FR :

La composition et la distribution spatio-temporelle des ciliés du Lac Municipal de Yaoundé (Cameroun) ont été analysées d’avril à décembre 1997, en relation avec des paramètres physico-chimiques du milieu. Trois stations pélagiques et l’herbier littoral ont été échantillonnés. 58 espèces de protistes ciliés ont pu être identifiées, parmi lesquelles environ 60 % d’espèces littorales et périphytiques et 37 % d’espèces pélagiques. Les espèces les plus courantes sont les espèces pélagiques Uronema nigricans et Coleps hirtus. L’abondance saisonnière des ciliés a atteint une valeur maximale de 21 800 ind•L^-1, les valeurs les plus élevées étant enregistrées dans la couche trophogène du lac, particulièrement au cours de la grande saison des pluies. Les variations de la biomasse totale des ciliés sont plus faibles, avec des maxima correspondant également aux fortes pluies. Les espèces dominantes en matière de biomasse sont Prorodon africanum, Coleps hirtus et Uronema nigricans. Enfin, les fluctuations saisonnières de l’abondance et de la biomasse des ciliés sont discutées en relation avec les paramètres environnementaux, parmi lesquels le régime pluvial tient apparemment un rôle central.

EN :

The Municipal Lake of Yaoundé (3° 51’ 37’’ N and 11° 30’ 40’’ E) is a shallow water body (Z_m = 4.3 m), situated in downtown Yaoundé, the political capital of Cameroon (Central Africa). The composition and the spatio-temporal distribution of ciliates in combination with physical and chemical water variables were measured in this lake from April to December 1997. Sampling was carried out weekly, at 5 depths (surface, 0.5 m, 1 m, 1.5 m and 2.5 m) and at 3 pelagic stations, A, B and C, representing the upper, middle and the lower parts of the lake, using a Van Dorn sampling bottle. In the littoral station, the periphytic fauna is collected by agitation of the vegetation in a bucket and filtration of water. The water temperature was measured with a thermometer and the pH was measured in the field with a portable pH-meter. The oxygen concentration was determined according to the Winkler method and the transparency of the water column was determined with a Secchi disk. Light availability was measured with a luxmeter. Water colour, suspended solids and ammonium-nitrogen (NH₄-N) were analyzed spectrophotometrically using APHA methods. Ciliate species were identified using appropriate and specific keys and were counted under a stereomicroscope at magnifications of 250X and 500X.

Results showed that the lake was hypereutrophic. Throughout the study, the temperature and pH values of the water remained around 25°C and 7, respectively. The dissolved oxygen concentration decreased with depth to almost zero at the lake bottom, while the Secchi disk transparency did not exceed 1 m. Spatio-temporal variations of these variables indicated that the middle zone (0.5 to 1.5 m) was the stratum with a critical role in the functioning of the lake. Indeed, it represented the transitional zone between the trophogenic upper layer where photosynthetic activity occurred and the bottom water layer, formed by the microaerophilic to anaerobic tropholitic layers where reducing processes were important.

Fifty-eight ciliate species, belonging to 10 orders and 28 families, were identified among which five (Holophrya sp., Lagynophrya rostrata, Lagynophrya simplex, Pseudoprorodon sp. and Histriculus sp.) were new records for Cameroon fauna. This population contained 62% of littoral species and 37% of pelagic species respectively. Within these taxa, dominant species were Uronema nigricans (present at all pelagic stations) and Coleps hirtus (present at more than 97% of the pelagic stations). Their abundance was greater than 2,000 ind•L^-1 during the study period showing variation with many peaks. Prorodon africanum occurred sporadically between October and November when rainfall decreased and their densities reached 5,700 ind•L^-1 in November at 0.5 m depth at station A. After November, this species was replaced by Prorodon sp. Most of the species collected were cosmopolite or characteristic of shallow tropical water bodies.

The total abundance of the ciliate community was high reaching 21,800 ind•L^-1 at station A, at 0.5 m depth. According to several authors, the abundance of ciliates in such tropical water bodies varies from 3.6 to 9.75 x 10⁴ ind•L^-1. The highest abundances were observed at depths of 0.5 m to 1.5 m at all 3 stations, especially during the rainy season. This corroborated the importance and the role of this stratum in the functioning of Lake Yaoundé. The lowest abundance of ciliates was observed at station A, at the bottom (2.5 m depth). The total abundance of ciliates showed one or several peaks during the period from July to November. With respect to the total biomass, the variation was more regular with generally one peak during the rainy season. At certain depths at stations B and C, several peaks were observed during the period of April to June and from September to November. The total biomass values obtained were higher than those reported by other authors. This can be explained by the fact that reported values in these latter studies were expressed as dry weight while values reported in this study were expressed as wet weight. With respect to biomass, the dominant species was Prorodon africanum (5,300 µg•L^-1), followed by Coleps hirtus (3,800 µg•L^-1) and Uronema nigricans (3,100 µg•L^-1). The highest biomass was 130 x 10⁵ µg•L^-1, and was observed at station A, at 0.5 m depth.

