The Hydrologic Cycle

Earth is a planet of water, with over 75% its surface covered by water. Without water most of the life on Earth would not be able to survive. Water is important for sustaining human civilization; we use it to grow our crops, purify it for our consumption, and flow it through turbines to generate electricity. Water is important as an agent of weathering and erosion (see my articles on Physical Weathering and Chemical Weathering), making possible much of the farmland on the continents. Water is in constant motion on our planet, falling as rain and flowing in streams and rivers to the ocean. We refer to this system of water transport as the hydrologic cycle.

The hydrologic cycle (diagram at right) is a generalization of the complex system of water transport on the Earth. The cycle involves many parts, but at its heart is the cycle of water from oceans to clouds to rain that flows over an under the ground back to the oceans where the cycle starts over.

We'll start our examination of the hydrologic cycle with precipitation. Whether in the form of rain, sleet, snow, or hail precipitation is important for getting the accumulated moisture in the atmosphere to the ground where it is used by all living organisms on the planet. The amount and type of precipitation that reaches the ground varies with different regions and seasons and sometimes the water evaporates before even reaching the ground. Once on the ground the water is either absorbed by plants, flows away as runoff, or percolates into the ground.

Plants remove a large part of the water that reaches the ground through a method called transpiration. Through transpiration, water is drawn up into the plant through its roots, flows through the stem and out to the leaves where the water then evaporates through the stomata. The water flows through the plant and is used to absorb minerals and reduce the temperature of the leaves. You can often tell times of drought by the way plants will droop and wilt because they are not getting enough water.

After a period of precipitation much of the water returns rather quickly to the atmosphere through evaporation and transpiration to again fall as precipitation at a later time. The water that remains percolates into the ground or flows as runoff in streams and rivers. The amount of water that flows as runoff is relatively small, being only about 15 to 20 percent of the water from rainfall. The runoff collects in streams and rivers and flows along the slope of the land until reaching the ocean. During its travel the runoff water continues to evaporate back into the atmosphere. It is also often diverted to be used by humans for irrigation and other uses. The water that percolates through the ground is filtered and enters the ground water system. Ground water also flows down slope until it too reaches the soil saturated by salt water from the oceans. The length of time for surface runoff to reach the ocean is often a few years, whereas it can take thousands of years fro ground water to return to the oceans. It is this extreme length of time that create the aquifers that are tapped by humans and used to irrigate crops and provide water for drinking, bathing, and other uses.

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