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

An Empirical Explanation of the Speed-Distance Effect

Understanding motion perception continues to be the subject of much debate, a central challenge being to account for why the speeds and directions seen accord with neither the physical movements of objects nor their projected movements on the retina. Here we investigate the varied perceptions of speed that occur when stimuli moving across the retina traverse different projected distances (the speed-distance effect). By analyzing a database of moving objects projected onto an image plane we show that this phenomenology can be quantitatively accounted for by the frequency of occurrence of image speeds generated by perspective transformation. These results indicate that speed-distance effects are determined empirically from accumulated past experience with the relationship between image speeds and moving objects

Combining Next-Generation Sequencing Strategies for Rapid Molecular Resource Development from an Invasive Aphid Species, Aphis glycines

Aphids are one of the most important insect taxa in terms of ecology, evolutionary biology, genetics and genomics, and interactions with endosymbionts. Additionally, many aphids are serious pest species of agricultural and horticultural plants. Recent genetic and genomic research has expanded molecular resources for many aphid species, including the whole genome sequencing of the pea aphid, Acrythosiphon pisum. However, the invasive soybean aphid, Aphis glycines, lacks in any significant molecular resources.Two next-generation sequencing technologies (Roche-454 and Illumina GA-II) were used in a combined approach to develop both transcriptomic and genomic resources, including expressed genes and molecular markers. Over 278 million bp were sequenced among the two methods, resulting in 19,293 transcripts and 56,688 genomic sequences. From this data set, 635 SNPs and 1,382 microsatellite markers were identified. For each sequencing method, different soybean aphid biotypes were used which revealed potential biotype specific markers. In addition, we uncovered 39,822 bp of sequence that were related to the obligatory endosymbiont, Buchnera aphidicola, as well as sequences that suggest the presence of Hamiltonella defensa, a facultative endosymbiont.Molecular resources for an invasive, non-model aphid species were generated. Additionally, the power of next-generation sequencing to uncover endosymbionts was demonstrated. The resources presented here will complement ongoing molecular studies within the Aphididae, including the pea aphid whole genome, lead to better understanding of aphid adaptation and evolution, and help provide novel targets for soybean aphid control

Identification of groundwater quality trends in a chalky aqufieer threatened by intensive agriculture

As a response to the continuous increase of use on fertilizers and pesticides in last decades related to agricultural practices, the European Union (EU) has adopted in 1991 the Nitrate Directive 91/676/EEC (EU 2000). More recently the “Water Framework Directive 2000/60/EC” (WFD) has been published, stating that a good status of water is required for all EU members. The methodology used in this research mostly follows the works of Grath et al. (2001) who proposed particular algorithms and techniques for the identification of pollutants trends in ground water. As suggested by Hirsch et al. (1991), a three-step procedure is considered: 1) normality test of the dataset (Conover 1980, Helsel and Hirsch 1995); 2) trend detection (Mann 1945; Kendall 1975); and 3) trend estimation (Hirsch et al., 1991; EPA 2000). The study was performed in the Hesbaye chalky aquifer, located in the Geer river basin, at the North-West of the Liège city (Belgium). The Hesbaye aquifer is an important ground water resource for drinking water supply for the city of Liège and its suburbs, which means around 30 million cubic meters per year (Brouyère et al., 2004a). The land use in the basin is characterized by a dominant agricultural component, covering about 65% of the catchment area, the remaining space remaining being divided between pastures (15%), housing (13%) and forests (7%) (Broers et al., 2005). Nitrate datasets used in this study come mainly from the Nitrate Survey Network (NSN) established by the Walloon Region water authorities and from VMW (Vlaamse Maatschappij voor Watervoorziening), the Flemish water supply company. From groundwater points used (24), a total of 17 sampling points are characterized by an upward trend (71 % of the points), the annual increase of nitrate concentration ranging between 0.3 and 0.8 mg•l-1•year-1. The remaining points (7 over 24), which do not show any evidence of upward or downward trend, generally corresponds to sampling points with limited nitrate records, irregularly distributed in time. The transit time of nitrates across the thick unsaturated formations (from 10 to 70 m) have been deduced at approximately 1 m•year-1 to reach the groundwater (Brouyère et al., 2004b). This means that measures taken today will have an observable effect with a delay of 10 to 70 years. It gives thus more than time to react. Because changes in agricultural practices have started recently, it can be considered that the trends observed nowadays are likely to remain for years. Based on this assumption, a “simple” trend extrapolation is relevant in order to estimate the time remaining before ground water be unusable for public water supply. A rough estimation of the time remaining before the threshold concentration of 50 mg•l-1 would be reached in various parts of the chalk aquifer, has been calculated based on a point-by-point extrapolation of nowadays nitrate contamination levels using nitrate trend estimates in this research

Application of the Hybrid Finite Element Mixing Cell method to an abandoned coalfield in Belgium

The Hybrid Finite Element Mixing Cell (HFEMC) method is a flexible modelling technique particularly suited to mining problems. The principle of this method is to subdivide the modelled zone into several subdomains and to select a specific equation, ranging from the simple linear reservoir equation to the groundwater flow in porous media equation, to model groundwater flow in each subdomain. The model can be run in transient conditions, which makes it a useful tool for managing mine closure post-issues such as groundwater rebound and water inrushes. The application of the HFEMC method to an abandoned underground coal mine near the city of Liege (Belgium) is presented. The case study zone has been discretized taking advantage of the flexibility of the method. Then, the model has been calibrated in transient conditions based on both hydraulic head and water discharge rate observation and an uncertainty analysis has been performed. Finally, the calibrated model has been used to run several scenarios in order to assess the impacts of possible future phenomena on the hydraulic heads and the water discharge rates. Among others, the simulation of an intense rainfall event shows a quick and strong increase in hydraulic heads in some zones coupled with an increase in associated water discharge rates. This could lead to stability problems in local hill slopes. These predictions will help managing and predicting mine water problems in this complex mining system

Le “Patriotisme constitutionnel” : une identité politique pour les citoyens de l’Union européenne ?

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