Modélisation mathématique de l'évolution, à long terme, des teneurs en nitrates dans la nappe aquifère des craies du Crétacé de Hesbaye (Belgique)
FR : La nappe aquifère de Hesbaye, logée dans les craies du Crétacé, est sollicitée à raison de trente millions de mètres cubes par an. Bien que naturellement protégée par une épaisseur de 5 à 20 mètres de limons, de nombreux indices montrent une dégradation de la qualité des eaux souterraines, notamment par les nitrates. Les concentrations en nitrates atteignent 15 à 25 mg.l-1 dans la partie semi-captive de la nappe et sont systématiquement supérieures à 35 mg.l-1 dans la partie libre. Malgré de fortes fluctuations temporelles, les teneurs augmentent en moyenne de 0,35 mg.l-1 à 0,7 mg.l-1 par an selon la situation semi-captive ou libre de la nappe.La détermination des paramètres hydrodynamiques et de transport de la craie par plus de 35 traçages répartis sur 11 sites, a permis de réaliser un modèle local (10 km2) de transport simulant la propagation des nitrates dans la nappe. Le modèle a montré que cette dernière est, malgré une certaine homogénéisation, très sensible aux apports de surface engendrant une très forte variation spatiale des concentrations. La nappe réagit de manière très différente selon que les apports de surface sont d'origine ponctuelle ou diffuse. Pour les pollutions ponctuelles, les concentrations fluctuent rapidement avec des valeurs maximales et minimales observées respectivement en périodes de basses eaux et de hautes eaux. Cette situation est liée à un phénomène de dilution de la pollution par les eaux en provenance de l'amont. En cas de suppression d'une pollution ponctuelle, la qualité de la nappe s'améliore rapidement (délai de 1 à 2 ans). Pour les pollutions diffuses, les concentrations minimales s'observent en période de rabattement de la nappe : le front de nitrates migre plus lentement (environ 1 à 2 m par an) que les vitesses de rabattement de la nappe (jusqu'à 5 m par an) et les intrants restent nuls durant des périodes pouvant aller jusqu'à 3 ans.Différentes simulations mathématiques ont montré que si la quantité d'intrants d'origine diffuse diminue de manière permanente, la nappe mettra une vingtaine d'années pour se rééquilibrer. Ces constatations sont primordiales dans le cadre de la mise en œuvre de mesures de protection puisque, si les résultats de la suppression des pollutions ponctuelles sont rapidement mais localement observés, ceux liés à la diminution des pollutions d'origine diffuse sont observés dans des délais nettement plus longs (une à deux décennies).Ces résultats montrent clairement que toute gestion qualitative des aquifères doit être basée sur des actions à long terme.
EN : The Hesbaye area is located in the northeastern part of Belgium. The aquifer formations consist of chalk deposits. Groundwater provides about 80,000 m3 d-1. Despite 5 to 20 meters of superficial loess deposits, the groundwater quality is threatened by increasing nitrate concentrations of 0.35 mg×L-1 per year in the semi-confined part of the aquifer to 0.7 mg×L-1 in the unconfined aquifer. Presently, nitrate concentrations are between 15 and 25 mg×L-1 in the semi-confined part of the aquifer but are more than 35 mg×L-1 (reaching locally 150 mg×L-1) in the unconfined part that covers 95% of the area. Nitrate concentrations have such a high spatial variation that various statistical treatments (such as kriging used to draw iso-concentration maps) have failed. This failure is due to the fact that the concentrations are highly influenced by surface land use (grass land, culture land, villages, point source pollutants, etc.). In addition, nitrate content in the aquifer varies vertically with decreasing values at depth (gradient of 0.7 mg×L-1 ×m-1).Aquifer parameters were determined by 38 pumping and tracer tests conducted in radial convergent or cylindrical flow at 11 sites. Results showed that hydraulic conductivity values ranged from 1 × 10-6 m×s-1 to 4 × 10-2 m×s-1 and effective porosities from 0.5% to 7%, showing that the aquifer was heterogeneous. Dispersivity values were affected by scale effects and varied according to chalk