Volume 17, numéro 4, 2004

FR :

Le contrôle de la qualité de l'eau est fondé naturellement et traditionnellement sur des mesures et des prélèvements in situ. Des images satellites étalonnées à partir des données mesurées in situ fournissent une information quantitative et continue sur le milieu aquatique et peuvent être employées pour estimer, avec une approximation raisonnable, les facteurs affectant la qualité de l’eau

L’objectif de cet article consiste à établir des corrélations entre les propriétés optiques de l’eau de mer et les paramètres physico-chimiques. Nous présentons des relations exprimant les variables indicatrices de la qualité des eaux du littoral d’Oran et la réflectance calculée de chaque pixel à partir d’un modèle physique de correction radiométrique. Les mesures in- situ sont effectuées pour des zones de différentes qualités d’eaux et leurs réflectances sont calculées à partir d’une image satellite à haute résolution IRS1-C.

Les meilleures corrélations sont obtenues sur le deuxième et le troisième canal visible. Pour la demande chimique en oxygène, le coefficient de corrélation atteint 0.84, pour les matières en suspension r² = 0.88, pour la demande biochimique de l’oxygène pendant cinq jours r² = 0.62, pour l’oxygène dissous r² = 0.77 et pour la turbidité r² = 0.90.

Finalement, des relations linéaires sont établies avec les réflectances. L’inversion de ces relations permet d’obtenir des images transformées à partir du logiciel de traitement d’image afin d’estimer pour chaque pixel le degré de pollution du milieu.

EN :

Population growth in developing countries has led to a rapid expansion of primary urban areas. Solid and liquid wastes coming from domestic consumption and industrial activities are discharged into potential water sources such as seas, lakes and other natural areas. In order to protect these areas and to control the pollution caused by such waste, it is necessary to continuously monitor these zones. Satellite imagery, such as that obtained with the IRS1C satellite, can be used to estimate, with reasonable accuracy, the factors affecting water quality. This technique allows for the necessary continuous monitoring of impacted areas and affords an overall analysis of their degree of pollution.

Waste disposal affects and alters the chemical and physical characteristics of water. Moreover, water quality could be altered by the decay products of extracellular release and death of aquatic organisms. In turn, these changes can cause an alteration in the appearance of water. It is therefore reasonable to look for relations linking variations in chemical and physical properties to variations in the spectral properties of water, or more precisely, to its reflecting power. The aim of the present study was to:

1. relate the reflectance of polluted water to its physico-chemical parameters;

2. show the significance of such relationships.

Water samples were collected from different sites:

1. two outlets where sewage of Oran City is being emptied into the sea;

2. far from these outlets;

3. far from the port;

4. far from two sites in a lake known to be subjected to both urban and industrial waste.

From each site, water samples were taken at the source and from different places far from the coast. The following water quality parameters were analyzed: temperature, acidity, turbidity, suspended material, dissolved oxygen, electrical conductivity, chemical oxygen demand and 5-day biological oxygen demand. The reflectance coefficient of water in each of the studied areas was calculated using the IRS1C image at four bands. The satellite observes the earth in four spectral channels: C1 (0.45- 0.52 m); C2 (0.52 - 0.59 m); C3 (0.62 - 0.68 m) and C4 (0.77 - 0.86 m) with a spatial resolution of 6 m. The radiance measured by the satellite sensor results from solar radiation affected by several processes including absorption and diffusion on both downward and upward paths by the atmospheric components, and reflection at the ground surface.

We first simulated the measurement achieved by the captor of our reference water (from the sea far from any pollution). Secondly, we used imagery treatment to determine the real value evaluated by the satellite for deep-sea water. We used both a simulated value and the real value to calculate the calibration factor for each channel. We took the image and transformed the digital account into radiance by linear relationships. For each channel, we use the reverse model to calculate the reflectance for each pixel. The substances that determine the optical properties of water surfaces, and thus influence their reflectivity, may be classified into three categories:

1. living phytoplankton and the associated detritus;

2. suspended particles;

3. dissolved organic matter.

