D. Bengeni, P. Lim et A. Belaud
Pendant deux années, des mesures et analyses d'eau ont été faites mensuellement sur une station de la Garonne et sur trois bras morts différant par leur communication avec le fleuve. La variabilité spatio-temporelle a été dressée à partir de 14 variables physico-chimiques susceptibles d'influer les équilibres chimiques de l'eau et la vie aquatique. Les données ont fait l'objet d'une Analyse en Composantes principales précédée par une analyse de variance entre saisons et entre stations de mesures afin de déterminer l'importance des hétérogénéités spatiale et temporelle des données.
L'eau du fleuve est soumise à un cycle climatique annuel de température et de débit. De brèves fortes eaux de printemps alternent avec de longues périodes de débits stables et inférieurs aux moyennes établies sur plusieurs décennies. Par rapport aux charges de sulfates et de chlorures prises comme référence du drainage du bassin versant, les flux de nitrates ont un pic accentué au printemps, résultant des activités agricoles. Les phosphates présentent aussi un accroissement automnal qui pourrait traduire un cycle annuel de minéralisation-déminéralisation. Les matières organiques s'élèvent en rapport au taux de chlorophylle a. L'eau de Garonne est de bonne qualité et conforme à la typologie habituelle, hormis des taux déclassants d'ammoniaque provenant de l'agglomération toulousaine et dont l'autoépuration est souvent incomplète. Par rapport à la Garonne, les trois bras morts sont caractérisés par un cycle thermique accentué en été. Mise en évidence par l'ACP, la minéralisation des eaux de ces trois bras morts évolue selon un cycle saisonnier parallèlement aux fluctuations de débit de la Garonne. Elle indique un gradient de minéralisation croissante de l'hiver au printemps. La qualité de l'eau lors des fortes eaux printanières est homogénéisée et imposée par le fleuve. En phases de faibles débits, la qualité de l'eau évolue parallèlement à celle de la Garonne (concentrations des substances) pour un bras mort ventilé par une communication amont et aval. En revanche, les deux bras morts en simple communication aval présentent un retard à la concentration de l'eau d'autant plus évident que la communication est étroite. Les substances fertilisantes (nitrates, phosphates, ...) augmentant de l'amont vers l'aval dans ces deux derniers bras morts, sont en été en concentration inférieure par rapport au fleuve, en raison : 1) du remplissage printanier par des eaux diluées, puis du retard estival à l’équilibrage par simple communication aval et 2) de la consommation par les organismes végétaux aquatiques. L'élude des différences spatio-temporelles met ainsi en évidence un gradient saisonnier de minéralisation, un gradient aval-amont de productivité et un gradient aval-amont de réchauffement estival. La productivité apparaît donc liée au réchauffement estival plutôt qu'à la minéralisation ou aux teneurs en substances fertilisantes.
For two years, water analysis and measurements were carried out monthly at one station on the Garonne and at three different ancient arms. Spatio-temporal variability was drawn up from 14 physical and chemical properties (water temperature, pH, electrical conductivity, oxygen, oxydability, total alcalinity, ammonia, nitrate, orthophosphate, sulphate, chloride, calcium, magnesium and chlorophyll a) which can have an effect upon the chemical balance of water and upon aquatic life. The data were studied by Principal Component Analysis preceded by Analysis of Variance between date and between station in order to determine the significance of spatio-temporal heterogeneity.
The river water is under the influence of an annual climatic cycle affecting temperature and flow. The flash floods in spring alternate with long periods of flow stability (these periods are lower than the established means for the flow over the last few decades). For each element specific water discharge-concentration relationships can be drawn, which may present time variations according to the hydrological regime. The particulate and dissolved soil-derived components increase with water discharge, whereas groundwater-derived elements are generally diluted during floods. In relation to the sulphate and chloride loads, which are used as a reference for the drainage of the basin, the nitrate levels have an accentuated peak in spring resulting from agricultural practices. Phosphates also display a high autumn level which could be due to an annual cycle of mineralisation-demineralisation. Organic matter increases correlatively with the chlorophyll a rate. The water in the Garonne is of a high quality conforming to the standards expected for this type, except for the ammonia levels which come from the Toulouse agglomeration, and mean that the self cleaning process in this case is offer incomplete. In relation to the Garonne, the three ancient, arms have thermal cycle which characteristically accentuated in the summer. Water temperature, electrical conductivity and pH value were not so varied from one to another ancient arms. This spatial homogeneity was coupled with the temporal homogeneity for the pH value. In the other hand, water temperature and electrical conductivity followed the different seasonal cycle. The temperature variations are parallel in all of the stations, but the amplitude of summer-winter cycle is important in three ancient arms. Within the ancient arms, the longitudinal profile of temperature was similar in summer, characterized by a little warming up according to the downstream-upstream gradient presented in Principal Component Analysis (PCP). This gradient was more pronounced in the ancient arms PV and GF than in ancient arm RB where subsists in summer a weak circulation of river flow.
The temperature presents a determinant role in the ancient arm productivity, reflected by the significant correlations between temperature on the one band and, the organic matter contents, chlorophyll a and pH values on the other hand. The increase of temperature in summer is accompanied by the production of organic matter, particularly in the upstream part of PV and GF. This phenomenon could be explained as a result of biological activity growth essentially phytoplankton because it was attached to an increase of chlorophyll a concentration. The biological activity stimulated by the increasing temperature contributes to reduce the pH value, accentuated by the diminution of oxygen saturation vigorously indicated in the morning, at the upstream part of the ancient arms. The percentage of oxygen saturation in the morning was essentially modified by the biological activity. It is stable and high during the spring period which corresponds to high water level of the river Garonne. Outwards this period where the river imposed the oxygen rate closely related to the air saturation, an upstream-downstream gradient occured in the weak water level periods (summer and winter). This is particularly indicated in the ancient arms which have not an upstream communication with the river. In this lentic ecosystem, it appears that the oxygen rate depends to the flora and fauna metabolic activity, confirmed by a highly significant correlation between the chlorophyll a concentration and the oxygen rate measured in the morning. The persistence of weak renewal water by the upstream part of ancient arm RB limited the summer warning up in this ancient arm compared to PV and GF. Thus, the productivity associated, evaluated by the chlorophyll a concentration, organic matter contents and the variations of pH and oxygen values are lower than the two last ancient arms where have only the downstream outlet.
The conductivity value in the ancient arms is closely allied to the river Garonne. It constitutes a first evidence to show that the water quality in the ancient arms was affected by the river. In spring, the regular re-covering of the ancient arms by the river water diluted induces an annual drop of conductivity.