weathering or fracture zones. They ranged from less than 5 m in fractures to more than 60 m in weathered chalk (as in the upper part of the aquifer) and in the chalk matrix. In the chalk, transport processes were influenced by the immobile water effect due to diffusive transfer from the moving to the non-moving fluid. Non-effective porosity filled by non-moving fluid was estimated between 8 to 42%. The transfer constant ranged from 0.98 × 10-7 s-1 to 10 × 10-7 s-1.The determination of the transport parameters allowed simulation of nitrate transport at a regional scale. The SUFT3D (Saturated and Unsaturated Flow and Transport Model), developed by the Hydrogeology Section of the Georesources, Geotechnologies and Building Materials Department of Liege University was used. The modelled groundwater zone was defined as a 2.0 x 4.5 km rectangle of 10 km2. The aquifer was subdivided into 6 layers of 3350 cells (50 x 50 m wide and 3 to 15 m thick). Boundary flow conditions were defined as a prescribed head (Dirichlet conditions) to the north and the south of the area modelled. As the model simulations run for a time period of 30 years, the northern Dirichlet conditions had to be adapted to the regional and seasonal water table fluctuations that were observed during this period. At the south boundary, as the aquifer is drained by the river Geer, the water table is fixed at the river bed altitude. The eastern and western boundaries were, according to the regional piezometry, assumed to be impermeable. For the transport boundary conditions, prescribed flux (Cauchy conditions) was used for the aquifer top. Elsewhere Neumann conditions were usedSimulations were run for the period from 1963 to 1992. Nitrate inputs were averaged yearly and estimated according to actual input conditions. These conditions were calculated by simulation of nitrate flows through the non-saturated part of the aquifer using the EPIC-Model and taking into account the amount of nitrate fertilisers used by farmers (given by the Belgian government Statistical Institute). Initial conditions were calculated according to the 1963 nitrate inputs.Simulations demonstrated that it is important to distinguish the origin of the pollution as either point or non-point (diffuse) sources. For point source pollutants (such as contaminated infiltration basins), aquifer nitrate concentrations increased during low water level periods due to weaker dilution linked with a poor regional water gradient. During high groundwater levels, dilution is more important and the nitrate concentration decreases. If a point source pollutant is suppressed, aquifer quality is improved within one to two years. This demonstrates the importance of protective actions that could be applied within the framework of the protection zones around collecting galleries and pumping fields.For diffuse contamination the mean input over the area (10 m depth below cropped areas) increased from 1.32 × 10-7 mg×m-2 ×s-1 in 1963 to 5.14 × 10-7 mg×m-2 ×s-1 (i.e., a factor of four). According to these values, concentrations ranged from 11 mg×L-1 to 22 mg×L-1 (i.e., increasing by 0.5 mg×L-1 per year) between 1963 and 1992. Predictive simulations, using 1992 input, show that it will take more or less 30 years for the aquifer to be in equilibrium with the 1992 input. At that time the mean concentration value will be around 30 mg×L-1.The main results of the simulations clearly show that if actions are taken to decrease nitrate inputs, even if the aquifer nitrate contents rapidly react to the new input, nitrate levels will decrease slowly and take about 30 years to be in equilibrium with the new inputs. This long delay is due to the immobile water effect that is characteristic of the chalk aquifer. Thus it is important to inform environmentalists who work on action programs (such as the water directive imposed by the European Community in the vulnerable zones) that the effects of their actions must be based on 10 to 20 year scenarios. To this estimation, based on the reaction time of the aquifer to a new input, one must also add the time transfer of the pollutant through the unsaturated part of the aquifer.