Generally large size ciliates were more abundant during the dry season whereas small size species were abundant during rainy season. Chilodonella uncinata and Prorodon africanum grew in surface layers (between the surface and 1 m depth) while Paradileptus conicus and Uronema nigricans preferred deep water (1 to 2.5 m depth). There was a great development of ciliates during the transitional period between the rainy and dry seasons. Moreover there was a close relationship between environmental parameters and the ciliate community. In fact, 2 to 3 physico-chemical predictors (dissolved oxygen, temperature and conductivity or pH) explained spatio-temporal distributions of different species. The variation of the total biomass in station A was explained by the dissolved oxygen concentration (r² = 0.366; p<0.001) and by the pH (r² = 0.274; p<0.001); at stations B and C, the variation was explained by ammonium-nitrogen (r² = 0.178 and r² = 0.294 for p<0.01). The most important abiotic factor that influenced the density and biomass variation of ciliate communities of Lake Yaoundé was the precipitation rate. Throughout the study, station B was the most characteristic of the structure of the ciliate community of this lake.

FR :

L’imagerie satellitaire dans le visible et l’infrarouge permet de cartographier le couvert nival à grande échelle, ce qui n’est pas facilement réalisable à partir des observations locales conventionnelles. Cependant, en raison de leur résolution spatiale inadéquate ou de la faible durée de leurs séries d’observations, les produits satellitaires actuellement disponibles sont inutilisables pour l’étude à long terme du couvert nival. Par conséquent, l’objectif de la présente étude a été de développer un algorithme opérationnel de cartographie de la neige à l’aide des données du capteur AVHRR (Advanced Very High Resolution Radiometer) embarqué à bord du satellite NOAA. Cette procédure doit permettre de suivre l’évolution spatio-temporelle de la neige au sol sur une longue période de temps et avec une bonne résolution spatiale. Les résultats de la cartographie ont été validés par rapport aux observations de l’occurrence et de l’épaisseur de la neige au sol. L’algorithme a été appliqué au territoire du Québec sur trois périodes spécifiques : 1998-1999, 1991-1992 et 1986-1987. L’algorithme a réussi à identifier la catégorie de surface (neige/non-neige) avec un taux de succès global moyen de 87 %. Les performances de l’algorithme ont été supérieures dans la détection de la neige (90 %) qu’elles l’ont été pour les surfaces sans neige (82 %). Également, l’algorithme a permis de situer le début des périodes de formation et de fonte de la neige, et ce tant au niveau local qu’à l’échelle du bassin versant.

EN :

This work is part of a multidisciplinary study designed to validate the elements of the hydrological cycle of the Canadian regional climate model simulations (CRCM) over Quebec (Canada). These simulations, carried out over a 20-year period (1979-1999), aim at examining the annual and inter-annual hydrological budgets of a dozen catchments. Snow cover is a key factor in the modeling of the hydrological budget as well as the climatic changes. The remote sensing component of the project involves the use of satellite data in order to validate CRCM simulations of snow cover characteristics (i.e., snow cover extent), which are impossible to validate using conventional in situ snow observations.

Satellite data in the visible and infrared spectra as well as passive microwaves represent an alternative source of information on snow cover. Various satellite snow products have been available since the middle of the 1960’s and a few are available in real time and online. However, their quality varies considerably with respect to sensor and platform characteristics, image processing procedures and snow classification techniques. Consequently, these operational products cannot be used for the validation of the CRCM simulations because of their limited spatial extent, or their coarse spatial resolution, or the lack of a continuous and homogeneous series of observations covering the targeted period (1979-1999). In addition, the coarse temporal resolution and the small areal coverage of high-resolution satellites limit their use for the temporal monitoring of snow cover on a regional scale. Consequently, it was decided to explore the potential of NOAA-AVHRR data for the space-time monitoring of snow on the ground and to produce snow cover maps. These maps would then be used to validate CRCM simulations. Among the 20 years concerned by the study (1979-1999), six winter seasons were targeted to be used in the validation process.

The objective of this work was thus to develop a simple procedure of space-time monitoring of snow cover over the province of Quebec using AVHRR images. The algorithm was calibrated and validated over three winter seasons: 1998-1999, 1991-1992 and 1986-1987. In order to monitor snow cover, especially during snow setting and melt phases, the daily images from October 1^st to December 15^th and from April 1^st to May 31^st of each of the three periods were used. Images at the beginning of the afternoon were preferred since they are less sensitive to topographic effects and variation in illumination conditions. Only the images presenting a minimal cloud cover were retained (164 images out of the 411 initially identified). These selected images were used for the calibration and validation of the snow cover mapping algorithm. Selected AVHRR images were calibrated and corrected radiometrically and geometrically. A sub-region (82°30’ W, 58°N; 60° W, 46° N) covering the territory being studied was therefore extracted from each image.

