The water cycle can be separated into six main processes: evaporation, condensation, precipitation, surface runoff, infiltration, and transpiration.(1)
EVAPORATION: Evaporation is the process in which a liquid is changed into a gas. It is important to realize that this is a physical change and not a chemical change. What this basically means is that the chemical composition of the liquid is NOT changed during this process. Evaporation is an endothermic process. This means that energy must be added for evaporation to occur. Calculation of Gibb's Free energy for this process reveals that evaporation is not a spontaneous process at twenty five degrees Celsius and one atmosphere of pressure.
Now we must consider what affects the boiling point (the boiling point is the temperature and pressure at which evaporation occurs). All gases condense (condensation is the reverse of evaporation) to a liquid under high pressure. This implies that evaporation is more favorable at lower pressures. From our everyday experience, we know that evaporation and condensation depends on temperature as well. Now using the equation for Gibb's free energy, we can deduce that temperature has a greater affect on evaporation than pressure.
CONDENSATION: As previously mentioned, condensation is the reverse of evaporation; it is the process by which water is changed into a gas. This too is a physical change. At a constant pressure, condensation is favored at cooler temperatures.
PRECIPITATION: When the temperature and atmospheric conditions attain a certain value, small droplets of water in clouds from larger droplets and precipitation occurs.(1)
SURFACE RUNOFF: "Much of the water that returns to Earth as precipitation runs off the surface of the land, and flows downhill into streams, rivers, ponds, and lakes."(1) Small streams join rives, which in turn flow into the oceans. (1)
INFILTRATION: Infiltration is a critical process. During infiltration, rain water soaks into the ground, goes through the soil and underlying rock layers. Some of this water eventually returns to the surface, and some of it remains underground. The water that remain underground is called groundwater.(1)
TRANSPIRATION: This last process is very important to the water cycle. Plants absorb water through their roots and eventually release it (they transpire) back into the atmosphere in the form of water vapor. to a lesser extent, transpiration also occurs in mammals through perspiration. Sweat released uses the heat from the body as an energy source and becomes water vapor.
Humidity refers to the amount of water vapor present in the air. It can be described in many ways, including relative humidity. Relative humidity is the amount of water vapor in the air compared to the amount of water vapor needed to make the air saturated at the current air temperature. (2) Recall that saturation implies equilibrium. That is, the maximum amount that can be dissolved or included in the mixture is present. For example. When the maximum amout of sodium chloride (table salt) that can be dissolve in water is present (in the solution), the solution is said to be saturated. Humidy levels can be calculated in several ways.
It is important to note that warmer air can hold more water vapor than colder air. This is consistent with general equilibrium trends- solubility, and thus the amount of solute needed to reach saturation, tends increases with increasing temperature. This means that a change in atmospheric temperature in the ABRE will affect the humidity levels of the ABRE. Because plants and animals (consider amphibians as anexample)rely heavily on humidity for certain physiological functions, it can be concluded that the net effect will include changes in plant and animal health.
The most influential condition in predicting the distinct mixture of organisms that can survive within the ABRE is the climate. (3) Specifically, the amount of precipitation limits the growth of primary producers which in turn support the rest of the ABRE. Although the amount of rainfall is not the same at each specific region in the ABRE, there still exists a minimun amout of moisture needed to sustain life in this ecosystem. This level is generally acceppted as 200cm of rain per year. (4) Temperatures in the forests are around 26 degrees celcius (78.8 degrees farenheit or 299.15 Kelvins). (5) Under these conditions, the humidity level ranges 65 to 96 percent. (6)As mentioned earlier, the temperature and humidity levels affect both the plant vegetation and animal biodiversity of the area.
One of the biggest threats to the humidity of the ABRE is deforestation. Essentially, deforestation destroys the canopy. Because the canopy is no longer there to protect the understory and floor from the sun's radiation, the soil there dries out. In addition, the temperature there also increases. This can have many long term affects. Consider for example a cool breeze coming from the ocean. Once it reaches the area where the canopy was destroyed, it pushes up the warm air. This is a commmon source of tornadoes, however, it is important to note that this condition does not guarantee the formation of tornadoes in the amazon- other factors, which also lead to the formation of tornadoes are not present in the ABRE. Rather, this leads to less cloud formation. Several different models and studies indicate that deforestation will lead to a reduction in average rainfrall and increaed surface temperature (7). This is mainly due to decreases in humidity levels. As Philip Ball mentions in his article "Making Waves", trees are responsible for returning huge amounts of water into the air as water vapor, which then condense to form cloud and rain. With out trees, far less water vapor reaches teh atmosphere, and as a result, there is less rainfall over the ABRE(8). This prediction is proven by a study performed by Willian E. Dietrich, Donald M. Windor, and Thomas Dunne.
Barro Colorado Island in Panama is a hill protruding 137m above Gatun Lake. The climate of this tropical forest is very similar to the climate of the ABRE. The average annual temperature is twenty- seven degrees celcius, and th eyearly rainfall averages aroun 260 centimeters. Barro Colorado also shares many characteristics of the land of the ABRE. Because of these similarity, Barro Colorado Island is a good model for the ABRE. Using mathematical formuls, the average daily solar radiation at teh earth's surface was calculated. What was discovered was that as this value increased, the relative humidity decreased, the average daily temperatue increased, and the evaporation due to mass tranfer decreased (these trend was general). These trends confirms that there exists a great probabily that the predictions made earlier are accurate.
(1) USA Today. Water Basics: Understanding Humidity. http://www.usatoday.com/weather/whumdef.htm
(2) Hall, Jennifer. The Affect of Tropical Farming Practices on the Amazon Rainforest http://www.personal.psu.edu/users/j/m/jmh280/title.html
(3) Ibid
(4) Ibid
(5) Ibid
(6) Hastenrath, 1991 adn Shukla 1990
(7) Dietrich, William E., Donald M. Windsor, Thomas Dunne, A. Sanley Rand, and Willian M. Rand. The Ecology of a Tropical Forest. Geology, Climate, and Hydrology of Barro Colorado Island, Variation in Rainfall of Barro Colorado Island. Leigh, Egbert G., A. Stanley Rand and Donald M, Windsor. The Smithsonian Institte, 1996.
(8) The Water Cycle. Copyright 1995-1998 by The Evergreen http://mbgnet.mobot.org/fresh/cycle/index.htm