Sub-Concept: Understanding Soil & Water / The Hydrologic Cycle
Agricultural Context: Agriculturalists depend on water for efficient production of food and fiber.
Exercise: Simulating the Water Cycle
Applied Principle: The amount of water on and around Earth remains fairly constant; however,
the availability of the water supply is constantly changing. This is because water is continually
moving from place to place in what is called the "hydrologic cycle", or water cycle.
Goals:
Materials: Part 1 (Demonstration conducted by instructor)
Part 2 (Terrariums)
Part 3 (Water Collector)
References: Klumpp, M. and Ruby, S. (undated). "Hydrologic cycle in action." Oklahoma
Aqua Times Teacher's Guide. Stillwater, OK: Cooperative Extension Service, Division of
Agricultural Sciences and Natural Resources, Oklahoma State University.
Camp, W.G. and Daughtery, T.B. (1991). Managing Our Natural Resources, 2nd edition.
Albany, NY: Delmar Publishers.
1. Gather all needed materials and set up the demonstration prior to class.
2. Place some ice cubes into the container and heat on the hot plate until the ice changes to water and the water begins to evaporate.
3. Place a piece of aluminum foil over the
boiling water so the students can see the vapor
condensing (see Figure 1).
Figure 1
4. Demonstrate "rain" by placing a shallow pan of ice cubes over the vapor from the boiling
water.
5. During the activity, lead the students into a discussion of the following points/questions:
a. In what forms does water appear in the natural environment?
b. In what three forms do we see water?
c. What is evaporation? water vapor? condensation?
d. What are the steps in the water cycle?
Part 2 -- Plant a Terrarium
[Note: Students may gather native plants as a part of this activity, or they may be provided in
class by the instructor. If making small terrariums (as from 2-liter plastic bottles), students should
work individually or in pairs; if using larger containers, such as aquariums, small groups of 4 to 5
students may work together. Larger terrariums may be used all year, adding animals and insects
later in the course.]
1. Lead the class in a discussion of the following concepts:
a. After seeing the water cycle demonstration, how does the water cycle work in nature? We can
demonstrate a more realistic situation in the classroom/lab using terrariums, which is the next part
of this exercise.
b. Plants play an important role in the water cycle...what are some plants (both native and from
greenhouses) that would work well for growing in a terrarium? Where can we find these plants?
2. If students are to be responsible for gathering plants:
a. Discuss where to find the desired plants.
b. Discuss the best way to collect the plants to be used, such as taking care to not damage the plants.
c. Divide the class into appropriately sized groups, and provide each group with paper sacks and gardening trowels.
d. Allow students to gather plants as previously discussed in class.
e. Return to the lab with the plants to complete the activity.
3. Gather the remainder of needed materials.
4. Cover the base of the plant container with 1" of gravel for drainage.
5. Cover the gravel with 2" of potting soil.
6. Make small holes in the soil. Place the plants in the holes so that the roots can be covered. Pack soil around the plants and press firmly. Be careful not to crowd the plants. (Refer to Figure 2.)
7. Water the soil lightly and cover the terrarium with
a lid or with plastic wrap. Your terrarium will need
only one or two teaspoons of water a month.
8. Place the terrarium in a well-lighted spot,
preferably near an east or west window.
9. After one week, examine the terrariums and discuss
the following points as a class:
a. Sometimes there is moisture on the sides
of the terrarium. What causes this?
b. What would happen if the terrarium did not
have a lid? Would you have to water it more?
Figure 2
Part 3 -- Make Your Own Water
1. Divide the class into 4 small groups.
2. Provide each group with the necessary materials, as well as an instruction sheet.
3. Assign each group to a different location to conduct the exercise.
[Note: By assigning groups to locations with varying
moisture and sunlight conditions, different results will
be achieved. Be sure that at least one group has a
location where the soil is fairly moist.]
4. Send students to their respective locations, making
sure to supervise each group's progress and techniques.
Figure 3
5. In the ground, dig a small pit slightly wider and deeper than the can.
6. Slope the surrounding soil toward the pit so the plastic does not sit directly on the ground.
7. Place the can in the bottom of the pit.
8. Cover the pit with plastic, securing the ends to suspend the plastic over the slope of the pit.
[Note: Black plastic works the best for this experiment.]
9. Place a rock directly in the center of the plastic over the can to form a slope.
10. The next day, the class is to visit each group's experiment site, and compare the contents of
each can.
11. Discuss the following points as a class:
a. Are there differences in the contents of each group's can? What accounts for these differences?
b. If different colors of plastic were used, did the color differences make a difference in the amount of water collected? What color works best? Why?
c. What causes the water to collect in the cans?
d. If the weather had been different (sunnier, rainier, colder, warmer), would the results be
different? Why/How?