The classification algorithm used herein was developed from published classification techniques. This algorithm is based on sequential hierarchical thresholds in order to classify the AVHRR images into three surface categories: snow, no-snow and clouds. It consists of a combination of six sequential thresholds. The thresholds go from least restrictive to most severe. A pixel that successfully passes through all the thresholds is classified as snow; if the pixel does not pass through all the thresholds, it is categorized either as clouds or no-snow. The thresholds were established empirically and are consequently specific to Quebec conditions. The classification results were validated at the temporal and spatial levels using ground observations, specifically snow occurrence at Environment Canada’s meteorological stations.

The algorithm was calibrated using pixel samples extracted from each selected image, above areas representing the three surface categories present within the scene. These areas were identified visually and delimited manually. Thereafter, radiometric data samples from all selected images were put together and their percentiles were calculated. The percentiles were used to build the values of the algorithm thresholds.

For each of the three studied periods, two dates were chosen for the spatial validation of the snow maps produced using AVHRR images: one during the snow cover setting period (at the end of October) and the other for the snow melt period in spring (at the end of April). For these six dates, ground snow occurrence at meteorological stations was compared to the classification results. For temporal validation, snow occurrence observations at 15 meteorological stations during each of the three winter seasons were used for the classification algorithm. Corresponding ground observations were compared to the occurrence of snow class within 3 x 3-pixel windows centered on each station and the total accuracy statistics were therefore calculated. When 50% or more of the 3 x 3-pixel windows were classified as cloudy, the results for the corresponding station were excluded from the comparison.

The classification results were quite accurate, with 87% of the pixels around validation meteorological stations being correctly identified. The algorithm successfully detected the presence of snow with a precision of 90% and 82% for no-snow surfaces. The algorithm performances in spring and autumn were similar. Also, the algorithm detected the presence of snow more accurately in open lands than in forested areas. We demonstrated that the algorithm allowed the location of the beginning of snow formation and melting periods at the local level as well as at the watershed scale, especially under clear sky conditions. The algorithm also captured interannual dynamics and spatial variations in the establishment and disappearance of snow cover. The use of high spatial resolution imagery (LANDSAT or SPOT) would improve the accuracy assessment of the algorithm results according to soil occupation types and pixel fractional snow coverage. The main limitation of the algorithm application is the presence of persistent clouds.

FR :

L’envahissement incontrôlé des plans d’eau par les plantes aquatiques est une problématique d’actualité, plus particulièrement dans les pays tropicaux, à cause du phénomène d’eutrophisation. Parmi les solutions proposées, une exploitation rationnelle de certaines plantes envahissantes comme source de biomasse valorisable, et comme agents de dépollution, est envisagée. Dans le cadre de cette étude, un accent particulier a été mis sur une sélection adéquate de plantes aquatiques visant un développement durable, se basant sur la capacité de cette plante à s’adapter à son milieu de culture avec un impact environnemental pratiquement nul. Pour cela, il est nécessaire que la plante sélectionnée puisse présenter des propriétés dépolluantes pouvant ramener l’écosystème à son état naturel d’origine et que le taux de croissance de cette plante soit contrôlé par une exploitation optimisée de la biomasse récoltée. La problématique envisagée s’est limitée à un nombre restreint de plantes aquatiques, fréquemment rencontrées dans les milieux eutrophisés, à savoir la jacinthe d’eau, les lentilles d’eau, la laitue d’eau, le papyrus et le chou de marais. Un choix et une hiérarchisation préalables des principaux critères d’évaluation ont permis, sur la base d’une analyse de satisfaction rigoureuse, d’opter pour la jacinthe d’eau, comme étant la meilleure alternative, présentant un grand pouvoir de fixation d’éléments polluants et un potentiel élevé en protéines et agents antioxydants, dans un contexte donné. Une exploitation judicieuse de cette plante dans un plan d’eau continuellement pollué ne posera plus de problèmes de rentabilité, étant donné que la productivité sera assurée par la récolte d’un excédent de jacinthes, deux ou trois fois par mois. Ceci permettra de mieux maîtriser le taux de recouvrement du plan d’eau par la plante aquatique, évitant ainsi une eutrophisation de celui-ci. La méthodologie adoptée peut être généralisée à d’autres plantes aquatiques et d’autres contextes.

