Volume 12, Number 2, 1999

FR:

Depuis quelques années, un modèle stochastique de génération de hyétogrammes horaires est développé au groupement d'Aix-en-Provence du Cemagref, pour être couplé à une modélisation de la pluie en débit, fournissant ainsi une multitude de scénarios de crues analysés statistiquement et utilisés en prédétermination des débits de crues. L'extension de la zone d'application du modèle de pluies horaires au-delà de sa zone de conception, a fait apparaître une hétérogénéité dans les résultats. Ce constat a entraîné certaines modifications du modèle comme : la recherche d'une loi de probabilité théorique peu sensible aux problèmes d'échantillonnage pour une variable du modèle (intensité d'une averse), la prise en compte originale de la dépendance observée entre deux variables du modèle (durée et intensité d'une averse), et la modélisation de la persistance des averses au sein d'une même période pluvieuse. Ces différentes modifications apportées au modèle initial ont entraîné une très nette amélioration de ses performances sur la cinquantaine de postes pluviographiques du pourtour méditerranéen français. On obtient ainsi un outil beaucoup plus robuste et validé sur une zone étendue, capable de fournir de multiples formes de hyétogrammes, couvrant toute la gamme des fréquences, permettant ainsi de s'affranchir des pluies de projet uniques. On aborde aussi une nouvelle approche du comportement à l'infini des distributions de fréquences des pluies qui semble parfois supérieur à une tendance strictement exponentielle. De plus, l'étude de plusieurs événements par an dont chacun présente plusieurs réalisations des différentes variables du modèle augmente la taille des échantillons analysés, semblant rendre la méthode plus rapidement fiable qu'une approche statistique classique basée par exemple sur l'ajustement de valeurs maximales annuelles.

EN:

A stochastic model for generating hourly hyetographs has been recently developed, in the Cemagref of Aix-en-Provence, to be coupled with a rainfall runoff conversion modelling. Thus, by simulation of very long periods (1000 years for example), we obtain a large number of hourly hyetographs and flood scenarios that are statistically studied and used in flood predetermination problems. The rainfall model studied is based on the theory that rainfall can be linked to a random and intermittent process whose evolution is described by stochastic laws. It is also based on the hypothesis of independence between variables describing hyetographs and on the hypothesis of the stationary nature of the phenomenon studied. Generating a rainfall time series involves two steps : descriptive study of the phenomenon (nine independent variables are chosen to describe the phenomenon and these variables are defined by a theoretical law of probability fitted to the observations) and creation of a rainfall time series using descriptive variables generated randomly from their law of probability. Initially developed on the Réal Collobrier watershed data, the model has been applied to fifty raingauges located on the Mediterranean French seaboard. The extension of the model applying area has shown heterogeneousness in the results. Therefore, modifications have been made to the model to improve its performances. Among these modifications, three of them have presented notable improvements.

A study of the sensitivity of the parameters has been made. Parameters of shape variables and of some other variables had only a slight influence on depth of generated rainfalls. But, the law of mean rainfall intensities clearly differentiates the stations. Then, a theoretical probability distribution for the storm intensity variable, less sensitive to the sampling problems, has been searched. An exponential distribution is fitted to the value smaller than four times the mean of the variable. A slope breakage was then introduced to generate all the values beyond this limit. The breakage at the value four times the mean of the variable and modelling this breakage were based on a study of so-called "regional" distributions of the storm intensity variable. These distributions were designed by clustering the variable's homogenized values for all 50 studied stations.

A second modification has been made to develop new model for the observed dependence between two variables (duration and intensity of the storm). The study of this dependence has been considered directly based on the cumulative frequency of the two variables. Then, an additional parameter was defined to model the dependence between the probabilities of the two variables. This parameter characterises the cumulative frequency curve of the sum of the probabilities of the two variables. This point, neglected during a long time, has been very important in the improvement of the model.

Finally, the modelling of storm persistence in a same rainfall episode has been studied to generate some high 24 hours maximum rainfalls. Persistence modelling is entirely justified by the fact that "ordinary storms" cluster together around the "main storm" (the "main storm" is the greatest storm of an episode and the "ordinary storms" are the other storms of the episode). When the study of this phenomenon is extended, it can be observed that there is a certain positive dependency between occurrence probability of the "main storm" and occurrence probability of storms which come before or after it. Two combined effects occur : within one rainy episode, the strongest "ordinary storms" are preferentially clustered together around the "main storm", and considering the number of "ordinary storms" throughout all the episodes, the strongest storms close to the "main storm" are preferentially associated with the strongest "main storms" and vice versa. This modification improves the performances of the altitude raingauges, which are characterised by high daily rainfall accumulations.

The different modifications added to the initial model, give very important improvements on the calibration of the fifty raingauges studied on the French Mediterranean seaboard. Its aptitude to generate rains observed in Mediterranean climate, strongly variables, consolidates us in the idea of its application on a zone much larger. The generation of hyetographs makes it possible to use the maximum the temporal information of the rain. Thus, we obtain a reliable tool, validated on a large area, for simulating hyetographs and hourly flood scenarios at all frequencies, and used instead of a unique design storm and design flood. The approach allows a new cumulative probability curve extrapolation, which seems sometimes greater than an exponential behaviour. Moreover, the study of many events per year, with many occurrences of the different variables of the model, increase the analysed sample size and seems to make the method more reliable than a statistical approach simply based, for example, on the fitting of annual maximum values.

FR:

A partir de résultats expérimentaux, des équations d'infiltration et de continuité, nous avons établi une équation permettant de prédire la profondeur maximale humidifiée par un goutteur à la surface du sol. Connaissant l'humidité initiale du sol Ø_i, la conductivité hydraulique K_f et la teneur en eau au niveau du front Ø_i ; l'équation établie permet de calculer la profondeur du front d'humectation Z_f(t) à partir du suivi du rayon du front d'humectation R_f(t) à la surface du sol. Pour tester la validité de cette nouvelle approche, des essais ont été menés sur deux types de sol (sableux et limono-argileux) et avec trois débits différents. Les valeurs de Z_f(t) mesurées (par deux techniques différentes) ont été comparées à celles calculées par cette équation, les résultats étaient, dans tous les cas, quasi-identiques (coefficient de corrélation R², dans tous les cas, supérieur ou égal à 0.94).

