River–Groundwater Interactions in the Brazilian Pantanal. the Case of the Cuiaba´ River

Journal of Hydrology 283 (2003) 57–66 www.elsevier.com/locate/jhydrol River–groundwater interactions in the Brazilian Pantanal. The case of the Cuiaba´ River Pierre Girard*, Carolina J. da Silva, Mara Abdo Projeto Ecologia Pantanal, Instituto de Biocieˆncias, Universidade Federal de Mato Grosso, Av. Fernando Correia da Costa s/n, Bairro Coxipo´, 78060-900 Cuiaba´, MT, Brazil Received 19 February 2002; accepted 23 June 2003 Abstract The Pantanal is a vast evaporation plain and sediment accumulation surface that floods annually. It is located in the Upper Paraguay River Basin, a major source of floodwaters to the Pantanal. The recent construction of a large dam in the upper reach of the Cuiaba´ River raises questions: What will be the dam influence on the flood area and duration? What will be the consequence for groundwater replenishment and permanence of flow in the floodplain channels during the dry period? This study of the Cuiaba´ River, within the Pantanal, describes water flow between the river channel and its adjacent floodplain, as well as relations between the surface water and groundwater near the river. Flooding of the plain adjacent to the Cuiaba´ River critically depends on the river stage and proceeds through a complex hydrographic network. No free water table was encountered; groundwater was confined below clay–silt layers. Two groundwater bodies were distinguished based on their piezometric behavior. In both cases the river stage variations appeared to control the piezometric heads and the flood was the main recharge source. The groundwater moved from the river towards the floodplain where it appeared to sustain channel flow and to maintain soil humidity in depressed areas during the dry period. q 2003 Published by Elsevier B.V. Keywords: Pantanal; Flood pulse; Groundwater; Recharge; Ecological stability 1. Introduction (Junk, 2000). This land is still in a rather pristine state and its ecological integrity is tightly linked to The Pantanal wetland is a vast evaporation plain its hydrology (Da Silva, 2000). Recently, however, and sediment accumulation surface (Junk et al., environmental disturbances such as deforestation due 2003) occupying an immense sedimentary to the expansion of the agro-industry and the depression. It is located south of the Amazon Basin consequent erosion and sedimentation have and east of the Andes, in the Upper Paraguay River increased. A new threat, the disruption of the Basin. Annually, the Upper Paraguay River and fundamental seasonal flood pulse by engineering its tributaries flood an area of about 140,000 km2 works such as waterways and large dams, is also emerging. One of these dams, which started operat- * Corresponding author. Fax: þ55-65-615-8264. ing at the end of 1999, is now regulating the flux of E-mail address: [email protected] (P. Girard). the Cuiaba´ River, one of the main sources of flood 0022-1694/$ - see front matter q 2003 Published by Elsevier B.V. doi:10.1016/S0022-1694(03)00235-X 58 P. Girard et al. / Journal of Hydrology 283 (2003) 57–66 waters to the Pantanal—the largest river after the is now extensively used for agriculture and cattle Paraguay River itself. ranching. Next, there is the ‘Depression’ at altitudes In spite of the environmental impact study that was ranging from 180 to 250 m. This is a small region with performed before the dam construction, the body of generally steep slopes that is covered by a dense forest knowledge concerning the flood hydrology of the locally called ‘cerradaõ’. The last unit is the Pantanal, Cuiaba´ River in the Pantanal is still small. The most from less than 100 to 180 m altitude; it is about half important study was realized by the Conservation the size of the Plateau. It is a low relief plain with Plan for the Upper Paraguay Basin Project (PCBAP, hydraulic gradient not exceeding 15 cm/km. Many 1997), whereby the available hydrological data were large rivers, such as the Paraguay, the Cuiaba´, the Saõ inventoried and summarized. Hamilton et al. (1996) Lourenço, the Piquiri, the Taquari, and the Negro also studied the seasonality of Pantanal rivers in cross this vast plain. This complex hydrographic relation to flood extent. However there is still no network in conjunction with diverse soil types gives description of how the flooding proceeds and how the rise to a variety of landscapes within the Pantanal. river and groundwater interact. These large sub-units are subjected to different The aims of this paper are to contribute to the hydrological conditions, and different plant commu- hydrological knowledge of a reach of the Cuiaba´ nities characterize them. River located within the Pantanal, to describe water The climate of the Pantanal is marked by a flow between the river main channel and its adjacent pronounced dry season from