Note sur la propagation de la crue le long de l'ile à Morfil et sur les inondations 1986, 1988

Ngoui et Salde sont situées à quelques kilomètres à l'aval de la défluence du Doué du Sénégal. Guédé et Podor sont elles situées à quelques kilomètres avant le retour du Doué du Sénégal. Ces stations encadrent l'île à Morfil. Leur étude devrait permettre de préciser le fonctionnement hydraulique des 2 bras, de connaître la différence de niveau entre les stations en parallèle (N'Goui-Salde et Guédé-Podor) et le temps de propagation entre Salde et Podor et entre N'Goui et Guéd

From code-switching to a mixed code: the role of parenthetical verbs in the emerging Polish-Russian mixed code in Lithuania

This paper investigates the role of parenthetical verbs in the emerging Polish-Russian mixed code in Lithuania. Parenthetical verbs share many similarities with discourse markers, since both are regarded as having little or no meaning and occurring outside the syntax of the utterance. The data analysis, based on recordings of spontaneous conversations in the Polish community in Lithuania, aims to test two hypotheses with regard to Russian parenthetical verbs in order to establish whether they should be treated as lexical elements that have been “fused” into the Polish-Russian mixed code, or as instances of ad-hoc switches triggered by identifiable factors. The data reveals that both options are permitted, which in turn suggests that fusion around parenthetical verbs has been “synchronically licensed” (Matras 2000: 525), in the sense that speakers have abolished the requirement to maintain a consistent separation between the Polish and Russian systems. However, the wholesale replacement of one system by another has not taken place

Indice des précipitations antérieures et prédiction des crues au Sahel

Une statistique de l'"Indice des Précipitations Antérieures" (IPA) est menée sur cinq chroniques pluviométriques journalières du Sahel. Sa dépendance vis-à-vis de la hauteur et de la date de la précipitation associée est soulignée. Puis, à travers les données disponibles d'une vingtaine de petits bassins sahéliens, l'auteur dégage une fonction de prédiction multilinéaire de l'écoulement journalier à partir de la lame précipitée journalière et de IPA correspondant. Il est cependant montré le faible rôle de cet indice dans la prédiction. (Résumé d'auteur

Etat des cultures dans la vallée du fleuve Sénégal en 1988, 1989

Description des crues de 1988 et 1989 dans la région du fleuve Sénégal et présentation de l'état des cultures de décrue et des cultures irriguées. Pour les cultures irriguées les taux de surface cultivée / surface aménagée sont données ainsi qu'une présentation des rythmes d'aménagement de ces cultures prévus pour l'horizon 200

Simulation of cell movement through evolving environment: a fictitious domain approach

A numerical method for simulating the movement of unicellular organisms which respond to chemical signals is presented. Cells are modelled as objects of finite size while the extracellular space is described by reaction-diffusion partial differential equations. This modular simulation allows the implementation of different models at the different scales encountered in cell biology and couples them in one single framework. The global computational cost is contained thanks to the use of the fictitious domain method for finite elements, allowing the efficient solve of partial differential equations in moving domains. Finally, a mixed formulation is adopted in order to better monitor the flux of chemicals, specifically at the interface between the cells and the extracellular domain

Construction de la courbe de remplissage d'un réservoir par bilan hydrologique et applications

L'équation de continuité appliquée aux données expérimentales de petits réservoirs ou de dépressions naturelles permet la définition de leurs courbes de remplissage. L'estimation est plus précise que celle déduite des fonds topographiques éventuellement disponibles. Deux exemples d'utilisation (reconstitution de crue, modélisation du remplissage d'une dépression) sont présentés. (Résumé d'auteur

Modélisation du ruissellement en relation avec l'évolution saisonnière de la végétation (mil, arachide, jachère) au centre Sénégal

