Gestion stratégique d'un système de ressources en eau : l'exemple du système NESTE

L'article que nous proposons s'inscrit dans le cadre des problèmes d'optimisation bimensionnelle (irrigation & salubrité) des ressources en eau durant la période d'étiage. Sur le cas du système NESTE, la résolution est effectuée selon deux approches :- un modèle de programmation dynamique avec état de dimension deux (niveau des réserves, niveau dans la rivière) où, dans la solution numérique, les variables sont discrétisées;- un modèle « synthétique » où l'on calcule une probabilité de non dépassement caractérisant l'état hydrique des ressources du système. Une règle empirique permet d'associer à cette grandeur une décision de consigne à effectuer.Les résultats numériques sont comparés sur une série de chroniques historiques. Les avantages et les inconvénients de chacune des deux approches sont mis en lumière sur le cas réel du système NESTE.This paper deals with bicreteria (irrigation & water quality) weekly operation of a water resource system during dry period. Two ways of handling the problem are assessed and compared on a real case study :- a stochastic dynamic programming modal with a two dimensional state (reservoirs level, river level) that is numerically solved by discretization ;- a more « synthetic » model where the state is expressed in term of a tail aera probability related to the consumption of all the present water resources in the future. A practical decision rule is based upon the associated critical value.Numerical results are plotted on historical varies for both methods.From the present application to the NESTE system, the conclusions are :1) Both procedures allow the system manager to formulate operating strategies in a rational way :- An operating rule can be derived to allocate water so as to meet a combination of the various objectives. It is expressed as a feedback law linking what we know from the state of the system to how we control its evolution.- Both methods need a parameter to be set up by stochastic simulation.- They give close results on the basis of the past data and can be conveniently proposed to system managers.2) The system analysis approach is based on stochastic dynamic programming. If can be efficiently used to derive optimal feedback ruses of operation and can routinely deal with complex decisions such as limiting irrigation when a shortage is to occur or take the risk to keep going and decrease output targets for water quality management. At the same time, this procedure entails heavy computing time, uneasy interpretation of the weighting coefficient between irrigation and water quality objectives, and a rather artificial elicitation of the global compromise.Such an approach is very well fit for simulation because it is composed of elementary blocks that are gathered in a transition relationship to describe the system's dynamic evolution. This approach also provides a means to get an optimal policy as long as the system manager accepts the necessity to formulate an objective function consistently with dynamic programming (i. e. stages are separable and additive). Of course this optimal allocation should be carefully examined because of modal uncertainties influencing both the system response and the hydrological behaviour.3) The synthetic method may appear more attractive from the engineering point of view for the following reasons :- the state is easily interpreted in terms of « dry year with a return period of 10, normal year, exceptionally wet year » and so on. The trade-off coefficient is the volume one wants to keep in the reservoirs at time T for a wet year. Consequently if the parameter is chosen with « good sense », no optimization scheme is needed.- there is no computation except a mass balance equation and a normal probability law adjustment which is very easy because it deals with cumulative quantities.4) Such models are designed to serve only as multicriterion decision making aids. In very dry days such as occurred in summer 1976 or 1989 in France they cannot create additional water resources... still, they can help the system manager by constant up dated multidimensional estimation of the risks that may be encountered when following different operation rotes. In the case of the NESTE system, a real-scale experiment began in 1989: in real time operation, both models worked on line as decision-making supports, and the system manager made a thorough study of the hydrological conditions when the two approaches did not agree on the same stragegy for the following week

Effect of disorder on superconductivity and Rashba spin-orbit coupling in LaAlO3/SrTiO3 interfaces

A rather unique feature of the two-dimensional electron gas formed at the interface between the two insulators LaAlO3 and SrTiO3 is to host both gate-tunable superconductivity and strong spin-orbit coupling. In the present work, we use the disorder generated by Cr substitution of Al atoms in LaAlO3 as a tool to explore the nature of superconductivity and spin-orbit coupling in these interfaces. We analyze the transport properties of three different samples whose only relevant difference is their elastic scattering time tau(e). A reduction of the superconducting T-c is observed with Cr doping consistent with an increase of electron-electron interaction in the presence of disorder. In addition, the evolution of spin-orbit coupling with gate voltage and Cr doping evidences a Dyakonov-Perel mechanism of spin relaxation (tau(SO) proportional to tau(-1)(e)) in the presence of a Rashba-type spin-orbit interaction

Gate-tunable giant nonreciprocal charge transport in noncentrosymmetric oxide interfaces

A polar conductor, where inversion symmetry is broken, may exhibit directional propagation of itinerant electrons, i.e., the rightward and leftward currents differ from each other, when time-reversal symmetry is also broken. This potential rectification effect was shown to be very weak due to the fact that the kinetic energy is much higher than the energies associated with symmetry breaking, producing weak perturbations. Here we demonstrate the appearance of giant nonreciprocal charge transport in the conductive oxide interface, LaAlO3/SrTiO3, where the electrons are confined to two-dimensions with low Fermi energy. In addition, the Rashba spin???orbit interaction correlated with the sub-band hierarchy of this system enables a strongly tunable nonreciprocal response by applying a gate voltage. The observed behavior of directional response in LaAlO3/SrTiO3 is associated with comparable energy scales among kinetic energy, spin???orbit interaction, and magnetic field, which inspires a promising route to enhance nonreciprocal response and its functionalities in spin orbitronics

