Formation des crues «cévenoles» dans des bassins élémentaires du Mont Lozère

Les théories classiques concernant les processus générateurs des écoulements rapides permettent mal d'expliquer la soudaineté de la montée de crue dans des petits bassins versants élémentaires, situés sur le versant sud du Mont Lozère (sud du Massif Central, France).Des études portant sur les conditions de genèse de ces crues « cévenoles » ont montré les conditions particulières d'apparition de tels événements: dans ces milieux de moyenne montagne granitique méditerranéenne, les formations superficielles sont peu épaisses et très filtrantes et les abats d'eau considérables; les surfaces saturées, seules capables de générer un ruissellement important, progressent, contrairement à ce qui s'observe habituellement, d'amont en aval des versants; ce type de fonctionnement empêche dans un premier temps toute arrivée d'eau jusqu'au drain principal et ce n'est que lorsque, de proche en proche, l'ensemble du bassin est quasi saturé que la crue se déclenche, entraînant immédiatement des débits importants.Ce modèle de fonctionnement hydrologique se vérifie bien lors de la crue du 22 septembre 1992, responsable de nombreux dégâts et de pertes en vies humaines dans le sud de la France.The southern and western edges of the French « Massif Central » are frequently subject to very high floods, known as « crues cevenoles ». They mainly occur during Autumn, and they are characterized by particularly sudden and high flows. Genesis conditions for these types of floods have been studied in a small experimental basin located on Mont Lozère.The Latte River basin (20 ha) is located between 1200 and 1400 m above sea level. Bedrock is granite, covered with sandy, weakly weathered soil, ranging in depth from 0 to 1 m, generally thinner on the upper part of the slopes. Infiltration capacity is very high, generally over 70 mm .h-1 (and locally more than 135 mm • h-1), as indicate by rainfall simulation (COSANDEY et al., 1990). The basin was previously covered with a 70 year old spruce forest but the forest was cut down between 1987 and 1990, without evident effects on flood genesis conditions (COSANDEY and BERNARD-ALLEE, 1992; COSANDEY, 1993).Rainfall is recorded by a network of three pluviographs. Facing the rainy winds coming from the Mediterranean sea, the basin receives about 1900 mrn of annual rainfall; large storm events, particularly in Autumn, can produce more than 400 mm of rain (e.g., in November 1986) with very high intensities (about 60 to 90 mm . h-1 during half an hour one time every year; an intensity of 170 mm . h-1 was observed during half an hour during an exceptional rainfall event of 22 September 1992). A V-notch weir was built at the outlet of the catchment, and the discharge measured using a hydrograph. During « cévenoles floods » discharge can reach 2 m3 • s-1 • km-2.Initial observations showed that the rising of the flood waters is always sudden and rapid, suggesting the existence of a threshold above which the flood can occur. Previous studies have shown that this threshold is not simply related tothe amount of rain. For example' in November 1986 discharge remained very slow, after more than 110 mm rain (fig.3). In another case, in October 1987, 36 mm was sufficient to initiate flooding (COSANDEY, 1993a). In fact this threshold depends on the total water storage within the basin; it can be calculated as the sum of potential maximum values of both the soil and ground water content and is equivalent to 270 mm (COSANDEY and DIDON-LESCOT, 1990). Flood generation processes include the following:The high infiltration capacity of soils does not allow the formation of Hortonian runoff, despite high intensities of rainfall. Rapid flow can occur only in conformity with the « saturated contributing areas » theory (DUNNE and BLACK, 1970). But here saturaton begins on the upper part of the slopes where the soils are thinner; runoff which occurs does not reach the stream because it can infiltrate downslope, where the soils are thicker. Saturated surfaces progressively extend downslope; from the moment where the saturated zones join the stream, runoff can contribute to rapid flow. At this moment, flood waters arise suddenly. Figure 4 shows the different steps of the extension of the saturated areas within the basin, and the model of streamflow generation:- When it does not rain, the water table is drained by the stream.- A small amount of rain can be sufficient to make the water table arise in the bottom of the valley; this saturated area becomes a source of rapid flooding, but the volumes remain very small.- If the rain continues, new saturated areas occur where the soil is thinner, mainly upslope. But runoff generated on these areas infiltrates downslope; this runoff may cause a rise in water level and an extension of saturated surfaces, but it cannot join directly to the stream, and it does not contribute to rapid flow. Discharge remains very low.- If the rain persists, saturated areas extend downslope, and finally join the saturated areas in the bottom of the valley. From this moment infiltration is impossible, and runoff generated by the whole slope contributes to the rapid flow directly or as piston-flow. The flood starts to form and increases rapidly; discharge becomes very high.This model allows us to understand why, when floods begin, they are immediately important if rainfall continues.This hydrological pattern has produced about 10 « crues cévenoles » during the studied period (1981-1992), with flood peaks around 300-500 l• s-1 (1300 to 1500 l • s-1 • km2). During the particularly high intensity event of 22 September l992, the peak flood reached 1500l • s-1 (7.5 m3 • s-1 • km2). The question is to assess if hydrological processes able to produce such a discharge are the same as those described here, and if the hypothesis of a threshold can be well verified.When the rain tregan on 2l September at 6 pm, the basin was under very dry conditions. The second part of August and September had been dry. Soil water content was quite low (about 9 mm) and the base flow discharge, 5 l • s-1, indicates, according to Maillet'law (ROCHE, 1967) a groundwater storage of about 5 mm (COSANDEY. 1993a): 256 mm of rain would be needed to reach the threshold of 270 mm. From Monday, 6 pm to Tuesday, 4:20 am the amount of rain was about 140 mm with medium intensities. Discharge remained very low (fig. 5; COSANDEY 1993b). Total soil water content within the basin was about 162 mm. Intensities then became very important, with 65 mm rain in half an hour. A flood occurred, with only a small peak (170 l • s-1) despite the high value of rainfall intensities. The water storage was about 227 mm, and the threshold had not been reached. The discharge decreased from the moment that rainfall intensities became lower.About 40 mm of rain were needed to reach the threshold, and this was exceeded at 5:52 am. A rainfall event with similar characteristics (65 mm in half an hour) then produced a peak flood estimated at 1500 l • s-1 • km2, seven times higher than previously reported. Rainfall stopped quickly, and the flow decrease was rapid. Nevertheless a small rainfall event of 10 mm over 15 min was sufficient to produce a peak of 334 l • s-1, This particularly high rainfall event demonstrated that, despite very high intensities, no important flood can occur when the basin is not completely saturated and the total water content is less than 270 mm.It is important to note that this hydrological pattern concerns a very small (20 ha) first-order basin. Flood genesis conditions are very different downstream, mainly due to the combination of tributary floods

