Note for parents and educators: This article contains material that works with the following NGSS standards:
2-ESS2-3: Obtain information to identify where water is found on Earth and that it can be solid or liquid.
5-ESS2-2: Describe and graph the amounts of salt water and fresh water in various reservoirs to provide evidence about the distribution of water on Earth.
MS-ESS2-4: Develop a model to describe the cycling of water through Earth’s systems driven by energy from the sun and the force of gravity.
Have you ever wondered where the water you drink comes from? You have probably noticed that water from different cities tastes different. I liked the water from my grandparents’ house in Highland, Utah the best out of any tap water I’ve had. I lived not too far away, in Provo, Utah while I was going to college. The water there didn’t taste as good. That water still tasted better than the water at my parents’ home in Northern Virginia, which often tasted like chlorine. Why does all the water taste different?
Where on Earth is all the water?
It is important to understand where the water on Earth is located if we want to understand where our water comes from. If you have seen any picture of the Earth you have probably noticed that there is more water on the Earth than there is land.
Water covers 71% of the Earth’s surface, and as you can see when you look at the above image of the Earth, the oceans hold most of that water. That’s where 96-97% of all the water on Earth is. The next largest reservoir for Earth’s water is ice. This includes the Earth’s ice caps, glaciers, and snow at the top of mountains. Ice makes up 2% of Earth’s total water.
The salty ocean water and the water frozen in ice aren’t available for us to drink. That makes almost 99% of all water unavailable for our use unless we develop an easy and cheap way to extract fresh water from the ocean. For now it is cheaper and easier to use other sources of water. Most of the water that we use for drinking water comes from the ground. Fresh groundwater is less than 1% of all the total water on Earth, but that is what we mainly use for agriculture and consumption. Many people think of lakes or rivers when they think of fresh water. However, you will find less than 0.05% of all Earth’s water in one of these locations.
How does Earth’s water move?
I’m sure that you have experienced rain or hail or snow at some point in your life. Rain, hail, and snow are all examples of precipitation, one of the ways that water can move around the Earth. Here are some of the other ways:
- Evaporation
- Transpiration
- Condensation
- Precipitation
- Groundwater
- Runoff
Evaporation
Earth’s seasons help the water move around. The Earth has a tilted axis, so half of it is closer to the sun than the other half is for 6 months of the year. The months of October through April are colder in the northern hemisphere and warmer in the southern hemisphere. The months of April through October are warmer in the northern hemisphere and colder in the southern hemisphere. Evaporation is easier during warm periods of time, when water on the ground, in the ocean, in rivers, and in lakes gets heated and allows some small particles of water to enter the air. These particles are small enough that we cannot see them, but sometimes we can feel them. Some days are more humid than others and on those days you feel sweatier and can feel the water in the air.
Transpiration
Transpiration is a word used for the water that is released by plants. We, as humans, drink water and eat foods that contain water. We also release water through sweat, urination, and as water vapor when we breathe. Plants also drink and then release water. Plants release water in vapor form, through tiny holes in their leaves. This is similar to us breathing!
Some water cycle diagrams combine the processes of evaporation and transpiration. This is called evapotranspiration, and it is the water that has evaporated from the surface of the earth plus the water released by plants through transpiration.
Condensation
Water in the air can condense, meaning the particles come together and the water is visible again. This depends on the temperature of the air, the concentration of water particles in the air, or both. Cold air cannot hold as much water as hot air can. An example of condensation because of temperature is when the air cools overnight and there is dew on the ground in the morning. This can also happen when you leave a glass of ice water on the table and water drops condense on the outside of the glass. We can also see condensation high in the sky. Water particles condense and join together in big groups to form clouds.
Precipitation
We know that clouds form by condensed water particles joining together. But what happens when too many water particles are in a cloud? When a cloud gets too heavy from having a lot of water particles in it, then it will cause precipitation. Precipitation is any form of water falling from the sky, such as rain, snow, or hail. When this water hits the ground, some of the it will go into the ground to join the groundwater. Some of the water will become runoff, which means that it will run along the surface of the earth until it gathers in a river, a lake, or the ocean. The water that is runoff on the surface of the Earth can also evaporate again before it reaches any of those locations.
Groundwater
Water that is stored in the ground is called groundwater. The chart earlier in this article showed us that 1.69% of Earth’s total water, and 30.1% of Earth’s total freshwater is stored in the ground. Many people around the world get their drinking water from groundwater. Groundwater moves around, just like the water on the surface of the Earth. As the groundwater moves, it can pick up minerals from the rocks or soil through which it is traveling. This is what causes tap water to taste different in different cities around the world.
Runoff
You have probably seen water rushing across the ground when it rains a lot. This water is runoff. Not all of the rain or snow water can be absorbed into the ground to join the groundwater, so some of it runs along the surface of the Earth until it finds a lake, stream, or ocean to join.
Sources:
Brigham Young University College of Physical and Mathematical Sciences, 2012, Physical Science Foundations, 4th ed., BYU Academic Publishing
Rutledge, K. et al, 2011, “Condensation”, National Geographic Resource Library, https://www.nationalgeographic.org/encyclopedia/condensation/
USGS, “The Water Cycle for Adults and Advanced Students”, Water Science School, https://www.usgs.gov/special-topic/water-science-school/science/water-cycle-adults-and-advanced-students?qt-science_center_objects=0#qt-science_center_objects
USGS, “Where is Earth’s Water?”, Water Science School, https://water.usgs.gov/edu/earthhowmuch.html
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