Classification, caractérisation et facteurs de variabilité spatiale des régimes hydrologiques naturels au Québec (Canada). Approche éco-géographique

Nous proposons onze nouvelles variables pour classifier, caractériser et analyser les facteurs de variabilité spatiale des régimes hydrologiques des affluents du fleuve Saint-Laurent au Québec. Ces variables se rapportent exclusivement aux débits mensuels et utilisent quatre (volume d’écoulement, période d’occurrence, durée et amplitude de variabilité intra-annuelle des débits) des cinq critères proposés par Richter et al. (1996) pour caractériser écologiquement les régimes hydrologiques.L’analyse en composantes principales de ces onze variables hydrologiques a permis d’extraire trois composantes principales significatives après rotation d’axes par la méthode varimax. La première composante principale est associée aux débits saisonniers hivernaux et aux mois d’occurrence des débits mensuels maximums et minimums. La seconde composante est associée aux débits saisonniers printaniers et au rapport entre ces débits et les débits estivaux. Enfin, la dernière composante est associée au coefficient d’immodération (rapport entre les débits mensuels maximums et minimums) et aux débits mensuels minimums. La variance totale expliquée par ces trois composantes, à part presqu’égale, est d’environ 83%. Sur la base des signes de notes factorielles sur les trois composantes principales, les 72 rivières analysées ont été groupées en huit régimes hydrologiques naturels non contigus dans l’espace. Les caractéristiques de chaque régime hydrologique ont été clairement définies.Quant aux facteurs environnementaux qui influencent la variabilité spatiale des régimes hydrologiques, il est apparu que les six variables hydrologiques associées aux trois composantes principales sont principalement influencées par la température de l’air ainsi que la superficie couverte par les forêts, les lacs et les marais.Several classifications of hydrologic regimes have already been proposed in Quebec. However, these classifications are exclusively based upon the magnitude of discharge (annual and monthly discharge, annual maximum and minimum discharge). This hydrologic parameter isn’t sufficient to describe the ecological hydrologic regime. Thus, Richter et al. (1996) suggested five fundamental characteristics to describe hydrologic regimes that regulate ecological processes in river ecosystems :1. The magnitude of the water condition at any given time. It is a measure of the availability or suitability of a habitat. It defines such habitat attributes as wetted area or habitat volume, or the position of the water table relative to wetland or riparian plant rooting zones.2. The timing of occurrence of particular water conditions can determine whether certain life-cycle requirements can influence the degree of stress or mortality associated with extreme water conditions such as flood or droughts.3. The frequency of occurrence of specific water conditions such as droughts or floods may be tied to reproduction or mortality events for various species, thereby influencing population dynamics.4. The duration of time over which a particular life-cycle phase can be completed or the degree to which stressful effects such as inundation or drought can accumulate.5. The rate of change (range) in water conditions may be related to the stranding of certain organisms along the water’s edge, in ponded depressions, or the ability of plant roots to maintain contact with phreatic water supplies.The application of these characteristics requires a daily discharge