DEMEETER : Dispositif Expérimental de Modulation du niveau d’Eau dans les Ecosystèmes TERrestres, un outil de recherche et de démonstration

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Development of an automatic bitemeter for grazing cattle

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Laboratory experiment of solute transport in a fracture with one porous wall

International audienceContaminant transport in heterogeneous fractured aquifers occurs mostly through the networks of intersecting fractures. The characterization and modeling of transport through these systems must take heterogeneity into account, both at the scale of the fracture network and at the scale of the fracture itself. The physical mechanims of solute transport in a single fracture with impermeable walls are well identified (Dronfield and Silliman 1993; Roux, Plouraboué et al. 1998; Keller, Roberts et al. 1999; Detwiler and Rajaram 2000; Loggia, Gouze et al. 2004): advection, Taylor-Aris dispersion, roughness dispersion, aperture variation dispersion and molecular diffusion. We address here solute transport through a fracture with a porous wall. To our knowledge, there is no fundamental description of the mass transfert coefficient between the region of high permeability (the fracture) and that of low permeability (the surrounding matrix). We present an analog experimental model setup in which we can focus on specific dispersion mechanisms, neglecting molecular diffusion, in order to extract descriptive laws that will be integrated in future numerical models. The planar horizontal fracture is 1 m long, 5 cm wide and its mean aperture is 5 mm. It is bounded by either two smooth parallel Plexiglass plates (impermeable walls configuration), or by one such plate and a porous medium consisting of 1 mm glass beads ("semi-permeable" configuration). A permanent laminar water flow is forced through the fracture at controlled mean velocity (~ 1mm/s), and a dye (patent blue) injection system simulates a point source of contaminant along the center plane of the fracture. The tracer plume is tracked using a visualization system based on lasers illuminating a series of vertical linear optical sensor arrays. It yields a quantitative temporal description of the tracer concentration, integrated over the fracture width and at several positions along the fracture length. We have first validated the setup using the impermeable walls configuration. The total amount of dye passing in front of each laser was monitored, and compared to classical advection-dispersion models (for different injection modes). Conservation of the total dye quantity was checked. Gravity effects are generally disregarded in theoretical studies; as recently suggested by Polak, Grader et al. (2003), Tenchine and Gouze (2005), they were observed to be significant. In a second stage, we have started addressing the impermeableporous configuration. Preliminary results on mass transfer between the fracture and the bounding porous matrix are presented

Heterogenous Impact of Water Warming on Exotic and Native Submerged and Emergent Plants in Outdoor Mesocosms

International audienceSome aquatic plants present high biomass production with serious consequences on ecosystem functioning. Such mass development can be favored by environmental factors. Temperature increases are expected to modify individual species responses that could shape future communities. We explored the impact of rising water temperature on the growth, phenology, and metabolism of six macrophytes belonging to two biogeographic origins (exotic, native) and two growth forms (submerged, emergent). From June to October, they were exposed to ambient temperatures and a 3 °C warming in outdoor mesocosms. Percent cover and canopy height were favored by warmer water for the exotic emergent Ludwigia hexapetala. Warming did not modify total final biomass for any of the species but led to a decrease in total soluble sugars for all, possibly indicating changes in carbon allocation. Three emergent species presented lower flavonol and anthocyanin contents under increased temperatures, suggesting lower investment in defense mechanisms and mitigation of the stress generated by autumn temperatures. Finally, the 3 °C warming extended and shortened flowering period for L. hexapetala and Myosotis scorpioides, respectively. The changes generated by increased temperature in outdoor conditions were heterogenous and varied depending on species but not on species biogeographic origin or growth form. Results suggest that climate warming could favor the invasiveness of L. hexapetala and impact the structure and composition of aquatic plants communities

Using optical index matching to visualize solute transport and perform particle tracking in a synthetic porous medium

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Visualization of solute transport and particle tracking in a three-dimensional porous medium, using optical index matching.

International audienceThe experimental study of preferential flow paths in real soil porous systems usually requires elaborate and expensive techniques such as X-ray tomography or MRI. When one aims at studying a particular feature of transport in such systems, one can utilize a laboratory experiment in which the real porous medium is replaced by a model porous system; in this case, it is sometimes possible, even for a three-dimensional medium, to rely on optical visualization techniques. We present here an experimental setup addressing the visualization of solute transport and particle advection in a synthetic porous medium consisting of glass beads. At fluid preparation, we obtain optical index matching between the glass beads and the fluid by mixing two miscible fluids whose refraction indices are respectively smaller and larger than that of the beads. The right proportions for the two fluids are found when the transmission of a laser Beam through a box containing a sample porous medium is observed to be maximal. The matched fluid allows monitoring of solute dispersion inside the porous medium. The influence of the index mismatch on the uncertainty of the measurements has been estimated using available theories[1]. Successful attempts at tracking colloidal particles advected inside the porous medium are presented. The simple tracking algorithm requires that particles not too close to each other be monitored at a large enough acquisition frequency