EN:

In trickle irrigation, the wetted soil volume fraction (P) is a fundamental parameter for both network design and irrigation scheduling. However, methods used to determine such parameter (P) are still limited and unaccurate. Further, numerical models established for predicting wetted soil volume (or bulb) dimensions remain onerous, complex, expensive, and therefore hardly used.

Experimental and numerical results performed on water infiltration from point source on the soil surface show a close relationship between the horizontal and vertical bulb dimensions.

This study which was based on continuity and cumulative infiltration equations allowed to establish a new and simple equation for predicting wetted soil depth. This equation enables, knowing initial and wetting front water content and K(h_f) (soil hydraulic conductivity on the wetting front), to calculate the front depth Z_f(t) beneath a point source with only measuring the front radius R_f(t) on the soil surface. The established equation verifies physical conditions of axisymmetric water infiltration into the soil.

The established equation was tested by performing trials on two different soil types and three discharge rates. Two different procedures were simultaneously used for determining R_f(t) and Z_f(t).

i) observing bulb dimensions evolution on the soil surface and along the symmetry axis.

ii) R_f(t) and Z_f(t) values were inferred from the horizontal and vertical suction profiles respectively.

The Z_f(t) values calculated, using such equation, are compared with those measured. The results obtained showed good agreement between calculated and measured data (R² >=0.94).

This study was performed during an axisymmetric water infiltration into bare, homogeneous, and isotropic soil. It needs to be extended to heteregeneous, anisotropic and cultivated soils.

FR:

La présente étude consiste à tester les potentialités d'un hélophyte : le roseau (Arundo donax) à épurer une eau usée urbaine sous un climat aride.

Le pilote expérimental est constitué de bacs de 115 litres de capacité, remplis sur une épaisseur de 5 cm de gravier et de 30 cm de sol (texture : limono-argilo-sableuse). Deux bacs sont plantés de jeunes tiges de roseaux. Deux autres bacs non plantés sont pris comme témoins. L'alimentation du système se fait exclusivement par des eaux usées urbaines en bâchées de 25 litres/jour et ce avec une fréquence régulière d'une fois par semaine. L'écoulement se fait par translation verticale à travers le substrat. L'eau de percolation est récupérée au moyen d'un drain placé à la base de chaque bac.

L'étude du fonctionnement hydrologique des pilotes testés, a montré que le débit moyen à la sortie du lit planté (254 ml/min) est cinq fois plus élevé par rapport au lit non planté (51 ml/min). Ce qui permet soit de traiter un volume d'eau usée plus important ou bien de réduire la superficie nécessaire par équivalent habitant. En plus, il apparaît que la présence du roseau aide d'une part à prévenir le colmatage qui est un problème fréquent dans le sol non planté et d'autre part de maintenir une porosité suffisante pour assurer la percolation des eaux en traitement.

Le suivi des performances épuratoires du système à roseau montre que ce dernier assure une élimination importante de la charge organique (MES : 91%, DCO : 72%). Il fournit des eaux épurées claires et limpides. Ces résultats sont comparables à ceux du sol non planté. La réduction de NTK est de l'ordre de 53% pour le lit planté contre 41% pour le sol nu. En général, le lit planté est plus efficace dans l'élimination de l'azote. Concernant l'élimination des ions ammoniums, le taux d'abattement moyen pour le lit planté est de 24%. Le sol non planté présente une surcharge de l'effluent à la sortie en ammonium (-17%). Le phosphore total et les orthophosphates subissent globalement un faible abattement avec des phases de relargage et d'autres d'abattement. L'abattement moyen obtenu par le système planté ne dépasse pas 28% et 10% respectivement pour le PT et les PO₄. Dans le système non planté, il se produit un relargage de phosphore qui se maintient sur une longue période durant la 1ère année et qui se répète également en 2ème année de fonctionnement avec une intensité moins importante. L'abattement moyen obtenu est de -8% et -33% respectivement pour le PT et les PO₄.

En tenant compte des pertes d'eau par évapotranspiration, la correction effectuée a montré une nette amélioration des performances épuratoires surtout pour le sol planté où les pertes de volume sont considérables. Dans ce cas l'abattement moyen obtenu pour le lit planté est de 68%, 48%, 52% et 39% respectivement pour NTK, NH₄, PT et PO₄. Pour le sol non planté, les abattements sont plus faibles (NTK : 51%, NH4 : 3%, PT : 12%, PO₄ : -8%).

La présence du roseau dans le système planté assure donc une nette amélioration de l'abattement de la charge organique et des nutriments par rapport au sol nu. Cette amélioration est faible (3 à 7%) pour la DCO et les MES, par contre pour les nutriments, elle est beaucoup plus importante (17 à 48%).

Concernant la charge parasitaire, les deux systèmes planté et non planté, assurent une élimination totale des œufs d'helminthes parasites. Pour les kystes de protozoaires, le système planté assure une réduction de 99,8%. Avec l'épaisseur du substrat testée, les risques de détecter des kystes de protozoaires à la sortie des systèmes plantés est à appréhender. Selon la norme de l'OMS, l'effluent de sortie du système est classé en catégorie B.

Le roseau produit une biomasse importante qui atteint 176,5 tonnes/ha. La biomasse aérienne facilement éliminée par faucardage est de 85,3 T/ha. Le taux d'azote et de phosphore exporté avec cette biomasse atteint 98,54 et 4,15 g/m² respectivement. Ces taux retenus au niveau des parties aériennes représentent 12% de la charge reçue par le système en azote et 3% de la charge reçue en phosphore.