Comparaison par simulation de Monte-Carlo des propriétés de deux estimateurs du paramètre d'échelle de la loi exponentielle : méthode du maximum de vraisemblance (MV) et méthode des moindres carrés (MC)
FR : La loi exponentielle est très répandue en hydrologie : elle est faiblement paramétrée, de mise en œuvre aisée. Deux méthodes sont fréquemment utilisées pour estimer son paramètre : la méthode du maximum de vraisemblance et la méthode des moments, qui fournissent la même estimation. A côté de ces deux méthodes, il y a celle des moindres carrés qui est très rarement utilisée pour cette loi. Dans cet article, nous comparons le comportement asymptotique de l'estimateur de la méthode des moindres carrés avec celui de la méthode du maximum de vraisemblance en partant d'une loi exponentielle à un seul paramètre a connu, puis en généralisant les résultats obtenus à partir de la dérivation des expressions analytiques. L'échantillon historique disponible en pratique étant unique, et de longueur généralement courte par rapport à l'information que l'on désire en tirer, l'étude des propriétés statistiques des estimateurs ne pourra se faire qu'à partir d'échantillons de variables aléatoires représentant des réalisations virtuelles du phénomène hydrologique concerné obtenus par simulations de Monte Carlo. L'étude par simulation de Monte Carlo montre que pour de faibles échantillons, l'espérance mathématique des deux estimateurs tend vers le paramètre réel, et que la variance de l'estimateur des moindres carrés est supérieure à celle de l'estimateur du maximum de vraisemblance.
EN : Exponential distributions are frequently applied in hydrology, for example: frequency analysis of the duration and severity of water flow conditions MATHIEU L. and al. (1991); regional frequency of storm intensities ARNAUD P., LAVABRE J. (1999); partial duration of hydrological droughts KJELDSEN T. R. and al. (1999) ; and daily rainfall modelling CHAPMAN T.G. (1997) ; KABAILI Z. (1983). This method has only one parameter, and it is easy to use. Its parameter is mainly estimated using the maximum likelihood estimator (MLE) or the method of moments estimator (MOME), but the least square estimator (LSE) can also be applied. For the one-parameter exponential distribution, MOME and MLE give the same expression for the parameter: Ex Σ xk k = 1X^0 = ________ ExUsing LSE requires a Ex size sample of exponential variables and involves the following steps:1. Sorting the Ex variables in the sample in ascending order2. Associating to each quantile xk whose rank is k in the sorted sample an empirical frequency F^k = k - 0.5 / Ex3. Plotting Ex against ln(1-F^k) and using LSE to calculate x^0 by :x^0 = - (ExΣk=1Xk ln(1-F^k)) / (ExΣk=1 [ln(1-F^k)]2In this paper we compare the asymptotic behaviour of the statistical properties (mean and variance) of the MLE and the LSE. These comparisons must be made by using a great number of sample parameter estimations. In practice, only one historical sample of variables issued from a known exponential distribution was available, from which only one parameter can be calculated. To overcome this difficulty, samples of variables whose original theoretical exponential distribution is known are generated using the Monte Carlo numerical method. Samples of estimated parameters (using the MLE or the LSE) are then created from the above samples of random variables, and the statistical properties of the two estimators are then calculated. These different successive steps are summarised below:1. Generate sample of finite size Ex for known exponential variables2. Use this sample to estimate one parameter using MLE or LSE3. Do steps 1 to 2 Np times to collect a Np size sample of parameter estimations4. Use this sample to calculate statistical properties (mean and variance) for the two estimators.According to this approach, sizes Ex and Np should influence the statistical properties of the two estimators. We have verified this with a one-parameter exponential law, with a known theoretical parameter X0=1. Samples of estimated parameters of size Np have been generated from virtual samples of size Ex issued from a population following the above statistical distribution. During this operation, one of these sizes, Ex or Np, has been held constant, while the other, Np or Ex, changed with a constant step. Statistical