The Water Cycle

Why do you think the Earth is often called the “water planet”? Perhaps an astronaut suggested this name. From space, an astronaut can see that there is much more water on land on planet Earth. Oceans cover nearly 71% of the Earth’s surface.

Water on Earth Figure 1 shows how Earth’s water is distributed. Most of the Earth’s water—roughly 97 percent-- is salt water that is found in the oceans. Only 3 percent is fresh water. Of that 3 %, about three quarters is found in the huge masses of ice near the North and South Poles. A fraction more is found in the atmosphere. Most water in the atmosphere is invisible water vapor, the gaseous form of water. Less than 1 percent of the water on Earth is fresh water that is available for humans to use.

Oceans To explore where Earth’s water is found, you can take an imaginary boat trip around the world. Your journey starts in Miami, Florida. From here, you can sail completely around the world without ever going ashore. Although people have given names to regions of the ocean, these regions are all connected forming a single world ocean. Ice How can you get back to Miami? You could sail all the way around South America. But watch out for icebergs! These floating chunks of ice are made of fresh water. Icebergs in the southern Pacific and Atlantic oceans have broken off the

massive sheets of ice that cover most of the Antarctica. You would also find icebergs in the Arctic Ocean around the North pole.

Rivers and Lakes. To see examples of fresh water in rivers and lakes, you’ll have to make a side trip inland. Sail north past Nova Scotia, Canada, to the beginning of the St. Lawrence Seaway. Navigate through the series of locks along the St. Lawrence River. Suddenly the river widens and you enter Lake Ontario, one of North America’s five Great Lakes. Together, the Great Lakes contain nearly 20 percent of all the water in the world’s freshwater lakes.

Below Earth’s Surface When rain or snow falls, some of the water soaks into the ground. This water trickles down through spaces between the particles of soil and rock. Eventually the water reaches a layer that it cannot move through. Water that fills the cracks ad spaces in underground soil and rock layers is called groundwater. Far more fresh water is located underground than in all Earth’s rivers and lakes. Figure 1.

The Water Cycle

Water is naturally recycled through the water cycle. The water cycle is the continuous process by which water moves through the living and nonliving parts of the environment. In the water cycle, water moves from bodies of water, land and living things on Earth’s surface to the atmosphere and back to Earth’s surface. The sun is the source of energy that drives the water cycle.

Water Evaporates Water moves continuously through the water cycle. The cycle has no real beginning or end. You can follow a water molecule through one complete cycle in figure 2. Think about a molecule of water floating near the surface of an ocean. The sun is shining and the air is warm. Soon the molecule has absorbed enough heat energy to change state. It evaporates and becomes water vapor. Evaporation is the process by which molecules at the surface of a liquid absorb enough energy to change to the gaseous state. Although the water comes from the salty ocean, it becomes fresh through the process of evaporation. The salt remains in the ocean. Large amounts of water evaporate constantly from the surfaces of oceans and large lakes. In addition, small amounts evaporate from the soil, puddles, and even from your skin. A significant amount of water is given off by plants. Plants draw water from the soil through their roots. Eventually the water is given off through the leaves as water vapor in a process called transpiration.

Figure 2

Clouds Form Once a water molecule has found its way into the atmosphere, warm air carries it upward. Higher up, the air tends to become much colder. Cold air holds less water vapor than warm air. Some of the water vapor cools and condenses into liquid water. Condensed droplets of water clump together around tiny dust particles in the air, forming clouds. This process is called condensation.

Water Falls as Precipitation As more water vapor condenses, the water droplets in a cloud eventually become so heavy that they fall back to Earth. Water that falls to Earth as rain, snow, hail, or sleet is called precipitation. Most water molecules probably spend only about 10 days in the atmosphere before falling back to Earth. Most precipitation falls directly into the oceans. Water in the ocean may stay there for many years before evaporating, thus continuing the cycle.

When precipitation falls on land, some of the water evaporates again immediately. Some called runoff runs over the surface of the land into rivers and lakes. From there it may eventually evaporate or flow back into the ocean. Some water trickles down into the ground and forms groundwater. Ground water may move underground until it reaches a river, lake, or ocean. Once groundwater reaches the surface, it can continue through the cycle by evaporating again. Precipitation is the source of all fresh water on and below Earth’s surface. The water cycle renews the usable supply of fresh water on Earth. For millions of years, the total amount of water on Earth has remained fairly constant. In the world as a whole, the rates of evaporation ad precipitation are balanced.

Water Used in the Home Task Water

Used (liters)

Showering for 5 minutes 95 Brushing teeth with water running

10

Washing hands 7.5 Flushing traditional toilet 23 Flushing “low-flow” toilet 6 Washing one load of laundry 151 Running dishwasher 19 Washing dishes by hand 114

Figure 3

How Water is Used

The water people use at home is just a small percentage of all the water used in the United States. In addition to household purposes, people use water for agriculture, industry, transportation and recreation.

Agriculture A constant supply of fresh water is essential to grow food. However, some parts of the United States don’t receive enough regular rainfall for agriculture. In such areas, farmland must be irrigated. Irrigation is the process of supplying water to areas of land to make them suitable for growing crops. In the United States, more water is used for irrigating farmland than for any other single use.

Industry Think about the objects in a typical school locker. Did you know that water is needed to produce all these objects? Even though water is not part of the final

products, it plays a role in the industrial processes that created them. Industries use water in many other ways. For example, power plants and steel mills both need huge volumes of water to cool down hot machinery. Water that is used for cooling can often be recycled, or used again for another purpose. Transportation and Recreation Oceans and rivers have served as natural highways for transporting people and goods since ancient times. You can probably think of a lot of ways water is used for recreation. And if you ski, snowboard, or skate, you are enjoying water in its frozen state. Water and Living Things Here’s a riddle for you: What do you and an apple have in common? You both consist mostly of water! In fact, water is a large part of every living thing. Water makes up nearly two thirds of your body. That water is necessary to keep you body functioning. Water is essential for living things to grow, reproduce, and carry out other important processes. Plants use water plus carbon dioxide and energy from the sun to make food in a process called photosynthesis. Another way living things use water is as a home. An organism’s habitat is the place where it lives and that provides the things it needs to survive. Both fresh and salt water provide habitats for many different types of living things.

Cornell  Notes  Topics  for  The  Water  Cycle  Reading  Packet  pp.  1-­‐4  

Note:  Draw  a  line  to  divide  the  page  into  two  columns.    Label  your  notes  with  a  title,  date,  your  name.    Leave  4  lines  between  each  note  item  to  fill  in  with  class  notes  later.    Number  your  notes.  

   

1.  How  much  of  the  Earth’s  water  is  salt  water  and  how  much  of  the  Earth’s  surface  does  it  cover?        2.      Where  is  most  of  the  Earth’s  fresh  water  found?    3.      Make  a  type  of  graphic  organizer  called  a  concept  map  showing  the  distribution  of  water  on  the  Earth.  Use  the  pie  chart  graphic  organizer  in  figure  1  to  get  your  information.      

4.    What  percent  of  the  Earth’s  water  is  usable  by  humans?      5.    What  are  three  different  forms  that  fresh  water  takes?    6.      What  causes  or  drives  the  water  cycle?    7.    evaporation    8.    condensation      9.        precipitation    10.      Does  the  amount  of  water  on  the  Earth  change  from  year  to  year?    11.      Runoff    12.    groundwater    13.    List  an  agricultural  use,  an  industrial  use,  and  a  household  use  of  water  that  you  relied  on  today.        13.    How  are  plants  a  part  of  the  water  cycle?