EN:

Aquatic plant - based system is increasing popular alternatives for treatment of various types of wastewater. Conventional wastewater treatment requires large capital investments and consumes large amounts of energy. While, aquatic plants are a simple and energy efficient means of removing some nitrogen and phosphorus quantities and other pollutants from wastewater. The present study consists of testing the efficiency of an helophyte Arundo donax to treat urban effluent under an arid climate and to assess the suitability of the treated effluent for irrigation purposes.

The experiment was conducted from August 1994 to September 1996. Fourth plots (capacity : 115 liters, diameter : 57 cm) were filled to depth of 5 cm and 30 cm with respectively gravel and soil (texture : 30% clay, 34% silt and 36% sand). Two plots were planted with reeds. Young shoots were taken from local and natural reed stand, cleaned, weighed and transplanted in August 1994 at a rate of 34 shoots/m². Two unplanted plots served as a control. The soil used has sustained the spreading of wastewater fore more than 60 years. It was an organic soil with a pH near neutrality. Alimentation was exclusively done by urban raw effluent with batch loads of 25 liters every 7 days. Water flowed vertically through the substratum.

In order to investigate the capacity of the systems for organic loads, phosphorus and nitrogen removal, the concentration in the inflow and outflow of each plots was determined over the whole period of experiment. At the end of experiment, the reeds biomass and nutrients analysis in plant material and soil were evaluated.

The control of system hydrology indicate that the retention time of planted system was very short. It varied between few hours to only few minutes in Summer. The unplanted system become completely clogged in Winter. So, the presence of Arundo donax in planted system maintained sufficient porosity in order to allow water pollution for treatment and to prevent the clogging which was a problem in the unplanted one.

Over the whole experimental period, results revealed significant performances of reed beds for organic load reduction. The mean elimination rates were 90% for TSS and 70% for COD. The TSS and COD elimination performance of unplanted soil was only little lower (TSS: 83% and COD: 90%). Removal of TSS and COD for the two systems, planted and unplanted, didn't varied significantly from season to another (p<0.05). TSS and COD elimination were almost entirely due to physical processes (filtration and sedimentation) associated to microbial community and the presence of reeds.

The nutrients reed bed removal efficiencies were relatively low (total phosphorus : 28%, orthophosphates : 10% and ammonium : 24%). The system was more efficient in removing NTK (53%). The unplanted soil carried away an overloading of effluent in phosphorus and ammonium particularly in Winter. The treatment could be improved if we took into account the evaporation and evapotranspiration phenomena in each systems. The planted bed appeared to reduce more phosphorus (TP : 52% and PO4 : 39%) and nitrogen (NTK : 68% and NH4 : 48%). The unplanted soil removed 12%, -8%, 3% and 51% respectively for TP, PO₄, NTK and NH₄.

The results showed that the presence of reeds can indeed improve the removal efficiencies in planted plots than the unplanted one. This amelioration was about 17 to 47% for nutrients and only 3 to 7% for organic loads.

Concerning parasitical load, there are several helminthic parasites that occur in wastewater. Two types of helminth eggs were recovered from the sample tested : Nematodes (Ascaris, Trichuris) and Cestodes (Hymenolepis, Taenia). Hymenolepis eggs alone represented 50% of the total number of helminthic eggs. For protozoa, the most important found in the inflow were the protozoan Entamœba and Giardia. The parasitical load presented an important temporal fluctuation at the influent with concentrations varying from 0 to 13 eggs/l and 55 to 1903 cysts/l respectively for helminthic eggs of protozoal cysts. In contrast, helminth eggs were never found at the outflow of planted and unplanted systems. On the other hand, except of the fourth occasions where protozoal cysts were found at the outflow of planted system, the effluents of all the systems generally were exempt of protozoal cysts. The protozoal reed bed removal efficiencies was about 99,8%. So, it will be necessary to increase the filter depth in order to prevent this problem. According to WHO guidelines (1989) for crops irrigation, the quality of effluent should be classified within category B which could be reused for irrigation of cereal crops, follder and trees.

On the other hand, conductivity, sodium and chlorides content increased in planted system more than the unplanted one when the effluent flowed through the substratum. So, when we compared the quality of treated water with FAO guidelines (relating to physico-chemical water quality designed for irrigation) it appeared that the risk of salinity must be taken into consideration. It is recommended that treated wastewater must be used in irrigation with caution and restriction on the type of crop planted. However, treated wastewater contains sufficient nutrients to reduce the need for other fertilization. The use of effluent in irrigation has, therefore, a positive economic impact.

Arundo donax presented a good resistance to irrigation with raw wastewater and it seems to be adapted to the soil rich in nutrients in which it was implanted. The reed above ground biomass produced was estimated at 85,3 tonnes dry weight.ha^-1. It represented 50% of total biomass of reed. Nitrogen and phosphorus removed by the aerial parts were respectively about: 985,47 and 41,51 Kg.ha^-1. This amount account for about 12% of nitrogen and 3% phosphorus with regard to their load at the influent. Plant harvest once a year may be necessary to achieve definitive removal of nutrients incorporated in plant tissue.

FR:

La nouvelle génération de logiciels destinés aux études d'assainissement dispose d'un nombre croissant de modèles hydrauliques et hydrologiques. Il en découle une augmentation des possibilités de choix parmi ces modèles qui complique la tâche des techniciens de l'assainissement. Pour limiter cette difficulté, nous suggérons d'introduire dans les logiciels des outils permettant d'aider les utilisateurs à choisir les modèles en adéquation avec le réseau à simuler. Dans cet article, nous nous intéresserons essentiellement aux modèles de propagation en conduite. Les modèles de propagation les plus usités sont un modèle basé sur les équations de Barré de Saint Venant et des modèles conceptuels nettement plus simples tels que le modèle Muskingum. Ces modèles présentent chacun des avantages et des inconvénients. Dans la pratique, plus un modèle est sophistiqué, mieux il est capable de représenter la réalité. En contrepartie, il est plus difficile à utiliser et nécessite davantage de données et des temps de calcul plus importants. Le problème qui se pose alors à l'utilisateur est de décider quel modèle utiliser. Pour régler ce problème, nous proposons un système d'aide au choix prenant en compte les caractéristiques du réseau étudié, les événements pluvieux simulés et le type d'étude réalisée. Les connaissances nécessaires pour cette aide au choix de modèles peuvent être de qualité variable. Pour mesurer la confiance à accorder à ces connaissances, il est nécessaire de prendre en compte les notions d'imprécision et d'incertitude. Cet état de fait nous a conduit, lors de l'élaboration de cet outil d'aide au choix des modèles de propagation en conduite, à définir un ensemble de règles utilisant la théorie des sous-ensembles flous.