Influence de la forêt sur le cycle de l'eau : conséquences d'une coupe forestière sur le bilan d'écoulement annuel

Des études menées dans deux petits bassins-versants expérimentaux présentant des couverts végétaux différents (pelouse paturée et forêt d'épicéas) cherchent à préciser le rôle de la forêt sur le déficit d'écoulement dans cette région de moyenne montagne méditerranéenne recevant des pluies abondantes. Après 4 années de mesures, les données sont modifiées par la coupe à blanc de la forêt. Les travaux durent trois étés. La méthode des bilans hydrologiques, qui est utilisée, ne permet pas de proposer avec certitude une valeur pour la différence d'évaporation entre les deux types de couvert végétal. Mais en contrepartie, il semble qu'on puisse conclure avec une certaine sécurité que la coupe à blanc de la forêt augmente l'écoulement annuel d'environ 130/150 mm. Cette valeur proposée de 130/150 mm représente l'effet du déboisement, qui inclut les perturbations du milieu dues aux possages des engins forestiers. Elle n'est évidemment pas assimilable à la différence d'évaporation entre la forêt et un autre type de couvert végétal. L'intérêt et les limites de l'outil bassin-versant pour ce type d'étude est également discuté. (Résumé d'auteur

Etude expérimentale du ruissellement sur des sols à végétation contrastée du Mont Lozère

Les conditions d'apparition du ruissellement de surface, et les valeurs limites que peut atteindre l'infiltration sont étudiés expérimentalement par simulation de pluie sur des sols à végétation contrastée du Mont Lozère (Sud du Massif Central, France), développés sur arène granitique peu profonde. Les résultats mettent en évidence la forte capacité d'infiltration du milieu naturel, mais aussi la grande variabilité spatiale de cette capacité d'infiltration, pour aboutir à un schéma de fonctionnement dans lequel, si la circulation hypodermique, à la limite de la roche saine, demeure le processus essentiel de transfert de l'eau entre son point de chute et le cours d'eau, le ruissellement sur les versants n'est pas exclu et peut jouer un rôle important dans la formation des crues. Par ailleurs il semblerait qu'une pelouse en bon état, couvrant parfaitement le sol, présente une aussi bonne protection contre le ruissellement et l'érosion que les litières forestières testées. (Résumé d'auteur

The potential of three whole blood microRNAs to predict outcome and monitor treatment response in sarcoid-bearing equids.

MicroRNAs (miRNAs) have been proposed as biomarkers for equine sarcoid (ES) disease. In this study, the suitability of three whole blood miRNAs to diagnose ES and to predict and monitor the outcome of therapy was explored. Using reverse transcription-quantitative polymerase chain reaction (RT-qPCR), expression levels of eca-miR-127, eca-miR-379, and eca-miR-432 in whole blood of ES-affected equids before and at least one year after therapy were compared to those of unaffected control equids. Associations of age, sex, species, diagnosis, and therapy outcome with miRNA expression levels were examined using general linear models. In total, 48 ES-affected equids and 47 control equids were recruited. From the affected animals, 31 responded favorably to treatment, and 17 demonstrated a failure of therapy. None of the tested miRNAs were influenced by age. Male equids showed increased expression of eca-miR-127 compared to females and horses showed higher expression levels of eca-miR-379 and eca-miR-432 than donkeys. Eca-miR-127 was confirmed as a diagnostic discriminator between ES-affected and control equids. No difference in miRNA profiles before therapy was found when comparing ES-affected equids with success vs. failure of therapy. Eca-miR-379 and eca-miR-432 decreased over time in horses where therapy was successful, but not in those cases where it failed. Biological variables influence equine whole blood miRNA expression, which may complicate biomarker validation. While none of the tested miRNAs could predict the response to therapy in ES-affected equids and eca-miR-127 showed poor diagnostic accuracy for ES, eca-miR-379 and eca-miR-432 miRNAs might allow refinement of monitoring of success of ES therapy