properties of the estimators have then been calculated for each of the two cases.Let Var Ex (x^0(Np)) and EEx (x^0(Np)) be statistical properties (variance and mean) of the two estimators for fixed values of Ex, and VarNp(x^0(Ex)) and ENp(x^0(Ex)), the same statistical properties for fixed values of Np.Plotting VarEx(x^0(Np)) for Ex=10 and Ex=100 shows that for large values of Np (1000 to 5000) variance tends towards a constant value, close to 0.1 for Ex=10 (Fig. 1a) and to 0.01 for Ex=100 (Fig. 1b), both equal to 1/Ex, when the MLE is used. When parameters are estimated with the LSE, variance tends towards a constant value, greater than the preceding ones (Fig. 1a and Fig. 1b). Plotting VarNp(x^0(Ex)) when Np=1000 is constant, the variance decreases as Ex grows whatever the estimator, but for a given value of Ex, the variance is always greater when the LSE is used (Fig. 3). These two calculations show that asymptotic variance depends only on size Ex of samples of known exponential distribution.Plotting EEx(x^0(Np)) when Ex=10, for important values of Np, the mean is close to the true parameter for the MLE, and greater than this true parameter for the LSE (Fig. 2a). When Ex=100, the mean is close to the true parameter for the two estimators (Fig 2b). From these calculations we notice that the asymptotic mean depends only on the size of Ex for known exponential variables and on the used estimator. The MLE seems to present no bias for the mean, while the LSE presents a bias for small values of Ex, but this bias disappears as Ex increases. To quantify this degree of dependence, we have plotted ENp(x^0(Ex)) for Np=1000 (Fig. 4). For the two estimators, the mean presents an initial bias, when Ex is low and the bias disappears when Ex becomes higher. The initial bias is more important with the LSE.In summary, the asymptotic statistical properties of the two estimators (mean and variance) depend only on the size of Ex for known exponential distribution variables.Empirical plots are unstable for low sample sizes, are sensitive to sampling, and are very difficult to explain. Analytical expressions for the asymptotic statistical properties of the two estimators are needed for realistic comparison. According to formulae (1) and (2), statistical properties depend on E∞(Xk) and E∞(X2k) respectively and the asymptotic mean of Xk and X2k.E∞(Xk) and E∞(X2k) have been derived using the density of probability of Xk through statistics of rank. Asymptotic statistical properties of the two estimators have then been evaluated using the expressions of E∞(Xk) and E∞(X2k).We let E∞[x^0(Ex)] be the asymptotic mean of estimator, Var∞[x^0(Ex)] be the asymptotic variance, and x0 be the theoretical parameter of exponential distribution. By plotting E∞[x^0(Ex)] for X0=1, we note that this expression has a constant value, equal to unity when the MLE was applied, and that it decreases quickly to unity when the LSE was applied (Fig. 6). By plotting Var∞[x^0(Ex)] for x0=1, we also note that the theoretical asymptotic variance diminishes as Ex grows, but is greater when the LSE was applied (Fig. 5). By comparing with empirical plots when x0=1, we establish the same trends.Theoretical derivations of asymptotic statistical proprieties have confirmed empirical experiences: · The MLE for a one-parameter exponential presents no bias · The LSE for a one-parameter exponential is a consistent estimator of the simple exponential parameter.
L. Zella, A. Kettab et G. Chasseriaux
FR : La microirrigation est une technique dont l'uniformité de distribution d'eau par les goutteurs est très sensible aux faibles variations de pression. Pour maîtriser ces variations, avec davantage de précision, le présent travail est basé sur une analyse hydraulique approfondie de l'écoulement aboutissant à des équations différentielles aux dérivées partielles dont la pression et la vitesse de l'eau sont des inconnues. Ces équations non linéaires sont résolues en utilisant la méthode d'intégration Runge-Kutta d'ordre quatre. Les modèles développés dans la présente étude permettent de simuler la dynamique de l'eau dans la rampe et dans le réseau et sont utilisés pour déterminer le dimensionnement optimal du réseau. Les résultats obtenus corroborent ceux publiés par d'autres auteurs ayant utilisé la méthode des volumes de contrôle ou la méthode des éléments finis.