The phytoplankton and the associated biogenic detritus generally have the same colour. In most oceanic waters, and in some coastal waters where terrigenous supplies are unimportant, the influence of phytoplankton is dominant. In natural conditions, it is very difficult to dissociate the influences of phytoplankton and those of biogenic detritus on the coefficient of absorption, for which only global estimations are made. The phytoplankton cells and the particles corresponding to biogenic detritus cause a Mie diffusion of light, which is relatively independent of the wavelength. Therefore, the colour of water gradually turns green with increasing phytoplankton concentration.

As expected, our results demonstrated that for polluted waters there was a good correlation between turbidity and concentrations of suspended material. Turbidity and suspended solids have a common effect in reducing light penetration, thereby suppressing primary production in the form of algae and macrophytes. This decrease, in turn, affects the available dissolved oxygen. Our results confirmed this situation by showing a highly negative correlation between turbidity and dissolved oxygen. The oxygen needed for chemical oxidation of organic matter and the accompanied minerals is expressed as COD. Therefore, higher values of this parameter means more organic pollution. BOD5 estimates the oxygen needed for biological oxidation of organic and inorganic matter by organisms that are actually present in the polluted water. Therefore, the DCO/DBO5 ratio refers to the capacity of organisms found in the water to oxidize the organic matter found in the medium. Our results showed that this ratio increased with increasing pollution and with reflectance in the different channels. For easier water quality monitoring we could use the satellite imagery to estimate, with excellent validity, the capability of the water to reduce organic pollution resulting from urban discharge. Moreover, biological parameters could be calculated from each other since there was a high correlation found among them.

The correlation between reflectance and the biochemical parameters was higher for channels C2 and C3 than for the other two channels. The following correlations between reflectance and the measured parameters chemical were obtained: oxygen demand, r = 0.84; suspended matter, r = 0.88; 5-day biological oxygen demand, r = 0.62; dissolved oxygen, r = 0.77; turbidity, r = 0.90. Finally, linear relationships were established between physico-chemical parameters and reflectance values. The inversion of these relationships offered the possibility to estimate for each pixel the degree of water quality. Figures showed clearly different distinct colour sub-areas in each of the studied areas. Each colour indicated a different degree of water quality or pollution. With this technique it was possible to construct, relatively rapidly, a global picture describing the degree of unknown pollution spread over a wide water surface.

FR :

La lagune de Khnifiss, située au sud du Maroc, est une réserve biologique d’intérêt mondial pour l’avifaune. Des études pluridisciplinaires portant sur la physico-chimie des eaux, la biologie, la sédimentologie et les courants ont été réalisées par l’INRH dans la lagune en septembre 1998 et février 2001. Les résultats du courant obtenus montrent que la circulation des eaux à l’intérieur de la lagune est gérée par un courant alternatif et bidirectionnel : les courants du jusant sont plus importants que ceux du flot et varient également en fonction du rythme de la marée vive-eau/morte-eau. Ce mode de circulation des eaux attribue à la lagune une richesse en éléments azotés et phosphatés, surtout en période de vive-eau. La biomasse chlorophyllienne, assez importante dans la lagune, est causée par la richesse du milieu en éléments nutritifs et le phénomène d’upwelling qui se manifeste en été et au printemps dans la zone sud du Maroc. Ces études ont montré que la lagune de Khnifiss est un milieu propice à la conchyliculture. Néanmoins, l’installation de tout projet aquacole dans le site devra tenir compte de sa capacité trophique et de son équilibre écologique.

EN :

The Moroccan coastal marine area has several zones that are targets for exploitation but must simultaneously be protected. Lagoons are considered to be among the most productive natural systems in the world, but they are also vulnerable systems due to natural and human constraints. Lagoons can also be a target of economic expansion. This is why an understanding about the state of these ecosystems is important in order to proceed with rational use of the resource. The Khnifiss lagoon, situated on the South Atlantic coast of Morocco, is a potential site for aquaculture and tourism if planned with an ecological awareness. The large surface area and high biological production for the avifauna qualify it to be among the four protected sites by the RAMSAR convention for humid zone conservation (lagoons of Khnifiss and Moulay Bousselham, lakes of Sidi Boughaba and Afennourir). The objective of this work was to complete previous studies and to estimate the nutritive richness of the Khnifiss lagoon. To understand the spatio-temporal variability in the water currents, and the physico-chemical and biological and characteristics of this lagoon, two surveys were organized in September 1998 and February 2001.