Ehrenfest's Principle and the Problem of Time in Quantum Gravity

We elaborate on a proposal made by Greensite and others to solve the problem of time in quantum gravity. The proposal states that a viable concept of time and a sensible inner product can be found from the demand for the Ehrenfest equations to hold in quantum gravity. We derive and discuss in detail exact consistency conditions from both Ehrenfest equations as well as from the semiclassical approximation. We also discuss consistency conditions arising from the full field theory. We find that only a very restricted class of solutions to the Wheeler-DeWitt equation fulfills all consistency conditions. We conclude that therefore this proposal must either be abandoned as a means to solve the problem of time or, alternatively, be used as an additional boundary condition to select physical solutions from the Wheeler-DeWitt equation.Comment: 20 pages, LATE

Conséquences de la forêt méditerranéenne sur les écoulements des crues.

Trois ensembles de bassins expérimentaux permettent d'étudier, en France méditerranéenne, les conséquences de la forêt sur les crues. Les résultats différent de façon considérable d'un bassin à l'autre, montrant la complexité des relations pluies-débits. Il est à noter que l'augmentation des crues extrêmes est loin d'être avérée dans tous les cas. Une réflexion sur le fonctionnement hydrologique des bassins permet de lever la contradiction apparente entre les résultats observés

Au/TiO2(110) interfacial reconstruction stability from ab initio

We determine the stability and properties of interfaces of low-index Au surfaces adhered to TiO2(110), using density functional theory energy density calculations. We consider Au(100) and Au(111) epitaxies on rutile TiO2(110) surface, as observed in experiments. For each epitaxy, we consider several different interfaces: Au(111)//TiO2(110) and Au(100)//TiO2(110), with and without bridging oxygen, Au(111) on 1x2 added-row TiO2(110) reconstruction, and Au(111) on a proposed 1x2 TiO reconstruction. The density functional theory energy density method computes the energy changes on each of the atoms while forming the interface, and evaluates the work of adhesion to determine the equilibrium interfacial structure.Comment: 20 pages, 11 figure

Bonding of gold nanoclusters to oxygen vacancies on rutile TiO2(110)

Through an interplay between scanning tunneling microscopy (STM) and density functional theory (DFT) calculations, we show that bridging oxygen vacancies are the active nucleation sites for Au clusters on the rutile TiO2(110) surface. We find that a direct correlation exists between a decrease in density of vacancies and the amount of Au deposited. From the DFT calculations we find that the oxygen vacancy is indeed the strongest Au binding site. We show both experimentally and theoretically that a single oxygen vacancy can bind 3 Au atoms on average. In view of the presented results, a new growth model for the TiO2(110) system involving vacancy-cluster complex diffusion is presented

Measuring Market Liquidity Risk - Which Model Works Best?

Market liquidity risk, the difficulty or cost of trading assets in crises, has been recognized as an important factor in risk management. Literature has already proposed several models to include liquidity risk in the standard Value-at-Risk framework. While theoretical comparisons between those models have been conducted, their empirical performance has never been benchmarked. This paper performs comparative back-tests of daily risk forecasts for a large selection of traceable liquidity risk models. In a 5.5 year stock sample we show which model provides most accurate results and provide detailed recommendations which model is most suitable in a specific situation

Global Diversity Hotspots and Conservation Priorities for Sharks

Sharks are one of the most threatened groups of marine animals, as high exploitation rates coupled with low resilience to fishing pressure have resulted in population declines worldwide. Designing conservation strategies for this group depends on basic knowledge of the geographic distribution and diversity of known species. So far, this information has been fragmented and incomplete. Here, we have synthesized the first global shark diversity pattern from a new database of published sources, including all 507 species described at present, and have identified hotspots of shark species richness, functional diversity and endemicity from these data. We have evaluated the congruence of these diversity measures and demonstrate their potential use in setting priority areas for shark conservation. Our results show that shark diversity across all species peaks on the continental shelves and at mid-latitudes (30–40 degrees N and S). Global hotspots of species richness, functional diversity and endemicity were found off Japan, Taiwan, the East and West coasts of Australia, Southeast Africa, Southeast Brazil and Southeast USA. Moreover, some areas with low to moderate species richness such as Southern Australia, Angola, North Chile and Western Continental Europe stood out as places of high functional diversity. Finally, species affected by shark finning showed different patterns of diversity, with peaks closer to the Equator and a more oceanic distribution overall. Our results show that the global pattern of shark diversity is uniquely different from land, and other well-studied marine taxa, and may provide guidance for spatial approaches to shark conservation. However, similar to terrestrial ecosystems, protected areas based on hotspots of diversity and endemism alone would provide insufficient means for safeguarding the diverse functional roles that sharks play in marine ecosystems