time series, but these data are not always available. To overcome this difficulty, we propose eleven new hydrological variables exclusively based upon monthly discharge data. These new variables describe four (magnitude, timing of occurrence, duration of time and the rate of change) of the five characteristics of hydrologic regimes suggested by Richter et al. (1996). The eleven new variables are as follows: seasonal discharge coefficients (%); monthly maximum and minimum discharge coefficients (%); median Julian day of occurrence of maximum monthly discharge; median Julian day of occurrence of monthly minimum discharge; spring and winter seasonal discharge ratios; spring and summer seasonal discharge ratios and monthly maximum and minimum discharge ratios.We have isolated, using principal component analysis (PCA), three significant principal components after varimax rotation. The first principal component was linked with the magnitude of winter discharge and the timing of monthly maximum and minimum discharge. The second principal component was associated with the magnitude of spring seasonal discharge and the spring and summer seasonal discharge ratio. The third component was linked with the coefficient of immoderation (monthly maximum/minimum discharge ratio) and the magnitude of monthly minimum discharge. The three principal components explain, almost weight for weight, about 83% of the total variance. On the basis of signs of loadings for these three components, 72 rivers were analysed and grouped into eight natural hydrologic regimes that are not geographically contiguous. For example, the first hydrologic regime was characterized by high winter discharge (> 12%), timing of monthly maximum discharge in April, high summer discharge (> 54%), high spring and summer seasonal discharge ratios (> 3.5), high monthly maximum and minimum discharge (> 12) and low monthly minimum discharge ( 0ºC. The correlation analysis revealed the following mean results:- The winter seasonal discharge was influenced by the forest surface area (negative correlation) and both annual and seasonal temperature (positive correlation).- The timing of the monthly maximum discharge was influenced by the length of rivers (positive correlation), the forest and lake surface area (positive correlation) and both annual and seasonal temperatures (negative correlation).- The spring seasonal discharge was influenced by the length of rivers (negative correlation), the mean basin slope (positive correlation), the forest surface area (positive correlation), the lake surface area (negative correlation), the annual precipitation (negative correlation) and the winter and summer seasonal temperature (negative correlation).- The spring and summer seasonal discharge ratio was negatively correlated with the drainage basin, the length of rivers, the mean basin drainage, the annual precipitation and the number of winter days with temperature > 0ºC, but was positively correlated with annual and seasonal temperature.- The monthly maximum and minimum discharge was positively correlated with forest surface area but negatively correlated with lake surface area, annual and seasonal temperature.- The monthly minimum discharge was negatively correlated with forest surface area but positively correlated with annual and seasonal discharge.From this correlation analysis, it appeared that temperature was the only factor that influenced the spatial variability of all hydrological variables, followed by forest and lake surface area. The influence of precipitation on this spatial variability was very weak

