The Hydrologic Cycle, or water cycle, consists of the following processes: evaporation, precipitation, groundwater seepage, surface runoff and transpiration. And the Amazon Rainforest is the prime example of the Water Cycle on Earth.

While some areas of the Amazon rainforest, near Iquitos, Peru, for example do not seem to have a very pronounced wet and dry season, most areas of the rainforest do. Dry seasons are typically defined as a month where there is less than 4 inches (10 cm) of rainfall in a given month; in the rainy season there will be between 10-40 inches (25-100 cm) or rainfall in the month (Kricher). The rainy season varies throughout the tropics with respect to the month in which it begins, the duration of the season and the intensity. During the rainy season the days are typically cloudy with intermittent rain throughout the day, the heaviest rains occurring in the late afternoons and evenings. In the dry season the sun can shine for up to 10 hours a day, even though there is usually a shower in the afternoon.

While as we mentioned there may be some areas of the rainforests with less clearly defined rainy and dry seasons, there are definite high and low water periods as the water level is not affected by rainfall alone. The rate at which flooded forests evaporate as well as transpiration rates of plants also account for flooding in rainforests. This creates unique environments for which the flora and fauna of the rainforest have adapted. For example, there are insect eating fish that thrive in the flooded areas of the forest. As the plants and trees become submerged as water levels rise, the insects that inhabit the trees are inundated as well providing a feast for the fish. There are also some fish in the Amazon whose main diet consists of terrestrial fruits and seeds. It is believed they locate their food by sight, smell and the sound it makes when it drops in the water!

In flooded forests there are fewer tree species than in well drained forest areas; the trees in the flooded forest also tend to be a bit shorter. This is due in part to the roots of the trees being almost constantly inundated by water; in some areas for as much as eight months of each year.

Trees flower more in the dry season as the insects tend to be active for longer periods due to less frequent heavy down pours, and, thus, will pollinate many trees. Seedlings pollinated in the dry season will then begin growing in the wet season thereby ensuring adequate water, providing an advantage for growth in an area where competition for scarce nutrients is high.

Insects graze on leaves more heavily in the rainy season because at this time many leaves are still young and have not yet developed chemicals and protective tissues to deter the insects’ feast. Even the birds of the rainforest respond to the conditions of the forest. Studies have shown that some species of birds do not breed in seasons where there are shortages of fruit, and will only breed in seasons where there has been adequate rainfall. Animals of the forest floor such as Giant anteaters (Myrmecophaga tridactyla) adjust their diet to cope with differences between the rainy and dry seasons. The anteaters switch their diets to termites in the dry season as the termites provide more moisture in the anteaters’ diet. Numbers of rodents such as the Paca (Agouti paca) are indirectly affected by the dry season. During the dry season they spend more time living off of their stored fat because foraging for food leaves them more vulnerable to predators.

Obviously all life forms are affected and respond to the seasons of the rainforest. In the Amazon, the forest creates half of its own rainfall. As humans continue to encroach upon the rainforests, it remains to be seen how much of an impact this will have on the dynamics of the rainforests.