Dressing by Principal Component Analysis, the mineral content in these three ancient arms shows a seasonal cycle which corresponds to the flow fluctuations of the river Garonne. The winter-spring gradient was indicated. This gradient was more evident for magnesium than for sulphate, chloride and calcium. The high spring water level reflected a low concentration of sulphate, chloride, calcium and magnesium due to water dilution. The water quality at this period is homogeneous because of the influence of the Garonne. When the water demands is weak, the quality of water in one of the ancient arms evolves parallel with that in the Garonne (concentration of matter) as it is alimented both up and downstream by the Garonne. On the other hand, the other two ancient arms have only the downstream outlet causing a delay in water concentration, especially as the connection with the main stream is narrow. The fertilising matter (nitrates, phosphates, etc.) which increases downstream, in summer, is at lower concentration in the ancient arms compared to the main stream for the following reasons :1) filling of the spring influx by diluted water, then the summer delay in equilibration of water via the single downstream inlet and 2) due to the absorption by the aquatic plants. Finally, the different spatiotemporal studies exhibit a seasonal mineralization gradient, a downstream-upstream productivity gradient and a downstream-upstream summer warning. The productivity therefore appears to be related to the summer warning rather than to the mineralization process or the presence of fertilising constituents.
Incidence de la source de pollution sur la dissolution et la rétention sélective d'hydrocarbures en milieu poreux saturé en eau
O. Razakarisoa, P. Muntzer, P. Rimmenlin et L. Zilliox
Des expérimentations menées sur des modèles-colonnes de milieu poreux, se localisant sur des études de lessivage du corps d'imprégnation formé par des mélanges d'hydrocarbures et sur des essais de propagation de leurs parties solubles, ont été réalisées au laboratoire. Il est montré que le comportement des hydrocarbures en solution pendant le lessivage est fonction de la nature de la source de pollution, et que le transport des traces solubles d'alcanes est fortement freiné par les effets de l'échange liquide-gaz masquant les propriétés adsorbantes de la matrice solide, ou pouvant entraîner une modification du processus de sorption des hydrocarbures par la phase solide.
La solubilité n'est pas un critère suffisant pour expliquer la dissolution sélective et progressive des constituants d'un corps d'imprégnation formé par un mélange de plusieurs espèces d'hydrocarbures, et lors du transport des parties solubles d'une telle source de contamination, l'influence des paramètres solubilité, constante de Henry et coefficient de distribution doivent être pris en compte simultanément pour étudier leur rétention sélective.
In the case of groundwater contamination hy petroleum products, the constituents of the immiscible pollutant source will be selectively solubilized and transporled over large distances by the water movement. Then, long-term pollution of aquifers is essentially due to the transfer and transport of dissolved hydrocarbon traces. The behaviour of soluble hydrocarbons and the extent of pollution depend on hydrodynamical conditions of the infiltration area and phase partitioning between water, oil, gas and solid in the aquifer.
This article deals with some aspects linked to the transfer mechanisms which take place during the selective dissolution of hydrocarbons and those relating to physico-chemicai exchanges accompanying the transport of soluble substances of the pollution source through saturated porous media. The experiments have been conducted with physical models constituted of glass columns : The glass column source of pollution (L = 75 cm, Ф = 10.4 cm) contained both natural quartz land and a motionless oil phase uniformly distributed in quantifies below residual saturation; it allows to study the selective dissolution of the constituents of the impregnation body. The exchange phenomena between phases were examined in a second glass column (L = 70 cm, Ф = 9.3 cm) which received the water charged with dissolved hydrocarbons coming from the glass column source, and filled with solid matrixes which were not contaminated at the beginning. The solid phase in the second glass column consisted of quartz sand or a mixture of agricultural soil and sand. The soil contained an average of 1.3 % of organic materials, 6.7 % of clay minerals and 27 % of limestone. The influence of the adsorbing properties of the solid material and the role of residual air on the transport and retention of soluble hydrocarbons were investigated. The variation of residual air content was obtained according to the filling method of the second glass column : “dry filling” favours the presence of the gaseous phase (water introduced by imbibition); “lifting by sedimentation” can reduce or partially avoid it (the second column is filled with the porous medium under water).
For a pollutant source made of a mixture of hydrocarbons at equal fractions and belonging to a same homologous series (alkanes and aromatics), the experimental results allowed to establish the importance of solubility parameter as well on the mode of selective leaching of the impregnation body, as on the selective retention of soluble elements of the pollutant source.
The behaviour of the constituents of the pollution source made of several hydrocarbon species appears differently and then, the solubility dues not constitute, for such a source, a sufficient criterion to apprehend the observed transfer and exchange mechanisms.
- Selective dissolution : the modifications of behaviour especially result to the fact that some components of the pollution source are represented at very low proportions then, they can’t respect, during the partition phenomenon, the gradation of solubility values as well at the beginning of the leaching of the impregnation body, as at the decreasing stages of concentrations. This situation must be regarded for studying the transfer of soluble hydrocarbons after accidental spill of petroleum products because it will affect the transport and the contamination prediction.
- Exchange phenomena : we observed that the liquid/gas exchange is the major factor of retention of soluble alkanes masking the effects of adsorbing materials. The influence of adsorbing character of the solid material appears clearly, only when the air content of the porous matrix is low. On the other band, the residual air content has no influence on the transport of soluble monoaromatics but the main phenomenon observed is in that case the liquid/solid exchange.
Moreover, the impact of the gaseous phase on the alkanes involves a modification of sorption process of hydrocarbons by the solid phase : the selective retention of various constituents of a pollution source made of several hydrocarbon species should be interpreted by the knowledge at their respective distribution coefficient Kd as it was admitted by several authors, when the air content of the porous matrix is low. The sorption process should be more complex when the residual air content is great because the gaseous phase plays a preponderant role for the alkanes; in that case, the simultaneous effects of the solubility, the Henry's constant and the distribution coefficient must be taken into account together for getting a best understanding of each component behaviour of the pollution source and assessing their fate in the case of alluvial aquifer contamination.
G. Morin et M. Slivitzky
Les résultats du modèle de circulation générale (MCG) à haute résolution du Centre climatologique canadien (CCC) sont utilisés pour estimer l'ampleur des impacts d'éventuels changements climatiques sur le régime hydrologique d'une rivière de la côte nord du Saint-Laurent.
Pour le Québec, le MCG du CCC prévoit des augmentations annuelles de l'ordre de 0 à 15 % pour les précipitations et de 4 à 5° C pour les températures, tandis que les variations saisonnières seraient beaucoup plus importantes, les températures hivernales (décembre à février) augmentant de 6 à 9° C et les précipitations 15 à 20 %.
Le modèle hydrologique CEQUEAU est appliqué au bassin versant de la rivière Moisie, pour simuler les débits dans le contexte climatique actuel et dans ce nouveau contexte climatique. Pour ce bassin versant, les précipitations annuelles seraient pratiquement inchangées alors que les températures annuelles augmenteraient de 4° C.
En appliquant, aux 24 dernières années (1986-1989), les changements mensuels de précipitation et température découlant du MCG, le débit annuel moyen serait réduit d'environ 5 % et l'écart-type augmenterait de 15 %. La probabilité des années humides serait pratiquement inchangée alors que pour les années les plus sèches enregistrées au cours de ces 25 dernières années soit 600 mm, la probabilité de non dépassement dans ce nouveau contexte climatique passerait de 0,12 à 0,28 ; les débits annuels, d'occurence décennale, diminueraient d'environ 10 %.
