The Amazon Basin is the world's largest drainage system. In fact, 1,100 rivers make up the Amazon system. The Amazon River carries one-fifth of all the river water in the world. The source of the Amazon can be traced to the Apurimac River, located at 5,200m above sea level. The 1,100 tributaries flow through nine South American countries: Brazil, Bolivia, Peru, Ecuador, Colombia, Venezuela, Guyana, Surinam, and French Guiana. The Amazon River and its tributaries drain most of the area of heavy rainfall. The area is called "Amazonia" and most of it is a sparsely populated rainforest. Most of the Amazon Basin is in Brazil (Amazonia Legal). The Amazon River represents 16% of annual global river runoff (Shiklomanov, 2001).

The Amazon River system is the single largest source of freshwater on Earth and its flow regime is subject to interannual and long-term variability represented as large variations in downstream hydrographs (Richey et al., 1989; Vorosmarty et al., 1996; Marengo et al., 1998a; Marengo and Nobre, 2001; Marengo 2004a,b). A better understanding of rainfall and river variability will depend on the physical mechanisms related to regional and large-scale atmospheric-oceanic-biospheric forcings that impact the temporal and spatial variability of the hydrometeorology of the Amazon Basin. The impacts could be felt on various timescales.

The implementation of field experiments in the region during the last 20 years— such as the ABRACOS (Anglo Brazilian Amazon Climate Observational Study) during the 1980s, the LBA (Large Scale Biosphere Atmosphere experiment in Amazonia), and the SALLJEX (South American Low Level Jet field experiment) during the late 1990s and early 2000s—has allowed for the development of new knowledge on climate and hydrology in the Amazon Basin, including the interaction between land surface processes in rainfall, and the development of regional and global climate models tuned with more realistic representations of physical processes for the region (Gash and Nobre, 1997; Silva Dias et al., 2002; Vera et al., 2006). Moisture transport into and out of the Amazon Basin has also been studied, and regional circulation features responsible for this transport and its variability in time and space have been detected and studied using observations from these field experiments and other global data sets (Marengo et al., 2002, 2004a, b; Vera et al., 2006).

On the basis of what is now known on climate variability in Amazonia and the moisture transport in and out of the basin based on observational studies and model simulations, the question that arises is: What are the possible impacts on the Amazon ecosystem of regional-scale deforestation or the increase of greenhouse gas (GHG) concentrations in the atmosphere and subsequent global warming. The issue of deforestation has been explored in various numerical experiments since the 1980s using atmospheric global climate models—general circulation models (GCMs)—all of which show that the Amazon will become drier and warmer (see reviews in Marengo and Nobre, 2001; Voldoire and Royer, 2004). Even though there are no clear signs of trends for reduction of rainfall in the basin due to deforestation—as suggested by climate models on deforestation scenarios—one study (Costa et al., 2003) has detected changes in the Tocantins River discharges as a result of land-use changes in its upper basin following the construction of the city of Brasilia in the 1960s.

Furthermore, since the early 2000s new developments in atmosphere-ocean-biosphere coupled models—by the Hadley Centre for Climate Research and Prediction in the U.K., the Institute Pierre et Simon Laplace at the University of Paris in France, and the Frontier Research Center for Global Change in Japan—have allowed for better simulation of future climate change scenarios. The new models include interactive vegetation schemes that more realistically represent the water vapor, carbon, and other gas exchange between the vegetation and the atmosphere. Projections for future climate change from the Hadley Centre model have shown that an increase in the concentration of greenhouse gases in the atmosphere will produce changes in vegetation such that Amazonia will become a savanna by the 2050s, and the region will become drier and warmer with most of the moisture coming from the tropical Atlantic such that normally produced rainfall in the region will not find the environment to condense above the savanna vegetation by 2050, and the moist air stream will move to southeastern South America producing more rainfall in those regions. Therefore, after 2050, the Amazon Basin may well behave as a "source of moisture" rather than a sink (like its present day climate) (Cox et al., 2000,2004; Betts et al., 2004; Huntingford et al., 2004).

Therefore, this chapter is focused on the role of the Amazon Basin in the functioning and modulation of the regional climate and hydrology, in both present and future climates, by means of (a) description of hydrological regimes and maintenance of humidity and import/export of moisture, (b) variability of climate and hydrology in various timescales; and (c) sensitivity of the Amazon system to changes in land use or climate change due to an increase in the concentration of GHG in the atmosphere.

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