Volume 11, Special Issue, 1998

FR:

Le succès d'une gestion des écosystèmes naturels requiert une connaissance approfondie des différents processus qui interviennent et de leurs échelles de temps et d'espace particulières. Pour cette raison, les décideurs ont besoin d'analyser une vaste gamme de données et d'informations géographiques. Les modèles mathématiques, les systèmes d'informations géographi-ques et les systèmes experts sont capables de produire cette analyse, mais seule une minorité de gestionnaires les utilise actuellement. Cet article identifie quelques unes des raisons à l'origine de l'hésitation des gestionnaires à adopter de tels outils d'aide à la décision pour la gestion des ressources naturelles et propose une structure qui pourrait faciliter leur utilisation pour le processus de prise de décision. Cet exercice est réalisé à l'intérieur du contexte de la gestion intégrée par bassin. Une revue des systèmes d'aide à la décision est également présentée.

EN:

Many methods of integrated or watershed management exist which account for the necessary biophysical and socio-economic factors at the watershed level. Some of these approaches are ecosystem oriented while others are socio-economically oriented. Whatever the definition, water management at the watershed level needs to account for a plenitude of variables related to the air, water, soil, biology, and economy. The successful management of natural ecosystems requires a thorough understanding of their characteristic time and spatial scales. Because of this, decision makers need to analyze a wide range of data and geographic information. Mathematical models, geographic information systems and expert systems are capable of performing this analysis, but only a minority of managers are currently using them. This paper identifies some of the reasons why ecosystem managers have been slow to adopt such decision support tools in natural resources management and proposes a framework to facilitate their use in the decision making process. This is done in an integrated watershed management context. A review of related decision support systems is also presented.

Four types of decision-support tools are introduced : mathematical models, expert-systems, geographical information systems (GIS) and decision support systems (DSS). Mathematical models have long been used for simulation, prediction, and forecasting, however, they are often task specific and were rarely developed for management uses. GIS are more and more commonly being used for decision support as they become more affordable and user-friendly and are very well-suited for managing resources at a spatial scale. There exist many kinds of software ranging from a simple viewer used for cartographic purposes to complex GIS oriented toward spatial analysis and modelling. Expert systems are also interesting for decision support when specific goals are being considered. Finally, DSS are perhaps the digital tools most applicable to management purposes, often integrating one or more models, a GIS or expert system functionalities. There are two types of DSS :

1. Environmental Information Systems (EIS), and

2. Integrated Modelling Systems (IMS)

EIS can be very user- friendly, relying heavily upon GIS and statistical functions.

IMS also use GIS capabilities, but integrates several mathematical models as well. The level of integration between models varies considerably and the complexity of IMS are generally high.

Two questions underlie the operational use of digital technologies for decision support. The first is whether or not such technology should be used at all, while the second is why such tools take time to be adopted by government and management agencies. The use of digital technologies is often required when the problem is complex and where there are a wide range of factors involved with different spatial and temporal scales. Three major constraints towards the implementation of decision support tools can be pinpointed :

1. technology,

2. data, and

3. working organization.

Technological constraints include cost, lack of user friendliness, and hardware problems, among other factors. Data constraints are mostly related to availability, cost, heterogeneity and volume. Finally, organization constraints pertain mostly to the manager's perception of the tool and the structural integration of the tool within the decision process.

This paper proposes a 4-step approach to optimize the use of decision-support tools. The first step requires that managers and decision-makers clearly define their project, goals and budget, as well as, decide whether to use an integrated watershed management approach or a more discrete approach. This leads directly to the second step, which consists of choosing the most appropriate digital support tool. This requires communication between managers and scientists, and at this point, data gathering and integration should begin. The third phase consists of the development of a new tool or adaptation of an existing one within the context of the agency's management structure. The final step is the operational use of the decision support tool by the agency, following an initial trial period. The successful use of a decision support tool for management purposes depends on proper planning that accounts for all factors related to management needs, budget, data, ease of use, and organization integration.

FR:

Cet article brosse un portrait de différents types de modélisation hydrologique développés à ce jour. Nous passerons donc en revue l'hydrologie, à l'érosion hydrique des sols, au transport et aux transformations des polluants et à la qualité de l'eau en rivière. Ce bref survol, nous amène à conclure que si le développement de la modélisation hydrologique s'est fait jusqu'ici essentiellement en affinant la description des processus et en considérant des échelles spatiales et temporelles plus fines, l'étape suivante passe par l'intégration de ces divers modèles. Cette intégration permettra dès lors de considérer un ensemble de problématiques directement liées aux aspects de gestion environnementale.

EN:

This paper presents an overview of physically-based hydrological modeling approaches and a look at the future of hydrological modeling within the context of water management. It extends beyond classical hydrological modeling by surveying the modeling of water contaminants transport in porous media and surface waters, as well as soil erosion.

Increasing concerns in predicting the impacts of land use management on the hydrological cycle have led researchers to construct two types of physically-based distributed models. The first type of model views the watershed as an ensemble of inter-connected reservoirs and mimics water routing with various types of discharge expressions and conceptual models (e.g., the infiltration models of Green and Ampt (1911), Holtan (1961) or Smith and Parlange (1978); the unit hydrographs of Sherman (1932) and Dooge (1973) and the geomorphological unit hydrograph of Rodriguez-Iturbe and Valdes (1979); the ground water discharge model of Beven and Kirby (1979); etc...). It is noteworthy that the pioneering Stanford Watershed Model of Crawford and Linsley (1966) led to the development of many currently used hydrological models including HBV (Bergstršm and Forsman, 1973), SLURP (Kite, 1978), TOPMODEL ( Beven and Kirby, 1979) and CEQUEAU (Morin et al., 1981), to name a few. The second type of model discretizes the watershed into an ensemble of control volumes and mimics water routing using combinations of partial differential equations for mass and momentum conservation and phenomenological models (e.g., Darcy's (1856), Dupuit's (1863), Boussinesq's (1904) and Richards (1931) equations for unsaturated and saturated flow in porous media; Saint-Venant's (1871) and Manning's (1891) equations for overland and open channel flows). Hydrological models such as SHE (Abbott et al.,1986a, b), IHDM (Calver, 1988), KINEROS (Woolhiser et al., 1990), THALES (Grayson et al.,1992) and HYDROTEL (Fortin et al., 1995), among others, represent classical examples of this type of modeling. It is noteworthy that recent advances in remote sensing and in digital elevation modeling have greatly facilitated and simplified the use of most of the hydrological models.

On another front, the adverse effects of agricultural, industrial and urban runoff on surface and ground waters have motivated the development and application of different approaches to predict the fate and transport of various water contaminants in the environment (i.e., eroded soil particles, adsorbed and dissolved nutrients and pesticides as well organic matter).

In soil erosion modeling, these concerns have led researchers to construct nonpoint source pollution models for evaluating the impacts of alternative land management practices on water quality. Based on the empirical Universal Soil Loss Equation (Wischmeier and Smith, 1978), the first nonpoint source models included CREAMS (Knisel et al., 1980), AGNPS (Young et al., 1987) and SWRRB (Williams et al., 1985). However, the lack of physical realism in these empirical formulations prompted the development of physically-based erosion models such as GUEST (Rose et al., 1983; Hairshine and Rose, 1992a, b), WEPP (Nearing et al., 1989), LISEM (De Roo et al., 1994) and EUROSEM (Morgan et al., 1992). The advantage of these models over the USLE resides in their ease of integration with physically-based hydrological models.

Because of its close ties with the hydrological cycle and the soil erosion process (adsorbed and dissolved contaminants), the development of physically-based models for nutrient and pesticide transport benefited directly from advances in soil erosion modeling, soil chemistry and soil physics. The modeling of nitrogen transport is a representative example of this. Early modeling efforts involved the coupling of first-order kinetics models for the nitrogen cycle (Mehran and Tanji, 1974) with two types of mass conservation equation in porous media: the convection-dispersion equation and the capacity transport equation. Well known soil nitrogen dynamics models include NCSOIL (Molina et al., 1983), SOILN (Johnsson et al, 1987), EPIC (Sharpley and Williams, 1990), LEACHN and LEACHA (Hutson and Wagenet, 1991, 1992, 1993), DAISY (Hansen et al., 1991) and AgriFlux (Banton et al., 1993).

The first attempt to model surface water quality goes back to the work of Streeter and Phelps (1925) who studied the impacts of a municipal waste water discharge on dissolved oxygen (DO) and biological oxygen demand (BOD) of an Ohio river. To predict DO and BOD dynamics, Streeter and Phelps assumed uniform and steady flow conditions and used first-order kinetics to model atmospheric supply of oxygen and oxygen consumption. The advances in computational power during the 70s and 80s allowed several researchers to substantially increase the complexity of the Streeter-Phelps approach. This was achieved by accounting for advection-dispersion phenomena, unsteady two and three dimensional flow conditions, as well as the effect of temperature on various chemical reactions. The QUAL2E model of Brown and Barnwell (1987) is a good example of a moderately complex water quality model where advection-dispersion and temperature effects on several water characteristics and contaminants are considered under one-dimensional steady flow conditions.

At present, the state of hydrological modeling and software engineering has reached a point where it is now possible to construct spatial decision support systems (SDDS) capable of simulating the impacts of various management practices (i.e., industrial, municipal and agricultural) on the water quantity and the quality of a watershed's river network. These systems, which idealy should be user-friendly for decision makers, will be both integrated modeling systems (including a database system, hydrologic, soil erosion, agricultural-chemical transport and water quality models) and spatial data analysis systems (including a geographical information system). Currently developed SDDS include PÉGASE (Smitz et al., 1997) and GIBSI (Villeneuve et al., 1996, 1997a,b). In a sustainable water management context, the use of such systems will provide decision makers with a complete tool for exploring a variety of integrated watershed management programs.

