The Water Cycle Water and energy are transferred throughout the hydrosphere, lithosphere and atmosphere during the water cycle, or the hydrologic cycle.
The amount of water on Earth remains constant, but it continuously changes forms as energy from the Sun drives the cycle. Movement of Water during the Water Cycle
The water cycle describes the continuous movement of water on, above, and below the surface of the Earth. Actually, there is much more water being stored in the cycle than is moving through the cycle. Water may be stored for a short time as water vapor in the atmosphere, for days or weeks in a lake, or for thousands of years in a polar ice cap.
Image courtesy of NOAA
The water cycle is a cycle with no beginning or end. It includes the following processes:
Condensation is the changing of gas to a liquid (water vapor to water) and is crucial for the formation of clouds. Clouds form in the atmosphere when air containing water vapor rises and cools. Water vapor can be present in the air even when clouds are not visible. Clouds become visible when water particles combine, along with tiny particles of dust, and form water droplets.
Water returns to the Earth as precipitation. Precipitation is the process by which water vapor in the air condenses to form drops heavy enough to fall to the Earth’s surface. During infiltration water fills the porous spaces in the lithosphere. Surface runoff occurs when no more water can be absorbed into the ground, and gravity pulls it downhill. Water flows over land and forms rivers. Rivers usually flow into the ocean, and the water cycle continues from there.
Evaporation and transpiration are similar in that they are both processes in which water is changed into water vapor. Evaporation often happens as a result of heat – liquid water is heated until it turns to a gas, water vapor, and is released into the atmosphere. Transpiration is the process by which water is carried through plants, from roots to leaves, where it changes to water vapor and is released to the atmosphere.
Sublimation is the changing of water from a solid directly to a gas with no intermediate liquid stage. The opposite of sublimation is deposition, when water vapor changes directly to a solid. Snowflakes and frost are examples of deposition. Transfer of Energy
Energy from the Sun drives the water cycle. As water changes from one form to another, energy is also transferred. The following table describes the different processes involved during the water cycle and explains how energy is involved in each process.
Process Description Energy Flow
liquid water is converted atmosphere → evaporation into water vapor hydrosphere
water vapor is converted hydrosphere → condensation into liquid water atmosphere Process Description Energy Flow
ice is converted into liquid atmosphere → melting water hydrosphere
liquid water is converted hydrosphere → freezing into ice atmosphere
ice is converted into water atmosphere → sublimation vapor hydrosphere
water vapor is converted hydrosphere → deposition into ice atmosphere
Energy can also be transferred from the atmosphere to the hydrosphere when wind causes formation of surface waves in the ocean and other bodies of water. The Importance of Water Earth is a unique planet because of the presence of water. Water is absolutely essential to support life on Earth. Plants, animals and humans all rely on water for survival. There would be no life without water. It serves us all well to conserve and protect the world’s water. Distribution of Water
Water exists in many different places and forms. Water found in oceans and seas is called salt water. Salt water has salts dissolved in it and it tastes salty. It cannot be used for human consumption. Fresh water does not have salt dissolved in it and is suitable for human needs. Fresh water may come from rain, glaciers, lakes, streams, or rivers.
Image courtesy of USGS.
Ninety-seven percent of Earth’s water is salt water and is “stored” in the oceans. Fresh water represents only three percent of Earth’s total water supply. Of that three percent, 68.7% is stored in icecaps and glaciers, 30.1% is found in ground water, and only 0.3% is found in surface water such as rivers, swamps or lakes. Watersheds
The area of land where water is drained downhill into a body of water is known as a drainage basin, or watershed. The components of a watershed include lakes, streams, rivers, wetlands, and groundwater.
Image courtesy of EPA.
The water collected in a watershed may drain into the ground or other bodies of water. Water flows between the different components of a watershed until it reaches an ocean. For example, precipitation may runoff into a stream. The stream may flow into a lake. The water may then flow out of the lake into a river which carries the water all the way to the ocean.
Watersheds are separated by hills or areas with a higher elevation. The high ground that separates two watersheds is called a divide. At the point of the divide, water that falls on one side of the divide will drain into one drainage basin, while water that falls on the other side will drain into another. In hilly areas, the divides usually lie along peaks and ridges. Dependence of Life on Water Plants need water for their cells to stay healthy. A wilted plant is one in which so much water left its cells that the cells have collapsed. A badly dehydrated plant cannot recover and will die.
Almost two-thirds of the human body is made of water. Our bodies use water to produce blood, to build healthy cells and organs, and to aid in body processes such as digestion. A human can live weeks without food, but only a few days without water. Water Usage
In addition to consuming water for life processes, humans use water in many other ways. Power plants account for almost half of the water usage in the United States. Coal-burning and nuclear power plants require water to generate power. Both types of power plants capture rising steam to create electricity.
The second largest use of water in the United States is agriculture. The use of water for crops, or irrigation, requires trillions of gallons of water each year.
Water Usage in the United States (2005)
Nuclear and coal-burning power plants use the largest percentage of freshwater in the United States. Irrigation for crops is the second largest category of water consumption. Data source: Estimated Use of Water in the United States in 2005. USGS.
Some uses of water do not actually remove it from our supply. Moving water can be used to generate power. Dams are built on rivers to harness the tremendous strength of water. Water is also used for transportation.
Large ocean liners transport cargo around the world; a canoe may move a person a few miles up a small river. Many people use water for recreation: water skiing, swimming, scuba diving, boating, and fishing. Water is used for cleaning bodies and buildings alike. Drought
Many parts of the world experience periodic droughts. A drought is an extended period of time during which there has been insufficient precipitation. A drought is detrimental because crops wither and die, drinking water sources dry up, and wild fires occur more frequently because plants and trees are so dry.
This animated image shows the effect of drought conditions on the water levels of Lake Mead.
Image courtesy of NASA/USGS.