Impacts des barrages sur les débits annuels minimums en fonction des régimes hydrologiques artificialisés au Québec (Canada)

Les débits annuels minimums des rivières déterminent le volume d’habitat minimum disponible pour assurer la survie des espèces aquatiques en période d’étiage. Dans cette étude, nous comparons les impacts de barrages sur les caractéristiques (période d’occurrence, magnitude, amplitude de variation et asymétrie) de ces débits dans trois régimes hydrologiques artificialisés d’une part, et les débits annuels minimums mesurés en aval des barrages aux normes de débits réservés pour protéger les habitats du poisson au Québec, d’autre part. Nous avons analysé 72 stations appartenant aux régimes artificialisés d’Inversion (26 stations), d’Homogénéisation (18 stations) et de Type Naturel (28 stations). Toutes ces stations appartiennent au bassin versant du fleuve Saint-Laurent. La présente analyse est fondée sur la comparaison des débits mesurés en rivières naturelles (75 stations) à ceux mesurés en aval des barrages au moyen des méthodes de proportionnalité et graphique. Il ressort de ces comparaisons les principaux résultats suivants.En régime artificialisé d’Inversion caractérisé par les débits mensuels maximums en hiver et les débits mensuels minimums au printemps, les impacts des barrages se traduisent par une hausse significative de fréquence des débits annuels minimums au printemps au moment de la fonte des neiges mais une baisse en été, une diminution significative de la magnitude des débits pour les bassins versants de taille 10 000 km2.Annual minimum discharges represent a crucial hydrologic parameter for the health of aquatic ecosystems. They determine the volume of available habitat for aquatic species and influence the concentration of pollutant within the fluvial system during low flows. They are also of importance for instream infrastructures and for the regulation of fluvial transport. For these reasons, the minimum discharges constitute the main hydrologic parameters for which clear regulation have been defined in several countries. In the province of Québec, albeit the large amount of dams on several important fluvial systems, there seems to exist a lack of studies examining their effects on the annual minimum discharges. This paper is aiming at highlighting the effects of dams (1) by examining their effect on the characteristics of annual minimum discharges for artificialised flow regimes in Québec, and (2) by comparing those discharges with recommended instream flows to protect fish habitats.Firstly, the effect of dams on annual minimum discharges is examined for the three types of artificialised flow regimes found in Québec. From the analysis of seasonal and monthly discharges, ASSANI et al. (2004) documented the three types of artificialised hydrologic regime downstream from dams: the inversion, the homogenization, and the natural type flow regimes. The inversion flow regime presents high monthly discharge values in winter and low monthly discharge values during spring. This type of regime occurs solely on the north shore of the St-Lawrence River and pertains to rivers with large reservoirs feeding in hydropower stations. The homogenization flow regime presents small annual fluctuations of the monthly discharge. The maximum monthly discharges are recorded during spring where- as the minimum monthly discharges frequently occur during fall. This type of regime is often associated with reservoirs created on large streams for which the storage of spring water is less important. This regime is observed mainly on the north shore of the St-Lawrence river. In the natural type flow regime, the maximum monthly discharges take place during spring snowmelt while minimum monthly discharges occur either during summer or winter. The annual natural flow characteristics are thus conserved albeit the existence of the dam. This regime pertains to dams with small reservoirs and it is found on both side of the St-Lawrence River.Secondly, annual mimimum discharges are compared with minimum instream flows recommended by BELZILE et al. (1997). These ones defined the minimum instream flows based on the different species of fish and their life cycle. Downstream from dams, the instream flows (Qr) can be estimated using the following relation:Qr = ek.Sawhere S represents the drainage area upstream from the dam; a and k are respectively regional and seasonal parameters. These parameters are associated to the ecohydrological region, to the season as well as to the critical phases of life cycle for the fish species found within the ecohydrological regions.From the Historical Stream Flow Summary of Environmental Canada, the distribution of discharge from 107 stations were selected and analysed. From those, 72 were located on rivers with dams and 75 on rivers with no regulation. On regulated rivers, 26, 18 and 28 were identified as belonging to the inversed, homogeneous and natural type regimes, respectively. All stations were located in the St-Lawrence drainage area. To highlight the effect of dams, we performed a comparison between the annual minimum discharges for stations on artificialised rivers to those from stations belonging to rivers with no regulation. The comparison is performed according to the size of the drainage basins (proportionality method) and uses a set of parametric and non-parametric statistical tests depending on the type of data. The proportionality method was chosen because of the non-availability of the discharges for the pre-dam periods. According to RICHTER et al. (1996), river flows can be described using several parameters relating to the daily discharges: the magnitude, the frequency, the duration, the timing and the rate of change (amplitude of the variability). The daily discharges required to compute these parameters were not available. The date of occurrence of annual minimum discharges, their magnitude, the interannual variability of the magnitude and the skewness of the distribution could however be obtained from the Historical Stream Flow Summary of Environmental Canada.The analysis of annual minimum discharges for the three types of artificialised flow regimes highlights several key elements associated with the effect of dams. For the inversion flow regime, the presence of dams increases and decreases significantly the occurrence of annual minimum discharges during spring and summer, respectively. For drainage area smaller than 10 000 km2, the magnitude of the annual minimum discharge is decreased significantly. Finally, the between-year variability is increased and the distribution presents a strong skewness. For the natural type flow regime, an increase in annual minimum discharges during the period between November and January can be observed as well as a significant decrease in magnitude for the small fluvial systems (drainage area 10 000 km2

Impacts des barrages sur les caractéristiques des débits moyens annuels en fonction du mode de gestion et de la taille des bassins versants au Québec

