Agua es vida (Water is life)—that statement adorned the logo of a water-well-drilling company I worked with many years ago in Puerto Rico, and to me, the phrase highlights the importance of water on Earth. Remember that 78% of a human baby's body is composed of water, as is 55–60% of an adult's body; obviously, water is a truly critical component of sustaining life on Earth.

A portion of the water cycle includes the flow of water across Earth's surface, as shown here from the Little River, Great Smoky Mountains, North Carolina. Flowing water not only sustains life, among other processes you will study in this lesson, but can also be visually and aesthetically pleasing.

Credit: Michael Collier. Image source: Earth Science World Image Bank

Water covers ~70% of Earth's surface. Most of this is in the oceans, a realm of Earth not covered in this course. However, the global ocean is more than peripheral to the Critical Zone. Ultimately, all of the water that bathes the Critical Zone was evaporated from the ocean surface, transferred to the continents as vapor via the atmosphere, and deposited on the land surface as liquid or solid precipitation, depending on regional climate. In this unit, we will learn about water's role in the Critical Zone. We will accomplish this in Lesson 6 by first considering in some detail the so-called water cycle. Because of the role of the atmosphere, we will revisit some aspects of atmospheric processes. This should serve to demonstrate the intimate link between the various "spheres" (our last unit with this one) that overlap in the Critical Zone!

The water cycle involves much more than the transfer of water from the ocean to the land surface. Once precipitated, water can flow across the land surface, infiltrate into the subsurface as soil pore water or groundwater (remember that the base of the Critical Zone is defined as the depth to which groundwater freely circulates), or be evaporated or transpired by plants (the top of the Critical Zone!) back to the atmosphere. In this lesson, we will consider the many processes involved in the flow of water through the Critical Zone.

Water is known as the universal solvent—rarely is it found in a pure state because many substances will readily dissolve in it. Thus, we will include brief considerations of water chemistry and quality (we will return to this topic again when considering landforms and biota) and the role human society has played in degrading this invaluable resource. In our landform unit, you will also learn about the role water plays in transforming and sculpting the land surface, while in the biota unit we will explore the many interactions between water and life in the Critical Zone, thus further linking the Critical Zone "spheres."

Finally, because of the somewhat mysterious nature (out of sight, out of mind) of groundwater, you will study various aspects of groundwater flow in Lesson 7, and complete the water unit by reading about links between water and Critical Zone science. Enjoy.

Salmon River estuary, Oregon. Estuaries are drowned river mouths where fresh water runoff from the continents intermixes with saline seawater. As such they represent an interface between the Critical Zone and Earth's largest water reservoir—the oceans!

Image source: Earth Science World Image Bank

In an earlier lesson, I described soil as the "heart" of the Critical Zone. With that analogy in mind, perhaps you can consider water to be the “blood” of the Critical Zone. Once precipitated onto a landscape, water can be stored at the surface in various reservoirs depending on the climate of a region, flow across the surface, or infiltrate into and flow through the subsurface domain. Along the various pathways, water can transfer dissolved constituents from one portion of the Critical Zone to another. Those constituents may degrade water quality, precipitate minerals, or provide sustenance for life, among many other processes. As in earlier lessons, you should not be concerned with memorizing and repeating various definitions for the many processes you will read about and study. Instead, begin by securely grasping the concept of the water cycle. Once you have mastered the water cycle, move on to learn about the chemistry of natural waters and human influences on water in the Critical Zone.

What will we learn about in Lesson 6?

• The role of the ocean in the water cycle
• The difference between evaporation, sublimation, and evapotranspiration
• The difference between condensation and precipitation
• Various reservoirs in which water is stored in and moves through the Critical Zone
• The difference between surface runoff and streamflow
• The distribution of water on Earth and in the Critical Zone
• Human influences on water distribution and quality
• The use of online surface water data
• Basic chemistry of some natural waters

Learning Objectives

By the end of this lesson you should be able to:

• Describe the water cycle using a schematic diagram
• Obtain and interpret online surface water data
• Discuss basic chemistry of some natural waters
• Discuss human influences on the availability and quality of water

What is due for Lesson 6?

Lesson 6 will take us one week to complete As you work your way through these online materials for Lesson 6, you will encounter additional reading assignments and hands-on exercises and activities. The chart below provides an overview of the requirements for Lesson 6. For assignment details, refer to the lesson page noted.

Please refer to the Calendar in Canvas for specific time frames and due dates.

Questions?

If you have any questions, please post them to our Questions? discussion forum (not e-mail), located under the Discussions tab in Canvas. I will check that discussion forum daily to respond. While you are there, feel free to post your own responses if you, too, are able to help out a classmate.

The term "water cycle" does not refer to water craft you can peddle across water—instead the water cycle describes the transit of water through all of the components or "spheres" of the Critical Zone (atmosphere, hydrosphere, lithosphere, biosphere, and soil) and the processes involved in that transit.

Want to learn more?

• To learn more about the water cycle and specifically the role of NASA satellite research in understanding the water cycle, see NASA's "The Water Cycle."
• A very simple interactive video called "Thirstin's Water Cycle" is also available on the EPA's Web site.
• The EPA site also has a collection of water-related K–12 resources!
• Teacher's Domain provides a lesson plan and multimedia resources that may be useful for your classroom. Though it is rated for grades 3–5, Cycling Water through the Environment has some interesting products including: "Observe precipitation," "Water cycle animation," and "The hydrologic cycle."
• The National Science Foundation provides a lot of information regarding water resources and humanity's dependence on and use of water, but this one in particular may be of interest.
• Several simple on-line model tools for understanding the water cycle and human influences on it are available from the Stroud Water Research Center, a co-host of the Christina River Basin CZO.