Sous climat soudanien caractérisé par une unique saison des pluies, les sols sont dénudés en fin de saison sèche suite au pâturage et aux travaux préparatoires au semis. Le ruissellement intense en début d'hivernage diminue progressivement avec la mise en place des couverts végétaux.L'influence du développement de la végétation sur le ruissellement est étudié au moyen des données pluie-débit de 4 parcelles (50 m2) couvertes en mil, arachide, jachère ou maintenue dénudée du centre Sénégal au cours d'une saison des pluies (1994). Un modèle analogique de ruissellement ‡ stockage de surface (BADER, 1994), dans lequel l'infiltration est une fonction croissante de la lame d'eau en surface du sol est ajusté sur les données. Le modèle présente 3 paramètres: un paramètre de transfert n, un paramètre de ruissellement Hl et un paramètre d'infiltration S. Une analyse de sensibilité menée sur les données de la parcelle de sol nu montre que le paramètre n est le plus sensible des trois.Le calage numérique des paramètres sur chaque crue au cours de l'hivernage permet d'étudier leur évolution temporelle. Cette évolution est cohérente avec l'occupation de chaque parcelle. Les paramètres n et S de la parcelle de sol nu sont invariants sur la saison tandis que ceux des parcelles en végétation s'écartent progressivement des valeurs obtenues sur sol nu. Pour les parcelles en végétation, les valeurs de S divergent de celles du sol nu lorsque l'indice radiométrique de végétation (N.D.V.I.) servant à l'estimation du couvert dépasse 0.30 - 0.35 environ. L'évolution des paramètres n et S des parcelles en végétation peut être reliée au temps écoulé depuis le semis (mil, arachide) ou le sarclage initial (jachère) et à l'état d'humectation du sol (pour S). On montre également que le paramètre Hl peut être estimé linéairement à partir d'un indice de rugosité de surface descriptif de la microtopographie.The Sudanese climate is characterized by a rainy season and a dry season (mean annual rainfall between 400 and 900 mm). At the end of the dryseason (June in the northern hemisphere), the landscape is completely bare under the effect of animal grazing or soil tillage. During the first rainfalls this leads to high runoff coefficients. These runoff coefficients decrease gradually as the amount of vegetation increases during the growing season (RODIER (1984-1985); ALBERGEL (1988)).This is particularly true in the Groundnut basin of central Senegal where millet and groundnut are cultivated every other year. As the vegetative cover increases, a system of macropores develops in the soil and preferentially induces water infiltration through mesofauna burrows and along root systems. Hence, many authors have distinguished matrix infiltration governed by the generalized Darcy's law, from preferential infiltration through macropores characterized by a strongly heterogeneous spatial distribution (GERMAN, 1990). These macropores are thought to be responsible for the proportional increase in infiltration with increase in rainfall intensity observed on several experimental plots (BOUCHARDEAU and RODIER, 1960; VALENTIN, 1985; COLLINET,1985; ALBERGEL, 1988). A more complete surface ponding or a differential distribution of the macroporosity in relation with the microtopography can contribute to this phenomenon.A conceptual runoff model accounting for surface storage, which views infiltration as a function of water depth on the ground surface, is proposed to describe the aforementioned phenomenon under three characteristic vegetative canopies of central Senegal (millet, groundnut and fallow). The model (BADER, 1994) is a distributed, three parameter model that accounts for transfer between spatial elements (parameter n), runoff (parameter Hl) and infiltration (parameter S). The model solves the equation of continuity according to an explicit scheme (forward time). The discharge exiting a spatial element is defined by a power function based on the water depth on the element. The value of the transfer parameter n (dimensionless) depends on the roughness and slope of the soil surface. Parameter Hl (meters) is equivalent to the water depth from which runoff occurs and is found in the discharge expression. Infiltration is defined as the product of the squareroot of the depth of ponded water of a plot and a S parameter (dimensionless) representing surface porosity.The experimental work took place on 4 rectangular 50m2 plots (10 m by 5 m) that were initially bare and weedy. At the beginning of the rainy season, two plots were cultivated in millet and groundnut, one left fallow and the fourth stripped by a powerful herbicide. The runoff was measured by a capacitive gauging system with each tank being equipped with a pressure transducer connected to a datalogger. A tipping bucket raingauge was also connected to the datalogger and rainfall and runoff were recorded simultaneously. The measurements were made to a precision of 4 mm in the tanks (0.16 mm uncertainty for surface runoff depth). With a total seasonal rainfall of 711 mm in 1994, the cumulative surface runoff varied between 40mm for the fallow plot to 150 mm for the bare soil plot. The cultivated groundnut and millet plots had cumulative runoff depths of 55 and 60 mm, respectively. The fallow plot would have had less runoff if it had been more than one-year old. The microtopography of each plot was evaluated using a profile meter. The surface roughness was estimated by the standard-error of measured relative elevations (GUILLOBEZ and ZOUGMORE, 1994). Measurements were taken after each significant rainfall and following tillage operations. The index of roughness varied following vigorous weeding of the groundnut plot to 5 mm for the fallow plot whose microtopography remained constant throughout the season. The development of the vegetative cover was indirectly followed by the calculation of a vegetation index (NDVI) derived from red and near infrared reflectances measured with a field radiometer. Although this index tends to saturate with full ground cover, it nevertheless remains a good indicator at the start of vegetative growth.The proposed model was used to reproduce measured runoff during several storm events. Calculations were undertaken with a 10-s time step on a 1m-long spatial element with a uniform set of parameters for each plot. A sensitivity analysis was performed for all runoff events on the bare plot. Hydrograph characteristics (runoff volume, peak discharge and time-to-peak) were particularly sensitive to variations in the transfer parameter (n) and to a lesser extent to changes in the infiltration (S) and runoff (Hl) parameters. For the 42 measured runoff hydrographs for all fourplots, the results were excellent: 70% of the simulated hydrographs had a Nash's coefficient greater than or equal to 0.90.For each plot, the seasonal chronicle of each parameter is coherent with the plot cover. The parameters for the bare plot were invariant throughout the rainy season. However, for the other plots, they varied with the vegetative cover. At the beginning of the growing season, they were similar to those obtained on bare soil and, as the vegetative cover increased, they varied until the NDVI exceeded 0.35 (approximately 20 days after seeding). The evolution of the n and S parameters for the cultivated plots was linearly extrapolated from past events (seeding for the cultivated plots and chemical weeding for the fallow plot) and for S to an antecedent precipitation index. Farming practices that modified surface roughness needed to be accounted for as well. For the transfer parameter (n) of the groundnut plot, an increase of approximately 0.4 was observed when a rainfall event followed weeding. No significant increase was seen for the millet plot. A linear relationship between the index of roughness and the roughness parameter (Hl) was also derived