EN :

Uncontrolled invasion of water bodies by aquatic plants, due to eutrophication, is a major environmental concern, especially in tropical countries. Most aquatic plants exhibit water-cleansing properties, but only some of them can be exploited as highly valuable sources of proteins and antioxidizing agents. In this regard, a rational exploitation of judiciously selected aquatic plants can provide low-cost technologies that combine increased biomass productivities and effective depolluting capacity, in a sustainable development context. In the present paper, we have considered some aquatic plants that usually grow in tropical countries, namely water hyacinth, duckweed, water lettuce, common papyrus and water spinach. To achieve a rigorous methodology that allows the rational exploitation of aquatic plants, attempts were made through a multicriteria analysis method to select the most adequate plant for this purpose. Furthermore, many criteria were selected and ranked, according to their impact upon the ecosystem in which the technology is to be implemented.

Among all the plants examined, water hyacinth displays the most interesting features, namely, a marked ability to remove pollutants from water (metallic cations, phosphate, nitrate, organic matter, etc.), and high contents in proteins and antioxidizing agents. It also displays an increased resistance towards eutrophic sites, along with an appreciable adaptability to environmental changes. Under suitable conditions, water hyacinth can also afford a considerable biomass productivity of about 200 tons dry weight/ha/year. Nevertheless, the main criterion in favour of this species is undoubtedly the high number of ways in which the water hyacinth biomass can be exploited. As a result, a rational exploitation of water hyacinth, and to lesser extent, of water lettuce or of duckweed, can be regarded as being a very convenient strategy. The main constraint in this regard must be a sustainable development that includes both environmental and socio-economical considerations.

In conclusion, the choice of an appropriate aquatic plant to depollute wastewaters in small settlements in tropical countries, using the satisfaction analysis method, seems to be a suitable tool for obtaining sustainable solutions, without major investments. Due to its high protein level, water hyacinth can be rationally exploited on the surfaces of eutrophic water bodies, by implementing a suitable control of the biomass excess, reducing thereby the environmental impact of such culture. The biomass excess can be optimized, regularly harvested (2 or 3 times per month) and then processed to obtain valuable protein extracts. In this way, the continuous eutrophication process of the water body will contribute to feed the plant growth.

FR :

L’assainissement des eaux usées par infiltration percolation appartient à la filière de traitement des rejets polluants à cultures fixées. Dans un contexte géographique spécifique et pour une population avoisinant 500 à 1 000 équivalents habitants, elle paraît bien indiquée. Filière dite rustique, elle n’en est pas moins complexe. L’objectif de cette étude est de contribuer, à travers une simulation numérique, à la compréhension des phénomènes physiques et biochimiques qui s’établissent au sein d’un lit d’infiltration percolation. Les aspects essentiels à l’activité bactérienne que sont l’hydrodynamique du milieu poreux, le développement de la biomasse active, le transport, la consommation et les transferts d’oxygène y sont abordés. À travers des essais d’une vérification méthodique du modèle effectuée à partir des solutions analytiques, il ressort principalement que la dispersion hydrodynamique et le taux de dégradation ont des effets contraires sur le rendement d’abattement des charges polluantes. En outre, un résultat significatif obtenu est la comparaison qualitative et quantitative des apports convectifs et diffusifs en oxygène au sein des lits d’infiltration percolation qui sont à aération naturelle.

EN :

Wastewater sanitation using infiltration/percolation is part of an approach that uses attached microorganisms to treat pollutant loads. It appears suitable for a specific geographical context, and for population equivalents of approximately 500-1000 people. The aim of this study was to improve, by means of a numerical simulation, the understanding of certain physical and biochemical phenomena observed within an infiltration/percolation bed. All the aspects essential to bacterial activity are examined, including: the hydrodynamics of the porous media; the development of an active biomass; transport; and oxygen transfer and consumption. The latter are of paramount importance in non-saturated porous media, where significant aeration can take place, whereas in saturated soils and aquifers containing nitrogenous and organic compounds, the oxygen in water is rapidly consumed.

The model we have formulated includes seven equations, which describe macroscopic transport, and are coupled and non-linear. The terms “wells/sources” are functions of unknown variables. The resolution of the equations, obtained after discretization of the equations using Euler’s finite difference method, was performed using Thomas’ algorithm and Fortran 95 programming. We used an innovative approach: analytical solutions developed for saturated porous media were modified to take into account a heterogeneous flow field in a non-saturated porous medium. In a systematic approach, we tested two problems that are part of a gradual verification process: one-dimensional convection-dispersion solute without a kinetic reaction; one-dimensional solute with a first-order decay.

The code we have developed insures a very good approximation of the solute transport within a non-saturated porous medium. For a given rate of flow and a given supply period, the greater the dispersion, the quicker the solute will become homogeneous. In other words, a very high dispersion will induce a very low residence time for the solute within the medium. In wastewater treatment within sand beds, the residence time or contact period between the pollutant matter transported by the effluent and the purifying biomass attached to the support is thus a parameter that is linked to the dispersion of the effluent within the medium. It also appears that the reduction in pollutant load is optimized within a biofilm with a high degradation rate, and for an effluent with a low dispersion coefficient. This result is coherent with the link between dispersion and residence time of the effluent in the system.

