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Lesson plan Grade 10 Social Studies & Civics

Who Owns the Rain? Alderhythe's Cistern Rebate Ordinance

A grade 10 lesson where students size roof runoff and cistern storage for a fictional town, then compare a rebate program with a public detention basin.

Objectives

  1. Students will calculate roof runoff volume from roof area, rainfall depth, and a standard conversion factor.
  2. Students will compare the cost per gallon of stormwater storage created by a rebate and by a public basin.
  3. Students will identify who benefits from and who is left out of a rainwater capture incentive.

Vocabulary

Cistern
A large tank that stores rainwater collected from a roof so it can be used later, usually for watering plants.
Impervious surface
An impervious surface is any hard surface, like a road, roof, or parking lot, that rain cannot soak through. The more impervious surface a town has, the more stormwater rushes off it.
Detention basin
A detention basin is a dug-out area that fills with stormwater during a storm and then slowly drains empty over hours or days. It holds the water back so streets and streams downstream do not flood.

Also: runoff, rebate, ordinance, downstream user

Teacher overview

Rain that lands on a roof is water nobody has yet paid for, and towns disagree about who may keep it. Some communities pay residents to install cisterns because every gallon held on a lot is a gallon the storm sewer does not have to carry, while in parts of the western United States captured rainwater has been treated as water already promised to downstream users, so capture is limited by law. The misconception this lesson addresses is the assumption that rainwater harvesting is always both legal and purely beneficial, when a program can be popular and cheap and still shift water away from a creek that other people rely on. Students work the runoff arithmetic for a fictional town, price a proposed rebate against a public basin, and mark up the draft ordinance.

Procedure

Hook (6 min): Ask students to estimate how much water runs off the school roof in a one-inch rain. Record two or three guesses. Give them the standard conversion used by rainwater harvesting designers, which is roof area in square feet times rainfall in inches times 0.623 to get gallons, and let them see how far the guesses were off.

Hydrology and law brief (10 min): Explain where roof runoff goes in a town with storm sewers, and note that the average household in the United States uses about 300 gallons a day, so a single storm on a single roof can be a meaningful volume. Then explain the legal wrinkle: rain that would have reached a stream may already be counted in someone's water right, which is why capture rules differ by state.

Handout calculations (22 min): Students complete questions 1 through 8 in pairs with calculators. The scenario is fictional throughout, so every figure students need is printed on the handout. Circulate and watch for the common error of applying the rebate cap of $900 before checking whether the uncapped rebate is smaller.

Ordinance markup (12 min): In groups of four, students reread the ordinance terms at the top of the handout and propose two amendments in writing, each with a one-sentence reason. Require one amendment to address a group the current draft leaves out and one to address maintenance or overflow.

Exit ticket (5 min): Students answer questions 9 and 10 individually and turn them in. If most answers name only cost, prompt the class to reconsider what happens to the creek in a dry summer.

Who Owns the Rain? Alderhythe's Cistern Rebate Ordinance — Grade 10 Social Studies & Civics
Name: ____________________________ Date: ______________

Student Handout

Alderhythe, a fictional town in Garrowby County. The town council has drafted an ordinance offering a rebate of $0.55 for every gallon of cistern capacity a homeowner installs, capped at $900 per household, on the condition that the cistern overflow is routed to a rain garden. The county engineer has offered an alternative: build one public detention basin holding 312,000 gallons for $265,000. Use 0.623 gallons per square foot per inch of rain for all runoff calculations.

  1. A house in Alderhythe has a roof measuring 1,650 square feet. How many gallons run off that roof in a storm dropping 1.4 inches of rain? Round to the nearest gallon.
  2. The homeowner installs a 1,200-gallon cistern. Using your answer to question 1, and assuming all of that water reaches the cistern, how many gallons overflow the cistern during that storm?
  3. The council expects 260 households to install 1,200-gallon cisterns. How many gallons of storage would the program create in total?
  4. The rebate pays $0.55 per gallon of capacity, capped at $900 per household. What rebate does a household with a 1,200-gallon cistern receive?
  5. Using your answer to question 4, what does the rebate program cost the town if 260 households take part?
  6. The public detention basin holds 312,000 gallons and costs $265,000. What is its cost per gallon of storage, to the nearest cent?
  7. Using your answers to questions 3 and 5, what does the rebate program cost per gallon of storage, to the nearest cent?
  8. How much less per gallon does the cheaper of the two options cost? Give the amount and name the cheaper option.
  9. Name one group of Alderhythe residents the rebate as drafted cannot reach, and explain why.
  10. Alderhythe's cisterns hold back water that used to run into Bellow Creek within a few hours of a storm. Describe one effect on the creek and one effect on the town's storm sewers.

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: 1,439 gallons. Multiplying 1,650 square feet by 1.4 inches gives 2,310, and 2,310 times 0.623 is 1,439.13, which rounds to 1,439 gallons.
  2. Answer: 239 gallons. The storm produces 1,439 gallons and the cistern holds 1,200, so 1,439 minus 1,200 leaves 239 gallons of overflow. The 1,439 gallons is the rain falling on the roof, and a real system collects somewhat less, so the actual overflow would be smaller.
  3. Answer: 312,000 gallons. Multiplying 260 households by 1,200 gallons each gives 312,000 gallons of new storage.
  4. Answer: $660 per household. The uncapped rebate is 1,200 times $0.55, or $660, which is below the $900 cap, so the cap does not apply.
  5. Answer: $171,600. Multiplying $660 by 260 households gives $171,600.
  6. Answer: $0.85 per gallon. Dividing $265,000 by 312,000 gallons gives $0.8494, which rounds to $0.85.
  7. Answer: $0.55 per gallon. Dividing $171,600 by 312,000 gallons gives exactly $0.55, which matches the rebate rate because every household stayed under the cap.
  8. Answer: $0.30 per gallon, and the rebate program is cheaper. Subtracting $0.55 from $0.85 leaves $0.30 per gallon.
  9. Answer: Accept any group that cannot install a cistern on a property it owns, most obviously renters and apartment residents, and also households without the money to pay for the cistern before the rebate arrives. A strong answer notes that these residents still pay for the storm sewer through taxes or fees.
  10. Answer: Accept any sound pair. For the creek, less water arriving quickly after a storm means lower and slower peak flows, which reduces bank erosion but can also mean less flow reaching the creek in the days after a storm. For the storm sewers, the smaller peak volume reduces the chance of street flooding and delays the point at which the town must enlarge its pipes.

Differentiation

Support

Give students the runoff formula written out with labeled blanks for area, rainfall, and the conversion factor, and complete question 1 together as a class before releasing pairs. Allow answers to questions 6 and 7 to the nearest ten cents.

Extension

Have students find the cistern capacity at which the uncapped rebate would exceed the cap, then explain in writing which households the cap protects the town budget from and whether that limit is a fair way to hold spending down.

ELL supports

Pre-teach "runoff," "cistern," "rebate," and "detention basin" with a simple cross-section drawing showing a roof, a downspout, a tank, and an overflow pipe leading to a garden. Provide sentence frames for questions 9 and 10, such as "One group that cannot take part is ___ because ___."