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How Evaporation & Transpiration Contribute to the Hydrologic Cycle

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  • 0:06 The Water Cycle
  • 1:38 Evaporation
  • 3:29 Transpiration
  • 5:22 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Evaporation and transpiration are important components of the water cycle, and while they are the same process they arise from different sources. In this video lesson you will learn about these two parts of the cycle and how they contribute to the overall movement of water on Earth.

The Water Cycle

Water is pretty special. Water is what makes Earth different than all the other planets in our solar system because it is what allows life to exist. It's also unique because it's found in all three phases: liquid water in oceans, lakes, and streams, frozen ice as polar ice caps and glaciers, and gaseous water in the atmosphere.

Water really likes to move, too. It moves around the world, but it also moves through different phases. As it travels around the planet it goes through the water cycle. This is the natural cycling of water through phases and locations on Earth, and it's one of the most important cycles we have on Earth. Each of the different components of the cycle contributes in its own way and is vital to the success of the overall cycle.

Water may condense in the atmosphere, which is when water vapor in the air turns back into liquid and forms clouds. Once in the clouds, that liquid water may fall back to Earth as rain, snow, and sleet, which is called precipitation. Once that precipitation hits the ground it may infiltrate into the ground, which is when it seeps into the soil in the ground, or it may flow across the surface of the ground as runoff.

Evaporation and transpiration are two other critical components of the water cycle because they are the processes that put water into the atmosphere in the first place.

We'll explore each more in detail in this lesson, but it's important to keep in mind that the water cycle is not circular, like running around a track. It's more like a web, where phase changes can occur in any order and in any location.

Evaporation from Earth's Surface

Evaporation and transpiration both put water into the air as a gas, but in different ways. Evaporation is a process you are likely familiar with because it is the changing of liquid surface water to a gas.

You've evaporated water every time you've boiled a pot of water on the stove for pasta. As you heat the water, it gets filled with energy. Once it gets energized enough, it's too excited to stay as a liquid, so it turns to a gas and flows up into the air. This is why if you boil the water long enough you will eventually run out of water in your pot. You see the same thing if you let water sit out long enough uncovered. You can even try this at home. Set a bowl of water out without a lid on it and wait a day or two. Eventually you will return to the bowl to see that the water is all gone - it has evaporated into the air as a gas! This just takes a little longer because you're not adding as much heat as when you cook it on the stove.

On Earth, evaporation occurs over the oceans, lakes, rivers, and streams on the surface. This is one reason we find the rainforests of the world near oceans, which provide a large amount of surface water. The air around the tropics is especially warm, which helps the water heat up and change phase, evaporating into the air as water vapor. It then falls over the rainforests as precipitation, and since the oceans are such a large source of water, the clouds in these areas have a lot of rain to drop back down to Earth.

Evaporation is affected not only by the surface area of the water body but also by the amount of sun exposure (as seen in the tropics), air temperature, and wind speed. You know that sunshine and warmer air temperatures speed up the evaporation process, but wind also helps because as it blows over the water it picks up water along the surface and puts it in the air.

Transpiration Comes from Plants

The other way that water gets into the air is through transpiration, which is water evaporation through plants. Plants 'drink' water from the soil, and as they take it up they use it for biological processes, just like your body does when you drink a glass of water. Both your cells and plant cells are mostly made up of water, so they need to keep hydrated in order to function properly.

Plants also need to take in carbon dioxide in order to photosynthesize, or turn sunlight into chemical energy. Basically this is how plants eat - they collect sunlight and convert it into a usable energy source. To take in carbon dioxide, though, plants need to open small holes on their leaves called stomata. This is where the plant 'breathes' as it takes in carbon dioxide and releases oxygen as a waste product from photosynthesis. Unfortunately, plants also lose water through their stomata as they exchange these gases, which makes the stomata like the sweat glands of a plant.

While plants don't like to lose water, we should be thankful that they do because it provides a lot of water to the water cycle. Moisture in the soil would just stay there without transpiration, and we need transpiration to occur so that water can be returned to the atmosphere and precipitate back down to Earth and recharge our surface waters.

The rate of transpiration is also affected by various factors. When air is more humid, plants will transpire less because the air already has water in it. Warmer air temperatures will increase transpiration because, like the pot of water on your stove, the water leaving plant stomata is heated as it hits the air and turns to water vapor. Wind has the same effect on plant transpiration as it does on surface water and increases the rate of liquid water turning to water vapor in the air. Wind easily picks up and carries water away from plant leaves as it blows by.

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