On assisterait à une modification plus importante dans la distribution mensuelle des écoulements. Les débits moyens des mois d'été (juillet à septembre) seraient réduits d'environ 35 % tandis que pour les mois d'hiver les écoulements moyens seraient plus soutenus.
The output of the Canadian Climate Center (CCC) General Circulation Model (GCM), coupled with the hydrologic deterministic modes CEQUEAU is used to evaluate the possible impact of a doubling of atmospheric C02 on the hydrologic regime of the Moisie river on the North Shore of the St Lawrence.
In regions where snow plays an important contribution to the annual runoff and the ground is covered with snow for periods from 4 to 6 months, the seasonal variations of climatic changes under a 2 x C02 scenario may have very different impact on hydrologic regimes; Figures 1 and 3 show the annual and winter distributions of temperature changes for Quebec under the CCC 2 x C02 scenarios, white figure 2 shows the possible changes in annual precipitation. The annual temperature will increase around 4 to 5° C, white winter temperatures may increase as much as 6 to 9° C ; annual precipitation will increase by 15 % to 20 %.
The CEQUEAU hydrologic model (MORIN et al., 1981 ; MORIN et COUILLARD, 1990) is a deterministic model which takes into account number of physiographic characteristics of the drainage basin (such as elevation and percent of forest and lake area) as defined in number of square grids. The model uses for input daily minimum and maximum temperatures and daily solid and liquid precipitation (rainfall and snowfall). As such meteorological information is usually available for a limited number of stations, the values are interpolated to each grid element of the drainage basin.
The CEQUEAU deterministic model uses the degree-days method (CORPS OF ENGINEERS, 1960) to estimate daily snowmelt under forest canopy and in the open and Thornthwaite equation to calculate daily evapotranspiration, for each square grid of the basin. Daily water budget, using linear reservoir storage for soil moisture and ground water storage is then used on each square grid element, to estimate daily runoff production ; this daily runoff production on each grid is then routed downstream to the basin outlet.
The CEQUEAU model is applied to the drainage basin of the Moisie river on the North shore of the Saint-Lawrence river. The Moisie river drainage basin covers an area of 19 248 km2, and we have used square grids of 20 km by 20 km to model this basin which is oriented roughly north to south with a length of 320 km and a width of 70 km.
Climatic Normals for the 1951-1980 period for the Sept îles and Wabush Lake climate stations are used to calculate the coefficients of the Thornthwaite formula for the present conditions.
Daily temperatures and precipitation for the Sept îles and Wabush Lake climate stations are then used as input to the CEQUEAU model, la calculate the flows for the 1966-1989 period. Even if calculations of flows have been made on a daily scale monthly values are used for the analysis of results. Table 1 presents a comparison of monthly and annual observed and calculated runoff for this period and show that the model satisfactorily reproduces the observed flows ; the annual difference is only - 1.2 % white monthly differences vary tram - 7 % to + 6 %.
Monthly flows (fig. 4) adequately represent the annual cycle and there is no important under or over estimation of mean monthly values. Frequency analysis of ail monthly flows (fig. 5) show that the observed sequence is adequately simulated even for extreme values. The coefficient of correlation between annual observed and calculated flows is 0.88 and the error of estimation of calculated values is only 48 mm.
Monthly changes in temperature and precipitation as output by the Canadian GCM under a 2 x C02 scenario are then used to simulate daily flows under changed climate conditions. The values at the grid points of the GCM are interpolated to the Sept-Iles and Wabush Lake Stations.
Climate normals for the two stations, table 2, are first modified by these monthly changes in precipitation and temperature to reflect new conditions for the application of the Thornthwaite formula under a 2 x C02 scenario. Then daily temperature and precipitation values for the 1966-1989 period are modified by these monthly changes and used as input to the CEQUEAU model.
The hydrologic results of this 2 x C02 scenario are presented in table 3 and figures 7 to 10. White annual runoff is reduced by only about 5 %, monthly values show much larger variations.
Figure 7 and table 3 present a comparison of average monthly flows under present and the 2 x C02 scenario. Winter flows are significantly increased (from 19 % to 210 %) due to much more frequent snow melts in the beginning and end of winter, and winter low flows are significantly higher. The monthly spring flow is about the same (fig. 7), but is concentrated in a shorter period. Summer fiows (June to October) decrease by 25 % to 40 % due to increase in temperature and therefore evapotranspiration but are still higher than winter flows.
Figure 8 shows the coefficients of variation of monthly flows. Like mentioned previously one notices a significant increase for winter months (December to Match), due to more frequent snow melts, white for the month of April the coefficient of variation is significantly reduced. For the summer months (May to October) the variation does not change very much.
Probability distribution of simulated annual flows (fig. 9) show that for wet years the probability of occurrence does not change significantly, white for dry years, i.e. runoff of about 600 mm, corresponding to the driest years in the 1966-1989 period, the probability of not exceedence goes from 0, 12 to 0,28; i.e. the years of low runoff will happen much more frequently.
For summer months the situation will be different as it is the wet years that will be more affected by changes in temperature; as an example for the month of August (fig. 10) the probability of occurences of dry years will not change as drastically as the frequency of wet years; increases in temperature and therefore possible evapotranspiration, will have no effect if no sufficient soil moisture is available.
C. Robert, O. Banton, P. Lafrance et J. P. Villeneuve
Afin de protéger l'eau souterraine des pesticides épandus à la surface des sols agricoles, on doit être capable de prédire et d'évaluer a priori les risques de contamination. La modélisation mathématique, qui est basée sur la représentation des processus, s'avère être un outil à privilégier pour une telle prédiction. Cependant, la fiabilité des résultats de ces modèles est fonction de la précision et de la représentativité des différents paramètres d'entrée. A l'aide d'une analyse de sensibilité, il est possible d'évaluer l'impact de la variabilité de ces paramètres sur les résultats de la modélisation. Une étude de sensibilité menée avec le modèle Pesticide Rosit Zone Model (PRZM) a permis d'identifier les paramètres physiques d'entrée dont la variation apporte le plus de changements au niveau des principaux résultats, c'est-à-dire les paramètres d'entrée auxquels le modèle est le plus sensible. L'utilisation du coefficient de sensibilité relatif s'avère être à cet égard un outil de comparaison fort efficace dans le cadre d'une telle étude. Les paramètres d'apport en eau (précipitations) et en pesticide (taux d'application) sont ceux dont les variations provoquent le plus d'impact au niveau des résultais de la simulation. Egalement, les résultats des simulations sont aussi sensibles aux variations de la capacité au champ et de la densité du sol. La température, le point de flétrissement, ta profondeur d'évaporation et la dispersion sont des paramètres auxquels la modélisation est peu sensible.
