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NOTE ON THE AMAZON REGION AND DROUGHTS IN BRAZIL’S SOUTHEAST ¹

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Some claim that deforestation will turn the Amazon into a desert.According to the definition given by the World Meteorological Organization, a desert is a region in which it rains less than 200 mm per year. Therefore, with an average precipitation of 2,400 mm per year, the Amazon would never be classified as a desert. The Atlantic Ocean is the source of water vapor (moisture) for its total rainfall. By calculating the water input and output, one sees that trade winds transport into the region, on average, approximately 1 million cubic meters per second (m3/s) of water in the form of water vapor, of which only 40% fall as rain, while the remaining 60% pass directly over the region.

Regional water balance studies show that 50% of the total rainfall goes out by the river, and the other 50% goes to the atmosphere through forest evapotranspiration. Half of the precipitation, which corresponds to 20% of the input flow, is returned to the Atlantic Ocean by the Amazon River (200,000 m3/s) and the other half (20%) is recycled by evapotranspiration and incorporated into the 60% making up 80% of the input flow that is transported and converted into rain by frontal systems or cold fronts in the rest of Brazil.

For 24 months in the 1983-1985 period, during the Amazon Region Micrometeorological Experiment Project (ARME)(*), we estimated forest evapotranspiration and obtained an annual average of 3.4 mm/day. Knowing that 1 mm corresponds to a volume of 1 liter/m2, if we multiply that by 5 million square kilometers, the area covered by the forest [(3.4 liters/m2/day * 5 * 1012m2)/8,64 * 104s/day] we will obtain an average evapotranspiration flow of approximately 200,000 m3/s. Interestingly, this number corresponds to 50% of the rainfall recycled by evapotranspiration and to 20% of the flow into the region.

The forest, a tree, does not produce water but only recycles the rainwater that fell previously and was stored in the soil. Therefore, deforestation does not affect rainfall in the rest of Brazil. In the year 2014, in particular, the annual moisture flow coming from the North region was the largest in the 1999-2014 period. In other words, the drought in the southeast did not occur due to a lack of atmospheric humidity but because of the persistent high atmospheric pressure system and associated thermal inversion, both of which inhibited the formation of rain.

However, it is obvious that large-scale deforestation would be highly harmful to the region, particularly with regard to biodiversity and environmental degradation. Surely, there are thousands of unknown species living in the region, and the large amount of rainfall in general causes erosion, soil degradation, and the silting of riverbeds, changing the quality of water and the life that depends on it. This, not the climate, is the reason why protecting the soils in the region is of utmost importance. But all this is well known and documented. And the native forest cover is a harmonious, beautiful way of protecting the tropical soils, although there are other, also effective techniques.

In summary

– Humidity input flow = 1 million m3/s

– Average discharge of the Amazon River = 200,000 m3/s (20% of the input flow)

– Average total evapotranspiration or water recycling by the forest = 200,000 m3/s (20% of the input flow).

– Output flow = 800,000 m3/s (20% of the flow recycled by evapotranspiration + 60% of the input stream that passes directly over the forest).

– (*) ARME = Amazon Region Micrometeorological Experiment, INPE/MCTI and Institute of Hydrology, UK

[1] 

https://panamazonsynodwatch.info/articles/commented-news/amazon-deforestation-and-droughts-in-the-southeast-of-brazil/

 

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