How well do meteorological indicators represent agricultural and forest drought across Europe?

Drought monitoring and early warning (M&EW) systems are an important component of agriculture/silviculture drought risk assessment. Many operational information systems rely mostly on meteorological indicators, a few incorporate vegetation state information. However, the relationships between meteorological drought indicators and agricultural/silvicultural drought impacts vary across Europe. The details of this variability have not been elucidated sufficiently on a continental scale in Europe to inform drought risk management at administrative scales. The objective of this study is to fill this gap and evaluate how useful the variety of meteorological indicators are to assess agricultural/silvicultural drought across Europe. The first part of the analysis systematically linked meteorological drought indicators to remote sensing based vegetation indices (VIs) for Europe at NUTs3 administrative regions scale using correlation analysis for crops and forests. In a second step, a stepwise multiple linear regression model was deployed to identify variables explaining the spatial differences observed. Finally, corn crop yield in Germany was chosen as a case study to verify VIs representativeness of agricultural drought impacts. Results show that short accumulation periods of SPI and SPEI are best linked to crop vegetation stress in most cases, which further validates the use of SPI3 in existing operational drought monitors. However, large regional differences in correlations are also revealed. Climate (temperature and precipitation) explained the largest proportion of variance, suggesting that meteorological indices are less informative of agricultural/silvicultural drought in colder/wetter parts of Europe. These findings provide important context for interpreting meteorological indices on widely used national to continental M&EW systems, leading to a better understanding of where/when such M&EW tools can be indicative of likely agricultural stress and impacts

Particle displacements in the elastic deformation of amorphous materials: local fluctuations vs. non-affine field

We study the local disorder in the deformation of amorphous materials by decomposing the particle displacements into a continuous, inhomogeneous field and the corresponding fluctuations. We compare these fields to the commonly used non-affine displacements in an elastically deformed 2D Lennard-Jones glass. Unlike the non-affine field, the fluctuations are very localized, and exhibit a much smaller (and system size independent) correlation length, on the order of a particle diameter, supporting the applicability of the notion of local "defects" to such materials. We propose a scalar "noise" field to characterize the fluctuations, as an additional field for extended continuum models, e.g., to describe the localized irreversible events observed during plastic deformation.Comment: Minor corrections to match the published versio

Estimation des niveaux d'inondation pour une crue éclair en milieu urbain : comparaison de deux modèles hydrodynamiques sur la crue de Nîmes d'octobre 1988