The ever increasing number of cases of groundwater contamination by pesticides has recently given rise to numerous experimental studies on the tale of these compounds within the soil-water system. In parallel with these experimental studies, the quick and intensive development of numerous simulation models bas emphasized the importance of the various factors and processes controlling the transport of pesticides in the unsaturated zone. The calibration and the validation of such deterministic models, which are especially used in groundwater management, requires the evaluation of several parameters related to the nature of the pesticide as well as to the pedologic and hydrogeological conditions of a given site. The reliability of results predicted by those models is mainly a function of the precision and the representativeness in the evaluation of those parameters at a specific site.
The aim of this study was to evaluate the sensitivity of a simulation modes of pesticide movement in the unsaturated zone with regards to the variation in soil physical properties. More particularly, a sensitivity analysis was performed in order to determine the importance of the variation in these parameters with respect to simulation results obtained from the Pesticide Root Zone Model (PRZM). The spatial variability of the composition and structure of the soil, which comes from the site pedogenesis, is the main cause of the variability of the soil water distribution and of the soil transient properties. The sensitivity analysis of a model with respect to the variation in these parameters allows the evaluation of the impact of their representativeness on the model output results. Such an analysis thus allows the determination of an acceptable level of precision (or error) for which an increase of precision in the evaluation of a parameter does not anymore correspond to a significant gain in the representativeness of the model results. It also permits the estimation of the impact of a potential variation of a parameter on the prediction of pesticide transport in the unsaturated zone.
The deterministic PRZM modal used in this study has been developed by the US-EPA and devoted to pesticide application on agricultural sites. This modal evaluates the pesticide leaching towards groundwater with respect to the type of culture and pesticide used, the climatic conditions, the soil characteristics and some agricultural practices. Output results from the model can he expressed in concentrations or masses of pesticide, in fluxes or cumulated quantities. Simulations were performed using characteristics and data of the Portneuf region (Quebec). This area of potato farming was until recently one of the Quebec most important sites for aldicarb application before the recommendation of its non use. Cultivated surfaces, which are rather homogeneous, consist of medium-size sand. Simulations were performed river a ten year period (1974 to 1984), implying one application of pesticide a1 the seed-time, and a trop for each year.
The parameters of water and pesticide inputs (rain and application rates) are those that induce the higher impact on the simulation results. The simulation results were also influenced by the variation of the field capacity and of the soil bulk density. Temperature, wilting point, maximum evaporation depth and dispersion are the input parameters for which the PRZM model is the less sensitive. The simulation results of pesticide transport, which ultimately consist in predicting the groundwater contamination, are very sensitive to the variations of some physical parameters for which the precision and the representativeness in the measured values are thus very important for the reliability of the results. Considering the spatial variability of a site characteristics, the representativeness of these results is very uncertain if a limited number of data is used in order to determine the mean value. A particular attention has to be focused on the parameters that induce the higher sensitivity of the model. Finally, such a sensitivity analysis shows that a stochastic approach in modelling the solute transport through soil can be a good alternative to take into account the variability of parameters encountered in field situations.
Délimitation des périmètres de protection des puits de pompage en zone agricole à l'aide de la simulation mathématique
O. Banton, P. Lafrance et J. P. Villeneuve
Un périmètre de protection de puits de pompage est la surface entourant le puits, dans laquelle des mesures sont prises pour empêcher des contaminants de migrer et de contaminer l'eau de ce puits. Dans l'établissement des périmètres de protection, de nombreux facteurs doivent être considérés, et une approche analytique systématique doit être adoptée. Les modèles mathématiques de simulation peuvent être employés en ce sens et sont souvent les seules méthodes capables de déterminer les périmètres de protection quand des critères quantitatifs sont utilisés. Une telle approche a été appliquée, en couplant un modèle de transport de contaminant en zone non saturée avec un modèle de transport en zone saturée. Le modèle en zone non saturée VULPEST évalue les concentrations de pesticide atteignant la nappe. Ces concentrations sont ensuite utilisées comme données d'entrée du modèle en zone saturée. Ce dernier considère les vitesses d'écoulement et l'influence de chaque puits. Les résultats quantitatifs permettent alors la détermination de périmètres de protection spécifiques à chaque contaminant potentiel. Cette application, réalisée sur un important site de culture de la pomme de terre du Québec, e permis de comparer favorablement les concentrations prédites à celles mesurées dans l'eau d'un puits, et de déterminer le périmètre de protection spécifique au pesticide utilisé. Le cas présenté est un exemple des applications possibles et futures d'une telle méthode pour la détermination des périmètres de protection des puits de pompage.
A wellhead protection area is the surface and subsurface area surrounding a waterwell through which contaminants are reasonably likely to move toward and reach. In the past, various approaches have been taken to delineate wellhead protection areas : fixed circles or rings around the well; simplified variable shapes based on geo-hydrologic mapping and classification ; zones with prescribed minimum travel times. However, in establishing wellhead protection areas, many factors need to be considered : zone of influence around the well; well recharge area; flow paths; transport velocities; travel times; sources and types of contamination. To determine a site-specific wellhead protection area, a systematic analytic approach must be taken. Mathematical simulation models may be employed and are often the only method capable to determine the wellhead protection area when quantitative criteria are used.
Such an approach can be used in agricultural zones, where pesticides are applied, by coupling a solute transport modal for the unsaturated zone with a saturated zone transport model. The (unsaturated zone) VULPEST model is an evaluation tool for the groundwater contamination by pesticides based on the transport modeling. Developed as a management tool, it permits the evaluation of the groundwater vulnerability to pesticides in terme of risk of contamination. It evaluates the concentrations of pesticide that reach the water table, taking into account the spatial variability of hydrodynamic, physical and physicochemical parameters of the soil. The variability of parameters is taken into account in the Monte Carlo approach. This approach consists of carrying out a sufficient number of simulations so that the distribution of values assigned to each parameter, these values being randomly selected from a chosen probability distribution, approximates the given distribution.
The concentrations obtained from the VULPEST model are used as input data in the model which simulates the transport and the fate of the contaminant in the saturated zone. This model uses the finite difference technique to simulate flow and solute transport. It considers the flow velocities and the influence of each well. In steady state conditions, the linked transport models in unsaturated and saturated zones may be considered independent. The quantitative results obtained by these means determine the vulnerability level of the well. Finally, they permit the delineation of the wellhead protection area for a specific contaminant, that is a given pesticide.
An application was performed to an important potato crop area in Quebec. Few years ago, this site has shown a contamination of the well water by the pesticide aldicarb. The cultivated soil consists of marine and fluvial sand with medium to coarse grain sizes, deposited on a sea clay with a thickness of about 20 m in some places. Potatoes are intensively grown in this region. In the eighties, a contamination by the pesticide aldicarb was noticed in some wells of this region. The granular form of aldicarb is applied during the sowing period (mid-May) at the recommended rate of 2.24 kg/ha. It has a high solubility (6 000 mg/l) and is leached by soil humidity. The aldicarb is transformed by oxidation to sulfoxide then to sulfone during its transit in the unsaturated zone. After a characterization of the soil physical parameters, calculations were run for both the unsaturated and the saturated zones. The depths of the well and the aquifer are 5 m and 3 m respectively. The thickness of the aquifer affected by pumping is about 2 m. The application of the pesticide aldicarb was done during 1982 and 1983.
