# Water Cycle

This course will guide you toward using physics to see water—and many other basic ingredients of the world around us—in a new light. Throughout this course, we will answer questions about water's many roles:

• How can water pressure supply water to your home without a pump?
• How does ocean water fuel hurricanes?
• How does water remove the waste from your toilet in a single flush?
• Does fluoride dissolved in water help or hurt you?
• How is a traffic jam like a mountain stream?
• How does water (and blood) form droplets and spatter?

To begin this course, we will peer into the physics of Earth's freshwater supply (i.e., water that is not too salty to drink). Animals need fresh water to survive, and animals living in different regions on Earth have different sources of fresh water. Although water covers $$71\%$$ of Earth, less than $$1\%$$ is accessible freshwater. Is this enough that every animal can drink its fill?

# Water Cycle

Water on Earth can be found in three different forms or phases: ice (solid), water (liquid), and water vapor (gas). Much of the water you drink has passed through all three.

Many land animals consume fresh water that flows in streams and rivers. This water falls to Earth's surface as precipitation (rain or snow) that flows in rivers and streams hundreds or thousands of kilometers across land.

In climates that are very cold, water is stored in snowpack and glaciers. In summer, some of this frozen water can melt. Is the liquid water that is produced fresh water or salt water?

# Water Cycle

Even in the harshest environments on Earth, animals survive on fresh water. In deserts, where rainfall is rare, fresh water can be difficult to find. In the Atacama desert on the western coast of South America, decades can pass without any precipitation. Yet, some animals thrive in this environment.

Most nights, clouds of water vapor form over the ocean are blown onto the land. This is the only source of water in any form. How do animals stay hydrated if these clouds don’t produce rain or snow?

# Water Cycle

In this quiz, we have been examining aspects of Earth’s water cycle, an ongoing natural process as old as Earth itself.

When water evaporates from the oceans, it can fall on the land as precipitation. This ice and snow form the fresh water that sustains Earth's animal life. It eventually flows back to the oceans, so that water on Earth continually cycles between its different phases: solid, liquid, and gas.

During this cycle, salt and other impurities are removed from the water supply. Which key step removes dissolved salt from the water, making it suitable for drinking?

# Water Cycle

In Earth's water cycle, the step that desalinates water for drinking is evaporation—an unavoidable consequence of the Sun's energy, which means it's free! As long as we can fill reservoirs with the precipitation that falls (or melts), we will always have a supply of fresh water.

The question we'll consider in the rest of this quiz is whether the precipitation that falls each year is enough to maintain freshwater supplies throughout the world.

How is the precipitation that falls related to the water humans consume? On one hand, rainfall predictions or observations are usually reported as a height $$H$$ in millimeters $$($$e.g. $$\SI{20}{\milli\meter}$$ or $$\SI{0.5}{in}).$$ On the other hand, water consumption is usually expressed as water volume in liters or gallons.

Fortunately, these are related by geometry, since volume is base area times height: $(\text{Volume of water})=H \times (\text{Area}).$ So, to calculate a volume of precipitation, we just need to multiply by the area it falls onto.

# Water Cycle

At any time, there’s a lot of water mixed up with other gases in Earth's atmosphere—about the same amount of water as in Lake Superior, one of the world's largest freshwater lakes. But this isn't a static collection: It periodically falls to the surface as precipitation and surface water keeps on evaporating.

If all of this water rained down at once over the entire Earth, it would cause an instant upshift in water levels of approximately $$\SI{3}{\centi\meter}.$$ Instead, it comes down gradually throughout the year, so that the average location on Earth receives approximately $$\SI{1}{\meter}$$ $$($$i.e. $$\SI{100}{\centi\meter})$$ per year in total rainfall.

Each molecule of water may go around the water cycle many times throughout the year. Estimate the amount of time that the average water molecule spends in clouds before falling to Earth as snow or rain.

# Water Cycle

The places on Earth where insufficient drinking water is a big concern are arid climates, which receive just a fraction of the average $$\SI{1}{\meter}$$ of rain per year. For example, Dubai, UAE, receives $$\SI{100}{\milli\meter}$$ (or about $$4$$ inches) of rain per year. It is home to more than five million people. To finish this quiz, let's assess to what extent collecting the little rain that does fall here can help offset human consumption.

Suppose you design a rain-collection system for the roof of a single-family home in Dubai. The roof has an area of $$\SI{200}{\meter\squared}.$$ If you purify all the water collected for consumption, how many people can the system supply with sufficient drinking water for a year—$$2$$ liters each day?

There are $$\SI{1000}{\liter}$$ in $$\SI{1}{\meter\cubed}$$ of water.

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