We also examined the impact that the hydraulic load and the substrate content in the effluent have on the oxygenation capacities of a filtering mass. A qualitative and quantitative analysis of the incoming oxygen flow was performed. Thus we show that, at the beginning of the supply period, convection is more influential than diffusion. We also present several results from the simulations of substrate reduction profiles, which were very closely linked to oxygen content profiles. Thus we observed a rapid decrease in oxygen content due to intensive bacterial activity in the upper part of sand filters, followed by an increase in oxygen towards the bottom of the filter due to the absence of substrate. The main conclusion of this part of the study was that in order to optimize the ability for oxygenation within infiltration/percolation beds, it would be preferable to connect them to the separate sewer networks, which yield more concentrated effluents than do combined sewer networks.

FR :

Nous examinons dans cet article, la répartition spatiale de la pluviométrie annuelle en vue de sa cartographie en combinant l’analyse en composantes principales (ACP) et la théorie des variables régionalisées. Deux vecteurs régionaux représentant la tendance pluviométrique la plus probable pour l’homogénéisation des données se sont dégagés. Les pluies annuelles probables de période de retour cinq et dix ans sont évaluées afin de compléter l’information donnée par les pluies annuelles. Les meilleurs modèles de la relation pluie-relief sont aussi recherchés. Par régression multiple, un certain nombre de paramètres morphométriques susceptibles d’expliquer les pluies est identifié, pour leur évaluation en tout point de l’espace. Le variogramme des résidus a présenté une structure spatiale nette. Par krigeage, les paramètres variographiques définis sont calés pour leur interpolation entre les postes pluviométriques. La cartographie automatique a permis d’élaborer les cartes des pluies moyennes annuelles et celles de période de retour cinq et dix ans.

EN :

The spatial distribution of annual rainfall in north-central Algeria was mapped by combining principal component analysis (PCA) and the theory of regionalized variables. The study of the spatial distribution of a natural phenomenon often requires a network of specific measurements. The extension of site-collected and fragmentary data over the whole study area is essential for mapping the variation of the parameter in space. The theory of the regionalized variables is used to account for the structural characteristics of a natural phenomenon in a suitable mathematical form. North-central Algeria, a region covering 390 km from west to east and 360 km from north to south, was the subject of this study. The region contains the Cheliff watershed, coastal Algeria and parts of the Isser and Zahrez watersheds. Ninety-seven rainfall measurement stations were retained, taking into account the quantity and quality of the precipitation data, which covered the time period from 1921/22 to 1960/61 and from 1968/69 to 1998/99, i.e., altogether 70 years but with some gaps. Using PCA, two regional vectors with a multiple correlation coefficient of 0.87, explaining 75% of the total variance, were identified. A linear combination between yearly precipitation and the two regional vectors was established, allowing us to fill the gaps in data among the 97 stations.

Two regional vectors, representing the most likely trend in rainfall used in the homogenization of the data, were identified. A rainfall return period of five to ten years was estimated in order to complete information given by the annual rainfall values. According to the literature, the annual rainfall distribution in Algeria has a positive skewness for which the root normal distribution led to the best fitting of extreme values.

The estimation of the variability in rainfall in the study area required field knowledge. The inadequate distribution of the rainfall network gave a poor representation of the rainfall measurement stations with respect to the altitude. For this reason, a model of the relationship between rainfall and altitude was developed. The altitude was initially analyzed using a digital elevation model (MNT) that samples the elevation using the nodes of a network with a square grid of 2 km x 2 km with the objective of identifying a certain number of morphologic parameters able to explain rainfall variability. The mapped zone was a rectangle oriented from east to west with 35,476 sampled elevations. We chose the model that identifies Lambert co-ordinates (X, Y) and hypsometric (elevation (Z), slope and orientation) parameters. To take into account the orientation of the slopes and the entrenchment of the site, six parameters (TG1, TGE, TGN, TGNE, TGW and TG3) were identified, the calculation of which was based on differences in level. They were identified on the basis of a regular grid (2 km x 2 km) of 25 nodes centered on the rainfall measurement station using a tangent (TG) function. The calculation of smoothed elevation (ZfS) was performed using the TG function and required the construction of a regular grid of nine nodes centered on a rainfall measurement station at an elevation of Z.

The best models of the relationship between rainfall and altitude were identified using multiple linear regressions, by comparing the coefficient of determination (r²) and the sum of residuals. The models obtained were validated by independence and residual normality tests. Some morphologic parameters explaining rainfall variability were also identified, to be used for the estimation of rainfall at any point in the study area using multiple regressions. These models allowed us to extend the series to the nodes (2 km x 2 km) for which the X, Y and Z coordinates are known. A functional relationship between observed and model estimated rainfall was developed. The observed variable «rainfall» can only be accurately defined with the nodes of the grid if the corresponding residuals are established, making the study of the node residuals essential. The knowledge of the regression residuals extended to all grid nodes remains necessary for the mapping of our parameters and allows us to better interpret the resulting variograms. The interpolation between nodes of the grid (2 km x 2 km) was carried out by kriging, following the investigation of their spacial structure by the calculation of experimental variograms. This spatial structure represented the variance of the differences between the residuals of regression between two distant points of h.