The predictive results obtained by modelling for the pesticide concentrations in the well water were favorably comparerd to the concentrations measured at the site. The concentrations of pesticide in the water reach their peaks 7 weeks after every application. The maximum concentrations reaching the water table were found to be about 0,5 mg/l. This level exceeds largely the water quality criterion of 9 µg/l set by Health and Welfare Canada, and the one of 10 µg/l of the US-EPA. The well concentrations are calculated by taking into account transport in the saturated zone and decay processes. The maximum concentrations obtained are near 24 µg/l for a decay rate of the pesticide in the aquifer of 0,003 d-1. This decay rate is the one corresponding of the hall-lite of 8 months found by other researchers for Florida soils. The analysis of the water well during this period shows concentrations of about 10 µg/l. Moreover, the leaching of the contaminant into the well, and its persistence in the soil and groundwater is still present over 3 years after the last application. Using the water quality criterion of 9 µg/l set by Health and Welfare Canada, calculations have provided the delineation of the wellhead protection area specific to the pesticide aldicarb. The boundary delineation of water well protection area is determined by the numerical technique of reverse path line. The maximum extension of the well protection area obtained by this mean is 110 meters. It corresponds to a peak arrival with a decay of 1.5 years after the application.
The case study shows an example of the possible and future applications for such a method for the delineation of the wellhead protection areas. Such an approach permits to council the best use of pesticides with an appropriate groundwater protection scheme, indeed, agricultural managers can safely decide on the pesticide application rate and date, as on the choice between various pesticides, with regard to the groundwater quality protection. Through this way, regulators and scientists can base their decisions for the registration of new pesticides by testing, before their use, their possible impacts on groundwater. Comparisons can be easily doge between water quality criteria and predicted quantifies, and regulatory decisions can be taken in light of these results.
Modélisation des effets hydrochimiques à long terme des dépôts acides et des reboisements dans les bassins versants du Mont-Lozère (Sud de la France)
P. Durand, E. Lelong et C. Neal
L'évolution de la chimie des sols et des eaux dans trois petits bassins-versants subméditerranéens granitiques recevant d'importants apports atmosphériques de S04 et de poussières sahariennes a été simulée, de 1845 à 2125, en utilisant le modèle MAGIC. Ce modèle biogéochimique global comporte une série de relations d'équilibre entre les phases gazeuse, liquide et solide (adsorption de soufre, solubilité de l'aluminium, échanges cationiques, système C02/carbonates, dissociation des acides organiques) et une comptabilisation des flux d'éléments (entrées atmosphériques, sorties hydrologiques, immobilisation biologique, altération). Les bassins versants diffèrent par l'historique récente de leur utilisation par l'homme : la pelouse pâturée qui les recouvraient en 1845 n'a subsisté que dans un bassin : elle a été remplacée vers 1930 par une pessière dans l'un des bassins, et colonisée, après abandon, par une hêtraie dans l'autre. Les simulations montrent que le bassin couvert de pelouse a peu souffert de l'augmentation de l'acidité des pluies au cours du XXe siècle, et pourrait subir les apports actuels pendant plus de 100 ans sans dommage. Les deux bassins forestiers s'acidifient fortement depuis 1970 par les effets combinés de la pollution et des reboisements. Pour la pessière, seule une réduction d'au moins 60 % des apports soufrés permettrait d'inverser le processus. Le vieillissement naturel de la hêtraie produirait une amélioration même sans réduction des dépôts acides, en diminuant le taux d'assimilation de cations. Les deux facteurs acidifiants agissent donc en synergie, mais dans le cas de la pessière l'augmentation des apports acides due à l'effet filtrant des frondaisons est prépondérant, alors que dans la hêtraie, la forte immobilisation cationique dans la biomasse perenne jolie un plus grand rôle.
Introduction. The combined acidifying effects of afforestation and acid deposition are well documented for sites in North-Western Europe and North America. In acid-sensitive mediterranean areas, acidification has been delayed by lower deposition rates and alkaline aeolian dust input to the ecosystems. Here, the evolution of stream water and soil chemistry, from 1845 to 2125, in three small submediterranean catchments of Southern France, is assessed using the MAGIC model.
Sites and methods. Three granitic catchments have been monitored since 1981 in the South-Eastern Massif Central. The mean elevation is 1300 m above sea level and the soils are rankers and acid brown earth. The catchments are submitted to heavy atmospheric deposition of industrial acidic oxides (mainly sulphate : more than 20 kg.ha-1•year-1 of S-S04) and of alkaline saharan dust. The land use was similar in all the catchments up to 1930, and consisted mainly of extensive sheep grazing on semi natural grassland. Then one catchment (19.5 ha) was afforested with conifers (spruce), another catchment (54 ha) was abandoned and progressively settled by a beech coppice, while the other catchment (81 ha) remained covered with grassland. MAGIC is a lumped, process-oriented biogeochemical model, where the soil physical and chemical characteristics are described by a single set of mean variables. The processes in soils are modelled by a series of equilibrium equations : Aluminium solubility, cation exchange, sulphate adsorption, C02 and carbonates equllibria, organic acids dissociation. The changes in the stocks of elements are calculated from the main input, output and internal fluxes : atmospheric deposition, river load, biological uptake and weathering. The calibration aims to get the best fit between measured and simulated values to the « target variables », i.e., the present day exchangeable cations amounts in soils and river chemical composition. Hindcast and forecast runs need historical and prospective scenarios for atmospheric deposition, dry deposition factor, nutrient uptake and discharge. Here, the growth of the forest was modelled by increasing dry deposition factor and nutrient uptake and decreasing the discharge according to the field observations. The sequence of sulphate deposition was derived from S02 emission data. Three scenarios were tested in the forecasts : a constant deposition al the present level, a 30 % reduction and a 60 % reduction within 2010. In both cases, the nutrient uptake of the spruce stand and grassland was kept constant, white those of the beech coppice was progressively decreased to simulate a natural ageing of the forest.
Results. The model successfully reproduced the chemistry of the catchments. The values of the optimised parameters suggest that :
- the soils of the area have a very high sulphate adsorption capacity;
- the weathering rates are similar in the three catchments except for the calcium (lower in the beech catchment) and the magnesium (much higher in the spruce catchment);
- the initial conditions calculated by the model for 1845 are slightly different; the alkalinity and cations concentrations are lower, and the base saturation higher, in the beech catchment.
The simulations show that for the grassland catchment, there has been little change due to increasing of S04 wet deposition over the last 100 years. The simulations for the other two catchments (beech and spruce stands) show that they have been acidifying since 1970 due to the combined effects of air pollution and afforestation. This trend could lead to a severe decline of soil base saturation and streamwater quality by 2050. Concerning the coniferous catchment, only a 60 % reduction of the sulphur input could allow a recovery. The natural ageing of the beech stand, resulting in a decrease of the biological uptake, would permit a recovery even under constant sulphur deposition.