EN : Micro-irrigation is recommended for use in arid and semi-arid countries such as Algeria. This method consists of accurately providing the right amount of water and mineral nutrients to the plant's root area. The goal is to provide water efficiently by applying it at the correct rate. However, irrigation efficiency is clearly a function of the uniformity of water application.Micro-irrigation is a technique in which a delicate instrument known as an emitter (a terminal element of the network) operating with low pressure is used. The emitter, designed and manufactured with high precision, is a system with hydraulic laws and norms considered as a black box model that discharges water at atmospheric pressure. The emitter is an element of a network that constitutes a unit called a system or physical model. Water and mineral elements are delivered to a localized place, to the level of each plant by the emitters whose discharge is a function of lateral pressure. The precision of the dosage of irrigation, which must exactly satisfy the requirement for cultivation, depends fundamentally on the design of the network. It takes into account the pressure variations, which are due not only to head loss in the lateral branches of the network but also to the land slope and to the characteristics of the emitters. Water and air temperature and the possible plugging of the emitter orifice also influence the discharge of an emitter.The network is designed to satisfy the water needs of all the plants. Uniformity of water distribution is a main criterion for network design. To understand the variations in water distribution with more precision, we based the present work on a hydraulic analysis focussed on the outflow. This approach yields differential equations in which the pressure and the velocity of water in the pipeline network are unknown; the uniformity of water distribution is largely dependent on these variables. The differential second-order equations obtained are non-linear and analytical resolution is impossible, due to the empirical relation of the discharge emitter and the energy loss relation. Thus, the solution is obtained by numerical methods using the Runge Kutta integration method. The conditions in the limit equation modelling the outflow in the lateral pipes are different from those for the submain pipe. For the lateral pipes, the velocity of water at the extremity of the downstream region is inevitably minimal, as the whole region of discharge in the last pipe section is delivered by the last emitter where the pressure is minimal (Hmin). The velocity and pressure are calculated step by step along the lateral pipe until the entrance of water into the network where the pressure is maximal (Hsmax). The algorithm developed to simulate the emitter discharge distribution from the lateral pipes is called the "RK" model, and when it includes the discharge in the submain pipe it is called the "RS" model. These two models are transcribed in Fortran language by a computer program that automates iterations and calculations. Twelve parameters are changed in turn, or per group according to the cases studied, and the choice of the optimal solution of the parameters includes: emitter coefficients (a, y and Cvf); length and diameter of lateral pipes; the submain and main network (Lr, Dr, Ls, Ds, Lp, Dp); the roughness of the pipes (C); the spacing between the emitters (Δxr); the spacing between the lateral pipes (Δxs); and the water temperature. From these data of discharge and available pressure to the level of the parcel, the model precisely describes the distribution of the pressure and the discharges to all network emitters. In this case, the total discharge and the total required pressure, the uniformity of pressure and discharges are determined for each pattern of design. The combination of structural, functional and environmental factors is applied to guarantee an optimal exploitation taking into account the limits imposed by the specific norms for the micro-irrigation and the technical limits of velocity and pressure tolerance.Parameters that influence variations in uniformity are numerous and variable, which is why it is not easy to integrate them into this phase of study. The proposed model has merit as it avoids the complex numerical method of finite elements, recommended by some researchers (BRALTS et al., 1993; KANG and NISHIYAMA, 1994). The finite element method based on matrix structuring requires an important volume of iterations and calculations that could constitute a major constraint in the case of a large network. The model of BRALTS et al. (1993) is of particular interest in this regard ; our results have been confronted with those obtained with their model. Thus, the models presented in this study permit the simulation of water dynamics in micro-irrigation networks and offer the opportunity to determine the optimal design for such networks. Optimization is based on the variation of twelve classical parameters plus the associated geometric structure of the network, which was shown to be a non-negligible parameter. Optimization would not only reduce irrigation water volumes, but also fertilizer use and pumping energy. The example illustrated in Table 4 shows that although the networks deliver the same total discharge and have some similar design characteristics, the consumption of pumping energy changes from one geometric structure to another. Once a network is installed, it is impossible to change its design, so it is important to assure precision of the design calculations.This work shows promise in the simulation of the optimal design of micro-irrigation networks and also constitutes an economic means of making decisions. Moreover, the modelling results can guide field experimentation to explore other methods. Micro-irrigation can potentially solve many water shortage problems, but it requires further research in the safe reuse of low quality water and wastewater, the development of long term sustainability and the minimization salt accumulation and drainage problems.