Lors des crues extrêmes en ville, une forte part des écoulements reste en surface. Pour simuler ces inondations, deux modèles sont présentés : le logiciel REM2 U unidimensionnel a pour objectif de simuler la propagation des débits de crue dans l'ensemble d'un réseau de rues alors que le logiciel Rubar 20 bidimensionnel vise à fournir plus d'information sur ces écoulements. Des calculs avec ces deux logiciels ont été menés sur la crue d'octobre 1988 dans un quartier de Nîmes. Lors de cet événement, les hauteurs d'eau maximales ont dépassé deux mètres en certains points et les vitesses 2 m/s ce qui entraînait des passages en régime torrentiel. A partir des données rassemblées sur les sections en travers des rues, des maillages de calcul limités au réseau de rues ont été construits pour les deux logiciels afin de permettre un calcul détaillé. La comparaison des résultats avec les laisses de crue montre des situations très contrastées d'un point à un autre pour une hauteur d'eau maximale moyenne sur l'ensemble de la zone inondée correctement simulée. L'écart sur cette hauteur est, en moyenne, de 1 m ce qui provient des incertitudes sur les observations, sur la topographie et sur les conditions aux limites, des approximations lors de la modélisation et de particularités locales non décrites. Entre les deux logiciels, l'évolution des hauteurs et des vitesses est généralement très proche bien que, comme pour la comparaison avec les laisses de crue, des différences locales importantes sont observées.The hydraulic models that are used to simulate floods in rural areas are not adapted to model floods through urban areas, because of details that may deviate flows and create strong discontinuities in the water levels, and because of the possible water flow running in the sewage network. However, such modelling is strongly required because damage is often concentrated in urban areas. Thus, it is necessary to develop models specifically dedicated to such floods. In the southern part of France, rains may have a high intensity but floods generally last a few hours. During extreme events such as the October 1988 flood in the city of Nîmes, most of the flow remained on the ground with high water depths and high velocities, and the role of sewage network can be neglected. A 1-D model and a 2-D model were used to calculate such flows, which may become supercritical. On the catchments of the streams which cross the city of Nîmes, the rainfall was estimated as 80 mm in one hour and 250 mm in six hours in October 1988, although some uncertainties remain. The return period can be estimated between 150 and 250 years. The zone selected to test the models was an area 1.2 km long and less than 1 km wide in the north-eastern part of the city. It includes a southern part with a high density of houses. The slope from the North (upstream) to the South (downstream) was more than 1 % on average and was decreasing from North to South. Various topographical and hydrological data were obtained from the local Authorities. The basic data were composed of 258 cross sections of 69 streets with 11 to 19 points for each cross section. Observations of the limits of the flooded areas and of the peak water levels at more than 80 points can be used to validate the calculation results. The inputs consisted of two discharge hydrographs, estimated from a rainfall-discharge model from rains with a return period of 100 years, which may result in an underestimate of these inputs. These two hydrographs correspond to the two main structures that cross the railway embankment, which constitutes an impervious upstream boundary of the modelled area. Whereas the western and eastern boundaries are well delimitated by hills above maximum water levels, the downstream southern boundary is somewhat more questionable because of possibilities of backwater and inflows from neighbouring areas.The 1-D software REM2U solved the Saint Venant equations on a meshed network. At crossroads, continuities of discharge and of water heads were set. The hydraulic jump was modelled by a numerical diffusion applied wherever high water levels were found. The Lax Wendroff numerical scheme was implemented. It included a prediction step and a correction step, which implied precise solving of these very unsteady and hyperbolic problems. The software was validated on numerous test cases (Al Mikdad, 2000) which proved the adaptation to problems of calculations in a network of streets.The 2-D software Rubar 20 solves 2-D shallow water equations by an explicit second-order Van Leer type finite volume scheme on a computational grid made from triangles and quadrilaterals (Paquier, 1998). The discontinuities (hydraulic jumps for instance) are treated as ordinary points through the solving of Riemann problems. For the Nîmes case, the grid was built from the cross sections of the streets. Four grids were built with respectively 4, 5, 7 or 11 points for every cross section and these points correspond to the main characteristics of the cross section: the walls of the buildings, the sidewalks, the gutters and the middle point. The simplest crossroads were described from the crossings of the lines corresponding to these points, which provide respectively 16, 25, 49 or 121 computational cells. The space step was about 25 metres along the streets but went as low as 0.1 m in the crossroads; due to the explicit scheme, which implies that the Courant number was limited to 1, the time step was very small and a long computational time was required.The computations were performed with a uniform Strickler coefficient of 40 m1/3/s. Both 1-D and 2-D models provided results that agreed well with observed water levels. The limits of the flooded area were also quite well simulated. However, locally, the differences between calculated and observed maximum water depths were high, resulting in an average deviation of about 1 metre. The reasons for such deviations could come from three main causes. First, the uncertainty of topographical data is relatively high, because of the interpolation between measured cross sections without a detailed complementary DEM (digital elevation model). Second, the observed levels were also uncertain and reveal local situations that are not reconstructed by the hydraulic models which provided maximum water levels averaged on one cell which may not coincide with the exact location of the observations. Finally, modelling means a simplification of the processes, which implies cancelling the level variations due to some obstacles, such as cars, which are not simple to identify.In conclusion, both software packages can model a flood, even a flash flood, in an urbanised area. Research is still necessary to develop methods to fully use urban databases in order to define details more precisely. The improvements to the 1-D software should include a better modelling of storage and of crossroads with an integration of adapted relations for the head losses. 2-D software has a greater potential but the difficulty to build an optimal computational grid means a long computational time, which limits the use of such software to small areas. For both software packages, methods still need to be developed in order to represent exchanges with the sewage network, storage inside buildings and inputs directly coming from rainfall

Collective Behavior of Asperities in Dry Friction at Small Velocities

We investigate a simple model of dry friction based on extremal dynamics of asperities. At small velocities, correlations develop between the asperities, whose range becomes infinite in the limit of infinitely slow driving, where the system is self-organized critical. This collective phenomenon leads to effective aging of the asperities and results in velocity dependence of the friction force in the form $F\sim 1- \exp(-1/v)$.Comment: 7 pages, 8 figures, revtex, submitted to Phys. Rev.

