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La variabilité hydrologique dans le bassin amazonien
Introduction
Durant les deux dernières décennies de nombreux evénements hydrologiques ca-tastrophiques ont et´ observés dans le basin amazonien. Callède et al. (2004) relève une baisse des étiages depuis le milieu des années 1970. Simultanément, les dernières quarante années sont caractérisées par des débits de crue fréquemment très forts : 14 evénements avec un débit supérieur `a 250 000 m /s `a Obidos (Figure 1.1) sont dénombrés depuis 1970, tandis qu’entre 1903 et 1970 n’ont et´ enregistrés que 5 evénements de cette importance (Callède et al., 2004). Cette augmentation de l’am-plitude des evénements extrˆemes dans le cours principal continue plus récemment : le plus grave étiage des quarante dernières années `a et´ enregistr´ en 2005 (Marengo et al., 2008; Zeng et al., 2008), tandis qu’une très forte crue l’a et´ en 2006. Différentes causes comme la variabilité climatique régionale et globale (Marengo et al., 2008) ou la déforestation dans le bassin (Callède et al., 2008) ou bien entendu, une com-binaison des ces deux facteurs sont evoqués pour tenter de donner une explication `a la variabilité hydrologique du cours principal de l’Amazone.
Ce chapitre vise `a documenter et comprendre la variabilité hydrologique régionale `a l’intérieur de l’Amazonie. Dans l’étude qui suit, nous nous appuyons sur un nou-veau jeu de données hydrologiques pour les principaux sous-bassins de l’Amazone, avec un intérˆet particulier pour les evénements extrˆmes (débits de crue et d’étiage).
Nous présentons les résultats sous la forme d’article soumis une première fois `a Journal of Hydrology le 21 mars 2008 et révis´ le 05 novembre 2008. Une version courte de cet article a re¸cu un accueil favorable de Nature Geosciences (10 octobre 2007) puisqu’il a et´ accept´ pour révision. Finalement, il n’a malheureusement pas et´ retenu.
Résumé de l’article
Dans cet article, nous essayons de donner une vision globale de la variabilité spa-tiale et temporelle des débits `a l’intérieur du bassin amazonien. Nous utilisons les données journalières de 18 stations contrˆolant les principaux sous-bassins pour par-venir `a une analyse régionale des valeurs moyennes et des extrˆemes hydrologiques. Dans la première partie de l’article nous décrivons les principales caractéristiques du cycle saisonnier dans les différents sous-bassins, ce qui permet de connaˆıtre en détail la genèse du cycle annuel du débit dans le cours principal. Les caractéristiques climatiques de chaque région qui donnent lieu aux différents régimes hydrologiques sont rappelées dans cette première partie.
Dans une deuxième partie, nous analysons la variabilité interannuelle de trois séries dans chaque sous-bassin : les débits moyens, les débits de crue et les débits d’étiage. Nous utilisons le coefficient de corrélation paramétrique de Pearson et des coefficients non paramétriques comme ceux de Spearman et Kendal pour mesurer la tendance des séries. Les résultats montrent que des tendances significatives op-posent le sud du bassin, où les débits, principalement ceux d’étiage diminuent, et le nord-ouest, où les débits de crue augmentent. Divers test de rupture appliqués aux séries temporelles montrent qu’`a partir de 1992, la diminution dans le sud est plus nette, tandis qu’après cette mˆeme date, les débits dans le nord-ouest sont plus elevés. Cette opposition s’intensifie dans les bassins versants andins : d’une part, la diminution des débits prédomine en Amazonie bolivienne et dans le sud de l’Ama-zonie péruvienne. D’autre part, l’augmentation des débits est plus importante dans le nord de l’Amazonie péruvienne et en Amazonie équatorienne et colombienne. Ces résultats sont confortés par des analyses en composantes principales sur les débits moyens et extrˆemes. Les liens entre composantes principales et indicateurs clima-tiques montrent des débits plus faibles les années El Ni˜no dans les sous-bassins du nord et du centre, un signal moins clair dans les sous-bassins de l’ouest, du sud et de l’extrémit´ nord, et un signal opposé dans le sud-ouest (Amazonie bolivienne).
