UNIT 1 Water Cycle The water planet The Earth has often been called the “water planet” as nearly three-fourths of our planet’s surface is covered with water. Water is continuously changing its form but the total amount of water on Earth remains constant. Water exists in three forms; solid (ice), liquid (lakes, oceans, rain, etc.), and gas (water vapor, clouds). Oceans and seas account for approximately 97% of the total water supply. The polar world water supply icecaps and glaciers account for about 2%, rivers and lakes (including salt lakes) make up less than .02%, and groundwater makes up about .6% of the water on Earth. The remaining water is in the form of water vapor and soil moisture. Water is generally classified by where it’s located. When it flows over the Earth through rivers and streams or collects on the Earth in oceans, glaciers, lakes or swamps, it’s referred to as surface water. Water found below the Earth’s surface is called groundwater. Atmospheric water is the term used for water vapor and clouds. Limited, renewable resource Water is a renewable, but limited, resource. It is renewable because the same amount of water is perpetually passing through a global cycle known as the water, or hydrologic, cycle. Water is a limited resource because its availability in the fresh, liquid form varies greatly throughout our planet, and water is not evenly distributed throughout the globe. For example, limited precipitation and high evaporation rates in the Sonoran Desert result in water scarcity. The majority of fresh water in arctic regions is frozen and therefore unavailable to living things. Many cycles in one The water cycle is commonly described as a series of processes occurring in a predictable pattern: condensation, precipitation, runoff, evaporation. This concept of the water cycle is a simplified model that falls far short of conveying the variety and complexity of water’s movement around the globe. The water cycle is actually many, many cycles requiring greatly varying times and distances to come full circle. An example of one unique cycle, which is common in the Sonoran Desert, is the cloud – virga (rain that never reaches the Earth) – water vapor – cloud cycle, where water droplets fall from a cloud and evaporate before they even hit the ground. This cycling of water may take only a few hours. At the other extreme is a cloud – snow – glacier – water vapor – cloud cycle at the North or South Poles. Snow falling and becoming part of a glacier may remain frozen for thousands of years before evaporating and once again condensing into a cloud. Watershed The water cycle, while occurring at the local level, is a global system. To better understand the movement of water on a local scale, one can focus on the watershed. The total land area that contributes water to a particular drainage channel (wash, arroyo, or stream) is called its watershed. For example, the rain or snowmelt from Mount Lemmon that

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flows south into Sabino Canyon belongs to the Sabino Creek watershed. Sabino Creek flows into the Tanque Verde Wash and becomes part of the larger Tanque Verde Wash watershed. The Tanque Verde Wash runs into the Rillito River and becomes partof the Rillito River watershed. And water from the Rillito River runs into the Santa Cruz River, entering an even greater watershed. Unless a system is established which imports water from outside a local area (e.g., Colorado River water via the Central Arizona Project), the water supply of a community comes from the immediate watershed. In recent history, long periods of time pass during which there is little or no runoff in the major waterways that pass through Tucson. It is important to remember, however, that the Santa Cruz River flowed perennially at one time, and that as recently as 90 years ago surface water from the river was Tucson’s primary water source. Extensive human use has caused surface water in our valley to largely disappear from above the Earth’s surface. Even though Tucson’s source of water has changed from surface water to groundwater over the years, the four mountain ranges surrounding the city still funnel water into the valley.

Source: UofA Water Resources Research Center

The groundwater/surface water connection Understanding the intrinsic connection between surface water and groundwater is essential to grasping why the Santa Cruz River doesn’t flow as it once did. When the aquifer beneath Tucson held a maximum capacity of water, the water table was close to the surface of the Earth and there were few unsaturated pore spaces in the upper ground layers. There was little physical space to allow runoff water to percolate. Therefore, the majority of runoff remained on the surface, flowing through the various washes and rivers. However, the rapid decline in the water table started in the 1940s, when groundwater pumping in the Tucson Valley began in earnest. Large quantities of groundwater have been removed from the aquifer, emptying pore spaces and allowing for greater, and deeper, percolation. This downward movement of the groundwater will continue as long as more water is pumped from the aquifer than is recharged, either naturally or artificially. So, water that used to flow has gone below.

Elevation of the water table and direction of groundwater.