Impact de la sécheresse sur l'évolution de la qualité des eaux du lac Mansour Eddahbi (Ouarzazate, Maroc)
M. Sadani, N. Ouazzani et L. Mandi
FR : L'objectif de ce travail est d'étudier l'impact d'une année hydrologique très sèche (98/99) sur la qualité physico-chimique et l'évolution de l'état trophique du lac Mansour Eddahbi. Ce lac est situé dans la région de Ouarzazate, au sud du Maroc, caractérisée par un climat aride continental, avec des écarts thermiques hiver-été importants, des précipitations très faibles et une forte évaporation. Ceci induit une réduction considérable du volume total du lac et par suite une baisse de 14 m de son niveau.À la lumière des valeurs de températures enregistrées au niveau de la colonne d'eau, le lac peut être classé dans la catégorie des lacs monomictiques avec une seule période de mélange hivernale. Le pH est légèrement alcalin. La réduction du volume d'eau au niveau du lac a engendré une augmentation de la salinité des eaux. Une corrélation significative est enregistrée entre les deux paramètres (r2 =0,60 pour n=13 et p<0,05).Un déficit marqué en oxygène dissous (7,3 mg d'O2 /l comme moyenne en surface et des valeurs inférieures à 2 mg d'O2 /l voire nulles en profondeur durant la stratification), les teneurs en Chl "a" (24 µg/l) permettent de classer le lac dans la catégorie des lacs eutrophes. Selon les teneurs enregistrées en azote (0,2 mg/l) et en phosphores (0,02 mg/l), le lac est hyper-eutrophe. Deux années auparavant, le lac était considéré comme mésotrophe (ONEP/BRL, 1998). Les conditions climatiques sévères qui ont sévi durant l'année hydrologique 98/99 ont contribué à une évolution accélérée de l'état trophique du lac.
EN : Eutrophication is one of the most important water quality problems in lakes. Due to the effects of excessive external inputs of nutrients (phosphorus and nitrogen), compounded with unfavorable climatic and hydrologic conditions, the lake undergoes major changes that involve both the physical-chemical properties of water and the aquatic communities living there. There have been many studies on lake ecosystems and their evolution (HENRY et al., 1984; AFDALI, 1993; HARRAK, 1991; LOUDIKI et al., 1994; SBIYYAA, 1998, MOUHRI et al., 1999). These studies relate especially to the role of nutrients (nitrogen and phosphorus mainly) in the evolution of lakes. The influence of climatic and hydrologic conditions on the structure and dynamics of these ecosystems has been often underestimated. Indeed, there are few studies on the relationship between climatic conditions and the trophic status of aquatic ecosystems.The aim of this paper is to study the behavior of the physical-chemical qualities of water in lake Mansour Eddahbi (Ouarzazate, Morocco) and its trophic status under severe climatic conditions, especially during the very dry hydrological year 1998/99. The Ouarzazate region is characterized by an arid continental climate with very hot summers and cold winters. Due to both great evaporation and increasing demand for water for irrigation, the water levels of the reservoir has been reduced by 14 m and its storage capacity has been considerably reduced. The approach used was to assess water quality in a station at the deepest area of the lake during the hydrological year 1998/99. Sampling was undertaken monthly and different parameters were analyzed including temperature, pH, total suspended solids (TSS), salinity, dissolved oxygen (DO), chlorophyll a, phosphorus (TP and PO43-), and nitrogen (TNK, NO3 - and NH4 +).The results of this study demonstrated important seasonal variations of water temperature in the lake. Temperature varied from 9.5°C in the winter to as high as 28.5°C in the summer. Based on this parameter, the reservoir could be classified as a monomictic hot lake with one mixing period occurring in winter. In the entire water column, the pH values were slightly alkaline and oscillated between 7.8 and 8.5 at the surface and 7.4 to 8.2 at the bottom of the lake. This reflects a well-buffered system with moderate benthic decomposition activity. The decrease in water volume by evaporation and intensive use for irrigation resulted in an increase in water salinity. These two parameters presented a significant correlation (r2 =0.60 for n=13 and p<0.05). A decrease in dissolved oxygen from 7.3 mg O2 /l at the surface to 0 mg O2 /l at the bottom of the lake occurred during summer stratification. However, water of the lake reservoir was well oxygenated during preceding years (concentration of dissolved oxygen oscillated between 2 and 