Three hundred twenty-six million trillion gallons – that is the total estimated amount of water on earth, taking into account water in various forms as it moves along the hydrological cycle. This number represents the most water the earth has ever had and as much as it will ever have.
In this section...
The Water Cycle
At any given time…
- Ninety-seven percent of the world’s water is seawater in oceans; the remaining 3% is water vapor in the air and freshwater on land.
- Of this freshwater, two-thirds is locked up in ice caps and glaciers. One-third is in liquid form either underground (in aquifers and rocks) or above-ground in lakes, soil, wetlands, rivers, and living organisms.
- This means that only 1% of the earth’s water is in a form usable by humans. Only .4% is easily accessible in the form of lakes and rivers.
Water moves through a hydrological cycle, constantly changing between liquid, gas, and solid forms:
- Water in the oceans is heated by the sun and evaporates into the air where it is cooled. It then condenses into precipitation in the form of snow or rain.
- Some precipitation that falls on land ends up as surface water stored in lakes, rivers, and streams, either directly or through run-off. Other precipitation is absorbed and stored at various depths under the ground as subsurface water. Both are considered freshwater. It is at this stage of the cycle that water is extracted by humans for a variety of uses.
- Water that falls as precipitation may be put into long-term storage in the form of glaciers, ice caps, and general snowpack. When these melt, they recharge freshwater stores. Freshwater may also be stored deep in the ground in rocks or aquifers where it is slowly released naturally or accessed with greater difficulty by humans.
- Ultimately, the water that falls on land either finds its way back to the ocean through a network of above ground and underground channels, or it returns to the air as it evaporates from lakes and rivers or through plant transpiration.
- The cycle repeats.
Human Impacts on the Water Cycle
Some water moves through the cycle relatively quickly; some water enters deep storage as ice or snow or seeps deep into the ground where it remains for tens of thousands of years.
Human activity impacts the movement of water through the cycle in numerous ways. Some examples include:
- Carbon dioxide emissions that alter air and ocean temperatures, affecting currents, evaporation rates, and weather patterns. Global warming of surface water intensifies the effect of contaminants.
- Dams that alter the flow of waterways.
- Concrete roads and buildings that impede water absorption and underground flows.
- Wells and boreholes that deplete wells at rates higher than they can naturally be recharged, and that tap into ancient aquifers that can never be recharged by precipitation.
- Destruction of plant ecosystems responsible for the health of the water cycle.
- Pollution that impacts water flows and usability.
When we talk about water, we are really talking about freshwater. Freshwater is the stuff of life for earth’s creatures (even sea life is dependent on freshwater as runoff or precipitation to balance out the salt content of water in the ocean).
Freshwater is subdivided by experts into a number of categories:
- Blue water moves above and below ground on its way back to the ocean. This is the primary source of water for human and animal life on earth.
- Green water is soil moisture that is absorbed by plants. It is consumed by humans in the form of crops containing water.
- White water is evaporated water in the air. It is not usable by humans.
- Grey water is wastewater, usually of poor quality but usable by humans.
- Black water is polluted water of such poor quality that it is considered unusable by humans.
Thus, included in the total figures for freshwater is water that is unusable by humans – either because of its form or because it has been degraded by human activity.
Freshwater supplies are unevenly distributed on the planet, with only six countries holding up to half the world’s supply of renewable freshwater (blue water) – Brazil (which significantly outstrips other countries), Canada, Russia, Indonesia, China, and the United States.
Next: A Framework for Understanding: Water Consumption