Nous avons comparé les impacts des barrages sur les caractéristiques (volume d’écoulement-fréquence, variabilité interannuelle et forme de courbe de distribution) des débits moyens annuels dans les trois régimes régularisés observés (inversion, homogénéisation et type naturel) au Québec. Nous avons appliqué la méthode de proportionnalité qui consiste à comparer les caractéristiques des débits des rivières naturelles et celles des rivières régularisées en fonction de la taille des bassins versants. En ce qui concerne le volume d’écoulement-fréquence et sa variabilité interannuelle, le changement a été observé seulement en régime d’inversion. Il se traduit par une baisse des débits moyens annuels durant les années hydrologiques sèches et une variabilité interannuelle relativement forte par rapport aux rivières naturelles. Ces changements sont attribués principalement au mode de gestion des réservoirs car on lâche moins d’eau durant ces années hydrologiques sèches. Enfin, les changements des coefficients d’asymétrie et d’aplatissement ont été observés surtout en régime d’homogénéisation. Cette étude démontre que les barrages peuvent modifier toutes les caractéristiques des débits moyens annuels contrairement à l’opinion couramment admise.We compared the impacts of dams on the characteristics (magnitude-frequency, inter-annual variability and distribution curve shape) of the mean annual flows in three regulated flow regimes (inversion, homogenization and natural type) in Québec. We applied the “proportionality method”, which consists of comparing the flow characteristics of natural rivers to regulated rivers according to watershed size. A change in the flow-frequency volume and its inter-annual variability was observed only in the inversion flow regime. This result translates into a decrease in average annual flows during dry hydrological years and a relatively high inter-annual variability relative to natural rivers. These changes are mainly ascribed to the reservoir management mode because less water is released during dry hydrological years. Finally, the changes of the coefficients of asymmetry and skewness are particularly observed in homogenization flow regime. This study shows that, contrary to the commonly accepted opinion, dams can alter all the characteristics of annual average flows

Analyse de l’influence de l’oscillation Arctique sur la variabilité interannuelle des précipitations dans le bassin versant de la rivière Saint-François (Québec, Canada) au moyen de la méthode des corrélations canoniques