Dans la dernière partie de l’article nous analysons la variabilité de la lame d’eau précipitée dans les régions nord-ouest et sud du bassin pour tester l’origine clima-tique de la variabilité hydrologique. Nos résultats montrent que dans le nord-ouest, la pluie de mars, avril et mai (MAM), saison la plus pluvieuse dans cette région, observe une variabilité décennale, avec de fortes valeurs dans les années 1970, une diminution ensuite et une nouvelle augmentation des pluies entre 1984 et 2003. De plus, dans le sud, une tendance a` la diminution des pluies est observée, avec une rupture au milieu des années 1980, et de faibles valeurs ensuite. Ces résultats contri-buent `a expliquer la variabilité pluriannuelle des débits.
Nos résultats montrent que durant la période 1974–2004 le débit moyen dans le cours principal de l’Amazone (`a Obidos) est relativement stable ; néanmoins, cette stabilité est expliquée par des tendances opposées, principalement dans les rivières andines. Une diminution des débits d’étiage très nette est observée dans le sud du bassin, tandis qu’une augmentation est enregistrée dans le nord-ouest du bassin, en particulier pour les débits de crue. Cette opposition spatiale des extrˆemes est plus marquée depuis le début des années 1990. De plus, les résultats suggèrent une im-portante influence du climat sur les modes de variabilité des extrˆemes hydrologiques `a l’intérieur de l’Amazone.
Contrasting regional discharge evolutions in the Amazon basin (1974-2004)
Espinoza Villar Jhan Carlo, Guyot Jean Loup, Ronchail Josyane, Cochonneau Gérard, Filizola Naziano, Fraizy Pascal, Labat David, de Oliveira Eurides, Ordo˜nez Juan Julio, Vauchel Philippe.
ABSTRACT
Former hydrological studies in the Amazon Basin generally describe annual discharge variability on the main stem. However, the downstream Amazon River only repre-sents the mean state of the Amazonian hydrological system. This study therefore uses a new data set including daily discharge in 18 sub-basins to analyse the va-riability of regional extremes in the Amazon basin, after recalling the diversity of the hydrological annual cycles within the Amazon basin. Various tests are applied in order to detect trends and breaks in the time series. We show that during the 1974-2004 period the stability of the mean discharge on the main stem in Obidos, is explained by opposing regional features that principally involve Andean rivers : a decreases in the low stage runoff, particularly important in the southern regions, and an increase in the high stage runoff in the northwestern region. Both features are observed from the beginning of the nineties. These features are also observed in smaller meridian sub-basins in Peru and Bolivia. Moreover we show that the changes in discharge extremes are related to the regional pluriannual rainfall variability and the associated atmospheric circulation and to tropical large scale climatic indicators. Key–Words : Amazon basin, discharge trend, rainfall and runoff variability, tropi-cal Atlantic, ENSO, Brazil, Peru, Bolivia.
The Amazon drainage basin is the world’s major hydrological basin. Its watershed covers about 6 000 000 km2 , almost 5% of all the above–water lands. Its average di-scharge is the greatest in the world (209 000 m3/s) (Molinier et al., 1996). Due to its size and its position astride the Equator, the Amazon basin includes very different regions with various discharge regimes. Some works have documented interannual va-riability on the main stem (Richey et al., 1989; Marengo, 1992; Callède et al., 2004, etc). However, regional scale discharge variability has been incompletely discussed in the whole Amazon basin at annual and pluriannual time scales. Futhermore, some recent dramatic events such as the 2005 drought (Zeng et al., 2008; Marengo et al., 2008) and the 2006 flooding show it is not sufficient to analyze mean annual discharge and it is important to pay attention to extremes values. That is why the aim of this paper is to investigate high and low water changes at annual and pluriannual time scales in the main stem and in all the main sub-basins of the Amazon Basin. This study is made possible thanks to the cooperation of the HYBAM program (Hydro-logy and Geodynamic of the Amazon Basin, www.mpl.ird.fr/hybam) between IRD (Institut de Recherche pour le Développement / Institute for Research and Develop-ment) and national hydrological institutions. This has permitted, for the first time, the integration of data from the different countries which form part of the Amazon basin.
