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
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.