Le bassin versant de la rivière Saint-François, situé sur la rive sud (rive droite) du fleuve Saint-Laurent (Québec, Canada), couvre une superficie d’environ 10 000 km2. Dans le but de déceler les facteurs climatiques qui influencent les précipitations dans ce bassin versant, nous avons analysé la succession des périodes pluviométriques sèches et humides par la technique des moyennes glissantes sur cinq ans, d’une part, et la relation entre quatre indices climatiques (oscillation arctique, oscillation australe, oscillation nord-atlantique et la température des eaux océaniques de surface) et ces périodes pluviométriques au moyen de l’analyse des corrélations canoniques, d’autre part. Nous avons analysé les données pluviométriques mesurées à trois stations représentatives des régimes pluviométriques du bassin versant : Sherbrooke, Disraeli et Drummondville. Ces données couvrent une période de 76 ans (1914-1990).En ce qui concerne la variabilité interannuelle des précipitations, nous avons détecté deux types de changement. Le premier type de changement, survenu autour de 1950, concerne la répartition des précipitations à l’échelle du bassin versant (changement spatial). Avant 1950, la succession des périodes sèches et humides des précipitations n’était pas synchrone (opposition des périodes) mais elle l’est devenue après 1950. Le second type de changement a été observé autour des années 1935 et 1970. Il correspond à un changement des totaux pluviométriques au niveau des stations (changement quantitatif). Avant et après 1935 et 1970, on passe ainsi des périodes sèches aux périodes humides ou vice versa selon les stations. En tenant compte de ces trois dates, nous avons observé la succession des périodes sèches et humides suivantes : 1) Avant 1950, entre 1914 et 1935, nous avons observé une période sèche aux stations de Disraeli et de Sherbrooke mais une période humide à la station de Drummondville. Entre 1936-1950, ces périodes se sont inversées : humide à Disraeli et Sherbrooke mais sèche à Drummondville; 2) Après 1950, entre 1951 et 1970, les précipitations étaient déficitaires aux trois stations. En revanche, elles sont devenues excédentaires après 1970.L’analyse des corrélations canoniques entre les précipitations et les indices climatiques a révélé les faits significatifs suivants : 1) Avant et après 1950, les précipitations sont positivement corrélées à l’oscillation arctique (OA), mais cette corrélation est plus faible après qu’avant 1950. Ainsi, l’augmentation des valeurs de OA entraînerait une hausse de fréquence des masses d’air en provenant du sud dans le bassin versant; 2) Lorsqu’on considère les périodes sèches et humides, OA est toujours positivement corrélée aux périodes sèches à la station de Sherbrooke.The Saint-François River watershed, located on the south shore of the St. Lawrence River (Québec, Canada), covers an area of about 10,000 km2. To detect the climatic factors that influence precipitation in this watershed, we analyzed the succession of dry and wet pluviometric periods by a method of simple moving averages computed over five years. In addition, the relationship between four climatic indices (Arctic Oscillation, Southern Oscillation, North Atlantic Oscillation and Sea Surface Temperature) and these pluviometric periods was analyzed by means of canonical correlation analysis. We analyzed the pluviometric data measured over a 76-year period (1914-1990) at three stations representative of the watershed’s pluviometric regimes: Sherbrooke, Disraeli and Drummondville.Two types of change in the inter-annual variability of precipitation were detected. The first type of change, occuring circa 1950, concernend the distribution of precipitation throughout the watershed, i.e. spatial change. Before 1950, the succession of dry and wet precipitation periods was asynchronous (opposition of periods), but became synchronous after 1950. The second type of change, corresponding to a change in the pluviometric totals at the stations, i.e. quantitative change, was observed circa 1935 and 1970. There was, therefore, a shift from dry to wet periods or vice versa, prior to and following 1935 and 1970, depending on the station. By accounting for these three dates, we observed the succession of the following dry periods and wet periods. First, before 1950 and between 1914 and 1935, we observed a dry period at the Disraeli and Sherbrooke stations and a wet period at the Drummondville station. Between 1936 and 1950, these periods were reversed: wet periods at Disraeli and Sherbrooke but a dry period at Drummondville. Second, after 1950 and between 1951 and 1970, there was a precipitation deficit at all three stations, which, however, moved into a surplus phase after 1970.The canonical correlation analysis of precipitation levels and the climate indices revealed the following significant facts: 1) prior to and following 1950, precipitation was positively correlated to the Arctic Oscillation (AO) indices, but this correlation was weaker after 1950 than before; and 2) with respect to the wet and dry periods, the AO index is still positively correlated with the dry periods at the Sherbrooke station

Analyse d'impacts d'un barrage sur le régime hydrologique de la rivière Matawin (Québec, Canada)