Gravity is an important force affecting the flow of water both above and below ground. Just as water in rivers and washes flows from higher to lower elevations, so too does groundwater. Because surface water moves overland with great exposure to the open air, its flow is much less restricted by friction than groundwater squeezing its way through the confines of tight, subsurface fissures and pore spaces. Nevertheless, subterranean water does flow, although the flow is so slow that it is measured in feet per year. Elevation change in the topography of the Tucson watershed results in a general northwest flow (towards Casa Grande) of both the surface waters and groundwater.

Now you see it, now you don’t Many of the water cycle processes are familiar and understandable to us largely because they are easily visible. We’ve all seen raindrops and snow falling out of the sky. Everyone has watched water running off over the desert floor after a heavy rain. When we irrigate plants we can observe water percolating into the soil. However, it is more difficult to fully understand (and teach) the processes of evaporation and transpiration because the movement of the water vapor is largely invisible. This is especially true in an arid environment such as the one we inhabit. Annual evaporation rates in the Sonoran Desert range from 70 to 84 inches per year. In contrast, Tucson’s average annual rainfall is a mere 12 inches.

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Many plants and animals of the Sonoran Desert have fascinating behavioral or physical adaptations which help them overcome the scarcity of water, due in part to the high evaporation rates in the desert. Transpiration is the process of plants giving off water vapor through the leaves or stems. All plants transpire to some extent during the processes of respiration and photosynthesis, as they convert water and carbon dioxide (CO2) to starch. In order for plants to exchange gases with the atmosphere they must open their stomates (small pore spaces), thus losing moisture. However, many plants adapted to desert regions transpire much less than plants adapted to regions where water is abundant.

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Activity 1.1 Invisible Water

*Activity adapted from “Transpiration/Weeping Leaves,” Educating Tomorrow’s Hydrologists. 1996. Arizona Hydrological Society. At a Glance By trapping water transpired by plants, students are able to better see the water cycle process of transpiration in action. This activity also involves identifying differences in transpiration rates between low and high water use plants. Arizona Department of Education Academic Standards Please refer to the Arizona Department of Education Academic Standards section for the ADE standards addressed by this lesson. Learning Objectives Students will be able to:

1) Explain the various processes of the hydrological cycle. 2) Explain how plants take water from the soil and transpire water into the atmosphere. 3) Interpret classroom data and graph the results.

Materials water cycle drawing (Student Activity Book) data collection sheet (Student Activity Book) For each pair of students: sturdy plastic bag permanent marking pen small, nickel-sized pebble masking tape or twist-ties cardboard disc ruler graduated cylinder Procedure Part 1 The first thing to do is to walk around your schoolyard to determine how many kinds of plants are available for this lesson. Make special note of those plants located “out of the way” as students will be leaving plastic bags attached to some plants for at least 24 hours. Also, it is important to notify maintenance personnel so that the experiments are left alone. Begin the lesson by asking your students to recall what they know about the water cycle. Ask student volunteers to come up to the chalkboard and list the different processes included in the water cycle (condensation, precipitation, evaporation, transpiration, runoff, percolation). Once the list is complete, hand out the Student Activity Books to all students and have them

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turn to the Water Cycle drawing. Ask them to write in the water cycle process names in the correct numbered spaces, and draw arrows in the appropriate place and pointing in the correct direction to convey the cycling of water. When the students have completed drawing in the arrows, tell them that this activity is called “Invisible Water,” and ask them if water can become invisible and under what circumstances. Ask them to circle the names of the most invisible processes on the Water Cycle page. See that everyone understands and agrees that evaporation and transpiration are the invisible processes. Conduct a brief discussion of evaporation and transpiration and the roles they play in Tucson’s local water cycle, using the Teacher Background Information. Consider asking your students:

“What do all living things require to survive?” “In what ways do plants use water?” “What plant structures are involved in the transport of water?”