EN:

Most hydraulic and hydrologic softwares offer an increasing choice of models, each with its advantages and disadvantages. Generally, the more sophisticated the model, the better it can represent larger aspects of reality, but also the more difficult it is to use and the longer the data acquisition and calculation times are. In fact, the real difficulty lies in selecting the appropriate model to use. To answer this question, two subproblems must be solved :

- what is the validity field for each of the models, with respect to the network structure, operating conditions, type of rainfall, nature of problem, etc. ?

- once this information is available, what is the best way to ensure its usability, even by people who are not experts in hydrology or hydraulics ?

The first problem can be dealt with by analyzing the theoretical validity field of the different models. Nevertheless, this method raises certain difficulties. It requires a particularly thorough knowledge of the equations, algorithms and calculating artifices used in the software package. But, for various reasons, software designers generally refuse to supply this information. Until now, the solution to the second problem has only been addressed by means of scientific reports, communications or papers. The published data generally gives a global introduction to the validity field of each model. Experience shows that most software users do not have enough information on this subject, or, even if they do, do not use it correctly.

To work towards solving these two problems, we propose to introduce into the software packages a decision support system which can help to choose the best model according to the simulated network. In this paper, only the models for flow simulation will be taken into account. Presently, the most commonly used models are the more or less complete Barre de Saint Venant equations and more simple conceptual models like Muskingum model.

The major difficulty in solving the first subproblem, was mainly the collection and reformulation of pre-existing knowledge. Considerable bibliographical work had to be supplemented by interviews of experts and by complementary studies (Semsar, 1995) (Mottie, 1996). The result of these studies was the identification of a set of criteria related to the network (slope, fractal dimension, loop index, etc.) or to the working conditions depending on the rainfall event (fullness rate, travel rate, etc.).

The answer to the second problem was to develop an "intelligent" man-machine interface able to analyse the background of the simulation (values of the criteria) and to advise the user on the model to select. The knowledge required to build this decision support system can vary in both source and quality, so assessment of its reliability involve the notions of uncertainty and inaccuracy. The problem of uncertainty has been solved by associating uncertainty degrees to the rules. These degrees define a proposal's level of reliability. The approach to inaccuracy is based on the theory of fuzzy sets, according to which the membership of an element of a given set is not settled but relative. The validity of a given fact is represented by a value between 0 (false) and 1 (true). This value can be related to a variable by fuzzy rules, represented by trapezoidal intervals. By this way, each of the criteria has been represented by qualitative decision variables which allow the elaboration of qualitative rules, leading to a "probably better" decision. One more decision variable - the kind of study - has been added because the hydrograph at the outlet of the network is not necessarily the only criterion to be taken into account. Each of the decision variable is represented by one, two, or sometimes more possible qualitative characterisation(s), associated with a degree of possibility (not probability because the sum of all the possibilities is not necessarily equal to 1). The decision support system uses these variables in a set of rules to determine the degree of adequacy of each model. The development of this system shows us that the use of fuzzy sets and qualitative rules seems to be well adapted to represent the used knowledge.

The decision support system will be installed in a software package called CANOE developed by INSA de Lyon and SOGREAH.

In the future we envisage adding an explanatory unit to the decision support system. This study showed too that there is some lack in the knowledge about flow simulation models, so it seems useful to continue to study the validity field of each model.

FR:

Nous présentons une revue bibliographique à propos des effets des herbicides inhibiteurs du Photosystème II (PS II) sur les communautés algales. Ces herbicides sont abondamment utilisés dans les pratiques phytosanitaires. Ils sont susceptibles de contaminer les milieux aquatiques et, étant donné leur mode d'action inhibitrice de la photosynthèse, ils peuvent agir directement sur les algues. De nombreuses études ont été réalisées afin d'évaluer l'impact des contaminations par ces herbicides sur les microphytes, en particulier leur effet sur la croissance et la physiologie de certaines algues (monocultures en laboratoire). D'autres études expérimentales et quelques rares in situ, ont porté sur l'impact des herbicides inhibiteurs de la photosynthèse sur la structure des peuplements algaux. Certaines tendances ont pu être ainsi dégagées quant à la sensibilité et la résistance aux herbicides des différentes espèces étudiées soit isolément, soit au sein des peuplements. Les herbicides inhibiteurs du PS II perturbent effectivement la structure des peuplements phytoplanctoniques de façon plus ou moins marquée. L'impact des herbicides sur les algues est variable selon la structure des peuplements (liée aux successions) et les paramètres environnementaux, notamment liés à la saison. Nous devons donc développer nos connaissances à propos des interactions entre toxiques et facteurs environnementaux sur des pas de temps correspondant non seulement aux rythmes des contaminations mais aussi aux rythmes des succesions alagles, car ces interactions sont susceptibles de réduire ou d'amplifier les conséquences d'une pollution par ces toxiques dans les milieux aquatiques.

EN:

The aim of this paper is to present a review about the impact of Photo system II (PS II) inhibitors on algae communities. A brief discussion of the use in agriculture, the different chemical families, the photosynthetic inhibition effect and the occurrence of these compounds in aquatics systems is followed by the presentation of the impacts these herbicides have on algae.

