Section 1.4: The Water Cycle

Outcomes

Students will:

  • Investigate the water cycle as a representation of energy flow and the cycling of matter through ecosystems.

  • Examine a visual model of the water cycle to illustrate how water moves through ecosystems.

  • Conduct an experiment that displays evidence of water vapors releasing into the atmosphere.

Key Terms

Condensation / Evaporation / Infiltration / Precipitation / Run-off / Transpiration

See content or Module Glossary for definitions

What is the Water Cycle?

The water cycle is a series of processes by which water evaporates from the surface of the earth, rises into the atmosphere and cools, condensing into water droplets held by clouds, and then is released to the earth’s surface as precipitation (rain, hail, sleet, or snow) before the process starts again (see Figure 21).

The water cycle is important to a tree as water is an essential ingredient to the process of photosynthesis. Water also helps move nutrients through various systems and are absorbed by the trees. Some of these nutrients help aid in the growth of trees through various systems; others can be detrimental to the growth. With that in mind, when it comes to analyzing your tree cores and soil at the Canadian Light Source (CLS) and the Mistik Askîwin Dendrochronology (MAD) Lab (see Module 5 for more information), certain elements of interest may show to be present in your sample. You may wonder why these elements are in your sample and how they got there. By understanding the water cycle, you should have a sense of how elements can move through the water cycle from various water bodies.

Water Cycle Processes

  1. Evaporation - The sun is the driving force of the water cycle. When the sun heats up bodies of water such as oceans, rivers, lakes, puddles, etc., the water changes from liquid to gas in a process called evaporation. The water vapour rises high into the atmosphere.

  2. Condensation - As water vapor rises in the atmosphere, it cools off as it passes through air with colder and colder temperatures. As the water vapour cools, it changes from gas to liquid in a process called condensation. The condensing water vapour forms tiny water droplets that make up clouds.

  3. Precipitation - As more water condenses into water droplets, the clouds will continue to grow, until they become saturated and too heavy. When this happens, the water droplets fall to the earth as rain, snow, or hail, in a process called precipitation.

  4. Run-Off - The rain or melted snow on the ground flows back into various bodies of water such as rivers, lakes, streams, and oceans. The term for this water flow is called run-off. The water will then be evaporated again, starting the cycle all over.

  5. Infiltration - Water that remains on the earth's surface is soaked up by the soil using a process called infiltration. Trees absorb water from the soil and transport it to the leaves through the stem in a process known as plant uptake.

  6. Transpiration - During the plant uptake process, the water and nutrients found in the soil are soaked into the tree by the roots and move upward into the trunk, branches, and leaves for use in photosynthesis. Eventually, the water makes its way to the leaves and stem and evaporates in the process transpiration (See Section 1.1 for transpiration and capillary action). Transpiration from trees and plants is closely related to the evaporation of water bodies. This leads us back to step one and the cycle continues.


Figure 21 shows the water cycle processes. Image by NASA.
Figure 21 shows the water cycle processes. Image by NASA.


Transpiration in Action Activity

Since the transpiration process cannot be seen by the naked eye, a critical thinker may question the validity of the transpiration process. Follow these simple steps to conduct an experiment that displays evidence of water vapors releasing into the atmosphere:

  1. Tie a clear plastic bag to the leaves of a classroom plant

  2. Leave the bag alone for roughly an hour

  3. Revisit the plant and witness water vapor droplets on the inside of the bag

Additional Resources & References for Section 1.4

Resources

References

Science Struck. (2019). A simple guide to the steps of the water cycle. Retrieved from https://sciencestruck.com/steps-of-water-cycle

U.S. Geological Survey. (n.d.). Evapotranspiration and the water cycle. Retrieved from https://water.usgs.gov/edu/watercycleevapotranspiration.html