To study anisotropy, the north-south and west-east directions were retained. The exponential model provided a good fit to the regression residuals. The variogram of the residuals has a net spatial structure. Variographic parameters were defined by kriging and fitted for interpolation between rainfall measurement stations. Starting with the elevation grid, as defined by the digital elevation model, the spatial rainfall pattern was obtained by combining the estimated rainfall grids, obtained from the digital elevation model, and the residuals grid. Automatic mapping yielded annual, five-year and ten-year isohyet maps.

FR :

Cet article a pour objectif l’estimation indirecte de la transmissivité (T, m²/s) de l’aquifère du Haouz (6 000 km²) au Maroc, à partir de la résistance transversale (R, Ωm²). L’aquifère du Haouz est constitué par une succession complexe de séries lenticulaires, argilo-marneuses ou formées d’éléments grossiers, d’âge plio-quaternaire reposant sur un substratum marneux d’âge miocène. Une importante base de données des valeurs de transmissivité (≈500) et de résistance transversale (≈2 500) a été compilée. Une recherche a ensuite été effectuée pour retenir les couples (T_i, R_i) caractérisant le même volume d’aquifère. Deux cas de résistance transversale sont considérés : 1) résistance transversale de l’ensemble mio-plio-quaternaire (R_A); 2) résistance transversale des lentilles perméables uniquement (R_B). La meilleure régression, de forme géométrique, est obtenue entre la transmissivité et la résistance transversale des lentilles grossières perméables. Cette régression est sensiblement améliorée lorsque les valeurs (T, R_B) d’un même couple sont ramenées à un même état piézométrique de la nappe, après correction de la transmissivité en fonction des variations piézométriques de la nappe. On procède ensuite à l’estimation indirecte du champ de transmissivité de l’aquifère à l’échelle régionale à l’aide de l’équation de régression précédemment établie. Le champ de transmissivité ainsi estimé est validé par comparaison de ses propriétés statistiques (tendance centrale, dispersion, loi de distribution) à celles de l’échantillon des valeurs de transmissivité mesurées par pompages d’essais.

EN :

This article aims at the indirect estimation of the transmissivity (T, m²/s) of the aquifer of Haouz (6,000 km²) in Morocco, using the transverse resistance (R, Ωm²). This aquifer consists of a complex succession of lenticular layers, argilo-marly or formed of coarse elements, of plio-quaternary age, lying on a marly substratum of miocene age. A significant database of the values of transmissivity (≈500) and transverse resistance (≈2,500) was compiled. A research was then carried out to select the pairs (T_i, R_i) characterizing the same volume of aquifer. Two cases of transverse resistance are considered: 1) transverse resistance of the mio-plio-quaternary unit (R_A); 2) transverse resistance of the permeable coarse lenses only (R_B). The best regression, of geometrical form, is obtained between the transmissivity and the transverse resistance of the permeable coarse lenses. This regression is significantly improved when the values (T; R_B) of a given pair are brought back to the same level of the water table, after correction of the transmissivity according to the variations of the water table. The indirect estimate of the field of transmissivity of the aquifer is then carried out at a regional scale using the regression equation previously established. The estimated field of transmissivity is validated through the comparison of its statistical properties (central tendency, dispersion, distribution law) to those of the sample of the values of transmissivity measured by pumping tests.

EN :

Humans harbour a variety of pathogens that are often transmitted from other animal species. A few are localized in tropical areas, but most enteric pathogens are present everywhere on the planet and they travel with their hosts to distant locations. Several of these microorganisms are transmitted by water that has been contaminated by fecal matter, whereas some are normally found in water but, given an opportunity, will cause disease (i.e., opportunistic pathogens). While waterborne outbreaks have been reported for many pathogens, assessing the proportion of the disease burden to a specific route and pathogen has proven quite elusive. Surveillance of disease in populations, even when actively done, is very inaccurate as it often gathers data on the most acute cases that are only a very small proportion of the true number of infected individuals. There are several issues discussed herein, focusing on the needs and gaps linked to waterborne pathogen monitoring. The benefits and weaknesses of current and emerging methodologies are discussed, in addition to the appropriateness of allocating resources to waterborne pathogen monitoring. The most critical gap is the lack of validation of most methods used in environmental microbiology for the detection of pathogens. Data generated by various laboratories are currently extremely difficult to compare and cannot serve as the basis for risk assessment or management. The issue of laboratory capacity is also raised, within the context of the availability of trained personnel, the application of QA/QC protocols, and accreditation on a national level. In closing, needs are identified for informed communication of the risks of waterborne pathogens, the training of highly qualified personnel, and the development and standardization of methods that will ultimately enhance water safety and public health protection.