Help students to identify that roots bring water up to the leaves where photosynthesis occurs. Mention that in some plants, including many Sonoran Desert plants, photosynthesis takes place in the bark or exterior tissue of the plant (e.g. palo verdes). Discuss that plants transpire water from the leaves or stems through openings called stoma. Ask, “Do all plants transpire the same amount?” “Do cactus transpire?” Discuss the fact that many native Sonoran plants have very small leaves or no leaves at all. Discuss the availability of plants on campus and take the class on a tour of the grounds, pointing out plants appropriate for the activity. Ask students to hypothesize what the outcome of the experiment will be. Write the hypotheses on a sheet of chart paper, an overhead transparency, or on the chalkboard and save until the end of Part 2. Divide the class into pairs. Explain that each pair of students is to choose a plant on campus to which they will attach a plastic bag to collect transpired water. Try to have as many different types of plants represented as are available to you on campus. Show students the approximate amount of leaf cover to place in each bag so that there is some consistency among the various samples. It may be helpful to hand out equal-sized cardboard discs to each pair that they may use to measure the proper amount of leaf cover. Remind students to drop a small pebble in the bag and create a tight seal with the masking tape or twist-ties, while being careful not to damage the leaves or stems. Warn them that spines puncturing the bags will allow water to escape. Have students write their names on their plastic bags using a permanent marking pen. Send students out to attach bags with masking tape. When the various teams are in the process of attaching the bag, have them make the necessary measurements and observations to answer the questions on the Data Collection Sheet for Part 1 in the Student Activity Handbook. Part 2 After one or two days, have students carefully remove the bags, being careful not to spill any of the collected water. Have students measure how much water their bag contained using the graduated cylinders, and then have them record the volume in the appropriate space on the Data Collection Sheet for Part 2 in the Student Activity Book. Have students post the results on the chalkboard or overhead projector by plant type or species. Students can then copy the class data and find the average for each plant type or species, making a bar graph of the results on the Data Collection page.

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Review the experiment with the class. Refer back to the water cycle drawing and review the part that transpiration plays in the cycle. Ask, “What did you discover?” Focus the students’ attention on differences in the amount of water collected from the various plant types or species. Refer to the hypotheses generated by the class in Part 1. Compare the students’ conclusions with their hypotheses.

Activity 1.1 Invisible Water Answers

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Activity 1.2 Miniature Mount Lemmon

*Activity adapted from "Arizona Water Sources," Arizona Water Story. 1998. Central Arizona Project. At a Glance By creating a simple watershed model and reading maps, students understand what a watershed is and become more familiar with the Tucson watershed. Arizona Department of Education Academic Standards Please refer to the Arizona Department of Education Academic Standards section for the ADE standards addressed by this lesson. Learning Objectives Students will be able to:

1) Define the terms “surface water” and “watershed”. 2) Understand the flow of water into and out of the Tucson watershed. 3) Identify the major drainages in the Tucson watershed on a map.

Materials tucson watershed map (Student Activity Book) heavy grade white paper sheets aluminum pie plates or other wide, shallow containers dixie cups blue tempera or poster paint film canisters water Procedure Explain to the students that they are going to create miniature models of a watershed in the Santa Catalina Mountains (the mountain range where Mt. Lemmon is). Explain to them that the total land area that contributes water to a particular drainage channel (wash, arroyo, or river) is called its watershed. In the models, the watershed will be revealed as the colored water accents the drainage pattern of water running downhill out of mountains to lower places in the valleys below. Generate discussion about the Tucson watershed using the Teacher Background Information. Discussion questions: “Do any of you live near any major rivers or washes, or is our school located near any?”" “Have you ever seen water flowing in Tucson’s rivers or washes?” “What is the source of the water that you’ve seen flowing in the rivers or washes?”" “Do you think groundwater flows in a particular direction?” Give each student a piece of white paper (heavier grades of paper work best). Direct them to place their index finger in the middle of the paper and crumple the rest of the sheet around their extended finger and hand. This should form a rough model of a mountain with the creases representing canyons and drainages on the sides of the mountain. Have the students place their mountain models over a dixie cup in an aluminum pie plate, and then place a drop of concentrated blue tempera or other water-soluble paint on the highest point of the “mountain.” Have students fill the film canister

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half-way and slowly pour water over where they placed the paint and ask them to observe the flow of water. The color from the paint will show the drainage pattern down sides of the mountain. Ask the students what they observed. If clarification is needed, explain to them that the water in the small valleys coming down from the mountain peak drains into creeks, the creeks run together into larger streams and the streams eventually run together to form rivers. The area drained from the highest point or ridges into a lower basin or valley is called a watershed. In the Student Activity Book have the students identify the listed waterways and mountain ranges on the Tucson Watershed Map. Ask them to draw arrows conveying their guess as to the general direction of flow of the Tucson waterways.

Activity 1.2 Miniature Mount Lemmon Answers

1. Santa Catalina Mountains 2. Tucson Mountains 3. Rincon Mountains 4. Santa Cruz River

5. Rillito River 6. Pantano Wash 7. Tanque Verde Creek 8. Cañada del Oro Wash

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