FR:

L'utilisation de l'information historique dans une analyse fréquentielle permet de mieux mobiliser l'information réellement disponible et devrait donc permettre d'améliorer l'estimation des quantiles de grande période de retour. Par information historique, on entend ici de l'information relative à des grandes crues qui se sont produites avant le début de la période de mesure (dite période de jaugeage systématique) des niveaux et débits des lacs et rivières. On observe de manière générale que l'utilisation de l'information historique conduit à une diminution de l'impact des valeurs singulières dans les séries d'enregistrements systématiques et à une diminution de l'écart-type des estimations. Dans le présent article on présente les méthodes statistiques qui permettent la modélisation de l'information historique.

EN:

Use of information about historical floods, i.e. extreme floods that occurred prior to systematic gauging, can often substantially improve the precision of flood quantile estimates. Such information can be retrieved from archives, newspapers, interviews with local residents, or by use of paleohydrologic and dendohydrologic traces. Various statistical techniques for incorporating historical information into frequency analyses are discussed in this review paper. The basic hypothesis in the statistical modeling of historical information is that a certain perception water level exists and that during a given historical period preceding the period of gauging, all exceedances of this level have been recorded, be it in newpapers, in people's memory, or trough traces in the catchment such as sediment deposits or traces on trees. No information is available on floods that did not exceed the perception threshold. It is further assumed that a period of systematic gauging is available. Figure 1 illustrates this situation.

The U.S. Water Resources Council (1982) recommended the use of the method of adjusted moments for fitting the log Pearson type III distribution. A weighting factor is applied to the data below the threshold observed during the gauged period to account for the missing data below the threshold in the historical period. Several studies have pointed out that the method of adjusted moments is inefficient. Maximum likelihood estimators based on partially censored data have been shown to be much more efficient and to provide a practical framework for incorporating imprecise and categorical data. Unfortunately, for some of the most common 3-parameter distributions used in hydrology, the maximum likelihood method poses numerical problems. Recently, some authors have proposed use of the method of expected moments, a variant of the method of adjusted moments which gives less weight to observations below the threshold. According to preliminary studies, estimators based on expected moments are almost as efficient as maximum likelihood estimators, but have the advantage of avoiding the numerical problems related to the maximization of likelihood functions.

Several studies have emphasized the potential gain in estimation accuracy with the use of historical information. Because historical floods by definition are large, their introduction in a flood frequency analysis can have a major impact on estimates of rare floods. This is particularly true when 3-parameter distributions are considered. Moreover, use of historical information is a means to increase the representativity of a outlier in the systematic data. For example, an extreme outlier will not get the same weight in the analysis if one can state with certainty that it is the largest flood in, say, 200 years, and not only the largest flood in, say, 20 years of systematic gauging.

Historical data are generally imprecise, and their inaccuracy should be properly accounted for in the analysis. However, even with substantial uncertainty in the data, the use of historical information is a viable means to improve estimates of rare floods.

FR:

La pratique de l'hydrologie, appliquée au milieu urbain, a beaucoup évolué depuis les années 1960. De nouveaux outils scientifiques, méthodologiques et technologiques ont été mis au point. En ce qui concerne les eaux pluviales urbaines, un changement de philosophie complet s'est produit. Aujourd'hui, des développements rapides continuent à se produire pour faire face de la meilleure manière possible aux graves problèmes qui se posent en zone urbaine : inondation et pollution du milieu naturel. Deux approches récentes illustrent ces développements : la gestion des risques et l'approche globale sur le bassin versant. Une évaluation de plus en plus précise de l'aléa et de la vulnérabilité s'avère nécessaire ainsi qu'une planification hydrologique bassin versant par bassin versant. Celle-ci devrait conduire à définir et à hiérarchiser les principaux objectifs que l'on se fixe dans les domaines du contrôle des inondations, du contrôle de l'érosion, de l'amélioration du milieu naturel et de la protection de l'environnement. Les actions à mener doivent utiliser les outils de modélisation des phénomènes hydrologiques sur les bassins versants. L'efficacité de ces approches est illustrée par un exemple pris sur le bassin versant de la rivière Beauport au Québec.

EN:

Urban hydrology practice has evolved a great deal, keeping up with the evolution of urban problems. Thus, professional engineers have had to keep up with this evolution, in order to understand the effects of urbanization on the hydrological behavior of the systems for which they have to recommend best solutions. Engineers have also had to evolve from the traditional approach of a limited vision of the problem, of its extent and of its impacts on the system. A global vision of the entire system, grouping every watershed element, such as forestry, agriculture or urbanization, is necessary if one is to circumscribe the problem, to analyze it and to reach to the best long-lasting solution. This article represents a synthesis of the hydrological disturbance phenomenon caused by uncontrolled urbanization, not properly planned, which is often characterized by numerous short-term solutions, often ineffective in the context of a long-lasting approach.

Examples of natural disasters caused by meteorological events are more and more frequent. Take the case of Vaison-la-Romaine, in September 1992, where the Groseau devastated the Vaison community. More recently, the 1996 summer floods in the Saguenay region, in Québec, demonstrate once more that water always tends to return to its original bed, now occupied by artificial structures. When these types of events are analyzed in detail, we notice that meteorological phenomena are not always the main cause of the disaster cause and that often part of the responsibility lies with the occurrence of human activity in the catchment. This observation shows the importance of using wisdom and humility towards these natural forces and of anticipating, during design, a "secure" passage for the inevitable flows that one day will exceed the structure capacity. Knowing all the interactions that exist in the heart of the stormwater management problems, it is risky to intervene in isolation without analyzing the impact of the action. The approach must then be global and coherent; the tool necessary for success is the stormwater master plan made on a watershed basis. The master plan allows us to properly describe the problematical elements, to identify the real causes and to optimize the location of the control solutions. It also allows us to manage the increase in stormwater due to urbanization, with the "flow set point" concept of each watercourse tributary.

This article presents an application example where the global analysis approach is used and where the solution involves different interventions and developments, which, when combined, effectively treat backwater effects, flooding and erosion in the presented area. The master plan of the watershed containing this sector had already been conceived and the specific flows of each of its tributaries had been identified; the global approach method therefore allowed us to settle the local problems in the studied area while respecting the flow granted by the master plan. Finally, a floodable plain, containing marshes, completes the intervention plan by combining the restoration of certain watercourse ecological characteristics with the need for flood control. This example demonstrates the necessity and the advantages of approaching urban drainage problems at the watershed level. However, watershed management concepts lead planners and designers to deal with antagonisms: urbanization or renaturalization? Obviously, in urban areas, the return to natural conditions is impossible and the disturbances to watercourses generated by urbanization cannot all be compensated for. Rather, we must aspire to the establishment of a balanced environment by controlling inflows produced by the watershed and by supporting the system to receive them. The notion of feasibility must always occupy the thoughts of the master plan designers and planners. The credibility and the continuity of the master plan depend on all these considerations; any unrealistic flow order, on an implementation level, can compromise the balance of the management plan and its applicability, which in turn can lead to a confused plan, possibly having disastrous consequences. The ultimate consequence of the management plan is the protection of the watercourse, the development of which can be designed to meet different criteria, such as flood control, erosion control, ecological potential enhancement, etc. It is at this stage that management becomes most complex: flood control, for example, does not apply to the same type of events as erosion control. The former requires management of major hydrological events, whereas the second needs implies control of frequent flows, which are at the origin of bank undermining caused by differential volume excesses. The management scheme must then handle multiple events and multiple criteria; consequently, it becomes more complex but also grows in value and justifies itself more adequately. Furthermore, the whole question of watercourse quality, in the broad sense of the term, must be managed from a frequent-event point of view, as this yields the best physical, ecological and aesthetic image of the aquatic environment. To this effect, present efforts in the control of combined sewage network overflows can be seen as a predecessor of what will later be extended to the entire stormwater network. This whole aspect of management has not been treated in this paper, as many others are dedicated to doing so. The evolution of the situation is similar in France and in Québec; coherent watershed management must rest on clear political goals regarding environmental use, risk protection and urban development. The stormwater master plan must, permanently, become part of the water resource master plan for the whole watershed, with its objectives of flood control, erosion control and maintenance of acceptable physical-chemical and biochemical water quality, as well as assured resource use (drinking water--, hydroelectricity, agriculture, industrial waters, etc.). This water resource master plan will have to be integrated in the same way as the French Water Agencies do today, and as Québec is preparing to do so.

FR:

L'étude des activités hydrologiques menées au cours des cinquante dernières années dans les pays francophones de l'Afrique sub-saharienne apporte un précieux éclairage sur les difficultés rencontrées, les modes de collaboration adoptés et l'ampleur des défis à relever dans le domaine de l'eau à l'aube du troisième millénaire.

Ceci est l'occasion de tracer les étapes marquantes de l'acquisition de la " connaissance hydrologique " depuis l'ère pionnière des années 50 jusqu'aux programmes scientifiques les plus récents faisant appel à des technologies avancées. Cette rétrospective permet également d'apprécier l'évolution des modes de partenariat entre acteurs scientifiques, techniques et économiques du Nord et du Sud.

Ces travaux et recherches ainsi entrepris et développés montrent à la fois l'importance du savoir acquis et la nécessité de poursuivre les études en cours. Ils dressent aussi un constat encourageant quant à l'avenir des projets régionaux africains en hydrologie grâce au renforcement des relations scientifiques entre pays de la région.

EN:

This article describes a study of hydrological research carried out in the French-speaking countries of sub-Saharan Africa over the past fifty years, by all the parties concerned.

The work sheds useful light on the difficulties encountered, the types of co-operation adopted, and the scale of the challenge now facing the region in the matter of water resources, on the eve of the third millennium.