Conclusion. This exercise show that although the effects afforestation and acid deposition are synergetical and difficult to distinguish, their relative importance in acidification processes varies according to the vegetation. In the spruce catchment, the enhanced deposition due to the scavenging properties of the canopy is the determining factor. If no emission reduction occurs and no mitigating measures are taken, the decline of soil and water quality is a serious threat. In the beech catchment, the dynamics of biological uptake prevail, because the trees store a large amount of base cations. If the stand is allowed to age, a recovery can be observed even if no sulphur emission reduction occurs. This is also partly due, in this context, to the scavenging of the alkaline aeolian dust.
J. Malavoi et Y. Souchon
La dynamique écologique des cours d'eau est fortement liée à leur régime hydrologique qui en façonne la structure morphologique à l'occasion de crues caractéristiques mais qui en rythme également l'habitat physique quotidien. L'hydrologie joue aussi un rôle majeur dans la modulation saisonnière des caractéristiques thermiques et chimiques, autres paramètres abiotiques fondamentaux.
Deux exemples concrets pris dans un cours d'eau alpin, la Séveraisse, démontrent l'intérêt d'une étroite connexion entre l'hydrologie et l'hydroécologie.
La quantification de l'habitat des poissons repose sur la méthode des microhabitats dont les principes ont été définis par l'US Fish and Wildlife Service (Instream Flow Incremental Melhodology, USA, Fort Collins, Colorado). Cette méthodologie combine une description détaillée de stations représentatives du cours d'eau par mesure puis modélisation hydraulique des paramètres de l'habitat tels que la hauteur d'eau, la vitesse du courant et le substrat avec des modèles de préférence de localisation des poissons à différents stades vitaux. Des valeurs d'habitat potentiel par stade vital pour les différentes espèces étudiées (ici la truite Fano, Salmo frutta fario) sont calculées pour différents débits.
Des chroniques hydrologiques de qualité sont par conséquent nécessaires pour analyser la dynamique de l'habitat et en reconstituer l'historique.
Les faibles débits lors des étiages estivaux correspondent souvent à un habitat limitant pour l'adulte de truite.
D'autres épisodes hydrauliques critiques peuvent aussi exister. Ce fut le cas d'une crue sévère de 7D m3/s sur la Séveraisse dont l'effet biologique a pu être évalué : iode diminution de la densité et de la biomasse de truite, structure de taille désorganisée.
Ces deux exemples soulignent l'importance des données hydrologiques en hydroécologie. Les évènements exceptionnels, les épisodes structurants, les étiages, le rythme saisonnier des débits définissent la variabilité spatiale et temporelle de l'environnement aquatique, qu'il faut ensuite relier à la dynamique biologique des hydrosystèmes.
Hydrobiology studies deal more often with pollution problems in running waters than with those concerning stream regulation or deterioration of physical integrity of streams and rivers.
It is more and more obvious that running water ecology is strongly related to hydrological characteristics which structure aquatic habitats.
This latter challenge requieres a better understanding of the relationships between physical and biological parameters in particular in natural sites.
We have summarized basic knowledge on the role of hydrological characteristics in aquatic systems :
- Hydrology structures the morphology of the river bed and banks : bankfull discharge is often considered as responsible for hydraulic geometry of river channels in association with sediment load. The channel forming flows correspond to the flood discharges of frequency 1.5 to 2.0 years in most stream types.
- Hydrology punctuates the dynamic of habitat available for aquatic biota. Physical habitat is discribed as a combination of spatial attributes : depth, current velocity, which are directly dependent on instantaneous discharge, and substrate and cover, resulting from the morphology discribed above. For example fish need different habitat for each physiological function to achieve a whole life cycle : reproduction and incubation of embryos, nutrition, test, and hiding. For the biologists, it is essential to develop knowledge on the autoecological requirements of species, in particular fishes in original ecological conditions.
- Hydrology influences the thermal regime determining the oxygen concentration and several associated physico-chemical processes. Severe abiotic conditions could exist in periods of prolonged low flows.
In a second part, we demonstrate the interest of a close collaboration between hydrological and hydroecological sciences with two concrete examples in an alpine stream, Severaisse (Hautes-Alpes).
We use a quantitative methodology derived from the Instream Flow Incremental Methodoly (IFIM) of the Fish and Wildife Service (USA, Fort-Collins).
One of the hypothesis of this methodology is that physical habitat plays a major rate in structuring fish populations, certain habitats acting as « bottlenecks ».
In Severaisse, a representative study reach was chosen in a braided part of the stream, including each identified geomorphotogical unit.
The physical characteristics are described on perpendicular transects (2 or 3 transects per unit). On representing homogeneous area, on each transect different measurements of depth, current velocity and substrate type are taken at irregular internals, with more points in heterogeneous portions. These points divide the represented area in cells. Topographic data complement the description and are necessary for hydraulic simulation at different discharges.
The physical characteristics of the reaches are illustrated on maps, thus it is possible to follow the evolution of one parameter or a combination of them with changes in discharge (500 ls-1, 1 m3 s-1, 2 m3 s-1, 10 m3 s-1).
Fish habitat requirements are obtained from preference curves for each physical parameter and life stage. The studied species is the brown trout (Salmo truffa Fario, L., 1759).
The available habitat is then calculated cell by cell by converting physical data into suitability criteria. The resulting Wheighted Usable Area (WUA) for brown trout is then computed versus discharge.
Adult WUA increases quickly between 0.5 and 1.0 m3 s-1 and is quite linear up to 10 m3 s-1 and a rapid decrease up to 4 m3 s-1.
In Severaisse the low discharge is in winter with a monthly average stream flow of 2 m3 s-1 (period 1959-1985). This value corresponds to the maximum WUA (120 m2) for adult and to a medium WUA (300 m2 for a maximum of 450 m2 at 0.5 m3 s-1) for alevins and juveniles. The lower values for adult are often encountered.
The summer average discharges, when fish are more physiologically active and in a period of active growth, give also good WUA (10 m3 s-1, 100 m2 for adult). When the hydrological events are close to the mean average interannual pattern, brown trout population tends to an equilibrium with a number of adults defined by the minimum WUA. This type of population structure has been observed in September 1986.
This first example shows the importance of hydrological data which can be translated into habitat values for fish in two ways : real date by chronics or statistical approach (variability between years).
The second example consist in a biological assessment of trout population before and after a sever flood (70 m3 s-1) in the same stream. The number of fish was depleted and the site structure severely impacted with no more fish greather than 240 mm (total lenght) and a drastic loss in 1 + year class.
The flood resulted also in a morphological modification with displacement of the channel. During the flood, it was not possible to measure current velocities but it could be assumed that they were to high for fish to maintain. Fish might have drifted downstream with no recolonisation after 9 months.
These two examples have emphasized the importance of hydrology in aquatic ecology. Flood regime pattern can be translated in potential habitat values for fish or other biota.