Many studies investigate the effects of PS II inhibitors on algae growth and physiology. The response to pollutants were studied by monitoring changes in terms of different parameters : chlorophyll fluorescence induction usually increases with PS II inhibitors. Concentration of pigments decreases with PS II inhibitors, but increases sometimes with low contaminations of these toxicants (it is probably an homeostasis effect). Pigment ratio can change with herbicide exposure. Primary production (measured by ¹⁴C incorporation or dissolved O₂) usually decreases with PS II inhibitors. But, the " excretion " of dissolved organic compounds may increase with PS II inhibitors. These herbicides may alter or change cell morphology of algae. In consequence algae growth is inhibited by PS II inhibitors exposure. But growth inhibition varies, depending on each species' (and strain) sensibility or resistance to each herbicide.

By this way, PS II inhibitors can affect the algae community structure. In consequence, herbicides exert a selection pressure when the exposure reaches a certain level, and this for a sufficient period of time. Since organisms vary in their resistance to toxicants, the selection pressure will exclude the sensitive ones which will be replaced by resistant ones. Sometimes, responses to pollutants which are measured by global changes in biomass, pigments, dissolved O₂... can recover after a lagtime. This apparent ability to recover from effects of herbicides can be explained by the following selection effect : resistant species are indirectly stimulated and develop in the contaminated environment. The result is an algae community which has an increased resistance to these toxicants as compared to a community which has not been affected by the toxicants. This difference in resistance, between the unselected and the selected communities, may be detected by comparison of results from short term physiological tests performed with the respective community and by comparison of each community structure (taxonomy). This methodological approach called Pollution-Induced Community Tolerance (PICT) is of a great interest as a biological marker of specific pollution in aquatic systems.

Indirect effects of algae response to PS II inhibitors occur in the polluted ecosystem according to changes of the physicochemical conditions (decrease of dissolved O₂ concentrations and pH, increase of dissolved organic and inorganic matters ...). Furthermore, the impact of herbicides on algae communities varies, depending on the community's species composition (depending on successions) and on environmental factors such as interspecific interactions and physicochemical parameters (depending on seasonal changes). Interspecific interactions implies competition for the limiting nutrients among algae and allelopathic interactions among algae, as well as interactions between algae and other trophic levels (microbial loop, grazing pressure ...). These interactions between herbicides and environmental factors may reduce or emphasize the consequences of such a pollution in aquatic systems.

Seasonal change of algae communities species composition (algae successions) occurs as a response to changing environmental factors by the way of interspecific interactions and physicochemical parameters. Therefore, algae succession is affected by the herbicide destructuration of the algae communities. At the opposite, interactions and successions may affect the response of algae communities to the toxic. In this sense, herbicides act as a supplementary factor of disturbance in algae successions. Structural changes, induced by these herbicides, are usually accompanied by the attributes which are typical of an early successional stage. But, according to the " Intermediate Disturbance Hypothesis " the species richness should be maximal at intermediate intensities of herbicide contamination and at intermeadite frequencies of contaminations. The question is to compare the algae successions rythms and the frequencies of herbicides contaminations.

Then, the time factor (or the persisting quality, which is difficult to assess in experimental studies) has to be taken in account in monitoring aquatic polluted systems, because of the seasonal variability in the response of algae to PS II inhibitors as well as the seasonal variability of water bodies' contamination by these herbicides through watersheds. Moreover, usually, low values of herbicides' concentrations occur in aquatic environments but are persistant. It results that aquatic organisms are exposed during long periods of time, meaning that indirect effects, via interactions between herbicides and environmental factors, may be emphazised.

To further investigate these interactions and the herbicide persistance in aquatic systems, we have to develop experimental studies. This approach, however, must be complemented with in situ studies monitored with a timing of investigation relative to the natural population fluctuations and " pulses " of herbicides in these systems. Investigation must take place on various aquatic ecosystems. If a greater effort is given to monitor natural systems, for both herbicides and herbicides-induced effects, this will provide greater confidence in future predictions regarding the safety of PS II inhibitors in aquatic environments.

FR:

La sécheresse observée depuis plus d'une vingtaine d'années dans les pays sahéliens se fait également ressentir plus au sud dans des régions d'Afrique aux climats plus humides. Cette baisse de la pluviométrie et la diminution des apports en eau de surface qu'elle entraîne y sont de nature à pénaliser les différents projets de développements liés à l'eau. Le programme ICCARE mené par l'ORSTOM a pour objet l'identification et les conséquences de cette variabilité climatique dans l'ensemble de la zone non sahélienne d'Afrique de l'Ouest et Centrale, en s'appuyant sur les données de deux cents postes pluviométriques et sur un ensemble de méthodes alliant représentations cartographiques et procédures statistiques de détection de ruptures dans les séries chronologiques, univariées et multivariées.

La simple étude des séries chronologiques de hauteurs précipitées annuelles fait apparaître une nette et brutale fluctuation du régime pluviométrique dans toute la région considérée, à la fin des années 1960 et au début des années 1970. D'une manière générale, il apparaît que ce sont les zones à régime pluviométrique extrême qui ont subi les modifications les plus importantes : les plus arrosées (de la Guinée à la Côte d'Ivoire) et les plus arides (la bordure sahélienne au nord de la zone étudiée). Entre les deux, le phénomène est d'intensité plus nuancée. Les différentes procédures statistiques appliquées aux séries de hauteurs annuelles précipitées soulignent l'existence d'une rupture survenue à la fin des années 1960 ou au début des années 1970, et donc en phase avec ce qui a été observé et étudié au Sahel.

D'autres variables permettant une caractérisation plus "qualitative" du phénomène ont également été étudiées. Elles apportent un complément d'information quant aux manifestations de cette variabilité pluviométrique et montrent que la variabilité climatique se traduit à différents niveaux (durée des saisons des pluies, quantités précipitées hors saisons des pluies, etc.).

L'examen des séries chronologiques depuis l'origine des stations a permis de resituer l'événement observé dans une perspective historique faite d'alternances de périodes sèches et de périodes humides. Le phénomène observé à la fin des années 1960 et au début des années 1970 apparaît, cependant comme le plus significatif du point de vue statistique.

