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Classroom activity Grade 9 Social Studies & Civics

Rennick's Next Acre-Foot: A Water-Supply Cost Ranking Activity

A grade 9 simulation where teams act as a fictional city's water board, ranking supply options by marginal cost to fill a demand gap at the lowest total cost.

Objectives

  1. Students will rank water-supply options from lowest to highest marginal cost.
  2. Students will fill a demand gap at the lowest total cost by buying cheaper sources first.
  3. Students will justify a supply choice using cost and reliability.

Vocabulary

Acre-foot
The amount of water it takes to cover one acre of land, about the size of a football field, one foot deep. Western states use acre-feet to measure large amounts of water.

Also: marginal cost, demand gap, supply portfolio, reliability

Teacher overview

Water planners rarely have one source of new water; they have a menu of options that each cost a different amount per unit. This activity turns that menu into a simulation for teams of three to four students, who act as the water board of the fictional city of Rennick. Each team receives cards for six supply options, ranks them by marginal cost, and then "buys" sources cheapest-first to fill the city's demand gap. The activity teaches marginal cost directly: because each option costs a different amount per acre-foot, the order in which a city buys them decides the total bill.

Activity Directions

  1. Before class, print one set of the six supply-option cards and one Student Handout per team. Arrange desks so each team of three to four students can lay the cards out in a row.
  2. Open by explaining the goal in the fictional city of Rennick: the city needs 10,000 acre-feet of new water per year, and the board must meet that demand gap at the lowest total cost. Note that one acre-foot equals 325,851 gallons, so these are large volumes.
  3. Round 1 (10 min): Rank the cards. Each team places its six cards in order from the lowest marginal cost per acre-foot to the highest. Tell teams to check the cost line on each card, not the yield, when ordering.
  4. Round 2 (12 min): Fill the gap. Starting from the cheapest card, teams add each option's yield until the running total reaches or passes 10,000 acre-feet. They set aside any options they did not need. The last card they add is the marginal source.
  5. Check for understanding: ask each team to hold up the card they identified as the marginal source. If teams disagree, have two teams explain their ordering, and confirm the correct marginal source before continuing.
  6. Round 3 (10 min): Deliberate. Pose one twist. Rennick's river is expected to run low in droughts. Each team decides whether to swap in a more expensive but more reliable source, and prepares a one-sentence justification to present to the "council."
  7. End the activity when every team has ranked its cards, filled the demand gap, and prepared its deliberation statement. Gather the class for the debrief.
  8. Debrief using the Student Handout questions, then reveal the correct cost ranking and marginal source from the Answer Key.
Rennick's Next Acre-Foot: A Water-Supply Cost Ranking Activity — Grade 9 Social Studies & Civics
Name: ____________________________ Date: ______________

Student Handout

Suppose you sit on the water board of the fictional city of Rennick. The city needs 10,000 acre-feet of new water each year. Your six supply options are below, each with a marginal cost per acre-foot and a yearly yield.

  • Leak repair: $150 per acre-foot, yields 1,500 acre-feet per year.
  • Conservation rebates: $300 per acre-foot, yields 2,000 acre-feet per year.
  • New groundwater wells: $600 per acre-foot, yields 2,500 acre-feet per year.
  • Water reuse: $900 per acre-foot, yields 4,000 acre-feet per year.
  • Imported water: $1,200 per acre-foot, yields 5,000 acre-feet per year.
  • Seawater desalination: $2,000 per acre-foot, yields 6,000 acre-feet per year, and uses about 11 to 15 kilowatt-hours of energy per 1,000 gallons.

Answer these questions after the rounds:

  1. List the six options in order from lowest to highest marginal cost per acre-foot.
  2. Buying cheapest-first to reach 10,000 acre-feet, which options does Rennick need, and which one is the marginal (last-added) source?
  3. Does Rennick need to build seawater desalination to meet this demand gap? Explain using your ranking.
  4. Why does buying options in order of increasing marginal cost lead to the lowest total cost?
  5. Name one reason the board might still choose a more expensive source over a cheaper one.

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: Leak repair ($150), conservation rebates ($300), new groundwater wells ($600), water reuse ($900), imported water ($1,200), seawater desalination ($2,000). That is the order from lowest to highest marginal cost per acre-foot.
  2. Answer: Rennick needs leak repair (1,500), conservation rebates (2,000), groundwater wells (2,500), and water reuse (4,000), which total 10,000 acre-feet. Water reuse is the marginal source, because the running total reaches the 10,000 goal exactly when reuse is added.
  3. Answer: No. The four cheapest sources already reach 10,000 acre-feet, so Rennick does not need to build the most expensive option. Desalination and imported water can be set aside for this demand gap.
  4. Answer: Because each acre-foot bought from a cheaper source costs less, filling the gap with the least expensive options first spends the fewest dollars for the same total water, which is the point of marginal cost reasoning.
  5. Answer: Accept any sound reason, such as reliability during a drought, protection if a cheaper source fails, faster availability, or greater public acceptance. A board may pay more per acre-foot for a source it can count on when the river runs low.

Differentiation

Support

Give teams that need support cards with the marginal cost printed in large type and the yields removed for Round 1, so students focus only on ordering by price before yields are added back for Round 2. Provide a number line to place the six costs.

Extension

Ask teams to add a seventh blank card for a new option they invent, assign it a realistic marginal cost and yield, place it correctly in the ranking, and explain how it would change which source is marginal.

ELL supports

Pre-teach "marginal cost," "acre-foot," "demand gap," and "reliability" with a simple picture of a row of price tags ordered from low to high. Allow students to present the Round 3 justification using the sentence frame: "We chose ___ because it is more ___ even though it costs more."