Facts of the Matter
NEW YORK TIMES
Earth is the ony place in the universe that we know of that has the right conditions for water to exist as liquid, solid and gas in the same place at the same time.
Hydrologic cycle is feature of Earth alone
EARTH IS a closed system as far as matter is concerned. The planet receives energy from the sun, but there is very little material gained or lost.
Small amounts of cosmic debris rain down on Earth daily. In total amount it might seem like a lot, but in comparison with what is already here, it is insignificant. Likewise, we lose small amounts of our atmosphere, mostly the light gases hydrogen and helium. Their thermal motion is fast enough to exceed Earth's escape velocity in the upper atmosphere, but again, the losses are insignificant.
Our Earth is the ultimate recycler. Every atom on Earth is recycled, to be used over and over again by some process, whether biological or physical.
For each type of atom there is one or more cycles that involve any number of chemical, biological and physical processes. In most cases the cycles overlap and interact in complex webs that make it impossible to predict where any particular atom will go or in which cycle it might find itself.
A cycle that is central to all of the others is the hydrologic cycle.
Water is the singular most characteristic physical feature of our planet. It is a fascinating substance in its own right, having many unusual properties that make it a necessity for biological processes.
Earth is the only place in the universe that we know of that has the right conditions of temperature and pressure for water to exist in all three physical states (gas, liquid and solid) in the same place at the same time.
It is this unique combination of the properties of water, the location of Earth's orbit and its atmospheric pressure that make the hydrologic cycle possible.
Without water there can be no carbon-based life, and without the hydrologic cycle water would not be available for the biosphere.
At least one molecule of water in each of our bodies has been part of most of the countless billions of organisms that have ever existed, from the lowliest single-celled creatures to dinosaurs. You and I contain many molecules that were in the bodies of Aristotle, Julius Caesar, Isaac Newton and Albert Einstein.
The total amount of water on Earth is in the neighborhood of 330 million cubic miles, enough to cover the entire planet to a depth of 7,000 feet if the earth was smooth like a billiard ball.
When we speak of any cycling of material on a planetary scale, there are two aspects to consider. One is the reservoirs where it is stored; the other is the processes that move it from one reservoir to another.
Earth's water is stored in the ocean, as ice, as surface water in lakes and rivers, as ground water beneath the surface, in the atmosphere as water vapor and in the living organisms of the biosphere.
More than 97 percent of all water on Earth is in the oceans, and just more than 2 percent is frozen in ice and snow, on the average.
That leaves just less than 1 percent for all of the other reservoirs, of which about two-thirds of 1 percent is stored underground in soil and pore spaces of rocks as ground water.
Those three reservoirs together account for more than 99.99 percent of all water on Earth.
Of that remaining one one-hundredth of a percent, most is in freshwater lakes, rivers and streams (90.3 percent), and the atmosphere (9.3 percent). The rest, an almost infinitesimal amount, is stored in living organisms.
The processes that move water between the reservoirs are quite simple, although the interactions between them are quite complex and are responsible for virtually all meteorological and surface geological processes.
WATER EVAPORATES from land and water surfaces and is carried upward by rising air currents, where it condenses into clouds. From there it can evaporate again or fall as precipitation in the form of rain, snow, sleet or hail.
That which falls on land either evaporates, runs off in rivers and streams, is stored temporarily in lakes or soaks into the ground to join the ground-water system. A small portion of it enters the biosphere, but most of that is given back to the atmosphere by evaporation or transpiration.
Ground water eventually makes its way back to the sea through slow infiltration through cracks and pores in rocks or is used by growing plants.
The ground-water system also interacts with the surface-water system where the surface dips below the local water table.
Most rivers receive at least as much of their flow from seepage through the ground as from flow over the ground. In rare cases the water table is so far below a stream or river that the water in them soaks into the ground, and in desert areas streams can disappear or "sink" into the ground.
The size of the reservoir in no way is a measure of its importance. Each plays unique and important roles in various ways.
Surface water is the chief agent of erosion, transportation and deposition of sediments, which can affect our lives on a human time scale and can become hardened into sedimentary rocks over geologic time.
Soil moisture is essential to plant life, and ground water is a vital resource to many areas of the United States and most of the world. Half of the people in the United States get their drinking water from the ground.
Surface and ground water are intricately linked to the transport and cycling of chemical nutrients (such as nitrogen, phosphorous and carbon), and they determine to a great extent what kinds of human activities and their intensities a particular location can support.
Human activities such as water resource exploitation, urbanization and deforestation also affect both the distribution and processes of the hydrologic cycle.
For example, agricultural and industrial uses withdraw about 87 percent of the world's freshwater resources.
Irrigation consumes fresh water in the sense that it can not be reused directly because it evaporates or soaks into the ground. Only a very small fraction of the water used for irrigation actually makes it into plants that are ultimately consumed.
Much industrial water is put back into the system but is laden with chemicals or sediments that either render it unsuitable for human consumption or drastically increase the cost of making it potable.
Although water is plentiful on Earth, it is still a valuable and scarce resource. Like all of Earth's resources, it and its cycling system need to be carefully managed as the planet's population and its standard of living increase.
Richard Brill picks up
where your high school science teacher left off. He is a professor of science
at Honolulu Community College, where he teaches earth and physical
science and investigates life and the universe.
He can be contacted by e-mail at firstname.lastname@example.org