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Lesson plan Grade 7 Science

Larch Basin's Water Cycle: Where Water Waits and How Long

A 45-minute grade 7 lesson where students model the water cycle driven by sun and gravity, then use a 1,000-marble jar to picture how Earth's water is split.

Objectives

  1. Students will model the water cycle as movement driven by the sun and gravity.
  2. Students will convert the percent split of Earth's water into counts in a 1,000-marble model.
  3. Students will explain what a long residence time says about how fast water moves.

Vocabulary

Water cycle
The water cycle is the continuous movement of water between the oceans, the air, and the land. The same water evaporates, forms clouds, falls as rain or snow, and flows back to the ocean before the process repeats.
Residence time
Residence time is how long a drop of water typically stays in one part of the water cycle before moving on. Water passes through the air quickly but can stay underground for a very long time.
Aquifer
An aquifer is an underground layer of rock, sand, or gravel that holds water in the tiny spaces between its grains. Wells reach down into aquifers to bring water up to the surface.
Evaporation
Evaporation happens when liquid water, often warmed by the sun, turns into an invisible gas called water vapor that rises into the air. It is how water leaves oceans, lakes, and puddles.

Also: runoff

Teacher overview

The water cycle is often drawn as a neat loop, but two ideas get lost in that picture: what keeps the water moving, and how long water pauses in each place. This lesson uses a fictional watershed, Larch Basin, to model the cycle as movement powered by energy from the sun and the pull of gravity, then uses a 1,000-marble jar to show how little of Earth's water is fresh and reachable. A common misconception this lesson corrects is that all water moves at the same speed; in fact water spends about 9 days in the air but can spend centuries in deep groundwater.

Procedure

Hook (5 min): Ask students where a single raindrop that falls on Larch Basin might be one year later, and one hundred years later. Collect three or four guesses on the board without judging them.

Direct instruction (12 min): Draw the Larch Basin water cycle on chart paper. Label the ocean, the air, clouds, rain and snow, the river, and the aquifer below the ground. Add two labels that students often forget: the sun, which supplies the energy that evaporates water, and gravity, which pulls rain down and moves rivers and groundwater downhill. State the split of Earth's water: about 97 percent sits in the oceans, about 2 percent is frozen in ice, and less than 1 percent is fresh water people can reach.

Guided practice (13 min): Introduce residence time as the average length of time water stays in one place. Give two facts: water spends about 9 days in the atmosphere, but water in deep groundwater can stay for centuries. Ask students, in pairs, to mark each part of their diagram as "fast" or "slow" and to explain their choice. Then model the first marble-jar calculation from the handout together: 97 percent of 1,000 marbles is 970 marbles for the ocean.

Independent practice (10 min): Students complete the handout, finishing the marble-jar counts and the residence-time question on their own.

Closure and exit ticket (5 min): Students answer one question on an index card: "Name the two things that keep the water cycle moving." Collect and scan for both "the sun" (energy) and "gravity" before dismissal.

Larch Basin's Water Cycle: Where Water Waits and How Long — Grade 7 Science
Name: ____________________________ Date: ______________

Student Handout

Use the split of Earth's water for a jar of 1,000 marbles: about 97 percent ocean, about 2 percent frozen ice, and the small amount left over as liquid fresh water in lakes, rivers, and groundwater.

  1. If 97 percent of the 1,000 marbles are ocean water, how many marbles is that?
  2. If 2 percent of the 1,000 marbles are frozen ice, how many marbles is that?
  3. How many marbles are left for liquid fresh water once the ocean and ice marbles are set aside?
  4. Water spends about 9 days in the air but centuries in deep groundwater. Which has the longer residence time, and what does that tell you about how fast that water moves?

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key
  1. Answer: 970 marbles.
    97 percent of 1,000 is 0.97 times 1,000, which equals 970 marbles for the ocean.
  2. Answer: 20 marbles.
    2 percent of 1,000 is 0.02 times 1,000, which equals 20 marbles of frozen ice.
  3. Answer: 10 marbles.
    The ocean and ice take 970 plus 20, which is 990 marbles. 1,000 minus 990 leaves 10 marbles for liquid fresh water (the ice marbles are frozen fresh water).
  4. Answer: Deep groundwater; it moves very slowly.
    Centuries is far longer than 9 days, so deep groundwater has the longer residence time, which means that water moves through the ground very slowly compared with water in the air.

Differentiation

Support

Give students a diagram of the Larch Basin cycle that is already labeled, so they can focus on marking each part "fast" or "slow" and on the marble-jar counts. Provide the first two marble calculations worked out.

Extension

Ask students to research, using only the class facts, why the small fresh-water share matters for a town that depends on an aquifer, and to write two sentences connecting slow groundwater residence time to the risk of over-pumping a well.

ELL supports

Pre-teach "residence time," "aquifer," "evaporation," and "runoff" with arrows on the diagram and a gesture for each (fingers rising for evaporation, falling for rain). Provide a sentence starter for the exit ticket: "The water cycle keeps moving because of ___ and ___."