials when horizontal, mild slopes (less than 2%) and concave aquifer floors were considered, but failed to reproduce coarse aquifer numerical model outflows, in contrast to the quadratic decay relationship, which better reproduce outflows in such domains. On the basis of the comparative analysis, it has been found that recession flows obtained with the considered analytical approximations yield similar values only for certain ranges of aquifer properties and geometries. [less ▲]

Integrated approach for assessing climate change impacts on a regional chalky aquifer in Belgium

in Changes in water resource systems: methodologies to maintain water security and ensure integrated management (2007)

An integrated hydrological model was developed in order to study the potential effects of climate change on groundwater resources. This model considers most hydrological processes in a physically ... [more ▼]

An integrated hydrological model was developed in order to study the potential effects of climate change on groundwater resources. This model considers most hydrological processes in a physically consistent way. More particularly, groundwater flows are modelled using a spatially distributed finite element approach. The river-aquifer interactions are explicitly taken into account in the model, as well as the spatial heterogeneity of the chalk geology characteristics. After a detailed calibration on the last 30 years and validation on recent periods, quantitative interpretations can be drawn from the ground¬water model results. Considering IPCC climate change scenarios, it appears that, on a multi-annual basis, most tested scenarios predict a decreasing trend in groundwater levels in the Geer Basin. These first results indicate that groundwater deficits may be expected in the future in Belgium. Moreover, at this stage of the study, this trend is computed for a very “optimistic” scenario, neglecting all other pressure changes on the groundwater resources (i.e. no change in land use and in pumping conditions). [less ▲]

Identification of groundwater quality trends in a chalk aquifer threatened by intensive agriculture in Belgium

in Hydrogeology Journal (2007), 15(8), 1615-1627

The European Union (EU) has adopted directives requiring that Member States take measures to reach a "good" chemical status of water resources by the year 2015 (Water Framework Directive: WFD). In order ... [more ▼]

The European Union (EU) has adopted directives requiring that Member States take measures to reach a "good" chemical status of water resources by the year 2015 (Water Framework Directive: WFD). In order to achieve the environmental objectives for groundwater, the identification and reversal of significant upward trends in pollutant concentrations are required. A very detailed dataset available for the Hesbaye chalk aquifer in Belgium is used to evaluate tools and to propose efficient methodologies for identifying and quantifying nitrate trends in groundwater. Results indicate that the parametric linear regression and the non-parametric Mann-Kendall tests are robust; however, the latter test seems more adequate as it does not require verification of the normality of the dataset and it provides calculated nitrate trends very comparable to those obtained using linear regression. From a hydrogeological point of view, results highlight a general upward trend in the whole groundwater basin. The extrapolation of the trend analysis results indicates that measures have to be taken urgently in order to avoid further major degradation of groundwater quality within the next 10-70 years. However, a good groundwater quality status cannot be expected in the Hesbaye aquifer for the 2015 EU WFD deadline. [less ▲]