Male tarantula spiders' reactions to light and odours reveal their motor asymmetry

International audienceMany vertebrates present behavioural asymmetries (i.e. functional and/or structural specializations of left and right sides of the brain); however, evidence for arthropods is scarce. Some behavioural asymmetries can be correlated with morphology. A better understanding of behavioural asymmetries would be a crucial step to understand the evolution of brain asymmetries. Here, we investigated behavioural asymmetries of adult males of a mygalomorph Brachypelma albopilosum. First, as the time budgets of these active hunting spiders had not yet been documented, we established the temporal distribution of males’ motor activity to be able to test them when they were active. Their motor activity peaks during the night and again early morning (around 3 h and 10 h local time). Then, choice tests in a T-maze assessed the effects of light and of odours separately. Our results revealed that male tarantulas’ activity increased when they perceived light or the odour of conspecific females. Latencies to enter into the T-maze were shorter when a light cue was present and even shorter when odour cues (of prey or of conspecifics) were present. Choice between two identical cues (light or female odours) in a T-maze revealed, for the first time, their right behavioural laterality. Surprisingly, no significant external morphological differences could be evidenced between left and right eyes, lengths of the first legs or densities of mechanoreceptors and chemoreceptors on the tarsi of males’ first legs to suggest perceptual asymmetry supporting this behavioural asymmetry. This is the first report concerning tarantulas’ behavioural laterality

Assesment of preferential flow path connectivity and hydraulic properties at single-borehole and cross-borehole scales in a fractured aquifer

International audiencePreferential flow path connectivity is generally cited to explain scaling effects in hydraulic properties [Hsieh, P.A., 1998. Scale effects in fluid flow through fractured geological media, Scale dependence and scale invariance in hydrology. Cambridge University Press, pp. 335­353; Illman, W.A., in press. Strong evidence of directional permeability scale effect in fractured rock. Journal of Hydrology]. However, this information is rarely available in the field. In this study, we present a characterization of flow paths connectivity at the Ploemeur fractured crystalline aquifer from cross-borehole flowmeter tests. We show that high transmissivity zones are connected over distances of at least 150 m all over the site. In parallel, we synthesize hydraulic properties estimates obtained at this site from field techniques having distinct scales of investigation: single borehole flowmeter experiments, cross borehole flowmeter experiments and long term pumping tests. We find that borehole scale variability of transmissivity estimates vanishes at larger scale and that the transmissivity converges towards the high values of the transmissivity distribution. This effect may be explained by the organization of the flow field in the subsurface, and particularly the good connectivity of the permeable zones all over the site

Cross-Borehole flowmeter tests for transient heads in heterogeneous aquifers

International audienceCross-borehole flowmeter tests have been proposed as an efficient method to investigate preferential flowpaths in heterogeneous aquifers, which is a major task in the characterization of fractured aquifers. Cross-borehole flowmeter tests are based on the idea that changing the pumping conditions in a given aquifer will modify the hydraulic head distribution in large-scale flowpaths, producing measurable changes in the vertical flow profiles in observation boreholes. However, inversion of flow measurements to derive flowpath geometry and connectivity and to characterize their hydraulic properties is still a subject of research. In this study, we propose a framework for cross-borehole flowmeter test interpretation that is based on a two-scale conceptual model: discrete fractures at the borehole scale and zones of interconnected fractures at the aquifer scale. We propose that the two problems may be solved independently. The first inverse problem consists of estimating the hydraulic head variations that drive the transient borehole flow observed in the cross-borehole flowmeter experiments. The second inverse problem is related to estimating the geometry and hydraulic properties of large-scale flowpaths in the region between pumping and observation wells that are compatible with the head variations deduced from the first problem. To solve the borehole-scale problem, we treat the transient flow data as a series of quasi–steady flow conditions and solve for the hydraulic head changes in individual fractures required to produce these data. The consistency of the method is verified using field experiments performed in a fractured-rock aquife

3D tracking of animals in the field, using rotational stereo videography

International audienceWe describe a method for tracking the path of animals in the field, based on stereo videography and aiming-angle measurements, combined in a single, rotational device. In open environments, this technique has the potential to extract multiple 3D positions per second, with a spatial uncertainty of <1 m (rms) within 300 m of the observer, and <0.1 m (rms) within 100 m of the observer, in all directions. The tracking device is transportable and operated by a single observer, and does not involve any animal tagging. As a video of the moving animal is recorded, track data can easily be completed with behavioural data. We present a prototype device based on accessible components that achieves about 70% of the theoretical maximal range. We show examples of bird ground and flight tracks, and discuss the strengths and limits of the method, compared with existing fine-scale (e.g. fixed-camera stereo videography) and large-scale tracking methods (e.g. GPS tracking)