Inhomogeneous elastic response of silica glass

Using large scale molecular dynamics simulations we investigate the properties of the {\em non-affine} displacement field induced by macroscopic uniaxial deformation of amorphous silica,a strong glass according to Angell's classification. We demonstrate the existence of a length scale $\xi$ characterizing the correlations of this field (corresponding to a volume of about 1000 atoms), and compare its structure to the one observed in a standard fragile model glass. The "Boson-peak'' anomaly of the density of states can be traced back in both cases to elastic inhomogeneities on wavelengths smaller than $\xi$, where classical continuum elasticity becomes simply unapplicable

Realization of a semiconductor-based cavity soliton laser

The realization of a cavity soliton laser using a vertical-cavity surface-emitting semiconductor gain structure coupled to an external cavity with a frequency-selective element is reported. All-optical control of bistable solitonic emission states representing small microlasers is demonstrated by injection of an external beam. The control scheme is phase-insensitive and hence expected to be robust for all-optical processing applications. The motility of these structures is also demonstrated

Hafnium Oxidation at High Temperature in Steam

To assess the potential impact of using hafnium as absorber material in LWRs in high temperature accidental situations, the oxidation behavior of hafnium was studied up to 1400 °C, i.e. at temperature conditions relevant to severe accidents. Different sample geometries were tested and oxidized in steam/argon mixtures, either in a furnace or in a thermogravimetric analyzer. Metallographic examinations, hydrogen measurements and EPMA oxygen profiles were then performed. For hafnium rods/discs, metallographic examinations showed the presence of a dense and protective oxide film after steam oxidation. No or little hydrogen was detected in the metallic part of the rod/disc specimens. The reaction rate can be described by a parabolic law in the tested temperature range in the mid-to-long term, and the value of the effective activation energy determined from the experimental data in steam is in good agreement with the ones published in the literature. The diffusion coefficient of oxygen in hafnium was estimated at each temperature by fitting the experimental oxygen profile obtained on hafnium rods and its temperature dependence is derived in the temperature range 700-1400 °C. The hafnium claddings produced for the application in integral bundle tests exhibited a lower resistance to steam oxidation than hafnium rods/discs. Metallographic examinations showed a non-protective layer and a significant hydrogen amount was picked up by hafnium claddings. Above 800 °C, the oxidation rate for hafnium claddings follows a cubic to quartic law and the effective activation energy was determined in the temperature range 800-1100 °C. These tests highlighted the influence of the surface conditions on the oxidation rate of hafnium in steam. However, hafnium oxidation rate remains well below the oxidation rate of zirconium alloys in the same temperature range

Continuum limit of amorphous elastic bodies: A finite-size study of low frequency harmonic vibrations

The approach of the elastic continuum limit in small amorphous bodies formed by weakly polydisperse Lennard-Jones beads is investigated in a systematic finite-size study. We show that classical continuum elasticity breaks down when the wavelength of the sollicitation is smaller than a characteristic length of approximately 30 molecular sizes. Due to this surprisingly large effect ensembles containing up to N=40,000 particles have been required in two dimensions to yield a convincing match with the classical continuum predictions for the eigenfrequency spectrum of disk-shaped aggregates and periodic bulk systems. The existence of an effective length scale \xi is confirmed by the analysis of the (non-gaussian) noisy part of the low frequency vibrational eigenmodes. Moreover, we relate it to the {\em non-affine} part of the displacement fields under imposed elongation and shear. Similar correlations (vortices) are indeed observed on distances up to \xi~30 particle sizes.Comment: 28 pages, 13 figures, 3 table

Acoustic radiation controls friction: Evidence from a spring-block experiment

Brittle failures of materials and earthquakes generate acoustic/seismic waves which lead to radiation damping feedbacks that should be introduced in the dynamical equations of crack motion. We present direct experimental evidence of the importance of this feedback on the acoustic noise spectrum of well-controlled spring-block sliding experiments performed on a variety of smooth surfaces. The full noise spectrum is quantitatively explained by a simple noisy harmonic oscillator equation with a radiation damping force proportional to the derivative of the acceleration, added to a standard viscous term.Comment: 4 pages including 3 figures. Replaced with version accepted in PR

From Individual to Collective Pinning: Effect of Long-range Elastic Interactions

We study the effect of long-range elastic interactions in the dynamical behavior of an elastic chain driven quasi-statically in a quenched random pinning potential and in the strong pinning limit. This is a generic situation occuring in solid friction, crack propagation, wetting front motion, ... Tuning the exponent of the algebraic decay of the elastic interaction with the distance is shown to give rise to three regimes: a Mean-Field (MF) regime, a Laplacian (L) regime and an intermediate regime where the critical exponents interpolate continuously between the MF and L limit cases. The effect of the driving mode on the avalanche statistics is also analyzed.Comment: 28 pages in RevTex, 17 figure