Water Avaliability and Use

The total water potential of the region can be represented by the sum of the mean discharge rates of the various tributaries, estimated in 101,441 m³/s. The specific mean discharge rate for the region is 26 l/s/km² in the Brazilian section, varying widely in the sub-basins, with values between 14 and 52 l/s.km² (Figure 2).

The current situation of the Hydrographic Region of the Amazon as regards availability and demand of water is shown in Table 3.

Figure 2 Specific discharge rates by sub-basin

The principal water demand in the region is for irrigation, followed by human and livestock use. (Figure 3).

Figure 3 Distribution of demand

The highest water usage occurs in the sub-basins of the Madeira, Tapajós and Xingu rivers, and is primarily for water for irrigation purposes (91.5 % of the total demand) (Figure 4). The human demand represents just 4.1%. Generally, the estimated levels of consumption are relatively insignificant given the water availability in the respective sub-basins. In fact, the highest demand, which occurs in the Tapajós/Juruena/Teles Pires basin, represents only 0.67% of the average availability. As such, no water deficits have been identified in the region, at this level of analysis.

An overview of the main consumption types for water use shows: estimated human public water use: 8.44 m³/s, which represents only 4,1% of total demand. Approximately 45 % of the population of the basin is concentrated in the sub-basins of the Negro and Madeira rivers. The city of Manaus alone has a population of 1,405,835 inhabitants; Industrial use: Total demand for industrial use is 1.66 m³/s. Although relatively insignificant in a national context, industry is responsible for around 5% of the regional GDP, thanks to fiscal incentives. The specialization of some centers to cater to exports markets is particularly noteworthy. With the exception of Santarém and Manaus, that produce goods for the local market, all the others are specialized; Irrigation: Demand for water for irrigation purposes, amounting to 190.34 m³/s, represents 91.5 % of the total water demand. The majority of this consumption is for the irrigation of rice fields, concentrated in the sub-basins of the Madeira, Tapajós and Xingu rivers. Of the three million hectares of cultivated lands in the Amazon basin, approximately 90% are in these sub-basins; Livestock: Water consumption for livestock is 7.61 m³/s, corresponding to 3.7 % of the total demand.

Figure 4 Distribution of demand by sub-basin.

As regards underground water, in the northern and southern regions of the basin there are igneous rocks, which are covered by a thick layer of deposits left from erosion, with intermittent aquifers in which the volume and circulation of water depends on the extent and depth of the low permeability layers in the rock. The productivity of the wells in this region is extremely variable, averaging around 10,000 l/hr.

The alluvial and fluviomarine sediment deposits are widely distributed throughout the area, particularly in the central and eastern sections. They form porous aquifers that are exploited mainly in the towns of Manaus, Belém, Santarém and the Island of Marajó, through piped wells, borehole systems and Amazon wells. The yield of these aquifers is extremely variable (10,000 to 100,000 l/hr) and the water often needs to be treated to correct the pH-level and the high iron concentration.

The main non-consumption use is Power Generation: The Amazon basin has considerable hydroelectric power generating potential that has yet to be explored, estimated in 485,426.8 gWh. Of this amount, only 0.46% is currently in operation or under construction and 32.8% is inventoried or with feasibility studies in progress. The main hydroelectric power plants in operation in the basin are those of Samuel (RO), Balbina (AM), Curuá-Uma (PA).

Considering the estimated hydroelectric power generating potential in the whole of Brazil (1,138,288 gWh), the Amazon basin represents 42.6% of this total. Despite its great potential, factors such as the wide dispersion of the urban centers in the basin, the great distances between the areas of high power generation potential and the main consumer centers in the other regions of the country, in addition to the environmental liability resulting from the flooded areas, mean that the region’s electricity is, at present, supplied by thermoelectric power.

Of a total of 377 power generating units that exist in the states that are totally or partially in the Hydrographic Region of the Amazon River, 331 are thermoelectric power plants, 37 are small hydroelectric units and only 9 are hydroelectric power plants.

In general terms, the Amazon River is highly navigable, with low declivities between 2 to 3 cm/km. This characteristic, typical of rivers in plains, with reduced declivity and low flow speeds, extends to many of its main tributaries, such as the Purus, Madeira Icá and Japurá rivers. Others, such as the Tapajós, Xingu and Trombetas, have the characteristics of plateau rivers.

The concentration of decomposable organic matter in the region is 277 tons BOD₅/day, which represents 3.7% of the national total.

Brazil has a network of approximately 40,000 km of permanently navigable waterways, of which 26,000 km are already barely navigable. Just in the Amazon basin, there are 25,000 km of waterways, representing over 60% of the country’s navigable waterway network. The close relationship between the size of the Amazon basin and the wealth of its hydrographical network becomes more evident when extreme hydrological events occur. In this context, droughts and low discharge rates during dry seasons compromise the navigability of some stretches of the waterway network. In the rainy season, vast areas of meadows become flooded, influencing the dynamics of diverse species of fauna and flora, with high environmental relevance and contributing to the conservation of the biodiversity of the Amazon basin.

Close to Manaus, the mean annual amplitude of the Amazon river is around 10 meters. The maximum annual quota is in the order of 29 m (May and June) and the average minimum quota around 19 m (October and November) The amplitudes diminish towards the estuary, given the greater regularity of the discharges, the vast floodplains and the larger drainage areas.