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The La Plata basin is basically composed of three large sub-basins, corresponding to the Parana, Paraguay and Uruguay rivers. The Paraguay river flows into the Parana river and when the Parana river and the Uruguay river join it becomes the La Plata river near Buenos Aires, flowing into the Atlantic Ocean:
- The Parana river sub-basin: The Parana river has its source at the confluence of the Paranaiba and Grande rivers in southeast Brazil. It has a total length of 2 570 km from the confluence onwards, or 3 740 km if the Paranaiba river upstream of the confluence is also counted. The Parana flows mainly through the plains of Paraguay and Argentina before joining the Uruguay river. The Parana's main tributary is the Paraguay river, which is described as a separate sub-basin below. The basin has a total area of 1.5 million km² (without the basin of the Paraguay river). The poor natural drainage of the region through which the Paraguay river flows has created the Pantanal wetlands. The Parana river is usually divided into Upper Parana, from its source up to the city of Corrientes in Argentina (close to the border with Paraguay), and Low Parana, from Corrientes up to its outlet in the La Plata river. In the Upper sub-basin, characterized by a quick runoff, it receives the contribution of numerous tributaries, among which the Paranapanema, the Iguazu and the Tietê rivers. The main tributaries of the Parana river flowing from Paraguay are the Acaray, Monday, Piratiy and Carapa river. In the Low Parana the course extends with a very slight decline to the La Plata river. This sector presents very low coasts, especially on its right bank, so that the very high discharges cause severe floods. Stretches of the river mark the boundary between Paraguay and Brazil, and between Paraguay and Argentina (Barros, Clarke and Silva Días, 2006; CIC, 2016; VAMOS, 2001).
- The Paraguay river sub-basin: The Paraguay river has its source in the central region of the Brazilian State of Mato Grosso, not far from the Plurinational State of Bolivia, on the oriental slope of the "Chapada do Parecis" and ends in the Parana river in the south of Paraguay at the border with Argentina, running along 2 550 km. Its basin area is 1.1 million km². The sub-basin of the Paraguay river is mainly in a great plain and, with few exceptions, has a small and uniform slope. Its elevation rarely exceeds 70 m above sea level. The sub-basin can be subdivided in four areas: Pantanal, High Paraguay, Middle Paraguay and Low Paraguay. The main tributaries of the Paraguay river are Pilcomayo river, originating in the Plurinational State of Bolivia and forming the border between Paraguay and Argentina, and Bermejo river, flowing through Argentina and reaching the Paraguay river not far upstream from the point where the Paraguay river flows into the Parana river. Other important tributaries of the Parana river are the Cuiaba, the Apa, Aquidaban, Ypane, Jejui, Tebicuary, Verde, Monte Lindo and Confuso rivers (Barros, Clarke and Silva Días, 2006; CIC, 2016).
- The Uruguay river sub-basin: The Uruguay river has its source in the State of Santa Catarina in Brazil in the convergence of the Pelotas and Canoas rivers, which have their origin in the Serra do Mar and Geral, at almost 2 000 m above sea level. The river has a total length of 1 600 km (Flinker, 2012) and the basin has a total area of 0.4 million km². The hydrological regime of the Uruguay river is very irregular due to irregular precipitation. The Uruguay river is the only large river of the La Plata system that is not a tributary of the Parana. Two of the tributaries of Uruguay river, the Cuareim river and the Pepiri-Guazu river, form a portion of the international border between Brazil and Uruguay and between Brazil and Argentina respectively. After receiving the flow of the Pepiri-Guazu river, the Uruguay river forms the border between Argentina and Brazil until its convergence with the Cuareim river where it becomes the border between Argentina and Uruguay. After the confluence with Negro river, its major tributary with 850 km and 71 000 km², the Uruguay river becomes wider and an extension of the Plata river. (Barros, Clarke and Silva Días, 2006; CIC, 2016; VAMOS, 2001). Other important tributaries of the Uruguay river are the Chapecó (248 km) and Ibicuí (290 km) in Brazil, the Aguapey (310 km) and Gualeguaychu (268 km) in Argentina and the Arapey (240 km) and Queguay (280 km) in Uruguay.
- The La Plata river sub-basin: The La Plata river is formed by the confluence of the Uruguay and the Parana rivers. The river is about 290 km long, and it widens from about 2 km at the confluence to about 250 km at its mouth between San Antonio cape in Argentina and Santa Maria cape in Uruguay, where it empties into the Atlantic Ocean. La Plata river sub-basin covers approximately 0.1 km². It forms part of the border between Argentina and Uruguay, with the major ports and capital cities of Buenos Aires and Montevideo on its western and northern shores, respectively. The coasts of La Plata are the most densely populated areas of Argentina and Uruguay.
Information on the average discharge of the La Plata river into the Atlantic Ocean varies from 23 000 to 28 000 m³/s. Table 3 below shows the contribution by country and by sub-basin to the annual discharge of the La Plata basin.