Si les causes premières d'apparition du phénomène sont, à l'heure actuelle, encore insuffisamment expliquées, et ce même si certaines activités humaines y ont, sans aucun doute, contribué, cette baisse de la pluviométrie a, bien entendu, des conséquences importantes sur la disponibilité des ressources en eau dans ces régions. Si la carence pure et simple n'est pas à craindre dans ces régions où les quantités précipitées restent importantes dans l'absolu, les effets de cette variabilité climatique peuvent, malgré tout, se révéler désastreux, en ce sens qu'ils modifient les données d'un équilibre déjà souvent mis à mal par ailleurs (pression anthropique et déforestation par exemple).

EN:

The drought observed for more than twenty years in the sahelian countries has also affected those located more to the South with more humid climates (SUTCLIFFE and KNOTT, 1987; NICHOLSON et al, 1988; MAHE and OLIVRY, 1991; OLIVRY et al, 1993 PATUREL et al, 1995). The decrease in rainfall and consequently that in runoff might penalise development projects linked with water supply. The ICCARE programme led by ORSTOM aimed at identifying and measuring the consequences of this climatic variability in the non-sahelian parts of the West and Central African region as a whole. The study was based on rainfall data from more than two hundred stations, break detection methods in the time series as well as cartographic tools were used.

This study allowed to highlight the manifestations of the climatic variability observed for nearly 25 years in West and Central Africa. Whereas it had been thought for a long time that the rainfall deficit was restricted to the sahelian region, this study showed that the forest covered regions were also affected and generally speaking the so-called 'humid Africa'.

The decrease in rainfall entails that in runoff and thus a change in water resources availability which is the cornerstone of a fairly great number of development projects. Hydrological regimes variability and possible modifications of rainfall-runoff relationship are to be the next stages of the ICCARE programme, partial results were already published (SERVAT et al, 1997).

A simple study of the time series showed straightforward fluctuations of the rainfall patterns (figures 2 and 3), which happened during the late sixties or the early seventies over the whole region.

The mapping of the time series analysis depicts a clear general trend towards a shift of the isohyets to the south-south-west from the 1950's to the 1980's. This shift reflects a sharp drop in the annual rainfall over the whole of the non-sahelian west and central Africa.

Generally speaking it appears that the zones with extreme rainfall patterns underwent the most important modifications, namely : the wettest ones from Guinea to Ivory Coast and the driest ones, bordering the sahelian region to the North of the studied area. Elsewhere changes are less drastic (SERVAT et al, 1996).

The different statistical procedures applied to the series of annual rainfall showed breaks taking place during the late 1960's or the early 1970's (figure 4), which was in keeping with what had been observed in the sahelian region. Rainfall deficits were in the order of 20% and they could reach values higher than 25% (table 1), in particular along the Atlantic Coast or in the North, which upholds the fact that 'humid Africa' was also severely affected by the rainfall variability.

Other variables which allow a more qualitative characterisation of the phenomenon were also studied (table 2) (figures 5 and 6) ( PATUREL et al, 1997, SERVAT et al, 1997). They brought complementary information about the ways the rainfall variability expresses itself. The pattern of the rainy season was slightly different from what it had been before the 1970's, its length was generally shorter either because it started later than before or because it ended earlier. Likewise, the rainfall distribution was modified, which resulted in a more 'homogeneous' pattern for the zone with only one rainy season and in a sensible change in the ratio of the rainfall heights of the two rainy seasons.

Some of the regions of the so-called 'wooded' savannah saw a modification of their climate with a shift from a 'guinean' climate toward a 'sudanese' one. In west Africa some regions saw also a decrease of the amount of precipitation occurring outside the rainy season, which led to a strengthening of the dry season and contributed, if need be, to the perception of the phenomenon by local populations. The decrease in the number of rainy days, where it was possible to study it, was in line with the rainfall deficit.

A complementary statistical approach was carried out, it consists of a spatio-temporal study using a multidimensional exploratory analysis (KHODJA et al, 1998). This led to using a multivariate test for detecting a shift in the mean value. This approach confirmed the results obtained with univariate analyses whether it is for the time location of the break (late 1960's, early 1970's) or whether for the heterogeneous character of the phenomenon both from a spatial or temporal point of view. A major characteristic of this persisting rainfall deficit seems to be the existence of two axes of heterogeneity along the north-south and east-west directions (table 3) (figure 7).

The survey of the rainfall time series from the origin allowed to place the drought in a historical perspective. So, it appears that, since the beginning of the century, the region underwent a succession of dry and wet periods, although it is difficult to speak of cycles. The phenomenon observed during the late 60's and the early 70's appears, however, as the most significant from a statistical point of view. Besides, the still lasting period of deficit has displayed a length and an intensity quite remarkable, in particular in the north and west sectors of the studied zone where the phenomenon presents an even more exceptional character (figures 8,9,10 and 11).

Even though what brought about this diminution in rainfall remains, until now, unexplained, certain human activities undoubtedly contributed to the aggravation of the phenomenon. Although deforestation cannot be held entirely responsible for the drought, the fact remains that overlogging helped to increase the rainfall deficit in numerous regions along the Atlantic coast and the Guinean gulf.

Of course, this rainfall deficit has important consequences on the availability of water resources in those regions. Agriculture, the filling of dams and therefore the hydroelectric production, to mention only a few domains, are strongly penalised by this decrease in resources.

If a real shortage is not to be feared in those regions where the quantities of precipitation remain high in absolute terms, the effects of that variability can still prove to be disastrous, as they modify the elements of a balance that is already threatened by other factors (anthropic pressure and deforestation, for example).

The ICCARE programme, which goes on with the study of the modifications of the river hydrological regimes, will give answers as to the effect of the rainfall deficit upon water resources availability.