4. - Gestion des eaux du système Neste

For over thirty years now, CACG has been in charge of the water management of a complex hydrographic system, the "Systeme Neste". Thanks to this system, it is possible to divert a part of the Neste natural flow for the benefit of the Gascony rivers which are completely dry in the summer and often in the autumn. Despite this natural disadvantage, Gascony rivers provide irrigation water to 40 000 hectares drinking water to 200 000 people and keep a satisfactory level of quality down to the Garonne confluence : this was made possible not only through the increase of water ressources by building reservoirs on these rivers but also thanks to constant efforts to make the best possible use of the existing water ressources, thus leading, year after year, to the development of specific management techniques with the main following characteristics : -Perfect knowledge on needs, both in terms of quantity and quality . -Régular concertation with the main water users to define, at the beginning of each summer, the volumes allowed to be diverted. -Weekly adjustment of the satisfaction degree to the level of reservoirs, needs and flow forecasts (strategic management). -Automatic real-time modulation of released dam water (tactical management).Depuis maintenant plus de trente ans, la C.A.C.G. est chargée de la gestion des eaux d’un système hydrographique complexe, le " Système Neste", permettant de dériver une partie des eaux de la Neste (elle-même réalimentée par des barrages de haute-montagne) au profit de la Gascogne, totalement dépourvue de ressources propres en période estivale. Malgré ce handicap naturel, les rivières de Gascogne ainsi réalimentées permettent aujourd’hui l’irrigation par aspersion d’environ 40 000 ha, l’alimentation en eau potable de près de 200 000 habitants et conservent une qualité satisfaisante jusqu’à leur confluence avec la Garonne : pour en arriver là, il a bien sûr fallu augmenter les ressources (par la création de barrages sur ces rivières), mais également faire un effort constant pour utiliser au mieux les ressources disponibles ce qui a permis, au fil des années, de développer un système de gestion original, reposant sur 4 points : -Bonne connaissance des besoins tant quantitatifs que qualitatifs. -Concertation régulière avec les usagers afin de définir avant chaque été les volumes d’eau prélevables (en fonction des nouvelles ressources mises en service, des prévisions sur les ressources naturelles ...). -Modulation hebdomadaire du degré de satisfaction des besoins en fonction de l’état des réserves et des ressources naturelles (gestion stratégique). -Mise en oeuvre de dispositif automatique de régulation des lâchures de barrage afin de respecter les consignes définies par la gestion hebdomadaire (gestion tactique).Hurand P. 4. - Gestion des eaux du système Neste. In: L'avenir de l'eau. Quelques réponses des sciences hydrotechniques à une inquiétude mondiale. Vingt deuxièmes journées de l'hydraulique. Paris, 15-17 septembre 1992. Tome 3, 1992

Automatic control of river flows : complex architectures with intermediate flow measurements and controller robustness facing varying time-delays

Natural rivers can be used for irrigation water transportation and distribution, respecting a minimal flow constraint downstream the river for environmental purposes. These "Dam-river" systems are SISO distributed and delayed systems, submitted to predicted and unpredicted disturbances. Long time-delays varying with the hydraulic flow regime imply the use of specify control techniques (internal model, pole placement, Smith predictor) with special attention to their robustness. To increase the responsiveness of the regulation managers often use intermediate flow measurement points which allow to treat the system as a serial of SISO systems. This requires the building up of specific control architectures in order to ensure precision , stability and robustness. This paper details the study of complex architectures and robustness facing varying time-delays through the analytical development and study of the global transfert functions of the regulated systems and analysis of their poles migrations. The lead to highly improved automatic control techniques for the regulation of river flows. / Cet article étudie le problème de la commande automatique des débits en rivière, en soutien d'étiage et/ou en réalimentation de rivières en vue d'irrigations. Des architectures complexes avec des points de mesure intermédiaires sont étudiées. La robustesse du contrôleur est testée face aux temps de retard variables apparaissant lorsque les débits transités varient

Effect of disorder on superconductivity and Rashba spin-orbit coupling in LaAlO3/SrTiO3 interfaces

A rather unique feature of the two-dimensional electron gas formed at the interface between the two insulators LaAlO3 and SrTiO3 is to host both gate-tunable superconductivity and strong spin-orbit coupling. In the present work, we use the disorder generated by Cr substitution of Al atoms in LaAlO3 as a tool to explore the nature of superconductivity and spin-orbit coupling in these interfaces. We analyze the transport properties of three different samples whose only relevant difference is their elastic scattering time tau(e). A reduction of the superconducting T-c is observed with Cr doping consistent with an increase of electron-electron interaction in the presence of disorder. In addition, the evolution of spin-orbit coupling with gate voltage and Cr doping evidences a Dyakonov-Perel mechanism of spin relaxation (tau(SO) proportional to tau(-1)(e)) in the presence of a Rashba-type spin-orbit interaction

A new etching environment (FeF 3 /HCl) for the synthesis of two-dimensional titanium carbide MXenes: a route towards selective reactivity vs. water

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