11 mg/l (ONEP/BRL, 1998)). Concentrations of chlorophyll a in the euphotic zone (8 m depth) were very important and varied from 12 µg/L (August 1998) to 49 µg/L (September 1999) with an average of 25 µg/L. Based on the chlorophyll a value, the lake Mansour Eddahbi could be classified as eutrophic.The lake has become shallower and the thermocline, beginning at 12 m below the surface, has moved closer to the deeper layers where exchange between sediments and the water column is important. These exchanges between the two compartments of the lake contributed to enrichment of the water column in summer, creating an internal nutrient load. During the hot period when the lake was stratified, there was a large loss of nitrogen from the system. During the mixing period, nitrates were present at substantial concentrations in the deeper regions, but in summer nitrates were consumed rapidly in the surface layer, leading to a limitation on phytoplankton growth in this period. This same phenomenon was also noticed by HENRY et al. (1984). Furthermore, these losses of nitrogen induce a strong reduction in the N/P ratio during this period.Orthophosphate concentrations decrease from the bottom layers to the surface, with almost total impoverishment in the upper strata. This impoverishment reaches 10 m during the summer period and these values are often limiting for the phytoplankton production. The correlation obtained between orthophosphate concentrations and chlorophyll a was significant, especially in summer (r2 =0.79 for n=13 and p<0.05).Nutrient exchange at the water and sediment interface was favoured by the anoxic conditions at the lake bottom that prevailed during the eight months of stratification. The release of these substances can alone, without external contributions, maintain important primary production. This was confirmed by the importance of phytoplankton development compared to the preceding years. The algae production seemed to be enhanced, even if the reported N/P ratio was not optimal, particularly during the period of stratification and had no significant correlation with chlorophyll a content (r2 =0.19, n=13 and p<0.05). This important production appeared to be the result of an internal enrichment of the lake in nutrients coming from the sediment. According to nitrogen (0.2 mg/L) and phosphorus values (0.02 mg/L), the lake could be classified as hypereutrophic whereas two years earlier it was considered mesotrophic (ONEP/BRL, 1998). This work shows clearly that the severe climate conditions during the hydrological year 1998/99 contributed to a rapid degradation of the trophic status of this lake. It changed from a mesotrophic to a hypereutrophic state, resulting from an important enrichment in nutrients and the consequent algal production.
Les différentes approches méthodologiques de reconstitution des paléo-inondations : une revue de la littérature
D. Saint-Laurent et L. Lavoie
FR : Les deux dernières décennies en recherche scientifique se caractérisent par la multiplication des travaux concernant le réchauffement planétaire et les conséquences appréhendées de ce phénomène dans les divers milieux du globe. Ces travaux de recherche touchent différents champs disciplinaires dont la géomorphologie, l'hydrologie, l'écologie et bien d'autres. Dans le contexte des changements climatiques, on voit apparaître un intérêt grandissant pour l'étude des inondations anciennes ou des paléo-inondations. On tente de reconstituer la chronologie des inondations du passé notamment en regard des changements climatiques anciens ou subactuels (derniers siècles). Devant l'émergence de nombreux travaux dans ce domaine de la paléohydrologie, il nous est apparu intéressant d'examiner à travers une revue de la littérature scientifique les différentes approches méthodologiques utilisées dans l'étude des paléo-inondations. On s'attarde à décrire les principales méthodes et techniques employées dans la reconstitution des anciennes inondations, en particulier pour les environnements fluviaux (rivières, fleuves). On passe en revue les différents indicateurs biophysiques utilisés, notamment l'analyse des séquences stratigraphiques et sédimentaires, l'analyse des macrorestes, les méthodes de datation radiocarbone (14 C) et les techniques comme la thermoluminescence optique (TLO) et la dendrochronologie. Aussi, on présente les principaux modèles mathématiques utilisés dans la reconstitution des paléo-inondations, lesquels servent notamment à évaluer les variations niveaux/débits et la fréquence des inondations anciennes.