The first stage in the acquisition of hydrological knowledge in the region was the pioneer period of the 1950s, when many different kinds of water demand were emerging very fast but hydrological information was almost nil. Hydrometric networks incorporating 1,500 instrument stations had to be rapidly set up for an overall survey of water resources. 200 representative or experimental catchments were equipped. The data obtained from the stations provided practical answers, meeting the operational needs of development, though in many cases the data acquired were used again, later on, for pure research purposes. The data gathering was a huge task, carried out on a range of different geographical scales. To achieve it, hydrologists had to develop appropriate methodologies for the region's tough climate and difficult field conditions; they also had to train highly motivated, seasoned hydrologists.

During the sixties, the first hydrological monographs on the major river basins were completed,, rue Lafayette - research was conducted on regimes and ten-year flood peaks, and findings on the small catchments were collated. All this provided a first sketch of the region's main hydrological features.

Two major upheavals marked the end of the 1960s: a long, disastrous drought and the information technology revolution.

The scale of the hydropluviometric deficits recorded over several decades, amounting in some catchments to 30%-40%, led scientists to wonder how stable water resources really are and whether the notion of "normal" in hydrology or rainfall has any validity. The question was especially relevant because the design of all major African water engineering schemes completed around 1960-65 was necessarily influenced by the flow rates recorded in the previous, wet period. Changes in ground surface states and the environment in general, under the impact of climate change and increasing human activity respectively, made a more multidisciplinary approach to the hydrological cycle indispensable.

Information technology also had a major impact, as the many numerical data gathered could at last be properly analysed and exploited. User-accessible reference databases were developed and processing software packages like Hydrom and Pluviom were widely distributed. It was now possible to model the relations between rainfall and discharges at a detailed scale; modelling improved steadily, first with the introduction of fine-mesh models, then of coupled surface water-groundwater models.

As regards water engineering work, flood routing models were significantly improved for the Niger and Senegal rivers, and dam operation simulation models made spectacular strides. Of course, data processing greatly facilitated the use of statistical laws for everything to do with engineering hydrology.

For several decades, metrology made little progress; but the instruments in use were robust and easy to use, and so well suited to local conditions. Not until the early 1980s was there significant technological progress in sensor technology; meanwhile civilian satellites made their debut and were used from the outset to facilitate hydrological data gathering, verification and transmission. Two applications illustrate this use locally. One is the Hydroniger project, involving eight countries bordering on the Niger and designed to produce an operational, real-time hydrological forecasting system on the river basin; the other is the Onchocerciasis Control Program, with WHO. In this program, discharge data in only slightly delayed time has made it possible to calculate the right dose of insecticide to add to infected rivers.

Over the past ten or fifteen years, hydrological research has considerably extended its scope, to include spatialised hydrological parameters, soil/water/plant/atmosphere relations, soil erosion and conservation, geochemistry, hydrochemistry, etc. Meanwhile, sub-Saharan Africa has not been left out of the international organisations' programs involving operational or pure research hydrology, and it is making its contribution to the great challenges that face us with regard to water resources and management. Examples are the WMO WHYCOS project to establish permanent monitoring systems; the AOC-FRIEND program, a regional research project, which has been growing in strength; and the Hapex-Sahel experiment, designed to provide more information on the thermal and moisture characteristics of soil and vegetation, for integration into general atmospheric circulation models.

Forms of collaboration among the partners involved in hydrology in this part of Africa have naturally changed over the course of fifty years. In the beginning, demand from local technical services set up a particular form of partnership with hydrology experts from the North. After independence, scientific and technical co-operation developed along with a policy for training management-level staff to build up national hydrology services. In many countries, however, the national services only gradually took full charge of their hydrometric networks, with technical assistance continuing for a long time. During the seventies and eighties, most countries acquired scientific or technical infrastructures that played host to hydrologists from the North for joint research programs and studies, with gradually increasing involvement by African water research scientists.

After long years of work gathering, interpreting and applying data, the hydrological characteristics of this part of Africa have been established and the groundwork for a sound scientific partnership between Northern and Southern partners has been laid. Many of the region's water problems are linked to the difficulty of meeting constantly rising demand from fluctuating water resources; unrelenting effort is needed to resolve these problems.

In this regard, there are promising signs for the new millennium: the growing community of African hydrologists is increasingly active in major international programs; links have been forged among those involved in Africa's various regional programs; and there are now thriving scientific associations like the Association of African Hydrologists.

FR:

L'article présente brièvement l'état de l'épuration des effluents des collectivités en France et au Québec. Il souligne ensuite quelques changements importants intervenus depuis dix ans dans le monde grâce aux progrès de la recherche et qui pourraient marquer l'avenir.

En France, 95 % des agglomérations de plus de 10 000 équivalents-habitants disposent d'une station d'épuration. Le rythme de construction a atteint 730 installations nouvelles dans l'année 1976 mais il est redescendu à moins de 300/an. Sur les 11 310 stations de plus de 200 équivalents-habitants recensées, 5 % réalisent seulement un traitement primaire et la moitié en nombre, représentant plus de 60 % de la capacité de traitement utilisent le procédé des boues activées.

Les investissements à réaliser entre 1994 et 2005 pour satisfaire aux exigences de la directive européenne du 21 mai 1991 sont évalués à environ 36 milliards de francs français, correspondant à la création de capacités de traitement supplémentaires de 17 millions d'équivalents-habitants et à des améliorations plus ou moins importantes d'un grand nombre de stations existantes.

Au Québec, la construction des stations d'épuration s'est faite essentiellement dans les années 80 et 90. Aujourd'hui, près de 80 % de la population est desservie par des installations d'épuration, soit environ 4,9 millions d'habitants pour un débit hydraulique de 5,3 millions de m3/j.

Parmi les 450 ouvrages municipaux ceux des Communautés Urbaines de Montréal et de Québec représentent, à eux seuls, près de 60 % de la capacité installée.

Les stations physico-chimiques, bien que peu nombreuses, sont les plus importantes (6 stations pour 2,28 millions d'habitants) suivies par les boues activées (40 stations pour 0,69 million d'habitants) et la biofiltration (9 stations pour 0,75 million d'habitants). Environ 320 stations, de dimension plus modeste utilisent des étangs aérés, desservant en moyenne une population de 3 500 habitants.

Sur le plan de l'évolution des techniques, la décennie écoulée a vu apparaître ou se confirmer des évolutions qui marqueront sans doute profondément la conception et l'exploitation des stations d'épuration dans les années à venir :

- l'objectif maintenant presque généralisé d'éliminer les nutriments azote et phosphore et le développement rapide des techniques correspondantes;

- la prise de conscience de l'importance des flux polluants véhiculés par les eaux pluviales et un début d'adaptation des stations d'épuration;

- le développement limité mais réel des traitements anaérobies qui ont survécu à la démobilisation des surlendemains de la crise énergétique;

- le fort développement des systèmes d'épuration biologique à culture fixée et notamment des biofiltres;

- l'apparition prometteuse des membranes dans les systèmes d'épuration biologique permettant d'envisager de nouveaux objectifs de traitement;

- la prise en compte de la fiabilité des systèmes d'épuration avec un poids de plus en plus important par rapport aux performances de pointe

- une vision plus intégrée de la prévention des pollutions tant dans l'industrie (technologies propres) que dans les agglomérations (gestion intégrée de l'ensemble réseau-station d'épuration).

EN:

This paper provides a brief discussion of the state of municipal wastewater treatment in France and Quebec. It then presents some important changes that have developed over the last ten years in the world and their potential influence on the future.

In France, 95% of towns of more than 10 000 residents have a wastewater treatment plant. Construction of new plants reached 730 installations in 1976, but has since declined to less than 300/year. Among 11 310 sites of more of 200 inhabitants, 5 % utilize only primary treatment and 50 % utilize the activated sludge process which represents 60 % of the treatment capacity.In order to satisfy the European directive of May 21st 1991, approximately 36 billion French Francs should be invested between 1994 and 2005. This investment represents new wastewater treatment facilities for 17 million people.

In Quebec, construction of wastewater treatment plants occurred mainly during the 80's and 90's. Today, nearly 80 % of the population is served by wastewater treatment plants, which represent approximately 4,9 million residents. This accounts for a flow rate of 5,3 million cubic meters per day.Among 450 municipal treatment plants, those of the urban communities of Montreal and Quebec represent nearly 60 % of the total wastewater treatment capacity of Quebec.

Physico-chemical treatment plants are the most significant (6 plants for 2,28 million residents) followed by activated sludge process plants (40 plants for 0,69 million residents) and plants using biofilter technologies (9 plants for 0,75 million residents). A total of approximately 320 small treatment plants, serving an average of 3 500 residents, use the aerated lagoon treatment.

Over the past decade technological developments have resulted in an evolution that will modify the design and operation of wastewater treatment plants in the future:

- the more widespread use of techniques developed for the elimination of nitrogen and phosphorus;

- the realization of the importance of polluting charges transported by pluvial waters; the beginning of the resultant wastewater treatment plant adaptations;

- the limited development of anaerobic treatments that have survived the energy crisis;

- the valuable development of biological fixed-culture systems for wastewater treatment and most notably, the biofilters;

- the appearance of promising membrane technology in wastewater treatment systems, which could facilitate the definition of new treatment objectives;

- the consideration the global reliability of the wastewater treatment systems instead of only peak performance;

- a more integrated vision for the prevention of pollution in industry (clean technologies) as well as in the domestic environment (integrated management of sewerage system and wastewater treatment plant)

FR:

L'objectif principal de cet article est de montrer que les travaux de recherche en chimie de l'eau sont absolument nécessaires à la compréhension des phénomènes régissant la qualité des eaux de consommation produites et distribuées, ainsi qu'au développement de technologies innovantes dans le domaine du traitement de ces eaux. Pour l'atteindre, il a été choisi de présenter trois exemples des recherches menées à l'Université de Poitiers et à l'École Polytechnique de Montréal : deux exemples de recherche fondamentale, sur les matières organiques naturelles et sur les mécanismes d'oxydation par les radicaux hydroxyle et un exemple de la recherche d'application, sur l'évolution de la qualité de l'eau lors de son traitement biologique par ozonation couplée à la filtration sur charbon actif en grains.