The Khnifiss lagoon (20 km long and 65 km² surface area) is situated between Tantan and Tarfaya (28°02'54'' N, 12°13'66'' W). It opens up into the Atlantic ocean by a narrow inlet called ''Foum Agouitir'', about 100 m wide. The lagoon continues upstream beyond a salt marsh, called "Sebkha Tazra". Physical, chemical and biological variables were monitored: temperature and salinity; dissolved oxygen, nitrates and phosphates; chlorophyll a. The current (direction and intensity) and the sediment were also studied. Samples were collected four times from the surface and bottom waters during spring tide (ST), neap tide (NT), low tide (LT) and high tide (HT) during two seasons (September 1998 and February 2001). Seven hydrological stations were sampled, representing the lagoon system and one reference station in a neighbouring zone (Hréf), which represents the marine zone of the lagoon. Temperature and salinity were measured with a multi-sensor probe. The chemical analyses were carried out according to methods suggested by AMINOT & CHAUSSEPIED (1983). Thirty (30) sediment samples were also collected. Currents were measured at two fixed points by two current-meters.

Based on the hydrological characteristics of the lagoon, especially temperature, salinity, water circulation and the nature of sediment, three zones were identified :

1. Zone I (downstream from the lagoon) was dominated by the ocean’s influence and included the Hréf, H1 and H2 stations. Maximum depths varied from 2.5 to 6.6 m, depending on the tide and location. The concentrations of the different hydrological parameters were controlled by the plug effect of the oceanic environment. The most important current in the lagoon was recorded in this zone in the spring tide (ST: 110 cm.s-1). The sediment was coarse and important water - sediment exchange was caused by the strong current recorded in this zone.

2. Zone II (middle of the lagoon) was a transition zone with an intermediate oceanic influence. It was situated between the H3 and H5 stations. The size of the inlet and the important exchanges governed by currents caused by strong tides resulted in concentrations of the various parameters being similar to those in Zone I. Depths varied from 2.64 to 8.7 m. Waters were more saline during the LT and NT periods (36-41 practical salinity unit, or psu). The current decreased relative to that is zone I (56 cm.s-1 in September 1998 and 78 cm.s-1 in February 2001). In this zone the current moved preferentially towards the right strand at high tide and toward the left strand at low tide.

3. Zone III (upstream in the lagoon) covered a different biotope compared to the first two zones. The waters were less deep (2.64 to 5.3 m) and included the stations H6, H7 and the upper part of the lagoon. This zone was characterized by a very low oceanic influence. Water temperature and salinity were higher, 24-26°C and 41-44 psu respectively, during the LT and NT periods.

On the basis of the hydrological zonation, the conjunction of the climate descriptors (moderate wind, rare precipitation, absence of storms, moderate air temperature) of the lagoon and the strong hydrodynamic influence in the lagoon suggest that the Knifiss lagoon is a relatively unconfined ecosystem. The hydrodynamics of this lagoon permit a fast water renewal rate and insure a good environmental quality. In addition, studies carried out on the healthiness of this lagoon demonstrated not only a healthy environment, but also healthy shellfish. This diagnosis seemed to be related to the geographical location and important rates of water exchange. However, the absence of continental freshwater sources leads to an important increase in salinity and temperature at the level of the upstream zone (zone III). Zone III therefore cannot be considered for shellfish breeding.

On the basis of the biological zones: With respect to the bathymetry of the lagoon (depths varied from 2.5 to 8.7 m), shellfish, in contrast to fish, require less water depth. Therefore, it is sufficient to search for a less turbulent zone, which offers sufficient planktonic food for the shellfish growth. In the Khnifiss lagoon, the taxonomy of the phytoplankton has not been yet studied. However, BENNOUNA (1999) demonstrated that the phytoplankton community was dominated by dinoflagellates and diatoms in the Oualidia and Sidi Moussa lagoons. RHARBI (2000) found that picophytoplankton were the principal breeding food source of oysters and clams in Oualidia lagoon. The source of shellfish for breeding is located in the neighbouring upwelling marine waters. Development projects planned for the lagoon must include an awareness programme for fishermen about the role that moderate and responsible shellfish breeding programs can play in the preservation and reconstitution of the marine resource as well as in the improvement of their incomes.