FR :

De nombreux microorganismes pathogènes entériques affectent l’homme et certains peuvent être acquis d’autres espèces animales. Certains sont spécifiques aux régions tropicales, mais la plupart des microorganismes entériques sont les mêmes partout sur la planète, voyageant avec leurs hôtes aux coins les plus reculés. Ils se retrouvent dans l’eau contaminée par les matières fécales excrétées. D’autres sont indigènes au milieu hydrique et sont des opportunistes, causant la maladie chez les individus susceptibles. Par la surveillance des épidémies associées à l’eau, on a pu facilement identifier ceux qui sont importants en santé publique. Il est cependant beaucoup plus difficile d’attribuer quelle part du fardeau de la maladie peut être attribuée à une voie d’exposition spécifique. La surveillance de la maladie dans les populations, même lorsqu’elle est active, est très imprécise puisqu’elle ne collige que les données sur les cas les plus graves, soit une faible partie du nombre réel d’individus infectés. À des fins de santé publique, il y a plusieurs aspects, incluant des besoins et des manques, associés au suivi des microorganismes pathogènes dans l’eau. Les bénéfices et faiblesses des méthodes courantes et émergentes doivent être présentés dans un contexte d’appropriation de ressources au suivi des microorganismes pathogènes dans l’environnement. Le simple fait de vouloir analyser des échantillons pour y détecter des microorganismes pathogènes implique des éléments que trop de chercheurs connaissent mal. Les conséquences publiques, légales, politiques et économiques ont été mises en évidence lors de plusieurs événements partout dans le monde, mais plus récemment à Sydney en Australie. Une erreur de laboratoire a conduit à la détection (fausse) de parasites (Cryptosporidium) dans l’eau potable : cette erreur a coûté plus de 37 millions de dollars et affecté trois millions de résidents sans qu’aucune infection ne soit observée. Les producteurs d’eau veulent connaître le niveau de pollution de leur eau d’approvisionnement afin de déterminer le niveau approprié de traitement : comme les méthodes sont imprécises, la marge d’erreur est très grande. Les recommandations présentes sont plutôt dirigées vers des paramètres physico-chimiques (turbidité, mesure de désinfectant en continu, etc.) et des plans de sécurité lesquels sont facilement applicables, mesurables et fiables.

Les méthodes actuelles de détection des microorganismes pathogènes sont peu fiables et leur diversité dans les différents laboratoires rend l’interprétation difficile. Le manque le plus important reste le peu de validation de la plupart des méthodes de détection des microorganismes pathogènes en microbiologie environnementale. Les données fournies par des laboratoires différents ne peuvent donc être facilement comparées et ne peuvent servir à faire des évaluations de risque ou de la gestion de risque. En général, peu de laboratoires offrent de telles analyses, le personnel qualifié est rare, les protocoles de QA/QC sont rarement présents et il n’y a aucune accréditation nationale. La formation de personnel hautement qualifié et le développement de méthodes standardisées ne pourront donc ultimement que mieux servir la santé publique. Les méthodes actuelles utilisées dans un cadre bien défini de certains projets de recherche écologiques commencent à porter fruit. Les données de positionnement géographique, climatologiques et microbiologiques doivent être évaluées afin que la qualité des données produites par les modèles écologiques soit valide. En résumé, les informations acquises sur la présence des microorganismes pathogènes dans les eaux de surface sont utiles mais ne peuvent être utilisées que si elles sont validées. Les laboratoires effectuant les analyses devront être accrédités et devront utiliser des méthodes standardisées si nous voulons comparer les données fournies. Ce n’est qu’à ce moment que l’analyse quantitative du risque microbiologique pourra se faire. Cette nouvelle approche est en émergence au niveau international et les modèles proposés doivent utiliser des données précises. Au Canada, l’Agence de santé publique du Canada s’intéresse à cette nouvelle approche. En l’absence de mesures précises, les chercheurs et les agences de contrôle doivent utiliser les indicateurs de traitement et de contamination fécale pour s’assurer de l’innocuité de l’eau de consommation. Alors que les protocoles d’analyse sont bien définis pour ces paramètres, ceux requis pour les microorganismes pathogènes ne sont pas encore établis. Or, des analyses occasionnelles ou mal ciblées ne sont pas valides pour des fins de santé publique.