EN:

The main purpose of this paper is to demonstrate that a better understanding of chemical reactions and a better technologies development for the drinking water treatment absolutly needs research in water chemistry. In order to reach this objective, two examples of recent fundamental research works in water chemistry and one example of research applied to drinking water treatment have been selected.

The first one concerns a fundamental research on characterization and analysis of natural organic matter. Nowadays, the main analytical tools used for quantification of NOM are DOC (BDOC for the biodegradable fraction) and UV-absorbance. However, a better knowledge of NOM would be useful to predict water quality and determination of doses of some treatment chemicals, such as coagulants and oxidants. This paper presents some data about (i) NOM fractionning by XAD macroporous resins and relationship between UV-absorbance and 13C-NMR, and (ii) study of chlorination of the isolated fractions. Based on these results, one can suppose that aquatic NOM could be soon easily fractionnated and characterized by simples analytical tools. However, many research works are still necessary, particularly on the field of relationships between data of "heavy" analytical tools (13C-NMR, pyrolysis/GC/MS, etc.) and more simple others (UVA/DOC, BDOC, fluorescence etc.).

The second paragraph of the paper relates oxidation mechanisms by hydroxyl radicals. In ozonation of drinking water, currently applied in the drinking water treatment, OH radicals production is a necessary consequence, considered as the main action of ozone by some authors. So, disinfection quality, bromate production, BDOC formation, pesticides removal will never be entirely understood as long as hydroxyl radicals participation in ozonation processes will not be really cleared up. To illustrate this purpose, the example of ozonation of a free aminoacid (glycine) by molecular ozone and by OH radicals is shown. Oxidation by "pure" OH radicals, produced by radiolysis, as compared to oxidation by molecular ozone, (at acidic pH in the presence of radical scavengers) shows that two mechanisms are really different. So, some works published some years ago, mixed up the two pathways. Such mechanisms on aminoacids oxidation should allow to understand some more descriptive data on water disinfection and pesticides oxidation.

The third example concerns the changes of water quality during ozone/GAC treatment. It is well known that ozonation of naturel waters leads to an enhancement of water biodegradability, in terms of BDOC increase. So, if this BDOC is not removed in the plant, it can be the origin of bacteria regrowth in distribution system. The solutions are either to include, after ozonation step, a biological treatment step on activated carbon (BAC), or to increase chlorination doses applied in final disinfection. The effects of BAC treatment on BDOC and ozonation by-products removals, in the case of the drinking water treatment plant of Sainte-Rose (City of Laval, Quebec, Canada), are described. Finally, taking into account the cost of such a process, the last paragraph of this paper gives some recommandations for conception on BAC treatment.

As for general conclusion, some new research topics in water chemistry for the drinking water treatment are enumerated and some ideas about organization and development of applied and fundamental researches in water chermistry for the drinking water treatment, are given.

FR:

Depuis la mise en évidence de trihalométhanes dans les eaux potables en 1974, de multiples travaux ont démontré la présence de nombreux composés génotoxiques dans l'eau de boisson. L'eau potable obtenue à partir d'eau de surface subit un traitement incluant généralement une étape de chloration. Il est aujourd'hui largement admis que l'activité génotoxique des eaux de boisson provient principalement de la chloration des substances humiques, composés organiques naturels contenus dans l'eau brute et issus de la dégradation des déchets animaux et végétaux. Les très faibles concentrations en composés génotoxiques dans les eaux potables nécessitent la concentration des échantillons, procédé qui risque toutefois de modifier la génotoxicité. Plusieurs tests mettant en oeuvre des cellules procaryotes ou eucaryotes, des plantes ou des mammifères, ont permis de mettre en évidence les effets génotoxiques dans des eaux potables chlorées. L'identification des composés génotoxiques est réalisée au moyen des données de la spectrométrie de masse et de la spectroscopie UV ou RMN (proton ou carbone). Ces agents sont généralement non volatils, acides et polaires. Bien que certains composés inorganiques interviennent parfois, la majeure partie de la génotoxicité est attribuée aux agents organohalogénés (bromés ou/et chlorés), les principaux étant les trihalométhanes, acides acétiques, acétonitriles, cétones, et hydroxyfuranones. La fixation de normes contribue à limiter l'exposition des populations aux agents potentiellement dangereux. La qualité des eaux de boisson peut être accrue en utilisant une eau brute moins chargée en matière organique, et en améliorant le traitement chimique tout en veillant à conserver la qualité microbiologique de l'eau produite.

EN:

In 1974, two independent studies - one in the Netherlands and the other in the United States - demonstrated the occurrence of trihalomethanes in drinking water. Following studies showed that these chemicals were common contaminants of drinking water and that chloroform, i.e. one of these trihalomethanes, was carcinogenic in rodents. Further investigations demonstrated that extracts of chlorinated drinking water induced significant mutagenicity in the Ames/Salmonella assay. In the present paper we will fist discuss the methods used to detect the genotoxic activity of drinking water and, then, the methods developed to identify the compounds responsible for this activity. After this, we will present the main genotoxic chemicals identified in drinking water, before finally considering several propositions to limit the exposure of populations to these genotoxic compounds.

Drinking water is usually produced through a multistage process which includes one or several chlorination steps. It is now widely accepted that the genotoxic activity of drinking water mainly originates from the reaction of chlorine with humic substances present in raw water. Humic substances are natural organic matters (resulting from the degradation of plants and animal tissues) of very complex structure with most chemical functions arranged in aromatic rings or aliphatic chains. The identification of a genotoxic activity in drinking water usually requires concentration of the water samples. Even though such a process implies a probable qualitative/quantitative alteration of the constituents of water samples, the extremely low amounts of genotoxic compounds in drinking water require concentration steps. Among the many genotoxicity tests carried out, the Ames test (which detects reverse mutations in bacteria Salmonella typhimurium) is the assay which was the most frequently used in the field of drinking water mutagenicity. Other tests were performed on eucaryotic cells. Assays detecting micronuclei or chromosomal aberrations in plants, or mutations in mold, yeast, or maize enabled the detection of genotoxic effects of drinking water extracts. Tests on mammal cells also showed that drinking water extracts induced point mutations, sister chromatid exchanges, chromosomal aberrations and micronuclei. In vivo tests on aquatic organims such as newt or mussels demonstrated the micronuclei inducing effect of unconcentrated drinking water samples.Regarding the identification of the compounds responsible for the genotoxicity, it is obviously not possible to identify all of the thousands of chemicals that may be involved. But such a process is important in order to evaluate the specific genotoxicity and the risk associated with (at least) the main chemicals occurring in drinking water. The identification process usually follows three steps:

1. concentration of the sample can be performed using reverse osmosis, freeze drying, liquid-liquid extraction, and/or adsorption on non ionic resin followed by extraction with organic solvent;

2. the purification step uses one or a combination of chromatographic techniques (TLC, packed column liquid chromatography, HPLC or GC);

3. structural identification of the chemical is performed using data from mass spectrometry, and proton and carbon NMR, or UV spectroscopy.

The analysis of the genotoxic compounds of drinking water showed that they are rather non-volatile, quite acid and not stable at high pH, rather polar, and with a mean molecular weigh around 200.

Turning now to the identity of these compounds, it is considered that the genotoxicity of drinking water is mainly due to organohalogenated chemicals. Some inorganic chemicals (this class of chemicals is usually not recovered in drinking water extracts) which induce genotoxic or carcinogenic effects must, however, be recalled. Arsenic, nitrates, bromates and radon are natural or human-activity-related drinking water contaminants which are responsible for cancers in rodents or in humans. Among the many genotoxic or carcinogenic organohalogenated compounds identified in drinking water, the most abundant chemicals are chlorinated and/or brominated trihalomethanes. Other important groups of compounds are chlorinated and/or brominated derivatives of acetic acids, acetonitriles, ketones, phenolic compounds. The chlorinated hydroxyfuranones, although present at concentrations lower than 0.1 µg/l in drinking water, can be responsible for more than half of the Ames mutagenicity. MX, the most potent of these chlorohydroxyfuranones, has been submitted to intensive toxicological studies worldwide and was very recently identified as a potent carcinogen in rats.

Now that the presence of genotoxic compounds in drinking waters is a well documented and accepted fact, the perspectives lies in the better identification of the impact of these drinking water contaminants. The development of more sensitive tests such as the Comet assay (detection of DNA strand breaks) or the 32P postlabelling assay (detection of DNA adducts) should be pursued. Moreover, the interaction between genotoxic compounds and DNA must be investigated more thoroughly, including the identification of adduct structures. More globally, it is of interest to better assess the impact of these agents on public health and on the occurrence of specific human cancers. At present, even though a few individual water contaminants are classified as human probable carcinogens, the chlorinated drinking water (in itself) is not considered as carcinogenic to humans. Exposure to these potentially harmful agents can be limited with

1. improving drinking water quality - i.e. decreasing the formation of genotoxins - by using raw water containing lower amounts of organic matter; and

2. modifying the water chemical treatment by using lower amounts of chlorine and/or combining chlorine with other disinfectants.

The public health can also be protected by the setting of guidelines for drinking water: each compound identified as dangerous would be given a concentration threshold which should never be exceeded. The Environmental Protection Agency in the U.S.A. and the World Health Organisation are authorities setting such guidelines. Finally, we believe it is important to limit the concentration of genotoxic compounds in drinking water as much as possible, and one way to do so is to use chlorine in smaller amounts and in a more efficient way. But it is of paramount importance to keep in mind that the disinfection process (in which chlorine still plays a major role) and the providing of a microbiologically safe drinking water should never be jeopardized.

