Overview
In the hydrologic cycle, or water cycle, there are a few obvious contributors to the cycling of water, like rivers, clouds, and lakes. However, there is another important player in the hydrologic cycle that is frequently overlooked. Evapotranspiration happens when plants absorb water through their roots and then evaporate water from their leaves.
Introduction
This activity explores the role of plants in the hydrologic cycle.
Grade Level
K-5, 6-8
Learning Objectives
Students will be able to identify the role of plants in the water cycle through the process of evapotranspiration.
Lesson Format
This activity can be done indoors!
Time Required
1 hour
Credits & Contact Info
Trisha Smrecak and Caroline Boyajian
Paleontological Research Institution, 1259 Trumansburg Rd., Ithaca NY 14850
climate@priweb.org
Instructions & Materials
Materials
- Diagram of the water cycle
- Celery stalks or white carnations
- Clear container filled with water
- Colored food coloring
- Paper towel tube
- Paper cut into a series of connected circles that is slightly longer than the paper towel tube and can fit inside the tube
- Clear plastic bag and twist tie for each group/youth
Preparation Needed
- Place celery or carnation in water for several hours until leaves or petals are colored with the food coloring.
- Print off, or draw on the board, a diagram of the water cycle with vegetation. Have students label the water cycle (they will usually forget evapotranspiration).
Instructions
Part 1.
- Show the youth the celery or carnation that has been soaking in colored water and ask how the leaves got colored.
- Ask the youth to consider a 20-foot tree; how do its leaves get water?
- Show the youth the paper towel tube with the cut-out circles placed inside and sticking out. The tube represents the part of the tissue inside a plant (xylem). The paper represents water molecules.
- Point out the water molecule near the top of the tube, explaining that this represents a molecule at the stoma (or pore) of a leaf. Heat energy from the sun is stronger than the forces holding the water molecules together.
- Rip off a circle to represent the bonds breaking the water molecule's hold. As you do, pull another water molecule out of the top of the tube. This represents a water molecule breaking away, which causes a pull on the remaining water molecules, drawing them further up the tree’s xylem.
Part 2.
(Works best on a sunny day after a rainstorm or after an area has been watered)- Divide into small groups; give each group an empty plastic bag and a twist-tie.
- Assign each group a plant (tree or shrub) outside.
- Have each group carefully place its bag over a part of a limb of a tree or shrub.
- Groups count and record the number of leaves in each bag. Have the youth develop a method to estimate the number of leaves on their tree (x leaves per branch, x branches on a tree, etc). How is a tree identified? This step can take 20 minutes, or leave the tree and engage in another activity until about half an hour has passed.
- After 30 minutes or more, have each group remove their bag from the limb, being careful not to remove any leaves or branches.
- Ask the youth to observe what has happened to their bags. Why do they now have water/ condensation in them? Explain that this is called transpiration, much like how humans experience respiration. If every leaf on every tree transpired as much as our leaves did, would that be a lot of water?
Discussion
The celery or carnation shows how water travels from the roots of a plant to the leaves. The forces used to cause the travel were demonstrated with the paper and the paper towel tube. Finally, using the bags and twist-ties, we demonstrated how water leaves the plant through transpiration.
Have the youth evaluate their drawing and labeling of the water cycle. Did they factor in the vegetation in their model? Where does the water go when it is pulled from the tree by heat energy? It evaporates! Youth should label this source of evaporation on their water cycles, and point out that it is called Evapotranspiration!
Background
Learn more about the water cycle here: https://science.nasa.gov/earth/earth-observatory/the-water-cycle/
It might be obvious that trees cool the environment by providing shade, but evapotranspiration provides another type of cooling. It takes energy to change water from its liquid to gas state, and when water evaporates from a leaf, this energy is taken out of the surrounding air. Since energy is removed from the air in this process, the air cools right around the leaf. Scientists estimate that in the summer, trees can cool the surrounding air by 2°F to 9°F (1°C to 5°C) on average.
Learn more in this video: Evaporation is Chill