After the introduction, we recall some features of the regional hydrological and climatic characteristics and their variability as described by former works. The di-scharge database and the methods used in this paper are then described. The annual cycles in the different sub-basins are depicted and we show how they contribute to the annual cycle in Obidos, the last gauged station on the Amazon River main stem. We then comment on trends and breaks in regional maximum and minimum discharge annual series, with special attention to the Andean rivers of Peru and Bolivia. The discharge time evolution at Obidos is then explained using former re-sults. Finally, discharge variability is related to rainfall in the Amazon Basin and to regional climatic indicators. A summary and concluding remarks are provided in the last section.
Hydro–climatic characteristics of the Amazon Basin
Rainfall regimes in the Amazon Basin show the strong opposition between the nor-thern and southern tropics, with a rainy season in June, July and August – JJA (in December, January, February – DJF) in the North (South), due to the alternating warming of each hemisphere and to American monsoons. Next to the Amazon delta, a March, April and May (MAM) maximum and a September, October, November (SON) minimum are associated with the seasonal migration of the Intertropical Convergence Zone (ITCZ). In the Northwest equatorial region a better rainfall dis-tribution within the year is observed with quarterly rainfall percentages close to 25%. Various intermediate regimes are described between equatorial and tropical regions (for more information see Figueroa and Nobre, 1990, Marengo, 1992, Espi-noza et al., accepted, among others).
Interannual discharge variability on the Amazon mean stem may cause inunda-tions or very low water stages. For instance, the recent 2005 drought that affected the western sub-basins (Solim˜oes and Madeira Rivers) during the low water stage (October and November) received important attention from the scientific community (Zeng et al., 2008; Marengo et al., 2008) as it had serious impacts on human activity (transport, fishing, water supply, etc) and on the biosphere. This event is attributed to high sea surface temperature (SST) in the tropical northern Atlantic (Zeng et al., 2008; Marengo et al., 2008), a feature that is also pointed out by Marengo (1992) and Labat et al. (2004). Ronchail et al. (2005b) specify that higher than normal low–flow associated with cold events in the North Tropical Atlantic are particularly strong and wide spread in the central regions of the basin. Interannual discharge variability is also related to the SST in the Equatorial Pacific : authors coincide in finding lower (higher) discharge during El Ni˜no (La Ni˜na) in the main stem (Richey et al., 1989; Marengo, 1992; Marengo et al., 1998). Some works analyzing the regional di-scharge variability inside the Amazon Basin show that the ENSO signal is particu-larly strong in the northeastern basins (Molion and Moraes, 1987; Uvo and Graham, 1998; Guyot et al., 1998; Uvo et al., 2000; Foley et al., 2002; Ronchail et al., 2005b). Ronchail et al. (2005a) also find an opposite ENSO signal in the upper Madeira Ri-ver (southern Amazon).
Richey et al. (1989); Marengo (1995); Marengo et al. (1998) point out that the Solim˜oes River discharge in Manacapuru and the Rio Negro in Manaus do not exhibit any significant trend during the twentieth century but they note that di-scharge increases at the end of the sixties. This feature is also noted by Callède et al. (2004, 2008) in the Amazon River in Obidos ; they find a break in the mean, maxi-mum and minimum discharge times series at the beginning of the 1970s, with higher values after that date. Afterwards, mean and maximum discharge remain high until the beginning of the XXI century while minimum discharge decreases since the mid–1970s. Consistently, Genta et al. (1998), Garc´ıa and Vargas (1998), Collischonn et al. (2001), Garcia and Mechoso (2005) and Krepper et al. (2008) all find an increasing trend since the early 1970s in the La Plata Basin discharge. At a regional scale, Rocha et al. (1989) highlight that rainfall and discharge in the Ma-deira, the Solim˜oes and the Negro rivers increase during the 1960s and the early 1970s, but the records return to the long-term average values in the late 1970s and 1980s (except in the Madeira River). Espinoza et al. (2006) support a significant diminishing trend in the Peruvian Amazon (at Tamshiyacu station, near to Iquitos) for the 1970–2005 period, especially in the low-level discharge series.