FR :

Dans cette étude, nous nous sommes intéressés à la défluoruration des eaux souterraines de Youssoufia (eaux d’exhaure) par percolation en colonne sur les cendres volantes de la centrale thermique d’El Jadida et à la désorption des fluorures par la suite. Les résultats obtenus ont montré que :

- le rendement d'élimination du F- est d'environ 97 % dans nos conditions expérimentales avec un temps de séjour supérieur à 72 heures ;

- la régénération des cendres est possible à l’aide de NaOH;

- le nombre de cycles successifs tolérés par les cendres permettant d’atteindre des efficacités convenables est d’au moins huit.

EN :

The layer of black phosphate in Youssoufia is characterized by the presence of underground water in the building sites. This drainage water must be removed to allow the exploitation of these layers. Observations of the tonnage/flow relationship during previous years allowed the prediction of more than 35000 m3 drainage water/day from the year 2000. This water has particularly high levels of fluorides, which represents a permanent risk for the rural population, which relies on groundwater (wells) for its daily consumption. To minimize adverse health affects and to build on earlier work, the current study was focussed on the sorption performances of coal fly ash in the dynamic mode and on regeneration tests of these ashes for possible revalorization. In this study, we were interested in both the defluoridation of subsoil waters in Youssoufia (drainage waters) by percolation through a column of fly-ash collected from the power station of El Jadida and in fluoride desorption from the ash.

The first results from the characterization of this water showed that fluoride concentrations were elevated, exceeding the water quality standard established by the World Health Organization (WHO), 0.7 mg/L for a semi-arid climate. This abnormally elevated content in fluoride comes from the raw phosphates, which have a high fluoride content. Chemical analysis of the fly-ash, carried out by x-ray fluorescence, demonstrated that the principal components were silica, alumina, oxide iron (Fe2O3), and calcium oxide.

With respect to the first objective of this study, the sorption experiments of water soluble F- ions from the drainage waters were carried out at 25 °C in a glass column (33 cm high, 2.2 cm in internal diameter) filled with a well-defined mass of fly-ash. The process involved: placing 250 mL of the solution in the higher tank (placed in top of the column) and the flows were adjusted to 4.6, 7.7 and 15.4 mL/h. The concentration of fluorides was measured in the effluent every 24 h, with a fluoride ion selective electrode, pH model Orion SA 520 according to a standardized method (AFNOR, T90-004). Each experiment was carried out twice. The drainage water used had a fluoride content of 2.50 mg/L.

With respect to the second objective, the study of the desorption of fluorides from the fly-ash was carried out in a static reactor. The choice was related to chemical desorption with soda, and tests were carried out to evaluate the parameters likely to support desorption (concentration of soda, volume to be used, kinetics of desorption). The method used for desorption consisted of brewing the fly-ash in a basic solution (1g of ashes /100mL of soda) during one hour, at a stirring speed of 300 rpm. The concentration of fluorides was then measured in this solution by a potentiometric method following filtration. The fly-ash was removed and rinsed with distilled water and then re-used for a new sorption. The desorption tests were carried out on fly-ash that had been saturated by contact with the drainage waters from Youssoufia ([F] = 2.5 mg/L, pH=7.86). The sorption stage consisted of putting 10 g of ash in contact with 1 L of the drainage water with a stirring rate of 300 rpm and the tests were carried out at ambient temperature.

The results of the defluoridation by percolation through the fly-ash showed that the concentration of fluorides in the effluent decreased to 0 mg/L « < 5 10-7 M », when the flow decreased from 15.4 mL/h (flow 1) to 7.7mL/h (flow 2) to 4.8 mL/h (flow 3), after 96 to 120 hours. Similar results were also obtained by Piekos et al (1998).