EN : The last two decades of scientific research have been characterized by an increase in the number of studies on global warming and its impact on the earth's various environments (e.g., terrestrial and aquatic ecosystems, river systems). The research concerns different fields such as geomorphology, ecology, hydrology and many others. In the context of climatic change, there is growing interest in the study of past floods or paleofloods. Researchers are attempting to reconstruct the chronology of past floods, especially with respect to past or subrecent (past centuries) climatic changes. The work involves using different methodological approaches borrowed from various disciplines including geology, geomorphology and ecology. The reconstruction of ancient hydrological events such as paleofloods in fact requires that different methods and techniques be combined in order to retrace the chronology of events as precisely as possible using different biological and physical indicators. The earliest research in this field was conducted in the United States, more specifically in the southern and southwestern parts of the country, which are regions characterized by an arid and semi-arid climate. Over the last few years, however, there have been an increasing number of studies from various parts of the world. Much of this research is based on previous work but with new elements of interpretation, which are mainly related to the many fluvial environments and climatic patterns associated with floods.A wide variety of indicators are used in the chronological reconstruction of ancient fluvial environments, whether in humid, sub-humid or desert regions. These indicators involve analyzing stratigraphic sequences and sedimentary deposits, organic matter and macrofossil deposits, as well as using radiocarbon dating (14 C), thermoluminescence (TL), and even dendrochronology. Some research uses statistical and mathematical models to evaluate river flow rates, which are transposed and adapted to ancient flood events. In fact, most of the work pertaining to the reconstruction of the frequency and magnitude of ancient floods uses several methods and techniques to obtain the long-term chronology of flood events in relation to the specific conditions (e.g. climate, geomorphology) of a region or study area.Many studies have analyzed slackwater deposits to reconstruct the paleoflood history of rivers (ELY and BAKER, 1985 ; JONES et al., 2001 ; KOCHEL and BAKER, 1982 ; WOHL et al., 1994). These kinds of deposits are mainly composed of fine-grained sediments (silt and fine sand) from river banks that were deposited during "large floods in areas of reduced flow velocity caused by ponding, eddying, or back-flooding up tributaries" (ELY and BAKER, 1985, p. 104). These deposits were found in different fluvial environments and for many researchers serve as adequate physical and geomorphologic indicators in the reconstruction of relic floods. Also, in many studies researchers used both slackwater deposits (SWD) and paleostage indicators (PSI) for the reconstruction of ancient floods. In addition to slackwater sediments, flood debris and silt lines (paleostage indicators) are often well-preserved along the river terraces (BAKER,1987; OSTENAA et al., 2002; WOHL et al., 1994) and facilitate the reconstruction of ancient flood events. The main problem in reconstructing paleofloods essentially lies in finding layers of organic matter in the sedimentary sequences, which makes it difficult to date the flood events using radiocarbon methods (14C). In arid or semi-arid environments, these organic layers are often rare because the prevalent climatic conditions do not favor the formation of a thick vegetation cover. However, this problem is not limited to these environments but also characterizes humid regions (YANG et al., 2000). The frequent absence of such organic matter layers in sediments is either due to an overly short vegetation cover formation period between phases of flooding or the erosion of such layers through the action of different natural phenomena (e.g. streaming, gliding, bioturbation, fluvial erosion). Other methods through which flood events can be dated include dendrochronology and thermoluminescence (TL). This last method can be used to obtain relatively precise dating of archaeological artefacts but at present is not very reliable for dating mineralogical samples (e.g. quartz or feldspath grains). The various problems involved in dating ancient floods make us aware of the importance of using as many physical and biological field indicators as possible in order to reconstruct the chronology of flood events as precisely as possible.
L'exportation de l'eau : comment concilier les exigences du commerce international et du développement durable
J. F. Morin
FR : La législation québécoise interdit les transferts massifs d'eau à l'extérieur des frontières politiques du Québec. Or, cette mesure pourrait être jugée contraire aux règles du commerce international par un tribunal d'arbitrage de l'Accord de libre-échange nord-américain ou de l'Organisation mondiale du commerce. De plus, on ne voit pas en quoi les transferts internationaux seraient plus dommageables pour l'environnement que ceux effectués à l'intérieur de la province. Une approche par bassins hydrographiques semblerait plus conforme aux normes du commerce international, aux objectifs environnementaux et à l'esprit de la nouvelle politique québécoise de l'eau.
EN : Quebec legislation prohibits bulk water transfers outside of its own political borders. This legislation could be judged contrary to international trade law by an arbitration panel of the North America Free Trade Agreement or the World Trade Organization. Additionally, it is not obvious why international transfers should be considered more environmentally damaging than those effected within provincial borders. A hydrographic basin approach would conform better to international trade norms, environmental objectives and to the spirit of Quebec's new water policy.
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