This new habitat methodology can also improve the quality of impact studies, in particular chose dealing with stream regulation.
Therefore stream flow variability and predictability are essential to define temporal and spatial patterns of lotic environments.
Le suivi du régime hydrologique des grands fleuves de l'Ouest Africain : apport de l'imagerie satellitaire NOAA/AVHRR
S. Geffard, J. M. Gregoire et S. Pieyns
Le Projet, « suivi de la végétation en zone tropicale » de l'Institut des Applications de la Télédétection au Centre Commun de Recherche des Communautés Européennes d'lspra, a mis au point et alimente en permanence une banque de données satellitaires NOAA/AVHRR. On présente ici la méthodologie développée pour une utilisation de cette banque de données, à des fins hydrologiques, et les résultats d'une première application dans le cadre d'une étude menée en commun, par le Projet et le Département des Eaux Continentales de I'ORSTOM.
Cette étude réalisée sur la Falème, affluent du fleuve Sénégal, le Haut Niger et le Konkoure donne des résultats encourageants quant à la possibilité d'utiliser des descripteurs d'états de la surface, tirés de la banque de données NOAA/AVHRR, comme données d'entrée pour la modélisation hydrologique, au pas de temps mensuel, des grands bassins de l'Ouest Africain.
Il serait peut-être ainsi possible de suivre les modifications des régimes hydrologiques de ces grands fleuves liées à des modifications éventuelles, naturelles ou anthropiques, de leur environnement.
The project « Monitoring Tropical Vegetation » (MTV) of the Institute for Remote Sensing Applications-Joint Research Center of the European Communities, Ispra Establishment, has created and continually updated a NOAA:AVHRR Imagery data bank. We describe here the proposed methodology to use this data bank for hydrological purposes and the results of an initial application, within a joint research, performed by the MTV Project and the Department for Continental Waters of ORSTOM.
The knowledge and the monitoring of the hydrological regimes of large watersheds, especially in the inter-tropical zone, are obviously major topics for hydrology. Interest shown by the scientific community of hydrologists catches up with the goals that are environment protection and « reasonable » sustainable socioeconomic development of these inter-tropical environments, for which a good understanding of the hydrological cycle and water balance is necessary.
A lot of studies, which have already been carried out in order to reach these issues, very often include rainfall runoff relationship modeling, about which it is well known that there is a strong influence of surface conditions. But if many research program dealt with that particular point, few things in fact have been done on small scale basis, for large river basins. Taking into account the kind of information needed for the characterization of surface conditions at the required time and space perception levels (i.e. seasonal and interannual dynamics, large geographical expanses) and due to the fact that it was possible to utilize classical tools and techniques for data collection, satellite-based remote Sensing techniques were obviously the only ones which could contribute to start in answering the question.
The aim of this study is to demonstrate that a real possibility exists to obtain, by an appropriate use of the NOAA imagery, relevant informations related to the time and space dynamics of the surface conditions, (more precisely about eh evolution of the vegetation cover status), which can be used as quantitative inputs for hydrological models for the West African large watersheds.
This study has been conducted through three phases.
First, five groups of basins have been selected along a north to south transect which intersects pile all the major ecological systems of this part of the African continent, from the soudano-sahelian zone to the guiean one. Using scientific and technical criteria, but also taking into account the fact that some of this basins are of particular interest for development projects conducted by the EC in West Africa, 3 watersheds have been selected. The Falkeme river, a tributary of the Senegal river, the Upper Niger and the Konkoure. The main characteristics of these basins are given in the text.
Then, we have defined the methodological approach with 3 major steps :
- creation of date banks for runoff and rainfall, satellite time series and cartographic information (basin boundaries),
- extraction, from the satellite imagery, of the indexes we selected to describe the surface conditions,
- joint analysis of hydrological and remotely sensed data for 2 years with important hydrological contrasts.
The hydrological data bank contains the monthly mean discharges and rainfall for 5 selected basins, from year 1980/1981 to year 1988/1989 (hydrological year).
The creation of the satellite data bank started with the preparation of a NOAA-AVHRR HRPT time series including the years 1987, 1988 and a part of 1989. Each of the image we choose (no clouds or as less as possible and with orbital characteristics allowing a 3° W flight over the Equator in order to mitigate the image distortion) has been processed using a processing chain (installed al MTV project).
Radiometric corrections have been made to avoid calibration differences between the AVHRR sensors of NOAA 7, 9 and 11.
The computation of the derived channels is done through the transformation of the original data provided by the AVHRR sensor into values which are easier to interpret in terms of surface conditions.
Three radiometric indexes have been kept :
- the vegetation index (NDVI), computed by combination of records in the red and mean infrared bands, which is a good indicator of the vegetal chlorophilian caver on the land surface (Townsed and Justice, 1986),
- the surface temperature (TS), computed by the « split window method, using the brightness temperatures in the AVHRR 4 and 5 channels,
- the signal, in numerical count, recorded by AVHRR 3 channel. This mean infra-red channel (3.55-3.93 µm) is of great interest. First it records the radiation emitted from the surface and, being very sensitive to the high temperature emissions, is very useful for bush fires detection. Secondly, it records a reflected composent which gives information on clearings within vegetated surfaces. By combining the 2 signals, it is possible to detect the bush fires which are known to affect, each year, large extents of vegetation cover, and to made the differentiation between different vegetation covers, particularly in forest areas or forest-savannah transition zone.
Resulting images have been geometrically corrected and resampling of radiometric signals done by cubic convolution method. The final product is a 5 bands image (TS, NDVI, albedo, atmospherical water contents and channel 3 inverted) of 2400 x 1600 pixels (pixel size is 1 km), with geometrical rectification and centered 7° W, 10° N. It must be borne in mind that the radiometric signals are not corrected for the atmospheric effects. That explains why we are rather more interested in the evolution in time of the indexes values than in their absolute values. From these 5 channels images, sub-images of 500 x 500 km are extracted to cover areas of interest for the study of the selected river basins.
The graphic data base contains the digitized basins boundaries. These files have been geometrically corrected using reference points, both on the 1/1 000 000 map and on the NOAA:AVHRR HRPT images, in order to be superimposable to these images. After that, the basins are located on the images and using the CHIPS software statistical descriptors of watersheds surface characteristics are derived.
In a third step, we have applied our methodology to the Faleme river basin. The main hydrological data for the 4 hydrological stations within the basin are given in the text. 1987/1988 an 1988/1989 years has been compared among to each others and to the long term average values. One can remark that, as far as rainfall is concerned, the selected years are very close to the long term average annual rainfall, while runoff during 1987/1988 is clearly lower than the long terra average discharge, and that is true for all the basins. Runoff increase for 1988/1989, compared for the year before, is more than 100 %, while annual rainfall are quite equivalent. That can he explained by a difference in the time distribution of rainfall during these two hydrological years. In fact it is observed that runoff for 1988/1989 starts and ceases earlier than for 1987/1988. So the high values of rainfall at the very beginning of the rainy season 1988/1989 have been propitious to runoff, whereas a more homogeneous rainfall repartition during the rainy season, the year before, has given a higher rate of infiltration and probably an easiest use of water by vegetation.