The first regeneration results were encouraging for several reasons. First of all, the regeneration of support was possible and desorption was very important. This demonstrated that basic media are probably favorable for desorption. In parallel, the kinetics of desorption with soda were very fast and they did not exceed one hour for the various soda solution concentrations tested. Indeed, it was noted that at the end of one hour at least 90% of fluorides were desorbed. The optimum conditions for desorption were 60 min of contact and a soda concentration 6 M.

Regenerated ashes were placed again in contact with drainage waters containing fluoride concentrations of 2.50 mg/L. A new sorption of fluorides was noted. This result led us to study the effect of cycle numbers on the adsorption-desorption of fluorides in relation to the effectiveness of regeneration. A histogram of the results demonstrated that the quantities adsorbed and desorbed for the same cycle were practically equal for the eight cycles carried out, thus the effectiveness of ash was practically the same.

FR :

Dans cette thématique concernant le transport solide des cours d’eau, il nous semble opportun de résumer le cadre général et d’y situer notre approche. Les formules classiques du transport solide évaluent le débit en matériaux du lit (charriage et suspension) à partir de ses déformations. Elles ne permettent pas d’estimer le débit des matériaux provenant directement du lessivage des versants et qui transite sans interaction avec le lit. Dans cet article, nous considérons uniquement la phase en suspension "MES" mesurée sans distinction à priori de l’origine des grains qui la constitue : provenance directe du bassin versant (phase directe) et (ou) reprise des stocks disponibles dans le lit (phase différée). Le bassin hydrographique du Timis-Béga (Roumanie) est particulièrement bien équipé pour le suivi des débits de 28 sous bassins et le contrôle des flux de MES de douze d’entre eux. De plus, son contexte physiographique nous permet de penser que la phase directe est prépondérante. Le protocole de mesure des flux de MES prévoit, entre autres, une densification variable des observations selon l’intensité des crues liquides. Ces considérations précédentes nous permettent d’envisager une modélisation statistique des apports solides en MES des sous-bassins du Timis-Béga. Celle-ci est directement inspirée des connaissances acquises sur la modélisation statistique "QdF" des régimes hydrologiques des bassins versants. Sur l’exemple du sous-bassin du Béga à Balint, qui draine une superficie de 1064 km², nous retiendrons deux principaux résultats issus de la transposition du concept QdF aux débits solides QMESdF : Les analyses statistiques des régimes liquide et solide montrent que les débits solides de MES ne sont pas simplement proportionnels aux débits liquides mais croissent plus rapidement. Les deux lois de distributions privilégiées, Pareto généralisée pour les MES et exponentielle pour les débits, permettent de le justifier. Le temps de montée des hydrogrammes de projet liquide ou solide est quasiment identique, autrement dit nous vérifions la quasi concomitance de leurs débits de pointe. Ce résultat n’est possible que si le débit solide de MES provient essentiellement du lessivage des versants, ce qui était supposé à priori.

EN :

With respect to sediment transport, we detailed the general framework and how our approach contributes to these developments. Starting from the single traditional relation for the bed material load, specialists in river hydraulics cannot assess sediment yield of basins, when it involves the auto-suspension of fine sediments coming mainly from slope erosion (wash load). This latter estimate is needed for simulating the transfer of sediments and possible deposition in certain areas, particularly when a strong slowing down occurs. The Timis-Bega drainage basin (Romania) is fairly well equipped for the monitoring of discharge and suspended materials (sediment discharge). The hydrometric network includes 28 stations, of which 12 allow a monitoring of wash load. Moreover, its physiographic characteristics led us to think that the wash load dominates. Thus we assumed that sediment discharge was correlated with the physiographic features of the catchment area. The protocol for the measurement of the suspended sediment load was intensified during the floods. Thus, statistical modelling of the sub-basin sediment yields could be performed.

