Assessing the effects of spatial discretization on large-scale flow model performance and prediction uncertainty

Large-scale physically-based and spatially-distributed models (>100 km2) constitute useful tools for water management since they take explicitly into account the heterogeneity and the physical processes occurring in the subsurface for predicting the evolution of discharge and hydraulic heads for several predictive scenarios. However, such models are characterized by lengthy execution times. Therefore, modelers often coarsen spatial discretization of large-scale physically-based and spatially-distributed models for reducing the number of unknowns and the execution times. This study investigates the influence of such a coarsening of model grid on model performance and prediction uncertainty. The improvement of model performance obtained with an automatic calibration process is also investigated. The results obtained show that coarsening spatial discretization mainly influences the simulation of discharge due to a poor representation of surface water network and a smoothing of surface slopes that prevents from simulating properly surface water-groundwater interactions and runoff processes. Parameter sensitivities are not significantly influenced by grid coarsening and calibration can compensate, to some extent, for model errors induced by grid coarsening. The results also show that coarsening spatial discretization mainly influences the uncertainty on discharge predictions. However, model prediction uncertainties on discharge only increase significantly for very coarse spatial discretizations.Peer reviewe

Characterizing groundwater flow and heat transport in fractured rock using Fiber-Optic Distributed Temperature Sensing

International audienceWe show how fully distributed space-time measurements with Fiber-Optic Distributed Temperature Sensing (FO-DTS) can be used to investigate groundwater flow and heat transport in fractured media. Heat injection experiments are combined with temperature measurements along fiber-optic cables installed in boreholes. Thermal dilution tests are shown to enable detection of cross-flowing fractures and quantification of the cross flow rate. A cross borehole thermal tracer test is then analyzed to identify fracture zones that are in hydraulic connection between boreholes and to estimate spatially distributed temperature breakthrough in each fracture zone. This provides a significant improvement compared to classical tracer tests, for which concentration data are usually integrated over the whole abstraction borehole. However, despite providing some complementary results, we find that the main contributive fracture for heat transport is different to that for a solute tracer

Towards best practice for assessing the impacts of climate change on groundwater

An essay is presented on the recommendations that need to be considered by hydrogeologists in assessing the impacts of climate change on groundwater. It says that the use of climate model projections must consider the utilization of multiple global climate models (GCMs) and regional climate models (RCMs), multiple emission scenarios, and the implication of downscale method. The different recommendations for developed hydrogeological coupling and socio-economic considerations are also mentioned

Management of sinkhole risks using long term ERT monitoring : a laboratory experiment

Accurate methodologies are required to manage risks linked to land-use planning in covered kart terrains, especially in densely urbanized areas. The main risk lies in the occurrence of sinkholes at the base of buildings or infrastructure. We conducted a laboratory experiment to evaluate the contribution of ERT monitoring in the long term management of such karstic risks. After presenting the design of the laboratory experiment, we detail the selected scenarios and the acquisition protocols tested. The methodology proposed to process the data and manage the inversion results relies on two steps: (1) we estimate the resistivity variations due to measurement and inversion errors based on Monte-Carlo simulations and (2) we define a resistivity changes index for every cell of the 3D model. The methodology is tested on a 3D surface survey including inline dipole-dipole, equatorial dipole and Wenner-Schlumberger arrays. A 10 cm in diameter plastic ball is used to model a 3.5 m sinkhole at depths ranging from the surface to 20 cm, using a water resistivity of 12 Ohm.m. Based on the proposed methodology, we show that this target can be suitably detected when its top reaches 10 cm or less in depth

A new single tracer test: the Finite Volume Point Dilution Method (FVPDM). Theory, field application and model validation

Quantification of pollutant mass fluxes is essential for assessing the impact of contaminated sites on their surrounding environment, particularly on adjacent surface water bodies. In this context, it is essential to quantify but also to be able to monitor the variations with time of Darcy fluxes in relation with changes in hydrogeological conditions and groundwater – surface water interactions. The Finite Volume Point Dilution Method (FVPDM) is a new tracer technique that generalizes the single-well point dilution method to the case of finite volumes of tracer fluid and water flush. It is based on an analytical solution derived from a mathematical model proposed recently to accurately model tracer injection into a well. After a brief description of the underlying concepts and mathematical model, an analytical solution is derived for calculating straightforwardly the evolution of concentration of the tracer in the injection well during and after injection operations. Based on sensitivity analysis performed on the concentration evolution in the injection well, the technique is described and interpretation methods are proposed. It is shown that this new tracer technique is easier to implement in the field than the classical point dilution method while it allows going further by monitoring changes with time of the magnitude of estimated Darcy fluxes, which is not the case for the former technique. The FVPDM was performed and validated in two experimental sites characterized by contrasted objectives, geological and hydrogeological conditions, and field equipment facilities. In site A, the objective was to estimate contaminant travel times in groundwater to a spring while assessing vertical variations in groundwater fluxes, using a combined FVPDM – classical tracer test, with “non-ideal” experimental conditions. In site B, the purpose was to estimate the groundwater fluxes flowing out from a contaminated site to a neighbouring river, in very well controlled experimental conditions. In both cases, field tracer concentrations monitored in the injection wells are used to fit the calculated modelled concentrations by playing on the ‘apparent’ Darcy flux crossing the well screens. Modelling results are very satisfactory and indicate that the methodology is efficient and accurate, with a wide range of potential applications in different environments and experimental conditions

Downscaling transient climate change with a stochastic weather generator for the Geer catchment, Belgium

peer reviewedThe coarse resolution of climate models creates the need for future scenarios which are downscaled to an appropriate spatial scale. Considerable effort has been devoted to the development of downscaling methods but a number of important issues remain in the development of robust, usable climate scenarios. These include the incorporation of various sources of uncertainty into future scenarios and the production of scenarios at timescales relevant to planners. This paper describes a new procedure which addresses these issues by producing a multi-model ensemble of transient climate change scenarios. This method couples an existing stochastic rainfall model to a new, transient implementation of a weather generator, using changes projected by an ensemble of regional climate model (RCM) experiments. The methodology is demonstrated by the generation of transient scenarios of daily rainfall, temperature and potential evapotranspiration (PET) for the Geer catchment in Belgium for the period 2010 to 2085. The utility of these scenarios is demonstrated by assessing the changes projected by the simulated time series of several temperature indices. The Geer is projected to experience a decrease in the occurrence of frost days with a corresponding shortening of the frost season and lengthening of the growing season. By examining a large ensemble of transient scenarios the range of uncertainty in these projections is assessed, but further, it is suggested that additional information on the projected timing of specified threshold events or system responses may be provided which could aid planners in assessing the likely timescales of required interventions and adaptation responses.FP6 IP AquaTerr