FR:

L'évolution des techniques analytiques est, la plupart du temps, liée à l'apparition de nouvelles normes concernant l'environnement ou l'eau potable.

En ce qui concerne les micropolluants minéraux, si dans le passé une attention toute particulière avait été apportée à la connaissance des formes sous lesquelles ils étaient dans les eaux, toutes les normes proposées depuis n'ont pas repris cette approche, et toutes concernent l'élément total non différencié. Les méthodes de spéciation ne seraient donc plus à l'ordre du jour.

Il n'en est rien, car les limites fixées étant de plus en plus basses, la connaissance des formes, des valences sous lesquelles se trouvent ces éléments est de plus en plus importante pour l'optimisation des traitements curatifs. C'est le cas par exemple, du fer, de l'arsenic, du sélénium et du chrome. L'arsenic, par exemple, ne peut être éliminé de façon convenable, que s'il se trouve à la valence V. Une oxydation préalable peut donc être nécessaire. Pour le chrome, c'est le contraire, à la valence VI, cet élément n'est pas éliminable, il faut donc le réduire pour le faire passer à la valence III.

Une autre évolution de l'analyse concerne les possibilités offertes par les couplages ICP-MS. En effet, la connaissance pour un élément des rapports isotopiques peut être primordial pour retrouver l'origine d'une pollution.

Deux cas sont présentés :

1. Un élément lourd, qui ne connaît pas ou très peu de ségrégations isotopiques, c'est le cas du plomb.

- Lors d'enquête sur la plombémie des enfants, il peut être mis en évidence des plombémies importantes (> 150 µg/l de sang). L'origine de ce plomb pouvant être multiple : pollution atmosphérique (naturelle, pollution automobile), les peintures anciennes (poussières), l'eau de boisson ayant transité dans des canalisations en plomb, seule l'analyse isotopique permettra de prendre position et de mettre en évidence une origine prépondérante. Des exemples sont présentés dans le texte.

2. Un élément léger, qui normalement devrait conduire à des ségrégations isotopiques, mais qui ne précipite pas et ne s'adsorbe pas ou très peu, c'est le cas du bore.

- Le bore peut aussi avoir plusieurs origines. Les produits lessiviels dont le bore ne provient que de Californie pour la plus grande partie et de Turquie pour le reste. Ce bore a une signature isotopique différente de celle du bore trouvé en France. Cette méthode nous permet donc de distinguer le bore utilisé pour l'agriculture ou le bore naturel, du bore utilisé dans les formulations de produits de nettoyage.

EN:

The development of analytical techniques is, for the most part, linked to the emergence of new standards concerning the environment or drinking water.

As regards inorganic micropollutants, particular attention has been focussed in the recent past on determining the forms in which these elements were to be found in water. This approach has not been adopted, however, in all of the standards proposed since then. In all of these standards the analyte of concern is the total undifferentiated element. Methods of determining trace element speciation would therefore no longer appear to be the order of the day.

In fact this is not so, for given that the limits set are increasingly low, a knowledge of the forms and valencies in which these elements are to be found is more and more important for optimisation of the treatment processes designed to remove these inorganic micropollutants. Such is the case, for example, with iron, arsenic, selenium and chromium. Arsenic, for instance can only be properly eliminated if found in the +V oxidation state; prior oxidation may therefore be necessary. In the case of chromium, the reverse is true, since the element in question cannot be eliminated if present in oxidation state +VI. It has therefore to be reduced to bring it to the +III oxidation state.

A further development in analysis concerns the possibilities offered by IPC-MS couplings. Indeed, knowledge of the isotopic ratios as regards a particular element can be essential in discovering the origin of a pollution episode.

Two cases are presented here :

1. A heavy element, which undergoes little or no isotopic segregation, as in the case of lead.

- Investigations into high lead level in blood of children can reveal significant contamination (>150 mg Pb /l of blood). The origins of this lead can be multiple: atmospheric pollution (natural, automotive), old paint (dust), drinking water that has passed through lead pipes. Isotopic analysis alone allows one to identify the preponderant lead source: examples are given in the paper.

2. A light element, which would normally be subject to isotopic segregation, but which does not precipitate and which adsorbs very little or not at all. Such is the case with boron.

- Boron can also have several origins. For example, in detergent products the boron comes for the most part from California and to a lesser extent from Turkey for the rest. This boron has a different isotopic signature from that of the boron found in France. This method enables us to distinguish boron used in agriculture, or natural boron, from the boron used in the formulations of cleaning products.

FR:

Cet article représente une évaluation de l'état actuel et des tendances observées dans les écosystèmes lacustres, ainsi que de leur état futur probable lorsque les réductions d'émissions requises dans le cadre de l'Entente Canada-États-Unis sur la qualité de l'air auront été effectives. Outre une synthèse des faits saillants de ce dossier pour l'ensemble du Canada, le présent article s'appuie aussi sur l'ensemble des données physico-chimiques récentes (8874 échantillons) observées sur 2779 lacs de l'est canadien, ainsi que celles recueillies (1012 échantillons) sur 252 lacs de l'ouest canadien depuis 1985. Des données biologiques (poissons, benthos, zooplancton et oiseaux aquatiques) ont également été inventoriées pour identifier l'ampleur des dommages biologiques.

Les nombreux lacs ayant subi une acidification anthropique récente sont situés pour la plupart dans l'est du Canada où les dépôts de SO- sont élevés. La sensibilité des sols influence également leur distribution spatiale. Durant la période s'échelonnant de 1981 à 1994, seulement 33% des 202 lacs faisant l'objet d'un suivi temporel dans l'est du Canada ont montré une amélioration significative de leur acidité (réduction) en réponse à la baisse des dépôts de SO- (11% des lacs ont subi une hausse d'acidité et 56% n'ont montré aucun changement). Plus de la moitié des lacs ayant récupéré se situent à proximité de Sudbury en Ontario. Plusieurs processus biogéochimiques sont responsables du retard dans la réversibilité de l'acidification. Pour cette raison, la récupération biologique a été très faible dans l'est canadien, exception faite de la région immédiate de Sudbury.

Trois scénarios d'émissions ont été considérés: scénario 1: niveaux d'émission canadiens et américains de 1985; scénario 2: émissions canadiennes de 1994 et émissions américaines de 1990 ; scénario 3: réductions d'émissions américaines et canadiennes complétées. Ces scénarios de réductions d'émissions, qui ont été utilisés comme données d'entrée à des modèles stationnaires simulant la chimie des eaux de surface et qui ont été appliqués à cinq grandes zones lacustres du l'est canadien, suggèrent que la proportion de lacs "endommagés" (définis comme étant des lacs de pH<6) diminuera conséquemment aux réductions d'émissions américaines et canadiennes.

De 11 à 49% des lacs acidifiés le resteront après l'ensemble des réductions prévues (scénario 3). Le Québec et l'Ontario, qui reçoivent actuellement les plus fortes retombées acides, bénéficieront le plus des réductions. Les gains environnementaux seront plus faibles dans l'est et dans l'ouest du Canada. De plus faibles dépôts acides et une contribution naturelle à l'acidité pourraient expliquer cette moins grande récupération.

Il est maintenant reconnu que le pH est le principal facteur d'influence de la diversité spécifique du poisson, bien que d'autres facteurs comme la morphométrie du lac, l'altitude et les concentrations de COD soient aussi en partie responsables. Une réduction des dommages biologiques (i.e.baisse des disparitions de populations de poisson) serait donc possible, mais cette amélioration ne surviendra qu'après la hausse du pH des eaux de surface. L'importance relative des gains au plan biologique suivra une évolution similaire à celui des aspects chimiques. Des dommages significatifs aux écosystèmes lacustres subsisteront néanmoins après réalisation de l'ensemble des réductions d'émissions. Des pertes de populations de poissons devraient subsister dans 6% (Sudbury) à 15% (Kejimkujik) des lacs. Compte tenu du grand nombre de lacs situés dans le sud-est canadien, les pourcentages précédents impliquent que les ressources piscicoles perdues pourraient être très élevées. La restauration des communautés piscicoles devra passer dans bien des cas par un ré-enpoissonnement. De nouveaux programmes de contrôle visant des réductions supplémentaires d'émissions seront dès lors nécessaires pour protéger correctement les écosystèmes sensibles.

EN:

This paper is an assessment of the current status and trends of Canadian lake systems, and their likely status after the effect of the emission controls required by the Canada/US Air Quality Agreement is fully realized. Many anthropogenically acidified lakes presently occur in that part of eastern Canada where SO- deposition is elevated. Terrain sensitivity also influences their spatial distribution. From 1981 to 1994, only 33% of 202 lakes monitored across eastern Canada showed a statistically significant improvement (reduction) in acidity in response to reduced SO- deposition (11% had increasing acidity and 56% showed no change). Over half of the improving lakes are near Sudbury, Ontario. Several biogeochemical processes are delaying de-acidification. As a result, there has been little biological recovery in eastern waters, except near Sudbury. Steady-state water chemistry modelling suggests that the proportion of "damaged" lakes (defined as having pH <6) will decline in response to both the Canadian and US emission controls. Reductions in biological damage (e.g. fewer lost fish populations) are expected also, but they will lag behind chemical improvement. Significant damage to aquatic ecosystems will remain after all chemical and biological improvements are realized. Further controls will be needed to protect sensitive ecosystems.

FR:

Par leur abondance et leur diversité taxonomique et fonctionnelle, les microorganismes jouent un rôle prépondérant dans les flux de matière et d'énergie au sein des écosystèmes aquatiques.