The contribution of Argentina to the La Plata basin is estimated by AQUASTAT at 115.98 km³, of which to the Paraguay river sub-basin 22.74 km³, to the Parana river sub-basin 77.00 km³ and to the Uruguay river sub-basin 16.24 km³. Inflow in Argentina from other countries in the La Plata basin accounts for 513.13 km³, of which 10.10 km³/year from the Plurinational State of Bolivia (Bermejo river in the Paraguay river sub-basin), 377.45 km³/year from Brazil (326.40 km³/year through Parana river and 51.05 km³/year through Iguazu river in the Parana river sub-basin); 125.58 km³/year from Paraguay (73.27 km³/year through Paraguay river and 52.31 km³/year through Parana river). The accounted flow of border rivers is estimated at 67.96 km³/year, of which 2.96 km³ (5.92/2) correspond to Pilcomayo river in the Paraguay river sub-basin, border river with Argentina, and 65.00 km³ (130.00/2) correspond to Uruguay river, border river with Brazil. The outflow to Brazil is estimated at 50 percent of the 20.00 km³ of the flow of Uruguay river generated in Argentina.
The contribution of the Plurinational State of Bolivia to the La Plata basin is only through the Paraguay river sub-basin with 16.02 km³/year, of which 10.10 km³ flow to Argentina through Bermejo river and 5.92 km³ to Argentina through Pilcomayo river, which comes from the Plurinational State of Bolivia to become the border between Argentina and Paraguay. Paraguay river borders the Plurinational State of Bolivia for about 35 km, but is not accounted for since such a short distance.
The contribution of Brazil to the La Plata basin is as follows: to the Paraguay river sub-basin 74.40 km³, to the Parana river sub-basin is 359.90 km³ and to the Uruguay river sub-basin 129.40 km³. The outflow from Brazil to other countries in the basin is: 70.00 km³/year to Uruguay (Negro river in the Uruguay river sub-basin and Uruguay river, which comes from Brazil to become the border between Argentina and Uruguay), 442.45 km³/year to Argentina (Iguazu river in the Parana river sub-basin, Parana river and Uruguay river, which comes from Brazil to become the border between Argentina and Uruguay), 73.27 km³/year to Paraguay (Paraguay river). The Paraguay river does border Plurinational State of Bolivia only over a very short distance (35 km) and therefore is not counted as flowing from Brazil to the Plurinational State of Bolivia.
The contribution of Paraguay to the La Plata basin is 117.00 km³, of which to the Paraguay river sub-basin 76.05 km³ and to the Parana river sub-basin 40.95 km³. The inflow from other countries in Paraguay is estimated at 73.27 km³/year, which corresponds to Paraguay river from Brazil. The accounted flow of border rivers accounts for 197.53 km³: (i) 50 percent of the total flow of Parana river (border between Paraguay and Brazil and between Paraguay and Argentina): 163.20 km³; (ii) 50 percent of Iguazu river in the Parana river sub-basin, at the confluence with Parana river: 25.53 km³; (iii) 50 percent of Pilcomayo river in the Paraguay river sub-basin, border river with Argentina: 2.96 km³; (iv) 50 percent of Bermejo river in the Paraguay river sub-basin, at the confluence with Paraguay river: 5.84 km³. The outflow to Argentina is estimated at 125.58 km³/year, of which 52.31 km³/year through Parana river and 73.27 km³/year through Paraguay river.
The contribution of Uruguay to the La Plata basin is 70.90 km³, of which Uruguay river accounts for 21.90 km³, Negro river in the Uruguay river sub-basin for 40.30 km³, Santa Lucia river in the Uruguay river sub-basin for 5.50 km³ and La Plata river for 3.20 km³. The inflow from other countries in Uruguay is estimated at 80.00 km³, of which Negro river from Brazil accounts for 5.00 km³, and Uruguay river accounts for 75 km³ or 50 percent of the total flow of 150 km³ (border river with Argentina).
In the upper Parana river and Paraguay river sub-basins, the rainy season occurs during summer. In the Uruguay river sub-basin, on the other hand, high-flow season occurs during winter (VAMOS, 2001).
The basin also encompasses the Guarani aquifer, one of the world’s largest groundwater reservoirs, with an estimated area of 1.2 million km², the majority beneath Brazilian territory (71 percent of the total area of the aquifer), followed by Argentina (20 percent), Paraguay (6 percent) and Uruguay (3 percent). The aquifer’s depth varies between 50 m and 1.800 m. The total volume of the aquifer is estimated at 45 000 km³. However, the exploitable volume corresponds to the natural recharge calculated at approximately 166 km³/year (CIC, 2016).
Another important groundwater system is the Yrenda-Toba-Tarijeño (SAYTT) Aquifer System, entirely contained within La Plata basin in the semiarid Chaco of Argentina, the Plurinational State of Bolivia and Paraguay. The annual recharge for this aquifer in the territory of Paraguay is 2 460 km³ (Monte Domecq y Báez Benítez, 2007).
The Patiño aquifer in the central area of Paraguay is also an important aquifer in the basin. However, it is overexploited. According to the 2005 water balance, the aquifer has an annual recharge of 0.175 km³ while total annual withdrawal is 0.249 km³ (Monte Domecq y Báez Benítez, 2007).
In Uruguay, besides the Guarani aquifer, the main aquifers in the La Plata basin are Raigon, Salto and Mercedes. The Raigon aquifer, with a total area of 1 800 km², is located in the Santa Lucia river basin and is strategic due to its use for water supply for Montevideo and for irrigation and livestock. The Salto aquifer, in the north-west of the country, is mainly used for irrigation of vegetables. The Mercedes aquifer, of approximately 20 000 km2, is located to the west of the Uruguay river and is mainly used for water supply.