Malgré la présence de nombreux barrages au Québec, peu d'études ont été consacrées à l'analyse des impacts de ces ouvrages sur les régimes hydrologiques des cours d'eau. La présente note a pour but d'analyser les impacts d'un barrage sur le régime hydrologique de la rivière Matawin en comparant le régime hydrologique de la rivière en amont (1390 km2) et en aval (4070 km2) du barrage pendant une période de 60 ans (1930-1990) et sur trois échelles temporelles distinctes. A l'échelle interannuelle, l'influence du barrage se manifeste par une persistance plus marquée des effets des épisodes humides ou secs ainsi qu'une hausse ou une baisse des débits moyens annuels respectivement durant ces périodes. Mais cette succession et cette persistance n'ont pas affecté significativement la stationnarité de la série hydrologique. Aux échelles mensuelles et saisonnière, l'influence du barrage se manifeste par une inversion du régime hydrologique caractérisée par une hausse des débits hivernaux et une baisse des débits printaniers. Cette influence se traduit aussi par une baisse significative du débit maximum mensuel mais une hausse du débit minimum mensuel. Il en résulte une diminution du coefficient d'immodération. A l'échelle journalière, le barrage modifie la période d'occurrence des débits extrêmes minimums et maximums. Il provoque la diminution significative des débits extrêmes minimums et maximums. Mais l'écrêtement des crues est modéré pour les débits de récurrence=10 ans. L'impact le plus significatif du barrage de Matawin est sans nul doute l'inversion du régime hydrologique dont les conséquences morphologiques et biologiques ne sont pas encore documentées dans la littérature scientifique canadienne. Cette inversion résulte du faible écoulement hivernal et d'une forte production de l'énergie électrique pendant la saison froide.Few studies have characterized the effect of dams on the hydrologic regime of rivers in Quebec. This is rather strange given the large number of hydroelectric dams that have been constructed in the province. To shed some light on the environmental impact of these dams, this paper aims at describing and quantifying the effect of the Matawin River dam on the hydrologic regime of the river on an annual, seasonal and daily basis. The Matawin River is located north of the St-Lawrence River and is a tributary of the St-Maurice River. The Matawin dam was built in 1929 by Shawinigan Water and Power Co. mainly to supply the Gabelle hydroelectric dam on the Saint-Maurice River. The dam is 26 m high and the storage capacity of the reservoir is 348,000,000 m3 when full. The catchment area of the dam is 4070 km2.To assess the effect of the dam, we used various statistical methods to compare discharge time series over 60 years as measured at two gauging stations on the river. One of the stations is located upstream whereas the other one is located downstream from the Matawin dam. The upstream drainage basin covers an area of 1390 km2. No major tributaries are found between the two gauging stations, thus allowing us to ascertain the effect of the Matawin dam on the natural hydrologic regime of the river at different time scales.On the annual scale, no difference in the mean annual discharge is observed upstream and downstream from the dam. The specific discharges upstream and downstream from the river's dam are respectively 17.2 and 17.1 l/s/km2. This is to be expected because the reservoir is used neither for irrigation nor for derivation. However, analysis of the interannual variability of mean annual discharges, using the Hanning low pass filter, reveals that wet and dry periods are far more persistent downstream than upstream from the dam. This persistence can be seen to occur within the two dry periods of 1930-1960 and of 1980-1990 and during the wet period from 1965 to 1980. This persistence does not affect the stationarity of the discharge time series downstream from the dam as no significant changes are detected from Mann-Kendall and Pettitt statistical tests.On the monthly and seasonal scale, the comparison of the time of occurrence of maximum and minimum discharges shows a strong inversion within the hydrologic regime. Upstream from the dam, the maximum and minimum discharges are measured respectively during the spring and the winter. Downstream from the dam, the regime is inverted, with the maximum and minimum discharges being measured in winter and spring respectively. This inversion is closely associated with the production of hydroelectricity during the cold winter season when large amounts of water are released from the reservoir. Furthermore, is worth noticing that the monthly and seasonal coefficients of maximum discharge are lowered downstream from the dam whereas those for the minimum discharge remain similar.On a daily basis, the comparison of dates of occurrence for the lowest annual discharge downstream and upstream from the dam shows these are found at different times of the year. Upstream from the dam, most of the minimum daily discharges are measured in August and September whereas downstream from the dam, they largely occur during April. On the other hand, the maximum daily discharges are recorded almost exclusively in April and May upstream from the dam but can occur throughout the year downstream from it, with a marginally larger number in January.These results are relevant for the assessment of the environmental impacts of dams on rivers in the province of Quebec. For example, the inversion of maximum and minimum discharges is likely to have an important impact on the winter habitat characteristics by increasing the area of suitable habitat, but also by increasing the likelihood of sediment being transported during periods where usually only sporadic transport events occur. In the future, it would be crucial to understand the exact effect of the inversion on the morphological and biological components of the river dynamics

Uniformity in the Wiener-Wintner theorem for nilsequences

We prove a uniform extension of the Wiener-Wintner theorem for nilsequences due to Host and Kra and a nilsequence extension of the topological Wiener-Wintner theorem due to Assani. Our argument is based on (vertical) Fourier analysis and a Sobolev embedding theorem.Comment: v3: 18 p., proof that the cube construction produces compact homogeneous spaces added, measurability issues in the proof of Theorem 1.5 addressed. We thank the anonymous referees for pointing out these gaps in v