Au cours de ces dernières années, les progrès réalisés au niveau des techniques d'identification, de dénombrement et de mesure d'activité métabolique, notamment en microscopie à épifluorescence et en biologie moléculaire, ont permis d'entrevoir l'extraordinaire diversité des microorganismes aquatiques, l'étendue de leurs conditions de vie et leurs abondances jusqu'alors largement sous-estimées. De plus, l'amélioration sensible des méthodes séparatives a permis de décrire, in situ, la composition biochimique des communautés et d'aborder les transferts de matière sous un angle qualitatif.

L'ensemble des résultats disponibles laisse apparaître que les relations trophiques entre les microorganismes forment un véritable réseau à l'intérieur duquel la boucle microbienne permet le transfert, au moins en partie, de la production picoplanctonique vers les niveaux trophiques supérieurs. Ainsi, des progrès conséquents ont été réalisés quant à la compréhension du rôle des communautés bactériennes dans les flux de matière, y compris au niveau d'environnements particuliers (biofilms, milieux extrêmes). Par ailleurs, alors qu'il a été longtemps admis que la régulation des communautés bactériennes était essentiellement liée à la disponibilité ou à la qualité des substrats organiques, il apparaît maintenant que la limitation par les éléments nutritifs minéraux, la prédation des protistes phagotrophes et du métazooplancton et la lyse virale sont également des facteurs susceptibles d'intervenir significativement dans ce contrôle.

Malgré ces progrès considérables dans le domaine de l'écologie microbienne, près de 90% des microorganismes présents dans l'environnement n'ont pas encore été décrits et la compréhension des relations entre les microorganismes et le fonctionnement des écosystèmes restent un enjeu majeur pour les années à venir.

EN:

Recent advancements in the ecology of aquatic microbial communities, i.e. from viruses to protozoa, are summarized in this paper. The abundance and both taxonomic and functional diversities of microorganisms in the sea and in inland waters indicate that microbes play a key role in nutrient cycling and energy flows in aquatic ecosystems. In recent years, aquatic microbiology has indeed undergone profound changes due to the improvement of methods for identifying, counting, and essaying biochemical composition and metabolic activities of aquatic microbial assemblages. Specifically, the impact of new developments in microscopy (e.g. epifluorescence, immunofluorescence...) and in cell and molecular biology has allowed to realize that microbes are omnipresent in aquatic systems (including extreme environments such as Arctic, Antarctic, Deep ocean, Hydrothermal vents...).

Derived from direct counting under epifluorescence microscope that is able to visualize cellular pigment autofluorescence, recent total numbers of pelagic microbes generally vary from 105 and 102 cells ml-1 in oligotrophic systems, to 107 and 105 cells ml-1 in productive waters, for heterotrophic bacteria and heterotrophic flagellated protists, respectively. These bacterial numbers are significantly higher than previous estimates, derived from the indirect method of growing bacterial cells in selective culture medium. The use of adequate fixatives has allowed the counting of ciliated protozooplankton (range: < 1 - 50 cells ml-1) under inverted microscope. The use of artificial or natural fluorescent tracers now allows, via direct microscopic observation of protistan digestive vacuoles or the alimentary tract of some metazoan, the quantification of matter flows in the aquatic microbial webs. Besides, the coupling between fluorochrome dying of aquatic microbes and flow cytometric analyses allows to measure the cell size and abundance of the tiniest planktonic single-celled organisms, and to characterize some cellular constituents (e.g. ADN, protein...) or functions (e.g. membrane potential, enzymatic activity...). Other methodological improvements of the identification, counting, and essaying the biochemical composition of aquatic microorganisms come (1) from the separative chromatography that allows to describe the in situ biochemical composition of microbial communities, and to have access to a qualitative measurement of matter flows within aquatic microbial compartments, and (2) from the use of rRNA-targeted oligonucleotide probes that allows, after amplification by the polymerase chain reaction, to detect and quantify individual species in natural assemblages of microbial organisms (i.e. bacterio- and protoplankton). Concerning the assessment of metabolic activity, a method based on short-term incubations of water samples in the presence of radiolabeled compounds, primarily 3H-thymidine and 3H-leucine, has been widely developed and used during the few past years, to measure bacterial production in aquatic systems.

In the light of the above recent methodological improvements, it appears that aquatic microbial communities form complex food webs in which heterotrophic bacteria play a key role. These organisms decompose particulate matter or polymeric dissolved compounds, and rapidly utilize simple organic molecules. However, depending on their qualitative characteristics, some of these simple dissolved substrates can accumulated during certain times or in some aquatic environments. This has been relied to several hypothesis, including the higher diversity of natural organic molecules compared to the bacterial enzymatic pool, chemical constraints during microbial incorporation of substrates, and the fact that organic molecules can become refractory before utilization by ambient bacteria. Because bacterial metabolic efficiencies can vary widely both intra- and interspecifically, it is propose that the ecology of aquatic bacteria should gain in substance if considering functional groups (separated by criteria such as energy sources, ...) rather than taxonomic groups.

The seasonal abundances of pelagic bacteria are regulated by several factors, including temperature, resources, and predation. The general significant correlation between bacterial and primary production in both fresh- and marine waters suggests that organic substrates from algal exudation regulate bacterial communities. However, uncoupling between bacterial and algal production has been reported for some aquatic environments and during some times. It is now well known that bacteria and phytoplankton can compete for the same substrate source (including mineral nutrients). The accumulation of biodegradable dissolved organic carbon (DOC) in the euphotic zone of oligotrophic marine systems imply that factors other than substrate availability can regulate bacterial production. For example, in situ and experimental studies have recently demonstrated that mineral phosphorus (PO4) can limit bacterial growth in lakes where both autochtonous and allochtonous DOC are prevalent. Limitation of bacteria by mineral P and N has also been reported in marine systems. Besides, the relatively constant ratio between bacterial and phagotrophic flagellate abundances in pelagic fresh- and marine waters has been interpreted as the result of the impact of bacterivorous protist (mainly represented by flagellated protists) activity on bacterioplankton seasonal abundance. Both in situ and experimental essays have indeed shown that a flagellate can ingest from 10 to 250 bacteria hour-1. Protozooplankton bacterivory is actually considered the root of a system, i.e. the microbial loop (dissolved organic matter --> bacteria --> protozoa...), that can act as a significant mediator of energy transfer to the upper trophic levels, by recovering part of the primary production that would otherwise be lost from the system. Part of protistan grazing activity is from mixotrophic protists whose, in some lakes and during certain seasons, can dominate the total bacterivory. In general, predator-prey interactions among protists are as complex as those among metazoa, and chemical communications may operate as well as behavioral and polymorphic adaptations.

Even though the contribution of water ecosystems for disseminating enteric viral pathogens has been known for decades, the importance of wild virions in structuring aquatic communities and food webs has only come to light relatively recently. Evidences of viral infections in both pro.- and eukaryotic phytoplankton, as well as in heterotrophic bacterio.- and protozooplankton, have recently brought marine biologists to question the impact of viroplankton on processes such as

1.the mortality of microorganisms,

2. the nutrition of heterotrophic protists,

3. the promotion of genetic material exchanges among microbial populations,

4. the maintenance of species diversity,

5. the induction of planktonic aggregates, and

6. the cycling of organic matter in aquatic ecosystems.

Viruses undoubtedly influence to various degrees the biological processes in aquatic ecosystems, although almost all studies on the ecology of pelagic viruses are done during a limited period of year, mainly in marine waters situated in temperate zones.

Finally, despite the manifest significance of new findings in the field of aquatic microbial ecology, about 90% of microorganisms present in the environment have not yet been described. Therefore, the understanding of the interactions within microbial communities in relation to the functional dynamics of ecosystems remains of major interest for the future. We conclude that this task is to be include on the agenda of both marine and freshwater biologists as a high priority concern for the near future, partly because aquatic microbes constitute a major compartment for the biogeochemical cycles of elements in the biosphere.

FR:

L'analyse comparative des réseaux trophiques lacustres est d'un grand intérêt pour le développement de la limnologie contemporaine et l'aménagement des lacs. L'analyse des mécanismes écologiques déterminant la structure et le fonctionnement des réseaux trophiques dans les lacs tempérés a permis l'émergence de plusieurs modèles, souvent contradictoires, et suscité d'intenses débats sur le rôle respectif des ressources et des prédateurs. Par contre, dans les lacs tropicaux, les études sont en majorité descriptives et la recherche de principes généraux et de concepts unificateurs y est rare. Cette synthèse présente l'état des connaissances, les approches méthodologiques, les modèles de régulation concernant la structure et le fonctionnement des réseaux trophiques lacustres. Les réseaux trophiques semblent varier selon un gradient de situations intermédiaires entre deux modèles extrêmes : (a) les milieux à cascades trophiques intenses et à effet atténué des ressources (lacs tempérés oligo-mésotrophes) caractérisés par la présence de poissons piscivores et de zooplancton herbivore de grande taille (tels Daphnia spp.) et (b) les milieux à régulation intermédiaire (lacs tempérés méso-eutrophes et la plupart des lacs tropicaux), caractérisés par la présence de poissons filtreurs microphages omnivores et de zooplancton herbivore de petite taille. Notre synthèse souligne aussi l'importance d'allier les approches expérimentales en enceintes ou par biomanipulation à des suivis à long terme et des modélisations pour avoir une bonne compréhension et des prédictions précises du fonctionnement des écosystèmes lacustres à différentes échelles spatiales et temporelles et pour différentes conditions climatiques, géographiques ou trophiques.