The current study was directly inspired by the knowledge obtained in the domain of statistical modelling that describes hydrological regimes. The approach adopted was based on the flood-duration-frequency (QdF) analysis that takes into account the temporal variability of floods. The QdF approach analyses maximum average flows (Vd) over various durations (d), equivalent to intensity-duration-frequency (IdF) curves commonly used for rainfall analysis. The proposed model allows QdF curves V(d, T) for a given basin to be estimated using a minimum number of parameters. When the statistical law is the exponential law, this model contains only three parameters, due to observed scale invariance properties. The ∆ parameter that informs about the shape of the flood hydrograph is consequently the flood characteristic duration of the studied basin. The two parameters of the exponential maximum flood distribution for d=0 (a₀ and x₀) and ∆ were fitted to sample discharges (Vd). This model is called a converging QdF model because of the observed convergence of distributions towards small return periods. This model is also useful for the determination of threshold discharges (Qd). The analytical formulation of the V(d,T) model can be derived according to d, in order to obtain a Q(d,T) model. This model then permits the calculation of the hydrograph for any return period (T) and any duration (d).

The regionalization of the sediment yield was achieved within the framework of the Riverlife European project, in collaboration with NIHWM (National Institute of Hydrology and Water Management of Romania). Initially, local models were built. As an example, starting from the Bega sub-basin at Balint, with a surface of 1,064 km², our intent was to present the transposition of the discharge-duration-frequency analysis concept (or QdF) to the wash load Q_MES dF. The latter relates to the measurement procedure, the statistical processing of the observed data Q_MES (t), and to the building of the discharge hydrographs of the associated projects.

The main results were:

- The statistical analyses of floods and sediment discharges show that the wash loads were not simply proportional to the discharge, but rather they increased more rapidly. The selection of the appropriate distribution laws (Pareto generalised for the Q_MESdF model (four parameters) and exponential for the QdF model) reinforced this result.

- The lag-time was the same for both hydrographs with respect to flood and sediment discharge. This result can be achieved if the sediment transport comes primarily from the scrubbing of the slopes (wash load), which was hypothesised a priori. However, falling limb of the sediment hydrograph decreases more quickly than for the discharge hydrograph (∆_MES is lower than ∆).

The Bega sub-basin example at Balint was a first test towards the regional modelling of the contributions to sediment discharge in the catchment area of Timis-Béga. This flood and sediment discharge regionalization is necessary for the study of the protection of the town of Timisoara against flooding.

FR :

En général, les résultats des bioessais d’écotoxicologie sont étudiés par des méthodes statistiques et les paramètres estimés n’ont pas de signification biologique. La modélisation est apparue plus récemment en écotoxicologie et bénéficie même ces temps derniers d’un regain d’intérêt. Son développement s’effectue actuellement dans deux directions complémentaires que nous avons voulu présenter ici en en montrant les principaux apports. D’une part les effets sur les individus font l’objet d’efforts de modélisation afin de donner un sens biologique aux paramètres des tests de toxicité pour pouvoir intégrer des facteurs confondant au cours des tests comme par exemple des variations de la concentration d’exposition ou pour pouvoir déterminer les modes d’action des composés. D’autre part, l’écosystème étant l’objet d’étude par excellence de l’écotoxicologie, la modélisation est utilisée pour déduire les effets au niveau des populations à partir d’essais réalisés sur les individus. Jusqu’à présent, des approches classiques, qui se fondent sur l’équation d’Euler ou la diagonalisation de matrices de Leslie, ont été utilisées et ont permis une meilleure définition des paramètres à rechercher au niveau des tests de toxicité. D’autres approches sont à développer pour gagner en pertinence vis-à-vis du terrain (notamment hétérogénéité spatiale de la pollution et des habitats).

EN :

Traditional analysis of toxicity tests provides toxicity parameters that are estimated with purely statistical methods. Consequently, these parameters do not have any intrinsic biological meaning and these methods provide no information about the mode of action of the tested chemicals. It is also difficult for these methods to change scale from the individual level to the population level, or to account for temporal and spatial heterogeneity. Modelling is an important tool in ecotoxicology and recently it appears to have gained more interest. Developments in modelling are currently expanding in two directions, modelling effects at the individual level and applying toxicity data obtained at the individual level to responses at the population level. The objective of the current study was to present these two complementary modelling approaches together with the opportunities they offer.