Analyzing Obidos discharge long time series, Labat et al. (2004, 2005) high-light low–flow interdecadal processes (15.5 years) and high-flow bidecadal variability that can be related to the northern Tropical Atlantic and Pacific variability (low– flow) and the Southern Tropical Atlantic variability (high-flow). Garcia and Mechoso (2005) find a 9 year period in the Paraguay–Paran´ River and a dominant quasi oscillation with a period of around 17 years in mean annual discharge in the Amazon at Obidos, in the Tocantins and S˜ao Francisco Rivers. In the southern part of South America, Pasquini and Depetris (2007) find a quasi-decadal variability in the tributaries of the La Plata River, in the Patagonia’s Colorado River and quasi-bidecadal periodicities in discharge of La Plata, Colorado and Santa Cruz Rivers. Robertson and Mechoso (2000) attribute the quasi–bidecadal variability to the 17– year cycle of the South American Monson System.
In conclusion, most authors mention the possible links between long-term di-scharge variability and climate shifts. They generally deny the role of deforestation on the 1970 change as it occurs when deforestation was just beginning in southern Amazonia.
Data and methods
Daily water level data are compiled and their quality is checked by the national insti-tutions in charge of hydrological monitoring in the different countries of the Amazon Basin : Agˆencia Nacional de Aguas (Water National Office – ANA, Brazil) and Servi-cio Nacional de Meteorolog´ıa e Hidrolog´ıa (National Meteorology and Hydrology Ser-vice – SENAMHI, Peru and Bolivia). The rating curves of about thirty stations have been determined using Acoustic Doppler Current Profiler (ADCP) gauging measures conducted by HYBAM researchers between 1996 and 2008. This methodology has been shown to be well adapted to large Amazonian rivers (Filizola and Guyot, 2004). In some stations (Manacapuru, Fazenda Vista Alegre, and Itaituba) discharge is not a simple function of water level due to the backwater effect (Meade et al., 1991). The gauging curves have been established using the normal drop method. It is based on a correction of the water level using the level difference between the given sta-tion and a downstream one and the distance between both (Jacon and Cudo, 1989). Monthly and annual discharge values have been computed using daily data.
In this study eighteen hydrometrical stations are selected based on their water-shed sizes, their mean discharges, their locations in the Amazon basin and their data periods. Some smaller Andean basins are also documented. Table 2.1 and Figure 2.1 display the locations and the main characteristics of the stations. Discharge data is available for different periods, depending on the stations. The common selected period in this work is 1974 – 2004 for the main basins and 1990-2005 for the smal-ler Andean basins. Four stations are located in the South and drain N-S rivers : from East to West, Itaituba (ITA) is on the Tapaj´os River, Altamira (ALT) on the Xingu River, Fazenda Vista Alegre (FVA) and Porto Velho (PVE) downstream and upstream on the Madeira River respectively. The data of two stations, Gavi˜ao on the Juru´a River and Labréa on the Purus River, in the central western Brazilian Amazon, are added in order to create a virtual station Gavi˜o-Labréa (G–L), which has size and discharge of the same order of magnitude as the other stations. Four stations are located along the Solim˜oes River : Tamshiyacu (TAM) on the Peruvian Amazonas River drains tropical and equatorial regions of Peru and Ecuador, Santo Antˆonio do I¸c´a (SAI) northwestern equatorial regions on the upstream Solim˜oes River, Acanau´ı (ACA) on the Japur´ River the equatorial Colombian Amazon and finally Manacapuru (MAN), the whole Solim˜oes River. Toward North, Caracara´ı (CAR) on the Branco River drains tropical regions in the northern hemisphere while Serrinha (SER) drains the northwestern Negro River basin. Finally Obidos (OBI) on the Amazon River main stem gather water from the Negro, the Solim˜oes and the Madeira Rivers. Two gauging stations put together water from the Andes : 22% of PVE basin area is in the Andes of Peru and Bolivia and 53% of TAM basin area is in the Andes of Peru and Ecuador. PVE and FVA gauging stations have part of their watershed in the Brazilian shield. ITA and ALT are completely in the Brazi-lian shield and finally CAR and SER watersheds are partly located in the Guyana Shield.
Five complementary stations in Peru and Bolivia are used to give more detailed information about the Andean sub-basins (Table 2.1 and Figure 2.1). San Regis discharge in Obidos for thirteen main stations (top), one virtual station (G–L), five Andean river stations (middle) and five residual stations noted with an * (bottom). Altamira and Itaituba sub-basins, on the Xingu and Tapaj´os Rivers respectively, are not part of the Amazon basin in Obidos.