May to September and flood plain, and relations between the surface water a rainy season from October to April. Mean monthly and the groundwater near the river. temperature near Cuiaba´ City varies between 27.4 8C in December and 21.4 8C in July. Short-term ingressions of polar air masses in winter may cause 2. Study area the temperature to drop as far as 0 8C. Annual rainfall decreases from 1250 mm in the northern Pantanal to The study area is located near a colonial bird 1089 mm in the south. Evapotranspiration, ranging nesting area, locally known as Ninhal Corutuba from 1100 to 1300 mm yearly (Hamilton et al., 1997; (Fig. 1). This Ninhal is on the margin of the Cuiaba´ Ponce, 1995), surpasses precipitation during at least 6 River in the northern portion of the Pantanal. In the months per year. Mean monthly air humidity varies in next paragraphs, the Pantanal, the Cuiaba´ River at the northern Pantanal from 84% during the rainy proximity of the study site, and the Ninhal are briefly season to below 60% at the end of the dry season, described. when the floodplain is dry (Tarifa, 1986). There is an emerging body of literature describing The annual flooding is caused by the sharp gradient the Pantanal (Ada´moli, 1981; Alvarenga et al., 1984; contrast between the Depression and the plain Da Silva, 2000; Da Silva et al., 2001; Hamilton et al., (Carvalho, 1986). Some geomorphological constric- 1996; Junk et al., 2003; PCBAP, 1997; RADAM- tions, namely rock outcrops, along the Paraguay River BRASIL, 1982) from which the following description reduce even more the ability of the river to drain away was assembled. The Pantanal is located along the floodwaters. In addition, local rainfall on the flood- course of the Upper Paraguay River, an important plains drains slowly, enhancing flooding. The Panta- tributary of the Parana´, one of earth’s largest rivers. nal is essentially a huge, gently sloped basin that The Upper Paraguay drains an area close to half a receives runoff from an upland watershed—the million square kilometers of which two thirds are in Plateau—twice its size and slowly releases the flood Brazil, in the states of Mato Grosso and Mato Grosso pulse of those waters through a single, downstream do Sul. According to topographical elevation this channel, the Paraguay River (Ponce, 1995). basin can be subdivided in three physiographic units. The annual flood pulse is mono-modal and presents First, the ‘Planalto’ or ‘Plateau’, 250–750 m a.s.l, is temporal and spatial variations. Along the main river the headwater region. A flat undulating plain con- channels the annual pulse is sharp and well defined, stitutes most of the Plateau and it is covered by a more driven mainly by river overflow. Farther from or less open savanna—locally called ‘cerrado’—that important channels, the flood pulse is more attenuated P. Girard et al. / Journal of Hydrology 283 (2003) 57–66 59 Fig. 1. The Ninhal Corutuba study site. The top RADARSAT image shows the study area during the rising waters (January 1999). The bottom view is during the low waters (October 1999). The insert in top view shows the location of the Brazilian Pantanal, the study site and Cuiaba´ City. The insert in the bottom view is a schematic representation of the study site with the location of staff gages (g) and piezometers (P4–P16). 60 P. Girard et al. / Journal of Hydrology 283 (2003) 57–66 (Penha et al., 1999). As the surface slope along the nesting area. At both sites staff gages were installed. Paraguay is about 3–5 cm/km, and because of the They consisted of four wood posts standing about 1 m rock outcrops mentioned earlier, the flood pulse above the ground level installed in a stairway fashion moves slowly southwards and there is a lag of 4–6 from the top of the left bank levee. The water levels months between the flood peaks in the north and in the were measured from the top of the post with a south. Most of the water enters the northern Pantanal, conventional meter. The accuracy is ^0.001 m. as the three major contributors are the Paraguay, the Standpipe piezometers were installed in both sites. Saõ Lourenço and the Cuiaba´ Rivers. The flood pulse They consisted of PVC tubes ðf ¼ 32 mmÞ that were maintains the biodiversity and health of the Pantanal manually slotted in the lower 0.5 m. The slotted ecosystem (Da Silva and Esteves, 1993; Espindola section was covered with a fine mosquito screen. et al., 1996; Penha et al., 1998, 1999; Resende et al., These instruments were inserted in manually bored 1996; Strussman, 1991). holes ðf ¼ 60 mmÞ: The slotted section was installed The vast fluviolacustrine plain of the Cuiaba´ River within the aquifer sand formation. The slotted section consists of actual alluvium that forms fluvial islands, was coated with local clean quartz sand and covered marginal levees, and bars.

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