Modelling at the individual level provides parameters that are biologically relevant. Modelling also facilitates the formulation and the testing of hypotheses concerning toxicity processes (physiological mode of action and kinetics). Confounding factors such as time, varying exposure concentrations, or feeding can also be incorporated into models. In this paper, two kinds of models were examined: biochemistry-based models (Hill models) and energy-based models (Dynamic Energy Budget models). In the Hill approach, effects are modelled as the interaction between chemicals and receptors in the organisms, which leads to a relationship between concentration and effects close to the logistic equation often used in toxicity test analysis. In the energy-based approach, models are built on the dynamic energy budget theory, in which energy derived from food is used for maintenance, growth and reproduction. The effect of compounds is then described as a change in one of the parameters describing these physiological functions. Kinetics are taken into account by a one-compartment model. The uptake rate is proportional to the exposure concentration, whereas the elimination rate is proportional to the concentration in the tissue. This model is simple but is relevant for many organisms and compounds (KOOIJMAN and BEDAUX, 1996). As time is taken into account through kinetic modelling, the estimation of the other parameters, such as the No Effect Concentration, does not depend on the exposure duration. An energy relevant model has many advantages. First, observed effect profiles are more in agreement with expectations (KOOIJMAN and BEDAUX, 1996). Second, it becomes possible to account for the fact that an effect on survival increases the amount of food consumed per surviving organisms, which in turn partly compensates for the negative effects of pollutants. Third, it allows for the examination of effects at the population level on density and biomass, complementary to the usual study of population growth rate.

Most of the recent modelling research is related to deriving effects at the population level from effects at the individual level, because ecosystems are the target of ecotoxicology. Until recently, classical approaches, like the Euler equation or Leslie matrices, were used with population growth rates as endpoints. They provide interesting tools to determine the impact of life cycle parameters at the population level and to assess which level of effects has to be assessed. Even a simple approach such as that proposed by CALOW et al. (1997), separating the population into two different classes, juveniles and adults, can produce very interesting results. For instance, the authors showed that in populations for which females die just after reproduction, juvenile survival had much more importance than for populations where females can reproduce several times during their lifetime. The opposite is true concerning adult survival. However, these approaches do have some limits that make complementary approaches necessary to fully understand the effects of pollutants at the population level. First, they do not account for effects on the carrying capacity. SIBLY (1999) pointed out that there is a need for ecological studies on the effects of pollutants that measure their effects on density dependence and carrying capacity. Indeed an effect on population growth rate only accounts for a risk of disappearance for the population, but cannot help in the understanding of effects on biomass or density. Effects on the carrying capacity can have substantial effects at the ecosystem level, especially when studying species that constitute a food resource for other species. Second, more complex tools have to be developed to take into account spatial heterogeneity of pollution and habitats in order to be relevant from an ecosystem point of view. Indeed, it has been shown that uncontaminated sites can be significantly disturbed if they are connected, through the migration of organisms, with contaminated sites (SPROMBERG et al., 1998).

FR :

La modélisation du comportement hydrologique des bassins versants est incontournable dès lors que l'on s'intéresse à des problématiques relatives à l’évaluation et la gestion optimale des ressources en eau. Ceci s’illustre par un aspect quantitatif, dans les pays comme l’Algérie où l’alimentation en eau est un facteur limitant. L’objectif principal de cet article est d’expliciter les relations entre les conditions hydrologiques et la disponibilité en eau de surface. A cet effet, un modèle pluie-débit à l’échelle annuelle qui prend en compte les paramètres physiques et climatiques a été mis au point. L’application porte sur l’Algérie du Nord dont la superficie est de 325 000 km².

La mise au point de ce modèle nécessite au préalable une base de données qui a été acquise lors des travaux antérieurs incluant la carte des pluies médianes, la carte des perméabilités et le modèle numérique de terrain. Afin de compléter cette base de données, la cartographie des pluies des différentes années pour lesquelles on dispose des débits a été réalisée en utilisant une méthodologie intitulée "cartographie de la pluie centrée réduite". Ainsi, les données de 467 postes pluviométriques ont été traitées et ont permis de tracer les cartes des isohyètes annuelles.

L’étude de la relation pluie-débit à l’échelle annuelle est basée sur les données de débits de 50 stations hydrométriques réparties à travers la zone d’étude. La démarche utilisée s’est inspirée de la fonction de production du S.C.S (Soil Conservation Service). Les résultats du modèle après calage ont permis d’obtenir un coefficient d’explication de 0,75, ce qui signifie que 75 % de la variance est expliquée par la pluie moyenne, la surface et un coefficient (a) qui correspond à la pente moyenne des bassins versants.