EN:

Comparative analysis of lake food webs is a focal point of research in contemporary limnology and lake management. The study of ecological processes determining foodweb structure and function lead to the emergence of constrasting hypotheses and intense debates on the relative role of nutrients and food web structure in regulating temperate lake ecosystems. In contrast, studies in tropical lakes are in general descriptive and the search for integrate concepts and models is yet in development. This review paper presents an overview and a critical analysis of actual knowledge, methodological approaches, regulation models and controversies on foodweb structure and function in temperate and tropical lakes. Our synthesis suggests that the apparent diversity in models of lake foodwebs could reflect a gradient (or contiuum) of intermediate foodweb situations, regulated by various environmental factors. The differences among lakes could be related to three main biotic factors, independently of the climatic, geographical and trophic conditions:

1. the important cascading effect of strictly piscivorous fish in temperate lakes compared to the weak cascades induced by opportunistic omnivorous fish in tropical lakes,

2. the primacy of omnivory and opportunistic feeding behaviour of tropical fish,

3. the key role of herbivorous macrozooplankton (cladocerans, mostly Daphnia spp.) in temperate lakes where they are both selective preys of planktivorous fish and efficient grazers of nanophytoplankton, and

4. the synchronous reproduction of fish with seasonal plankton succession in temperate lakes, compared to continuous reproduction of fish and lack of seasonal coupling in tropical lakes.

Consequently, food webs regulation ranges along a gradient of situations with two extreme models:

1. a model with intense cascading (top-down) regulation and attenuation of bottom-up effects typical of oligo-mesotrophic temperate lakes, characterized by the dominance of piscivorous fish and large herbivorous zooplankton (Daphnia spp.), and

2. a model with intermediate regulation encountered in eutrophic temperate lakes and most of tropical lakes, characterized par the dominance of filter omnivorous fish and small size zooplankton.

Our synthesis also emphasizes the importance of coupling experimental approches in mesocosms or whole-lake biomanipulation with long-term monitoring and modelisation to fully understand and predict the functionning of lake ecosystems over different spatial and temporal scales

FR:

La transmission des maladies par la voie hydrique est sous contrôle dans la majorité des pays industrialisés. Malgré tout des maladies épidémiques ou endémiques sont encore observées. Plusieurs microorganismes sont en émergence, et Cryptosporidium a été impliqué dans des épidémies importantes dans plusieurs pays. Le conrôle de ces maladies transmissibles par la voie hydrique requiert des autorités des nouvelles approches qui allient le contrôle des risques de cancer dûs aux sous-produits de la désinfection au contrôle des micro-organismes les plus résistants . Aux Etats-Unis, l'objectif proposé est l'absence de microorganismes dans l'eau potable. Cet objectif ne peut être contrôlé par les indicateurs usuels et l'on recommande donc un niveau de traitement équivalent. Le traitement est alors contrôlé en temps réel par des moyens physico-chimiques tels la turbidité ou la mesure des particules, et un contrôle a posteriori par de nouveaux indicateurs telles les spores des bactéries sporulantes aérobies.

Le vieillissement des installations, des populations immunocompromises et une urbanisation grandissante sont autant de causes de l'émergence de nouvelles maladies infectieuses dont certaines transmissibles par la voie hydrique. La proportion des maladies gastro-intestinales qui est attribuable à l'eau de consommation est encore très grande et elle contribue à maintenir ces infections en circulation dans la population. Le dilemme du contrôle des risques de cancer dus aux sous-produits de la désinfection ne doit pas conduire à une réduction de l'efficacité des traitements, car le niveau de risque à partir duquel ont été fixées les concentrations maximales admissibles de ces sous-produits dans l'eau (10-6 cas de cancer par vie entière d'exposition) est bien plus faible que celui de contracter une maladie infectieuse d'origine hydrique en absence de traitement adéquat.

La situation en matière de pathologies induites par la consommation d'eau est extrêmement contrastée selon les pays. En effet la transmission de maladies infectieuses par la voie hydrique a été maîtrisée dans la plupart des pays industrialisés par la mise en place d'installations de traitement et d'un contrôle sanitaire s'appuyant sur une réglementation abondante. A l'opposé la situation des pays en voie de développement reste souvent très mauvaise dans ce domaine et l'Organisation Mondiale de Santé estime que 1,5 milliards d'habitants ne disposent pas encore d'eau potable dont cent millions en Europe et que 30 000 morts journalières sont dues à l'absence d'une eau en quantité et qualité satisfaisantes (Ford et Colwell 1996).

En revanche les pays développés voient la qualité chimique des eaux distribuées de plus en plus souvent mise en cause par les associations de consommateurs. Outre le progrès très rapide des techniques analytiques qui permet de découvrir la présence de traces dont on ne soupçonnait guère la présence dans l'eau du robinet, la pollution croissante de la ressource, les traitements de désinfection et le contact avec les matériaux des réseaux de distribution apportent des molécules dont la toxicité à moyen et long terme mérite d'être évaluée.

La mise en oeuvre de traitements de désinfection dont l'utilité est indiscutable et l'effet sur la morbidité et la mortalité par pathologie infectieuse chez des populations desservies parfaitement significatif, s'accompagne de la formation de sous-produits. Certains de ceux-ci étant cancérigènes et/ou mutagènes en expérimentation de laboratoire et des études épidémiologiques ayant pu montrer une légère augmentation du risque de cancer dans la population, l'impact médiatique de cette information peut conduire à une mauvaise appréciation dans la gestion des risques pour la santé. Ainsi l'arrêt de la chloration pour éviter la formation de sous produits et quelques cas de cancers aurait conduit un pays d'Amérique du Sud a enregistrer une importante épidémie de choléra et des centaines de décès.

Il n'est pas facile de gérer ce paradoxe entre sophistication du traitement lié à la pollution de la ressource entraînant la présence de sous produits de désinfection et la persistance d'éléments traces et de divers microorganismes dans une eau de qualité conforme aux critères de potabilité mais que le consommateur ne veut plus consommer. Dans cet article nous tenterons de faire le point sur le risque hydrique pour la santé lié d'une part aux contaminants biologiques et d'autre part aux contaminants chimiques. Sa meilleure connaissance est la clef d'une stratégie de gestion efficace et d'une reconquête du consommateur que la publicité a trop tendance à orienter vers les eaux embouteillées.

EN:

The transmission of waterborne diseases is now controlled in most developed countries but a residual level of both epidemic and endemic diseases can still be observed. Recent observations have involved emerging pathogens such as Cryptosporidium which has been implicated in several very large outbreaks worldwide. To control these waterborne outbreaks many countries are proposing treatment goals that would achieve a significant reduction in the risk to the population without increasing the risks of cancer due to disinfection by-products. In the United States, the objective is zero pathogen in drinking water. This objective can be approached by appropriate treatment but monitoring cannot be done using the current biological indicators. Reliance on continuous measurement of physico-chemical parameters such as disinfectant residual and contact time, turbidity and particulate measurement in real-time are proposed solutions. Microbiological indicators remain an active mean of controlling afterwards water quality : spores of aerobic or anaerobic bacteria are extremely resistant to treatment and ofter means of assessing removal of pathogens over a range of several order of magnitude.

Numerous pathogens have been involved in waterborne outbreaks and some are just emerging. Urbanisation, aging of water treatment plants, the increasing number of immunocompromised individuals are potential causes for increased risk of waterborne infectious diseases. The endemic level of gastrointestinal disease due to drinking water consumption is still significant and could contribute to pathogens in circulation in the populations affected. The dilemma of balancing microbial and cancer health risk remains a difficult one to resolve but it should not result in a reduction of treatment efficiency, because of the low risk-level for cancer used for the Maximum Admissible Concentration (MAC) values (10-6 fall life) as compared to the risk of waterborne infectious disease in absence of adequate water treatment.

For setting these MAC values in water it is necessary to consider all sources of exposition of the considered compound and according to is mode of action two ways of evaluation may be followed. The first one, for the molecules with a deterministic effect (or non stochastic effect) the dose-effect relationships dose-effet on individuals and dose responses relationships on populations, are considered, and only no indesirable effect in the consumming population may be accepted. For the second one, for the molecules with a probabilistic (stochastic) effect, (absence of dose-effect relationship but increase of the incidence of cancer or genetic abnormalities in population according to levels of exposure), MAC values are setted by computation off all exposures and considering an accepable risk of apparition of the pathology ranging from 10-5 (WHO) to 10-6 (North America and Europe) for consumption of two liters of drinking water during 70 years.

Because of the lack of information about these very conservative approaches, associations of consumers are often misinterpreating : in this situation water is not in accordance with drinking water standards but may be used during a short period without any risk increase for the population. Examples are developped in this paper with description of hazards due to nitrate, nitrite, pesticides and desinfection by-products which are actually frequently associated with debates within water specialists, politicians and consumers.If situation is now more clearly defined for nitrates and pesticides, the lack of scientific information for the effects of bromates combined with the conservative approach for the molecules with "probabilistic" effect, leads to a severe MAC value in comparison with the analytical capacities of laboratories and technical data of water desinfection both with ozone and bleaching agent.

This situation may not lead to the diminution of adequate desinfection water treatment because of the consequences on the increase of the risk of waterborne infectious disease. More progresses are needed, both in terms of knowledges in toxicological and epidemiological data and technological ways of treatment, for being able to produce safe drinking water, with a taste and a price acceptable for the consumer. But the levels of risk considered for setting standards insures that drinking water is one of the safer products offer for consumption.

FR:

La biodiversité des eaux continentales se caractérise par une forte endémicité de nombreuses espèces résultant de la nature insulaire des milieux. Alors qu'elles n'occupent que 1-2 % des terres émergées une proportion importante de vertébrés (environ 1/3) vit dans les eaux continentales ou en dépendent étroitement pour réaliser leur cycle biologique. Certains systèmes aquatiques, comme les lacs dits anciens, sont de véritables laboratoires naturels pour étudier l'évolution, et il faut protéger ce patrimoine.