FR:

La contamination des eaux naturelles par les micropolluants organiques tels que les herbicides, nécessite le recours à des procédés très performants, en particulier l'adsorption sur charbon actif pour satisfaire aux normes de potabilité sur les eaux distribuées. Si le charbon actif en poudre (CAP) présente une bonne efficacité pour l'élimination des herbicides seuls dans l'eau distillée, la présence dans l'eau d'autres composés organiques et / ou minéraux aura pour conséquence la modification des paramètres de l'équilibre de l'adsorption du système initial et la modification des performances du matériau adsorbant. Cette étude a montré que le CAP utilisé présente une bonne affinité pour les deux herbicides étudiés (atrazine et diuron), dont les capacités d'adsorption, déterminées par le modèle de Langmuir, sont 1,42 et 1,72 mmol.g^-1 respectivement. La présence de composés organiques tels que les tensioactifs anionique le dodécylsulfate de sodium (DSS), et cationique le bromure d'hexadécyl triméthyl ammonium (BHTA) provoque une forte diminution de la capacité d'adsorption des deux herbicides par le charbon actif. Le BHTA provoque l'effet le plus marqué, par contre, les ions calcium et bichromate n'ont pas d'influence sur l'adsorption des deux herbicides. Les essais de désorption ont montré que l'introduction des coadsorbats organiques provoque la désorption d'une partie de l'herbicide. Ce phénomène est accentué dans le cas du DSS en présence d'ions calcium. L'application de modèles simples de coadsorption, a permis de mettre en évidence l'existence presque exclusive de sites spécifiques d'adsorption pour chacun des substrats étudiés et une inhibition non compétitive due a une interdépendance possible entre les sites d'adsorption et une altération des interactions de chaque composé à la surface du charbon.

EN:

Contamination of natural waters by organic micropollutants like pesticides requires the use of powdered activated carbon (PAC) during the flocculation step of water treatment. One of the main factors affecting the efficiency of this process is the presence in natural waters of organic and mineral compounds. The aim of this study was to determine the influence of two surfactants [(i) anionic (sodium dodecyl sulfate : SDS) and (ii) cationic (hexadecyl-trimethyl ammonium bromide : HTAB)], and two mineral ions (calcium and dichromate) upon atrazine and diuron adsorption on powderd activated carbon resulting from Chemviron Filtrasorb-400. The adsorption capacities on PAC of atrazine and diuron alone were determinated according to the Langmuir model for the isothermes (table I). They were estimated to be 1.42 and 1.72 mmol.g^-1 respectly at pH 5.5. Coadsorption results pointed out that the herbicide adsorption was inhibited in the presence of surfactants whatever their nature (fig. 1 and 2).This inhibition was most importante with HTAB (45%), according to the zeta surface potential and the streric dimensions of molecules, than with SDS (23%) for atrazine . The same result was noted for diuron with an inhibition respectly of 15% for SDS and 27% for HTBA. Calcium and dichromate ions, each one with one herbicide had no effect. In any case, Langmuir isothermes were drawn and adsortion capacities were calculated with a good correlation (tables II and III). The influence of coadsorbats was always greatest for atrazine than for diuron. All the experiments were performed at pH=5.5, without influence upon the adsorption, even for the dichromate ions.

Desorption tests showed that a part of adsorbed pesticides was desorbed with surfactants introduction (fig. 3) : with SDS 12.3% of atrazine and 8% of diuron against respectly 23% and 17% with HTBA. The phenomenon was increased with SDS in presence of calcium ions (fig. 4 and 5) according to the increase of SDS adsorption (table II and III). The use of two simple coadsorption models was reinforced by the calculated values obtained for adsoartion capacities (fig. 6 and 7) which were very closely with the experimental ones.

The calculation of constantes given by the models confirmed the existence of specific sites of adsorption for every compound and a non competitive inhibition due to a change in the nature of the interactions between pesticides and activated carbon surface (table III).

FR:

L'objectif de cette étude est de vérifier le potentiel des images radar à synthèse d'ouverture (RSO) pour estimer l'équivalent en eau du couvert nival sur le bassin de la rivière La Grande (Baie de James, Québec). Il s'agit d'un milieu dominé par une forêt ouverte d'épinettes noires, des brûlis et des tourbières. Cette information intéresse grandement Hydro-Québec qui gère plusieurs installations hydro-électriques dans cette région subarctique. Durant deux ans, six campagnes de terrain ont été réalisées sur le bassin de la rivière La Grande et une dizaine d'images RSO du satellite européen ERS-1 ont été acquises, étalonnées et géoréférencées, afin de déterminer la relation entre les coefficients de rétrodiffusion des images radar (hiver et automne) et la résistance thermique du couvert nival. Cette relation constitue la première partie d'un algorithme d'estimation de l'équivalent en eau. Elle utilise plus spécifiquement le rapport de rétrodiffusion, qui est la différence entre une image avec neige et une image sans neige. La deuxième partie de cette algorithme déduit l'équivalent en eau du couvert de neige à partir de sa résistance thermique et de sa densité, en se basant sur la relation physique établie par les mesures de terrain. L'équivalent en eau du couvert nival a donc été estimé pour quatre images de février et mars 1994 et 1995. L'erreur moyenne sur l'estimation de l'équivalent en eau de la neige au sol est de 2% à 3% (-5 à 7mm) sur l'ensemble des sites d'échantillonnage avec un écart-type de 14 à 19% (-35 à 45mm). Ces résultats ont encouragé Hydro-Québec à poursuivre la recherche avec les données du satellite canadien RADARSAT (opérationnel depuis le 1er avril 1996) et à développer un prototype pour la cartographie de l'équivalent en eau du couvert nival à partir d'images radar.

EN:

The goal of this study was to evaluate the potential of synthetic aperture radar (SAR) images for estimating the snow water equivalent (SWE) on the La Grande river watershed (James Bay area, Québec). This information is of major interest for Hydro-Québec, which exploits many hydroelectric complexes throughout this subarctic region. The La Grande watershed is composed of moderately dense to opened black spruce forests, opened areas, burned areas and peat bogs. Over two years (1994-1995), six field campaigns were carried out on a study site located between the LG4 and Laforge1 reservoir, in the center of the La Grande river watershed. The field measurements were of two types: 20 snow lines (depth, snow water equivalent (SWE), density) and 8 snow profiles (depth, density, grain size, temperature, dielectric constant). With these data, the thermal resistance of the snowpack was calculated for every test-site, using the depth, density and thermal conductivity of each layer.