12. Gather all materials, placing in a location designated by the instructor.
TEACHER BACKGROUND SHEET
Water might be called our most recycled resource because the water on earth is constantly moving in a cycle. The water molecules here today are the same ones that were on earth millions of years ago. Consider, for example, that the water you bathed in last night may have contained the same water molecules that were part of the bath water for an ancient Egyptian princess, or they might have even rained on a (live!) dinosaur. The distribution of the Earth's supply of water changes in time and space, but the amount remains basically constant. This constant movement of water molecules is kept in motion by a phenomenon known as the "hydrologic (or water) cycle." This cycle is powered by the sun's energy and is aided by the force of gravity. (See Figure 4.)
As a rule, the water from our faucets comes from "groundwater" or "surface water." Ground and surface water combined makes up less than 1% of the Earth's total water. Approximately 97% of the water is in the world's oceans and about 2% is found in ice and snow primarily in polar ice caps.
The "water" we can see is made up of billions of molecules of water. These tiny molecules (H20) are made up of 2 atoms of hydrogen and 1 atom of oxygen. It is these molecules that are constantly circulating throughout the cycle.
Snow, sleet and rain which fall from the sky by force of gravity is one part of the water cycle. This falling water comes from land and surface water which was evaporated by the sun's energy. The water was heated by the sun, rose into the atmosphere, was then cooled sufficiently to condense into clouds and finally fell back to Earth as precipitation. Some of the precipitation which falls on the land is used by plants and animals, some percolates down through the soil, some runs off and eventually reaches the ocean and some lands on the ocean and other water surfaces.
About 30% of the rainfall in the U.S. falls into our lakes, ponds, and streams. This surface water is the most important element to the conservationist. It is used over and over again by people as it makes its way toward the ocean, fulfilling agricultural, industrial, and domestic needs. It is here that pollution may become a problem, rendering much of the water unsuitable for our needs.
The water that does not either evaporate or run off percolates down through the soil becomes
groundwater. This groundwater eventually flows into our rivers, streams, and lakes where it is
then exposed to the sun to be evaporated once again. The evaporated water cools as it rises until
it condenses into clouds once again, thus continuing the cycle. Water molecules are also given off
from plants through a process called "transpiration" (plant sweat).
Part 2 -- Plant a Terrarium
Terrariums serve as reminders of the constant motion of water in nature. As the terrarium plants
give off water vapor through transpiration, the glass lid and sides of the container trap the vapor,
so the plants use the water over again. There may be some moisture on the glass when the
terrarium gets warm. That means the plants are transpiring more, and the water droplets on the
glass are an example of condensation. A terrarium does not need very much water. You may not
need to water your terrarium for several months if the soil is moist from the start.
Transpiration and evaporation, together termed "evapotranspiration," are important steps in the
water cycle. "Transpiration" is actually evaporation of water from plants - 95% of all the water
absorbed by plants is transpired. The remaining 5% is used in plant processes. Up to 80 gallons
of water per day can be transpired from an average sized tree.
Water enters the air either by evaporation or transpiration. Heat from the sun aids the change
from liquid water to gaseous water (vapor). This can happen directly, as when water from the
ocean, a lake, or a river enters the air (evaporation), or indirectly via plants (transpiration). The
condition of these two is termed "evapotranspiration."
When the air temperature is cool enough, water vapor condenses and returns to Earth as
"precipitation" (rain, sleet, snow). Precipitation percolates into the ground and becomes available
to plants, or falls directly on rivers, streams, and lakes to begin the cycle again.
Part 3 -- Make Your Own Water
Making a water condenser is another way to show how water changes constantly.
Evaporation takes place because of the sun's heat. This experiment works best if it is done over a
period of time when the temperature changes considerably (overnight is best). The water found in
the can at the end of the experiment was formerly held in the soil.
The dark plastic heats the soil during the day. At night, the temperature of the plastic drops faster
than the soil temperature, causing moisture to condense on the plastic. The slope of the plastic
causes the droplets to flow into the can.
GROUP INSTRUCTION SHEET
Using the plants and materials you have gathered, complete the following steps in constructing
your own terrarium (refer to Figure 1):
a. Cover the base of the plant container with 1" of gravel for drainage.
b. Cover the gravel with 2" of soil.
c. Make small holes in the soil.
d. Place the plants in the holes so that the roots can be covered.
e. Pack the soil around the plants and press firmly. Be careful not to crowd the plants.
f. Water the soil LIGHTLY.
g. Cover the terrarium with a lid or plastic wrap. [Your terrarium will only need one or two
teaspoons of water a month.]
h. Place the completed terrarium in a location designated by the instructor. This location should
be well-lighted, but should not receive direct sunlight, and preferably in an east or west window.
2. Make Your Own Water
After gathering the necessary materials and location assignment from your instructor, complete
the following steps to construct a water condenser (refer to Figure 2):
a. Dig a small pit in the ground, slightly wider and deeper than the can.
b. Slope the surrounding soil toward the pit so the plastic does not sit directly on the ground.
c. Place the empty can in the bottom of the pit.
d. Cover the pit with the plastic, securing the ends to suspend the plastic over the slope of the pit.
e. Place a rock directly in the center of the plastic over the can to form a slope.
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