The Hydrologic Cycle: The Hydrologic cycle is also known as the water cycle. It's another example of how Earth recycles its resources constantly. There is a limited amount of freshwater on the surface, and beneath it, and it continually goes through a cycle which purifies and restores this valuable resource. The water here on Earth is all about 4.6 billion years old, formed just about when the rest of the Earth formed. We like to think that pure mountain streams and deep wells have been saving their water all of these years just for us, but, it's simply not true. The water that we use each day, has been used by countless creatures and living things in the past. The water only stays in a living organism for a limited time. When the organism is done with the water it is "returned to the environment", (seems like, usually during Earth Science class!) This water proceeds through the hydrologic cycle, and ends up fresh and clean, ready to be used by someone, or something, again. Think of it! That glass of water that you just drank may have been previously consumed in the past by a dinosaur, Cleopatra, a "caveman", or just about anything or anyone else that has ever lived upon this wonderful world of ours! On the day you were born, you became a part of this cycle. Consuming, utilizing and eventually replacing the water of the Earth.

Much of Earth's freshwater is found in moving water sources such as lakes, ponds, rivers and streams. Water moves among these sources of freshwater, to the oceans, and to the air in an endless cycle.

The first step of this hydrologic cycle involves the heat energy from the sun. This causes the water on the Earth's surface to change to a gas (water vapor). The process is called EVAPORATION. The water that is evaporated comes from animals, the soil, and Earth's surface. The water vapor is carried by the winds over the land and the oceans. The second step in the cycle occurs when the water vapor loses heat and changes back to a liquid in the process called CONDENSATION. Most of the water condenses into the droplets of water that form our clouds. The third step of the process occurs when this liquid water in the atmosphere falls back to the Earth in the form of rain, snow, sleet or hail. This process is called PRECIPITATION. After this liquid or frozen water returns to the Earth, the process begins all over again.

When water falls to the Earth, some remains on the surface and may run off into lakes, ponds, rivers, streams and the ocean. Some water may soak into the ground and become GROUNDWATER (water that is stored in pore spaces in the ground). Eventually this groundwater will flow underground back into the oceans. Some of the water that falls to Earth may be re-evaporated!

Water that enters a river or a stream after a heavy rain is called SURFACE RUNOFF. Runoff is also created in the spring when the snow and ice of winter melts and flows across the land. It is interesting to note that approximately 40% of all precipitation flows into the oceans as surface runoff.  Surface runoff dissolves minerals from the land, and may also pick up any pollutants with which it comes into contact.  In this way, even if you live miles from the ocean, chemicals used around your household often find their way back to the ocean, or into the groundwater supply.  This may be the most dangerous kind of pollution, given the importance of our oceans, and our water supply, to our lives.

When surface runoff follows the same path time and time again, RIVERS AND STREAMS are formed. These rivers and streams become important sources of running water. Many cities and towns are built near rivers and streams. The water can be used for irrigation, power generation, drinking, transportation,fishing, boating and swimming. Rain and snow that does not evaporate or soak into the ground flows into rivers and streams.

A WATERSHED is a land area ion which runoff drains into a river or a system of rivers and streams. A watershed can be very small or very large and they are extremely important, controlling and often preventing both floods and droughts in an area. Essentially, watersheds organize the water in an area, giving it a place to go.  Maine's major watersheds are the Kennebec, Androscoggin, St. Croix, Penobscot, Union and Saco. The St. John watershed is also quite large, occupying the northern part of our state, extending into Canada.  Watersheds help to ensure a constant supply of water in an area.  Sometimes, humans can disrupt a watershed by clearing timber from an area, digging up the landscape, paving roads, or through construction of buildings.  Local building codes often restrict development near shorelines in hopes of preventing this very real danger to our watersheds.  We humans need to be ever vigilant to minimize the "footprint" that we leave upon the land.  We must temper our desire to build and construct with an appreciation for maintaining the balance of nature in an area.  We shouldn't forget that we don't really own this land, we're merely the current tenants.  More people should be concerned about making sure that future generations have the same wonderful world to enjoy that we do!

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Section 3
Another brief excerpt from Bill Bryson in his book "A Short History of Nearly Everything".

"Only 3 percent of the Earth's water is "fresh".  Of that amount, most exists as ice sheets.  Only the tiniest amount 0.036% is found in lakes, rivers and reservoirs, and an even smaller part, just 0.001 % exists in clouds or as vapor.  Nearly 90% of the planet's ice is in Antarctica, and most of the rest is in Greenland.  Go to the South Pole and you will be standing on nearly two miles of ice, at the North Pole, just fifteen feet of it.  Antarctica alone has six million cubic miles of ice--enough to raise the oceans by a height of two hundred feet if it all melted.  But if all the water in the atmosphere fell as rain, evenly everywhere, the oceans would deepen by only an inch."

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