Concurrently, more than 10 SAR images (European Satellite ERS-1) of the study site were acquired, calibrated and georeferenced. The backscattering coefficients of all winter images were extracted. Using a reference image (snow-free), backscattering ratios were calculated. They are the difference between a winter image and a snow-free image. This process is used to reduce the impact of vegetation and topography. Then, the relationship between the backscattering ratios and the snowpack thermal resistance of february and march 1994 are established, as the first part of an algorithm developed to estimate the snow water equivalent. The second part of the algorithm infers the snowpack water equivalent from its thermal resistance and density, based on the physical relationship established with field data. This approach is based on studies conducted by INRS-Eau in a southern Quebec agricultural area (BERNIER and FORTIN (1998)). The hypothesis are based on the following:

- The snowpack characteristics influence the underlying soil temperature;

- The dielectric constant of the soil varies with the soil temperature under 0°C;

- The radar signal is influenced by the soil dielectric constant;

- Thus, the snowpack characteristics (thermal resistance) influence the radar signal.

However, due to variations of soil humidity on the date of the reference image (september 1994), two slightly different relationships were obtained. One for open areas and open forests and one for burned areas and peat bogs. This shows the importance of using a good reference image, with homogeneous soil conditions. It could be better to obtain an image later in the fall, when the soil is frozen.

The relationships established here are preliminary, as they use a small dataset. It is estimated that a better regression should be obtained with the acquisition of more images and with a greater range of snow characteristics. However, the algorithm is applied to test the applicability of the method.

First, the algorithm was applied on the test-sites, using the images from February and March of 1994 and 1995. The mean error on the estimation of the snow water equivalent is 2% to 3% ( 5 to 7mm), with a deviation of 14% to 19% ( 35 to 45mm). The results are comparable for both years, even if the algorithm is based on 1994 data only. Secondly, the algorithm is applied on the whole images. A classification of a Landsat-TM image is used to identify the land cover of every pixel, which determines the regression and the snow density to be used in the algorithm. Four maps of the SWE are produced and resampled to a resolution of 500m. These are compared with the field measurements from the four nearest Hydro-Quebec snow survey sites. The SWE measured by Hydro-Quebec are all within the most dominant SWE class of each map. Further validation of the results will be possible when the algorithm can be applied on a sub-watershed, which is the actual scale used by Hydro-Quebec.

However, the results of this study were sufficiently promising to Hydro-Quebec to support a follow up research with data from the canadian satellite RADARSAT (operational since april 1996). Meanwhile, to improve the algorithm, it is important to obtain a good and homogeneous reference image, to better assess the impact of the land cover and to acquire a dataset with a greater range of snow characteristics.

FR:

Depuis une dizaine d'années, la communauté scientifique s'est beaucoup intéressée à l'hypothèse d'un réchauffement à l'échelle planétaire. De nombreuses études ont porté sur l'analyse de ces modifications climatiques éventuelles ainsi que sur la modélisation de leurs impacts sur les ressources en eau. Cependant, malgré l'attention croissante que reçoit le sujet des modifications climatiques, très peu de travail a été accompli pour mettre en place des réseaux de mesure spécialement conçus pour l'étude des modifications climatiques et leurs impacts sur les ressources en eau, et pour créer des bases de données adaptées à cet objectif. Cette tâche est encore plus nécessaire dans le cadre des réductions budgétaires auxquelles sont soumis les réseaux hydrométriques dans certains pays développés. Cet article présente les bases d'une étude dont l'objectif est la conception d'un réseau hydrométrique pour le suivi des modifications climatiques dans la province de Québec, Canada. Le but est d'identifier, afin de les conserver, les stations de jaugeage les plus adéquates pour accomplir cette tâche. L'article présente aussi une brève revue des types de modifications climatiques qui peuvent être observés et de certains tests qui existent pour leur détection et leur quantification. Une procédure bayésienne de détection des sauts de la moyenne a été sélectionnée sur la base de ses avantages théoriques, et appliquée aux séries de données des stations retenues au Québec.

EN:

The 1980s and 1990s contained most of the warmest years since the beginning of worldwide temperature recording nearly 140 years ago. Widely accepted estimates project that the earth's average temperature might increase by about 2°C over the next 100 years. It is also expected that, as a result of global warming, the frequency and intensity of floods and droughts may change. However, despite the increasing attention that the issue of climate change receives, there has been little effort to develop a systematic approach for the collection of relevant data, and to establish observational networks specifically designed for the analysis of climate variability and change and their impact on hydrologic regimes and water resources in general. This task is particularly important given the major network reductions that result from recent cutbacks in the funding of monitoring programs in Canada and other countries. This paper presents the results of a rigorous study that was carried out recently, and aimed at establishing a hydrometric network for the study of the attributes of climate change and variability across the province of Quebec (Canada) and their impact on water resources. The approach is based on identifying and maintaining stations that can help provide an understanding of the physical processes within the hydrological cycle and account for climate variations across the province. This network will be of fundamental importance in establishing scientific evidence of the magnitude and direction of possible shifts in climate patterns across the province. These aspects are of global significance and must be considered during the rationalization of monitoring networks. The results of the application of this procedure to the hydrometric network of the province of Quebec are presented. The paper presents also a brief review of the various types of non-stationarities that can be observed in hydrologic data series, and some of the current approaches that can be used for the detection of these non-stationarities. Several statistical tests and procedures have been proposed in the literature for the analysis of the characteristics of data samples and for hypothesis testing, for various types of non-stationarity. A Bayesian procedure, proposed by Lee and Heghinian (1977) and generalized by Bernier (1994), for the detection of shifts in the mean of hydrological and meteorological time-series is selected based on its theoretical advantages (Faucher et al., 1997). The procedure is then applied for the analysis of all streamflow series of selected stations in the province of Quebec with the objective of extracting information on possible climatic changes. Results indicate the presence of significant non-stationarities for a number of the series analyzed. For five stations, the most probable date for the shift in the mean level falls in the period 1983-1985. Recommandations are made for future research activities.