EN :

Modelling the hydrological behaviour of drainage basins is very important for solving problems related to the evaluation and optimal management of water resources. This is illustrated quantitatively in countries such as Algeria where water supply is a limiting factor. The principal aim of this paper was to explain the relationship between hydrological conditions and the availability of surface water. A model of rainfall-discharge was developed on a yearly scale, taking into account physical and climatic parameters. The application was carried out in northern Algeria where the total land surface is about 325 000 km².

The development of this model required a database, which was acquired during previous studies where maps of median rainfall and permeability as well as the digital elevation model were developed. In order to complete this database, the cartography of rainfall for the years for which we have discharge data was carried out using a methodology entitled "mapping standardized rainfall". To estimate and map annual rainfall, the kriging method was used. Two problems were encountered:

- The presence of a drift highly altered the variogram and made it very difficult to infer a sub- structure function;

- The variogram is significant only if the hypothesis of ergodicity is valid, which was not easy to assume for any given year.

In order to resolve these difficulties, a homogeneous random and secondary stationary order function (same mean at all points and same covariance function) must be calculated. A previous study by ANRH (1993) allowed us to know the statistical parameters of the distribution at each point. These parameters were mapped, taking into consideration the topographical relief and distance to the sea. For every year and at each rainfall measure point, the standardized rainfall could be deduced. The correlogram gave information about the spatial variability of the phenomenon and its range, and subsequently the standardized rainfall was then interpolated. Annual rainfall was calculated by combining the grids of the means, the variances and the centered reduced rainfall (TOUAZI and LABORDE, 2000). Thus, the data of 467 rainfall gauges were used in order to create maps of the yearly isohyets.

The rainfall-discharge relationship on an annual scale was based on 50 hydrometric stations distributed throughout the study area. The methodology used was derived from the production function of the S.C.S (Soil Conservation Service). This production function was part of modelling, which transformed total rainfall to net rainfall. This method was very representative of the natural hydrological processes. Indeed, it takes into account rainfall and the maximum infiltration capacity (S), which depends on the nature of the soil (lithology), vegetation and soil moisture content. In the current study, the basin surface and a regional parameter (a) were introduced in order to calibrate the model. This production function was implemented by supplying different values for the parameter (S). The values (n+1) were obtained by increasing the previous value (n) by 10 %. We evaluated the different values of (S) in the same way to obtain the last value (i). We calculated for these different values of the parameter (S) the square of the difference between the measured and estimated discharges for each year by measuring the discharge at different stations. For each station, we calculated the sum of these values for all the years and we retained the value of (S) that gave the minimal value. The results demonstrated that the values of (S) obtained were not significant because they tend to the infinite. For this reason, (S) was considered as a constant. In order to improve the model, we repeated the same operation, but instead of (S), we used the parameter (a) and performed the same calculation. After calibration of the model the results gave a coefficient of determination of 0.75, which means that 75 % of the variance was explained by the mean rainfall, the surface and the parameter (a).

To explain the parameter (a), we calculated the correlation between its value at each station with the corresponding geology. This latter variable was characterized by the average storage capacity, which corresponds to the weighted average of the surfaces of the basin assigned to each permeability category (TOUAZI, 2001). The results demonstrated a coefficient of determination of 0.1. The correlation with the topographical relief was not necessary because it was taken into account in the cartography of the rainfall. We then proceeded to the cartography of the parameter (a). The results demonstrated an east-west gradient that was constant and a north-south gradient that decreased from north to south. With the digital elevation model, we used a geographical information system to deduce the slopes. For each basin, the average slope was calculated by taking the average of the values of the slopes of all the pixels that constituted the individual basin. The correlation between slopes and corresponding values of the parameter (a) gave a coefficient of correlation of 0.6.

The results obtained by this model after calibration gave a coefficient of determination of 0.75, which means that 75% of the variance was explained by the mean rainfall, the surface and a coefficient (a), which corresponds to the average slope of the drainage basins.