The chemistry of natural water, aqueous chemistry (or aqueous geochemistry), is a complex subject well beyond the reach of this course. However, some very basic concepts of water chemistry are important to help understand the role of water in the Critical Zone.

A hot spring in Yellowstone National Park.

Credit: Karan Witham-Walsh, 2008.

Humanity's use of water has a long history for obvious reasons—agua es vida! Civilization is thought to have first arisen in the Tigris-Euphrates River Valley, where irrigation canals have been used for at least 6,000 years. In addition, the site of vast water bodies (lakes, oceans) or flowing water in rivers and streams, provide aesthetic appeal and comfort for many, as well as providing sources for food, waste disposal, and recreation. For these reasons among others, humans have lived and worked near water, exerting strong and important influences on the water cycle, water quantity, and quality.

When Yellowstone National Park was established this feature was known as Solitary Spring and it did not erupt. Water was diverted from the spring to a swimming pool which lowered the water level sufficiently to start eruptions. Even though the diversion channel was filled and the water returned to its original level in the late 1940s, this thermal feature has not returned to its stable hot spring condition. A temporary change in the water level has led to a long-term change in the nature of this portion of the geyser basin, illustrating the delicate nature of these geothermal systems.

Credit: Karan Witham-Walsh, 2008

USGS has placed much of its surface water gauging data online using a Web-based tool called Streamstats. In this exercise, you will learn to use Streamstats to gather information about a study site. Ideally, this study site will be the same as the soil site you identified in Lesson 2. However, you will see that not all of the states have fully implemented this tool, so if you are from a state other than Pennsylvania and chose a soil site in that state, you may not be able to use Streamstats to evaluate your soil site. Read on.

Activity

Directions

1. Begin by visiting USGS Water Data for the Nation.
2. In upper left-hand corner select "Current Conditions" - see if the state where your soil study site is located (from our previous activity) is “Fully Implemented.” If it isn’t, choose a state that has fully implemented online data availability and use it for the remainder of this activity. Click on the state.
3. When you arrive at the state map you will see color-coded gauging stations plotted on the state map.
4. Begin to write a two-page report (described further in steps below) by answering the following questions (your responses will serve as the introduction for the report):
   • How does real-time streamflow in your state compare to historical data?
   • How does real-time streamflow in the gauging station nearest to your study site compare to historical data?
5. Identify the closest station to your study site. If you had to switch to another state in Step 3, then just pick a site of interest to you (explain why you chose it when writing up this part of the activity). Use the zoom function and you can use your cursor to hover over each site to learn the name and characteristics of the gauging station. If you click on the spot you can gather more information about the site, and if you click on the name of the site in that information box you will arrive at current conditions - and be able to determine what data is available from the site. What data is available from this nearest gauging station to your study site?
6. From the current conditions, plot as many data parameters as are available (up to 3) over a 60-day duration. Include the resulting plot(s) and describe your results in the report you began earlier in this activity. In your report, include responses to these questions:
   • Do any unusual events or anomalies exist in the data?
   • If so, to what do you attribute the anomalies?
7. Finally, go to Streamstats and visit and read the Documentation section.
8. Zoom in to the soil site you identified in Step 5 and delineate the drainage basin at the site and obtain the relevant information for your site.
9. As a third page to your report, include responses to the following questions along with a map of your basin:
   • What are the characteristics of your basin?
   • How do the flow data compare to the nearest gauging station?
   • To what do you attribute the similarity or difference (think about the location of your site relative to the nearest gauging station within the drainage network and basin).
10. Save your report as either a Microsoft Word or PDF file in the following format:

    L6_surfacewaterstudies_AccessAccountID_LastName.doc (or .pdf).

    For example, student Elvis Aaron Presley's file would be named "L6_surfacewaterstudies_eap1_presley.doc"—this naming convention is important, as it will help me make sure I match each submission up with the right student!

Grading criteria

You will be graded on the quality of your writing. You should not simply write responses to the questions and submit them to me. Instead plan on writing a short stand-alone paragraph (or page or whatever you decide is necessary considering any constraints I might have placed on you) so that anyone can read what you've written and understood it. You should strive to be specific and complete in responding to the questions. Your answers should be analytic, thoughtful and insightful, and should provide an insightful connection between ideas. The writing should be tight and crisp with varied sentence structure and a serious, professional tone.

The importance of water in the Critical Zone cannot be overstated. Water plays a primary role in physical and chemical weathering, erosion and transportation of sediments and dissolved ions, and the sustenance of life, to name a few processes. You should understand the water cycle and the various reservoirs in which water is stored and transported through the Critical Zone. You should also understand basic concepts linking water flow to the natural chemistry of water, and human influences on water resources. Finally, you should be confident in your ability to access widely available river and stream gauge data and should have used this data to learn about the surface water characteristics of your study site.

Want to learn more?

Here are some links to some Teacher's Domain resources (developed by Penn State!) that you might even want to use in your own classrooms:

• Lesson Plan: Acid Rock WebQuest
• Lesson Plan: Raising the pH

Reminder—Complete all of the lesson tasks!

You have finished Lesson 6. Double-check the list of requirements on the Lesson 6 Overview page to make sure you have completed all of the activities listed there before beginning the next lesson.