(SRE) and Borja (BOR) are located on the Mara˜n´on River that drains northern Peru and part of Ecuador. Requena (REQ) on the Ucayali River drains southern Peru. The Ucayali and Mara˜n´on Rivers form the Amazonas River where is located TAM. Upstream PVE on the Madeira River, Guayaramer´ın (GUA) on the Mamoré River and Cachuela Esperanza (CAE) on the Beni River drain lowlands and moun-tain regions in Bolivia and Peru.
Five supplementary stations are created in order to give information about the contribution and the variability of parts of large sub-basins. They are called “re-sidual” stations. Their discharges are the difference between a downstream and an upstream station discharges. In the residual Santo Antˆonio do I¸c´a station (SAI*), it is the difference between discharge in SAI and TAM, and in the residual Fazenda Vista Alegre (FVA*) station, it is the difference between discharge in FVA and PVE. The residual Manacapuru (MAN*) discharge is the difference between MAN and the sum of SAI, G-L and ACA discharges. The residual Obidos (OBI*) corresponds to the difference between OBI and the sum of CAR, SER, MAN and FVA discharges. Finally, in the Andean sub-basins, the discharge in the residual San Regis station (SRE*) is the difference between discharge in SRE and BOR (Table 2.1 and Figure 2.1).
In order to compare the discharge in the different basins, the runoff in milli-meters (mm) is computed for each station. Particular values such as the monthly maximum and minimum annual runoff (Qmax and Qmin respectively) are individua-lized and complement the mean annual runoff (Qmean). The interannual variation coefficients, ratio between the standard deviation and the average of annual data values, are calculated for each hydrological series (iVCQmean, iVCQmax and iVC-Qmin respectively). The seasonal variation coefficients (sVC) is the ratio between the standard deviation and the average of monthly values. The seasonal amplitude is computed by subtracting Qmin to Qmax. Maximum and minimum discharge data are not available for residual basins because the difference between extreme values in downstream and upstream stations may be negative, as there is a time lag bet-ween the occurrences of extremes in the different sub-basins. SAI* is an exception, as it is close, for the Amazon basin standards, to the upstream TAM station (1000 km).
The Southern Oscillation Index (SOI), the standardized difference of pressure between Tahiti and Darwin, comes from the Climatic Prevision Centre of the Na-tional Oceanic and Atmospheric Administration (CPC-NOAA) www.cdc.noaa.gov/. SST data also comes from the CPC-NOAA. Monthly SSTs (1950-2000) are provi-ded for the northern tropical Atlantic (NATL, 5-20◦N, 60-30◦W) and the southern tropical Atlantic (SATL, 0-20◦S, 30◦W-10◦E). The standardized SST difference bet-ween the northern and the southern tropical Atlantic (NATL-SATL) is computed to feature the SST gradient in this oceanic basin.
Table des matières
CHAPITRE 1 Introduction
1.1 Contexte général
1.2 Les programmes de recherche dans lesquels s’inscrit la thèse
1.3 Objectifs de la thèse
1.4 Organisation
CHAPITRE 2 La variabilité hydrologique dans le bassin amazonien
2.1 Introduction
2.2 Contrasting regional discharge evolutions in the Amazon basin
2.3 Résultats complémentaires
CHAPITRE 3 La variabilité pluviométrique dans le bassin amazonien
3.1 Introduction
3.2 Principaux éléments de la circulation atmosphérique
3.3 Variabilité spatio–temporelle de la pluie dans le bassin amazonien
3.4 Spatio-temporal rainfall variability in the Amazon basin
3.5 Résultats complémentaires
3.6 Variabilité pluviométrique et relations avec les débits
3.7 Variabilité pluviométrique et relations avec la SST
3.8 En résumé
CHAPITRE 4 Types de temps et désagrégation dans le bassin amazonien
4.1 Introduction
4.2 Concepts généraux
4.3 Cartes topologiques auto–organisatrices
4.4 Application dans le bassin amazonien
4.5 Conclusions
CHAPITRE 5 Types de temps : facteurs explicatifs de l’hydrologie
5.1 Evolution des types de temps et relations avec la pluie
5.2 Types de temps et événements hydrologiques extrˆemes
CHAPITRE 6 Conclusion générale et perspectives
6.1 Conclusions
6.2 Perspectives
CHAPITRE Annexe
La variabilité du débit du Rio Amazonas au Pérou
Bibliographie