Les organisations internationales, telles l’Organisation pour la coopération et le développement économique (OCDE) et l’Organisation mondiale de la santé (OMS), s’intéressent aux méthodes récentes qui allient la biologie moléculaire et la bioinformatique pour obtenir des réponses rapides et fiables sur la contamination des eaux. Ce sera par l’éducation et la communication que les risques pourront être établis et que nous pourrons en informer correctement le public en général, la communauté scientifique, les agences gouvernementales et les producteurs d’eau. Les recherches futures devront répondre aux questions de méthodologie, sensitivité, spécificité, et surtout aux questions de valeur prédictive des résultats de détection de microorganismes pathogènes.

FR :

L’Algérie, caractérisée par un climat semi-aride, est menacée par l’érosion des terres agricoles qui provoque l’augmentation du transport solide et l’envasement croissant des barrages. Cet article décrit une nouvelle méthode d’estimation des flux de matières en suspension (MES) au niveau d’un barrage algérien (barrage de Beni Amrane) basée sur la logique floue. Cette dernière utilise des termes flous tels que « faible », « moyen » et « élevé », pour décomposer le processus débit-MES en plusieurs sous-ensembles flous et d’en déduire les quantités de matières solides en fonction du débit observé de la rivière. Les performances de cette méthode ont été évaluées en période de calage, mais aussi en période de validation, pour mieux juger de la capacité prédictive du modèle à ces deux échelles. En comparant la logique floue avec un modèle empirique régressif utilisant une relation de puissance, nous avons démontré la robustesse du modèle flou en tant qu’outil de quantification du transport solide.

EN :

Sediment transport and erosion is a complex natural process that is strongly influenced by human activities such as deforestation, agriculture and urbanization. In particular, suspended sediments play a key role in controlling water quality and they can cause a major reduction in the capacity of a stream for handling floods. In Algeria, increasing erosion and suspended loads are responsible for serious problems in agricultural land and hydraulic reservoirs, since the suspended load and its sedimentation lead to flooding and dam silting. Water and soil conservation practices, such as contour ridges and areas of reforestation, were introduced in many regions of Algeria in order to decrease erosion and to collect runoff in hill-slope catchments.

Relationships for water discharge and suspended sediment load can be divided into three types: empirical models that allow quantification of erosion on annual time scales, such as the Wischmeier and Smith soil loss equation; conceptual models, which include several reservoirs estimating sediment load on different time scales; and finally, physically-based models, which introduce physical laws such as the Saint-Venant equation. These models represent another category, and allow the estimation of sediment load in different areas of the watershed and supply spatial results. These models also take into account numerous variables that are difficult to obtain on a regional scale.

The objective of this research was to develop runoff-suspended sediment models for the Beni Amrane reservoir. This reservoir is located in the Isser watershed, situated in northern Algeria. This basin covers an area of 4,000 km² and is characterized by a semi-arid climate and a very high soil erosion rate, exceeding 2,000 tons/km²/year. The Beni Amrane reservoir represents an important dam as it supplies the Keddara dam, which in turn supplies the town of Algiers with drinking water.

In the present study, two approaches to suspended sediment simulation were applied on hourly time scales for suspended sediment concentrations, and on daily time scales for water discharge and solid discharge analysis. The first approach is an empirical regression model based on a rating curve and uses a relationship between the observed runoff and the sediment concentration values. The model uses only two parameters, with the second being based on fuzzy logic. Fuzzy logic is already used in many scientific domains, and represents a new simulation technique based on artificial intelligence. Fuzzy variables were used to organize knowledge that is expressed ‘linguistically’ into a formal analysis (for example ‘high suspended sediment’, ‘average suspended sediment’ and ‘low suspended sediment’). The simulation results confirm the performances and robustness of the fuzzy logic model for the two time scales. In fact, the Nash criterion, which is the principal validation criterion for the models, displayed high performances in calibration and validation periods. The neurofuzzy model (fuzzy logic with neural supervised learning) offers a simulation advantage. On an hourly time scale, while increasing the number of fuzzy rules, the model results in good precision with the observed suspended sediments.

The fuzzy logic model results showed that the Nash criterion for two periods (calibration and validation) was greater than 88%, and the peaks of suspended sediments were generally correctly reproduced for the four episodes studied. This is in contrast to the empirical model, where the Nash efficiency was generally weak and decreased during the validation period. In this latter period, the Nash criterion was often negative, the global error was high and the maximum concentration peak was underestimated.

On a daily time scale, knowing the complexity of the runoff-suspended sediment process, we have analyzed these two models for solid discharge simulation. The study was carried out on daily solid discharge data collected from the gauging station on the Isser River (1986 to 1989). While based on the same validation criteria, i.e. the Nash efficiency and the global error, the fuzzy logic model appeared more robust than the empirical model. The fuzzy logic model produced better estimates of the daily sediment yield than the empirical model during calibration and validation periods, and it represents a high prediction power. Thus, we have validated the fuzzy logic model as a tool for simulation of runoff of suspended sediments, and it can be explored to predict sediment loading and silting in Algerian reservoirs.