Modes de variabilité temporelle des débits moyens annuels et leurs liens avec les indices climatiques au québec (canada)

La variabilité interannuelle des débits moyens annuels (1970-1995) de 70 stations hydrologiques réparties dans les trois grands bassins versants du Québec a été étudiée au moyen d’une analyse en composantes principales et d’un lissage par une moyenne mobile simple. Cinq modes de variabilité ont été ainsi identifiés selon la succession des phases de baisse et de hausse des débits. Les trois premiers modes caractérisent les rivières du bassin du fleuve Saint-Laurent. Le premier mode, qui regroupe le plus grand nombre de stations situées sur les deux rives du fleuve, montre une période de baisse des débits (avant 1980), suivie d’une longue phase de hausse modérée des débits. Ce mode est positivement corrélé à l’oscillation australe. Le second mode, qui regroupe les rivières situées au nord de la rive sud du Saint-Laurent, est caractérisé par des débits qui diminuent entre 1975 et 1985, puis augmentent. Il n’est corrélé à aucun indice climatique. Les stations qui forment le troisième mode sont principalement localisées en rive nord. Ce mode est caractérisé par deux phases de hausse séparées par une phase de baisse des débits. Certaines stations de ce mode sont corrélées aux oscillations arctique, australe et nord atlantique. Les deux derniers modes caractérisent les rivières situées au nord du 55e parallèle, dans les bassins de la Baie d’Ungava et de la Baie d’Hudson. Ces modes montrent une phase de diminution continue depuis la seconde période des années 1970 ou une phase de diminution précédée d’une longue phase normale des débits. Ils sont négativement corrélés à l’oscillation arctique et nord atlantique. Il se dégage de cette étude que la variabilité interannuelle des débits n’est pas synchrone à l’intérieur du bassin du fleuve Saint-Laurent.The temporal variability of the annual average discharges (1970-1995) of 70 hydrological stations distributed among Québec three main watersheds was studied by principal component analysis and smoothing by a simple moving average. Five temporal variability modes were thus identified according to the succession of decreasing and increasing discharge phases. The first three modes characterize rivers of the St. Lawrence watershed. The first mode, which includes the greatest number of stations located on both shores of the river, shows a period of decreasing discharges (before 1980), followed by a long phase of moderately increasing discharges. This mode is positively correlated with the Southern Oscillation. The second mode, which includes rivers located in the northern part of the south shore of the St. Lawrence, is characterized by discharges decreasing between 1975 and 1985 and then increasing. It is not correlated with any climate index. The stations forming the third mode are mainly located on the north shore. This mode is characterized by two increasing phases separated by a decreasing discharge phase. Some stations of this mode are correlated with the Arctic, Southern and North Atlantic Oscillations. The last two modes characterize rivers located north of the 55th parallel, in the Ungava Bay and Hudson Bay watersheds. These modes show a continuously decreasing phase since the second period of the 1970s or a decreasing phase preceded by a long normal discharge phase. They are negatively correlated with the Arctic and North Atlantic Oscillations. This study shows that the interannual discharge variability is not synchronous within the St. Lawrence River watershed

La sécurité alimentaire : perspectives d'amélioration des bananiers par voie biotechnologique

En complément des travaux d'amélioration génétique par hybridation et des nouvelles approches biotechnologiques, la connaissance des bananiers s'est considérablement accrue au cours de la dernière décennie. Les outils ainsi forgés ont déjà permis une meilleure diffusion du matériel végétal ainsi que la création de nouvelles variétés. L'ensemble de ces techniques contribue et contribuera sans nul doute à l'amélioration tant qualitative que quantitative de la production bananiere. En ce sens elles participent significativement au renforcement de la sécurité alimentaire de la planèt