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Lesson plan Grade 10 Science

Longmeadow Watershed: How Long Water Stays in Each Reservoir

A 50-minute grade 10 lesson where students calculate residence time by dividing stored water by throughflow and compare reservoirs across the water cycle.

Objectives

  1. Students will calculate the residence time of a water reservoir by dividing stored volume by throughflow.
  2. Students will compare residence times across streams, reservoirs, and groundwater and explain the consequences for pollution and supply.

Vocabulary

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.
Reservoir
A human-made lake, usually created by a dam, that stores water for later. Reservoirs supply cities and farms and help control floods.
Recharge
Recharge is water soaking down through the ground to refill an aquifer, the way rain refills an underground sponge. Aquifers stay healthy only if recharge keeps up with pumping.
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.

Also: throughflow

Teacher overview

Every part of the water cycle stores water for a different length of time. A raindrop passes through a mountain stream in hours, but a molecule that soaks into a deep aquifer may stay for centuries. That average storage time is called residence time, and it equals the water stored in a reservoir divided by the rate water flows through it. This lesson uses a fictional watershed to make the idea concrete and quantitative. A common misconception the lesson addresses is that all water "moves" at the same pace, when in fact residence time can range from part of a day to a thousand years across reservoirs in the same watershed.

Procedure

Hook (5 min): Project three images: a cloud, a fast stream, and a well tapping deep groundwater. Ask students to rank which water is, on average, the "oldest," and record their votes without confirming an answer.

Direct instruction (12 min): Define residence time as stored volume divided by throughflow. Model one worked example on the board using a fictional pond that holds 600 million gallons and passes 200 million gallons per day: residence time = 600 divided by 200 = 3 days. Stress that the units of the answer are time, and that a larger store or a slower flow both lengthen residence time.

Guided practice (13 min): Work handout questions 1 and 2 together, one about the Longmeadow Reservoir and one about a small stream, so students practice both a slow and a fast reservoir before working alone.

Independent practice (15 min): Students complete handout questions 3 through 6 on their own, covering a deep aquifer, a comparison across reservoirs, and a scenario in which withdrawals change the flow.

Closure and exit ticket (5 min): Each student answers one card question: "A reservoir stores 800 million gallons and passes 400 million gallons per day. What is its residence time?" Collect the cards; if fewer than 80 percent divide correctly (answer: 2 days), reteach the setup before the next class.

Longmeadow Watershed: How Long Water Stays in Each Reservoir — Grade 10 Science
Name: ____________________________ Date: ______________

Student Handout

The Longmeadow watershed is fictional, and every value below is given for it. For each item, residence time = stored water divided by the rate water flows through, and the answer is a length of time.

  1. Longmeadow Reservoir holds 9,000 million gallons and passes 150 million gallons of water through each day. Find its residence time in days.
  2. A mountain stream in the watershed holds 20 million gallons and carries 40 million gallons per day. Find its residence time in days.
  3. The valley aquifer stores 3,650,000 million gallons and recharges and discharges 10 million gallons per day. Find its residence time in days, then convert it to years (divide by 365).
  4. Rank your three residence times from shortest to longest, and explain why a pollutant spilled into the aquifer is a much longer-lasting problem than one spilled into the stream.
  5. On average, water stays in the atmosphere about 9 days. Is the water in Longmeadow Reservoir, on average, younger or older than atmospheric water? Use your item 1 answer to explain.
  6. Suppose Longmeadow doubles its withdrawals, so the reservoir's throughflow rises from 150 to 300 million gallons per day while its storage stays at 9,000 million gallons. Find the new residence time and explain what a shorter residence time means for how quickly the reservoir refreshes.

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: 60 days. 9,000 million gallons divided by 150 million gallons per day = 60 days.
  2. Answer: 0.5 day. 20 million gallons divided by 40 million gallons per day = 0.5 day, or about 12 hours.
  3. Answer: 365,000 days (1,000 years). 3,650,000 divided by 10 = 365,000 days; 365,000 divided by 365 = 1,000 years.
  4. Answer: Stream (0.5 day), reservoir (60 days), aquifer (1,000 years). A spill in the aquifer lingers because the water there is replaced only once in about a thousand years, so contamination is flushed out extremely slowly, while the stream replaces its water twice a day.
  5. Answer: Older. The reservoir's residence time of 60 days is far longer than the atmosphere's 9 days, so on average its water has been stored much longer.
  6. Answer: 30 days. 9,000 divided by 300 = 30 days. A shorter residence time means the reservoir's water is replaced twice as fast, so it refreshes more quickly but also draws down faster if inflow cannot keep up with the higher withdrawals.

Differentiation

Support

Give students a partially completed table with the storage and flow already placed in the correct columns so they only perform the division, and provide the worked pond example from the direct-instruction step as a reference card.

Extension

Have students solve an inverse problem: given a target residence time and a fixed storage, calculate the maximum daily withdrawal a reservoir could sustain, then explain how a water manager could use that number to set a drought withdrawal limit.

ELL supports

Pair "residence time," "reservoir," and "throughflow" with a simple picture of a tank with water flowing in one pipe and out another. Provide the sentence frame "The water stays about ___ because the store is ___ and the flow is ___," and allow the ranking in question 4 to be shown with a number line.