Les recherches sur l'origine et la dynamique de la biodiversité aquatique (inventaire, phylogénie, biogéographie) doivent se poursuivre, notamment pour les milieux tropicaux encore mal connus. Les relations fonctionnelles entre les espèces et les écosystèmes constituent un nouveau centre d'intérêt qui nécessite à la fois de revisiter les résultats déjà obtenus, de mettre en place des recherches spécifiques, et de développer des approches expérimentales. Quelles relations existent-ils entre la biodiversité et la stabilité ou la productivité des écosystèmes par exemple ?

La biodiversité est un médiateur entre les systèmes écologiques et les systèmes sociaux dont la conservation s'inscrit dans la problématique du développement durable. La valorisation économique de la biodiversité pose de nouvelles questions à l'économie de l'environnement malgré les difficultés méthodologiques rencontrées. Les questions qui intéressent les gestionnaires portent principalement sur l'état de santé des écosystèmes et les moyens de la caractériser (indicateurs biologiques par exemple). La gestion durable des ressources vivantes, et une meilleure évaluation des conséquences des introductions d'espèces (biomanipulations) constituent également des objectifs finalisés auxquels les recherches dans le domaine de la biodiversité aquatique se doivent d'apporter des éléments de réponse.

EN:

As a result of their island-like nature, inland waters are usually characterised by a high level of endemicity for several animal groups. This patchy distribution also results in great genetic variability between populations. While inland waters occupy only 1-2 % of emerged lands an unusual high proportion of vertebrates (around 1/3) inhabits or is strongly dependant on inland waters to complete their biological cycle. This remarkable concentration of vertebrate biodiversity is also extremely vulnerable, given that inland water aquatic resources world-wide have undergone severe deterioration.

The fragility of inland water ecosystems and their species has been recognised all over the world. These resources have been exposed to a variety of increasing pressures such as water extraction for domestic, agricultural and industrial uses, pollution (organic and inorganic), fishing, introduction of exotic species, habitat alterations in relation to water management, etc. These stressors to the inland water environments have affected, and will continue to affect life in all inland water ecosystems. One of the major causes of change in biodiversity has been, and most likely will continue to be long-term, climate change.

More knowledge is needed about the identification of species, how biological diversity is distributed, and what the trends observed on the short- to long-term biodiversity changes are.

In many tropical countries there are lakes and rivers lacking even the most basic research on fauna and flora. Pertinent information on biological diversity in most developing countries, when it exists, is too sparse or scattered to be of practical use. Therefore, efforts should be devoted to the development of computer data bases and information networks for the collection and storage of information dealing with the identification and distribution of freshwater biodiversity. Development of these systems would benefit from the expertise and collaboration of taxonomists all over the world.

From the perspective of sustainable development, multiple-use management of aquatic ecosystems can provide the framework for achieving long-term sustainability of resources and for maintaining biodiversity. The future of biodiversity is closely linked to the management options at the watershed scale and a key issue is to promote an integrated basin management approach of aquatic. However, a wise management of aquatic systems and the conservation of their biodiversity requires not only technical inputs, but also a good knowledge of social and economic issues. Valuation is a fundamental step in informing planners and resource managers about the economic importance of biodiversity in national development objectives, and to demonstrate the importance of different areas for the biological resources they contain. Research programs have also to be intensified on the complex relationships between biodiversity and aquatic ecosystems functioning which are still poorly understood. Some major ecological issues have been identified : how is system stability and resilience affected by species diversity, and to what extent could the integrity and sustainability of ecosystems be maintained in spite of species deletions resulting from degradation of environmental conditions ? The relationship of species richness to such processes as biological productivity is still an open question.

The key to successful monitoring programs is to maintain the ability to detect general changes in the status of biodiversity and to identify tools that are adequate and appropriate for assessing the effectiveness of measures taken for the conservation of biodiversity through the sustainable use of aquatic systems. A variety of indicators of biological diversity in aquatic systems have been developed and may be employed. The precautionary principle, as adopted for fisheries by FAO, emphasises growing awareness that fisheries management cannot be seen in isolation and must fit an integrated context which satisfies the requirement both for long-term resource sustainability and environmental conservation.

An understanding of the effects of invading exotic organisms on freshwater systems should receive high priority in regard to their documented consequences on native biodiversity in ecosystems where they have been introduced. The knowledge required relates primarily to predictable effects of introduced species.

FR:

Les sciences de l'eau connaissent actuellement un développement accéléré. Plusieurs facteurs contribuent à cet élargissement de la base de connaissances explicatives et instrumentales sur l'eau. On note, par exemple,

1. les investissements accrus dans la mise au point de systèmes de mesure permettant l'étude approfondie des propriétés de l'eau,

2. l'expansion considérable des approches mathématique et systémique à l'interprétation des données, ou encore

3. les progrès récents des outils informatiques qui ont favorisé le développement et l'usage des modèles de prédiction et ainsi, l'amélioration significative des connaissances sur la chimie, la biologie et la toxicologie.

D'un autre côté, la croissance et la diversification des problèmes sociaux reliés à la raréfaction de l'eau viennent multiplier les domaines d'application des connaissances en vue de trouver des solutions durables aux problèmes.

Dans cet article, on s'interroge, dans un tel contexte d'élargissement, sur l'évolution des sciences de l'eau au cours des prochaines années en mettant en évidence les problèmes socio-économiques dont la solution fait appel aux connaissances actuelles, à leur raffinement par les applications ou encore, à de nouvelles capacités techniques d'interprétation des phénomènes hydrologiques. On y distingue entre les activités qui seront entreprises pour résoudre des questions scientifiques fondamentales pouvant se justifier par des retombées possibles pour la société (la poussée scientifique) de celles qui seront engagées pour élaborer des solutions à des problèmes socio-économiques d'importance (les besoins socio-économiques de connaissances). On met ensuite en évidence les facteurs qui interviendront pour favoriser l'épanouissement des initiatives scientifiques, et on évalue l'effet de ces facteurs sur l'orientation de ces initiatives. On pose ainsi l'hypothèse que ce serait surtout la solution des problèmes socio-économiques, en conjugaison avec les aptitudes scientifiques actuelles, qui orienteront les développements des sciences de l'eau dans l'avenir. Enfin, on présente une approche émergente pouvant aider à comprendre l'évolution des sciences de l'eau. Ce modèle de représentation de la dynamique des initiatives scientifiques est caractérisé par deux pôles d'attraction relevant de la solution des problèmes sociaux reliés à la ressource : l'un en relation avec les besoins de connaissances pour la gestion de l'eau et l'autre lié aux besoins spécifiques de connaissances pour l'administration publique de l'eau.

EN:

The water sciences are now entering a process of accelerated development. Numerous factors can explain this rapid evolution : a) the important investments in measuring systems that now allowed the characterization of water properties, b) the considerable expansion of mathematical and systemic approaches to the interpretation of data, c) the recent progress in interface tolls for computer modeling and the subsequent diversification of simulation models and the remarkable development in water chemistry, biology and toxicology that followed, have all largely contributed to the actual broadening of the theoretical and applied knowledge base on water. Furthermore, the scientific and technical efforts unfolded in order to explain social problems related to water shortages and to find sustainable solutions have also contributed to the diversification and deepening of this wide knowledge base.

In this article, taking into consideration the deepening and diversification of the related knowledge base, we question ourselves on the evolution of water sciences in the future. We first underlined the socioeconomic problems that can be solved either by the application of actual knowledge, its refinement by learning from applications, or by the development of new technical ability for the interpretation of hydrological phenomena. We then distinguished between the activities undertake to solve scientific problems justified by long term social benefits (the science push) from those that aim to find solutions to important socioeconomic problems (the social needs for scientific knowledge). We then look at the different factors that help the achievement of research enterprises and explain the effect of those factors on the orientation of scientific projects. Doing this, we formulate the hypothesis that it is the search for solutions of socioeconomic problems that, on the basis of actual scientific ability, that will be the prime factor for the evolution of water sciences in the future, its dynamic and orientation. Finally, we propose a general approach that can help the understanding of the evolution of water sciences. This model represent the dynamic of scientific initiatives as affected by two attracting poles : the first pole is related to the needs for scientific knowledge for water management problems (i.e. the rational and engineering approach to water problems), and the second being the needs for the specific knowledge required for public administration of water (i.e. the policy and political approach to water problems).

In general, we may conclude that the water sciences can be conceived as the scientific constructs generated by the application of particular scientific basic knowledge to water and its relations with natural and human systems. Those scientific constructs on water and its systemic interactions with terrestrial and human systems develop from this process are not as well structured as the sub-domains that emerge under traditional domains like biophysics, biochemistry, basic hydrology, political economy, or so. They are coherent sets of inter-disciplinary constructs elaborated to explain or predict complex natural processes or systems of relations between human and nature, mostly in response to real or perceived social needs.

Is this to say that the scientific works on water will not succeed in the establishment of well-structured scientific subdomains like hydrology for example ? In spite of the evident progress, natural water and its relation with nature and human systems will remain for a long time applications domains of the fundamental knowledge that have been developed in the basic or applied sciences. Those applications will certainly produced new theories or original basic knowledge with high explicative or predictive values. In this manner, the object of the applications (water) and its context (natural and human systems) are the prime determinant of knowledge development, while in comparison, in basic sciences, it is the knowledge per se and its related instrumental capacities that mostly determine its own evolution.

The development rhythm of technical and scientific knowledge on water is strongly influenced by the attention that society brings upon the resource. In the future, social preoccupations about water should increase considerably in light of its growing scarcity and the collective obligations to cope with higher probabilities of related extreme events. The type of knowledge that should developed will depend upon the specific approaches to social problem solving retained by political and administrative authorities, while in turn, those approaches will be influenced by research and development done in the field of management and public administration of water.