These differences in terms of total annual rainfall and in time distribution along the year, should have bar, an effect on vegetation dynamic for the two years, and subsequently on the radiometric responses of the surface.
In fact, analysis of dynamics of the signal in the NDVI and the medium infrared band shows that :
- a minimum is reached for the NDVI value during February for 1987/1988, December for 1988/1989,
- a clear shifting between the curves related of the different basins. At the same date, the mean value is always higher for the upstream parts of the basin, with a denser vegetation cover, than for lower part of the basin integrates downstream flat areas,
- a very low mean value for the NDVI as early as December 1988, lower than 0.10, which is to be compared to the threshold value 0.05 given as the normal value corresponding to a very sparse herbaceous cover (1 % coverage),
- much steeper slopes immediately at the end of the rainy season and beginning of the dry season 1988/1989 than for 1987/1988. This indicates a much faster drying up process for the herbaceous stratum.
When looking at the dynamics of the signal in the mean infra-red band, one can see that for the Faleme basin at Kidira during the year 1987 the shape of the distribution histogram changes as we are sinking deeper into the dry season. Starting from a bimodal form it reaches finally an unimodal one. This phenomenon indicates a gradual drying up of the entire basin with as a result a homogenization of the radiometric values.
A date la date comparison of the 1987 and 1988 histograms shows that, as early as November 1988, the distribution is unimodal with a mean value higher than in 1987. The fact that homogeneity kept on during the warn-up period indicates that the main part of the vegetation cover, and particularly the herbaceous stratum, was already dry in November 1988.
All these observations tend to show that it is possible to perceive, on one hand space-time distribution of rainfall and on the other hand the relative importance of grass in the vegetation.
Finally this study has been extended to the Upper Niger and the Konkoure basins. All the above observations related to the Faleme river basin are corroborated by the temporal evolution of the standard deviation values of NDVI.
In conclusion, we can say that this study bears encouraging results as regard as to the possible use of descriptive indicators of the surface conditions derived from the NORA AVHRR data bank, as inputs for the hydrological models, on a monthly basis, of large West African watersheds.
Thus, it may be possible to evaluate the changes in these large rivers regimes, in relation with changes, either natural or due to human activities affecting their environment.
Étude sur le choix du seuil de troncature en analyse des séries de durées partielles : application au Canada
D. Caissie et N. El-Jabi
L'estimation des débits de crues est d'une importance majeure pour la conception des ouvrages d'art et la gestion des ressources hydriques. La mauvaise évaluation de ces débits entraîne un surdimensionnement ou sous dimensionnement des ouvrages hydrauliques, induisant ainsi un investissement excessif ou un risque démesuré d'inondations. Un modèle de données chronologiques de durée partielle est retenu pour l'étude des crues. Ce modèle nécessite d'abord l'estimation du seuil de troncature qui définit la série chronologique intermittente à analyser. Ce paramètre joue un rôle majeur dans la solution du problème malgré le peu d'importance que lui reconnaît la littérature. Le but de la présente étude est d'élaborer une méthode graphique comme guide dans le choix du seuil. Par la suite, cette méthode est appliquée à 238 stations hydrométriques au Canada. Une fois le seuil obtenu pour ces stations hydrométriques, une régionalisation utilisant l'analyse de régression a été réalisée. Ces équations régionales peuvent servir à estimer le seuil d'une nouvelle série hydrologique sans avoir à passer par la méthode graphique.
Accurate forecasting of flood flows is required for the efficient design and construction of hydraulic structures in rivers as well as for the effective management of water resources. Underestimation of flood flows can result in tragic consequences while overdesigned structures are expensive.
One method for estimating flood flows is the partial duration series analysis. In this approach a truncation level defining the intermittent time series is chosen. All flows above this level (exceedances) are analyzed by assuming the time of occurrence of these floods to represent a Poisson distribution. In addition, exceedances are considered to be independent random variables identically distributed over a one-year time interval. The selection of the truncation level is somewhat problematic and not very well defined in the literature. This study presents a truncation level estimation technique based on a series of regional regression equations for 9 distinct regions in Canada, and documents their graphical derivation.
According to previous research, the truncation level can be obtained in two ways. First, it is selected in accordance with physical criteria such as the overflowing of a river, the critical flow for flooding of a crop, etc. The second method of truncation level selection is primarily mathematical such as satisfying the analytical fit or the assumptions of the model. In the present study, the truncation level was selected based on the mathematical approach using a graphical technique and using the Chi-square test. The graphical truncation level selection technique is based on the equality of the mean and variance of the Poisson distribution. Given this property of the Poisson distribution, one can study the mean-to-variance ratio as a function of truncation level.
Truncation levels were selected graphically from 238 gauging stations across Canada. The Chi-square at a level of significance of 5 % was used for their validation. It was observed, upon examination of the mean-to-variance ratio, that selection of the truncation level was easier in the eastern and western regions of Canada where the ratio varied very little around unity. In the prairies and northern regions, a small variation in the truncation level led to a large variation in the ratio, making the selection of a level more difficult.
Following the selection of a truncation level for each hydrometric station, homogeneous regions were selected based on previous countrywide hydrological studies. The regression analysis was then carried out to explain this truncation level using several variables (including bath physiographic variables and streamflow characteristics). Physiographic variables included area of drainage basin (km2); area of lakes and swamps (km2) ; area of forests (km2) ; mean elevation (m) ; slope of drainage basin (%) ; slope of principal watercourse (m/km, %) ; length of principal water course (km) ; area controlled by lakes and swamps (km2) ; and drainage density (km/km2). The Streamflow characteristics included the mean annual flow and mean annual flood. The mean annual flood was included because previous research has shown a strong correlation between the truncation level and the two-year flood estimated by a log-Pearson type III distribution function. For ease of application, the mean annual flood was used as a low return flood estimate as it represents the two-year flood calculated by a normal distribution function. Low return floods of any given distribution function are demonstrated to be similar in magnitude.
Multiple regression was carried out using original data and logarithmically transformed data. Equations were derived by selecting one variable, two variables, and so on until all explanatory variables were accounted for in a single equation. Among the selected parameters, the mean annual flood was the best parameter to explain the truncation level for all of the regions across Canada. The coefficient of determination, R2, of the equations calculated using mean annual flood varied between 0.847 and 0.987 from original data, and varied between 0.824 and 0.988 for logarithmically transformed data. In practice, the regional equation can eliminate the graphical estimation technique and a first truncation level can be obtained. This truncation level can be used in the partial duration series analysis with the different tests involved. If this first estimated level does not meet all the statistical tests, a second usually higher level is selected. It is important to note that when the selected level meets the statistical test requirements, the estimation of QT should be relatively insensitive to levels near the selected truncation level.
Tous droits réservés © Revue des sciences de l'eau, 1992