The Water Cycle

The water cycle, also called the hydrologic cycle, operates similarly to biogeochemical cycles in which plant and animal nutrients move through the atmosphere, the earth, and through living things. The water cycle includes water in the forms of solid, liquid, and gas, and the Sun provides the energy needed to power the constant cycle of water among these three forms. The water cycle occurs in oceans, bays, lakes, ponds, rivers, and streams. The ocean covers most of the Earth’s surface and so contributes the largest share to the water cycle; more than 85 percent of the water that evaporates from the Earth’s surface into the atmosphere does so from the ocean. About 80 percent of the world’s precipitation enters oceans.

The water cycle contains five components that transfer moisture either from the Earth’s surface to the atmosphere or from the atmosphere back to Earth. These components are the following:

• condensation—conversion of water vapor into droplets of liquid water

• precipitation—water that falls from the atmosphere to land or to surface waters as rain, snow, sleet, or hail

• infiltration—downward movement of liquid water through soil

• evaporation—conversion of liquid water into gas, or water vapor

• transpiration—movement of liquid water from plant roots, upward in vessels, and into the atmosphere from the leaves as water vapor

Forests serve in the water cycle in two critical ways: transpiration and water storage. Both animals and plants transpire water vapor into the air, but animals transpire as part of aerobic respiration when they exhale moisture from their lungs. Plants and trees possess special cells on the underside of their leaves, called guard cells, that release water vapor from the plant into the atmosphere. The transpiration process begins when trees draw water from the soil through their roots and transport it upward in vessels—xylem carries water and nutrients upward, phloem distributes water and nutrients throughout the plant. Trees, depending on size and species, transpire from 5,000 gallons (18,921 l) to almost 50,000 gallons (189,210 l) of water per year, more in warmer weather and less in colder weather. Low relative humidity and increased air movement caused by wind or breezes also increase the transpiration rate. During drought or in the desert where only drought-tolerant plant life lives, a tree’s transpiration rate decreases so that it can conserve water. Soil also affects water transpiration. When soil moisture levels are low, as in drought, trees slow their transpiration rate to conserve water. Trees undergoing the normal aging process, called senescence, give way to new trees ready to take their place. Prolonged drought, however, puts all trees into a premature senescence, an event that may eventually kill a forest. Global warming has exerted a critical effect on forest health because it increases the incidence and severity of droughts worldwide, and in doing so it accelerates the death of drought-vulnerable trees.

Forests’ second major activity involves the capacity of trees to store water. Trees act as a watershed by absorbing water during floods and storing and slowly releasing water in times of low rainfall. Surface waters, groundwaters (or aquifers), and plant life comprise the Earth’s total watershed. Part of trees’ role in the watershed involves regulation of the water table, which is the area underground where water has completely filled the spaces between rocks and soil particles. Beneath the water table lies an area of higher density where all the air has been squeezed out to make room for water. This location, called the zone of saturation, holds water undisturbed for longer periods than the water table. Therefore, water moves through three layers in the earth: the upper unsaturated zone, where soils hold varying amounts of moisture (also called the soil zone); the water table, where water exchanges from the saturated layer below to the unsaturated layer above; and the zone of saturation. Tree roots pull water from either the unsaturated zone or the water table.

Human activities interfere with the water cycle in the three following ways: by drawing large amounts from surface sources and groundwaters, by polluting water, and by removing or damaging the world’s forests. Part of the problem of water use by the Earth’s human population resides in the fact that water is a very poorly managed resource. Water covers 71 percent of the Earth and makes up at least 60 percent of living cells, but only 0.014 percent is available for people’s use. Yet humans continue to waste water, pollute it, or otherwise treat it as if it were free. Benjamin Franklin once said, “When the well’s dry, we know the worth of water.” The world has been progressing toward ever greater water shortages since the acceleration of global warming. Some countries have already entered a dangerous condition known as water stress, meaning their water requirements exceed the water they have available.

Since every living thing needs water and cannot exist without it for more than a few days, maintaining a clean supply is paramount to maintaining biodiversity. Forests play a vital role in maintaining a continuous supply of available water, so this represents one way in which forests maintain biodiversity. As the next section and the sidebar “The Forest Canopy” point out, forests add to the world’s biodiversity in other immeasurable ways.


The Sun’s energy powers the water cycle, which is critical for conserving Earth’s water. Within this cycle, oceans account for more than 80 percent of evaporation, and more than 80 percent of rainfall lands in the oceans.


Groundwater serves as a major source of drinking water in most of the world, but high water demand due to growing populations draws down the reserves, lowering the water table. Pollution further threatens groundwater reserves. Metals, pesticides, and organic chemicals have infiltrated many once pristine aquifers.

Source of Information : Green Technology Conservation Protecting Our Plant Resources