Classroom activity Grade 11 Social Studies & Civics
Vireo's Next Acre-Foot: A Grade 11 Water Supply Planning Activity
A grade 11 simulation where teams plan a fictional city's new water supply by marginal cost, choosing among conservation, reuse, and desalination options.
Objectives
- Students will build a least-cost water supply portfolio that meets a fixed target.
- Students will distinguish the marginal cost of the last acre-foot from the average cost of the whole plan.
- Students will defend a supply choice using cost and at least one non-cost factor.
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.
- Desalination
- Turning salty water, like ocean water, into fresh water people can use. It works, but it takes a lot of energy and leaves behind very salty water that must be handled carefully.
Also: marginal cost, average cost, water reuse, portfolio
Teacher overview
This is a team decision simulation for groups of three or four, run in a single class period. Each team acts as the water-planning staff of the fictional city of Vireo, which must secure 20,000 acre-feet of new water each year. Teams choose among four supply options, each with a stated yield and a stated cost per acre-foot, and the activity teaches the core economic idea directly: because the cheapest options are limited, meeting the target forces teams up a cost ladder, so the marginal cost of the last acre-foot is higher than the average cost of the whole plan. A council round then adds a real-world complication and makes teams revise.
Activity Directions
- Set up (5 min): Divide the class into water-planning teams of three or four. Give each team the supply-option handout and recording sheet for the fictional city of Vireo. Explain the goal: assemble supply options that total exactly 20,000 acre-feet per year at the lowest annual cost.
- Teach the core move (5 min): Explain marginal cost with a quick example: if a team has already bought the cheapest water available and still needs more, the next acre-foot must come from a costlier source, so each additional block of supply tends to cost more than the last.
- Build the portfolio (12 min): Teams select options in any order but should discover that starting with the lowest cost per acre-foot minimizes total cost. They record the yield taken from each option (a source may be used partially) until the selected yields sum to 20,000 acre-feet.
- Compute (8 min): Each team calculates the total annual cost of its plan, the average cost per acre-foot (total cost divided by 20,000), and the marginal cost of its final acre-foot (the cost of the most expensive source it had to tap).
- Council round (10 min): Announce a complication for the fictional city of Vireo: a drought cuts the conservation option's available yield in half, so teams must replace that lost water from a costlier source. Teams revise their portfolios and recompute the total and marginal cost.
- Debrief (10 min): Teams report their plans. Close with two questions tied to the activity: why did the marginal cost of the last acre-foot end up higher than the average cost, and which non-cost factor would most change your ranking of the options? Use the answer key to confirm the least-cost figures.
Student Handout
Your team is the water-planning staff for the fictional city of Vireo. Secure 20,000 acre-feet of new water per year at the lowest annual cost. You may take all or part of any option's yield.
| Option | Yearly yield (acre-feet) | Cost per acre-foot |
|---|---|---|
| Conservation program | 6,000 | $700 |
| Leak repair and main replacement | 5,000 | $1,100 |
| Water reuse (recycled water) | 7,000 | $1,900 |
| Seawater desalination | 12,000 | $2,800 |
Recording sheet: for each option, write the acre-feet you take and multiply by the cost per acre-foot. Sum the yields (must equal 20,000) and sum the costs for your total annual cost.
Answer these questions on your recording sheet:
- List your least-cost portfolio. How many acre-feet do you take from each of the four options so the yields add up to exactly 20,000?
- What is the total annual cost of your plan, in dollars?
- What is the average cost per acre-foot? Divide your total annual cost by 20,000.
- What is the marginal cost of your final acre-foot? Use the cost per acre-foot of the most expensive source you had to tap.
- Why is the marginal cost of the last acre-foot higher than the average cost of the whole plan?
- Council round: a drought cuts the conservation option's yield in half, from 6,000 to 3,000 acre-feet. Rebuild your least-cost portfolio so it still totals 20,000 acre-feet, find the new total annual cost, and name one non-cost factor that would change how you rank the options.
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Answer Key
- Answer: Least-cost portfolio: 6,000 acre-feet conservation, 5,000 acre-feet leak repair, 7,000 acre-feet reuse, and 2,000 acre-feet desalination, which sums to 20,000 acre-feet. Taking the cheapest sources first minimizes total cost.
- Answer: $28,600,000 total. The costs are 6,000 times $700 ($4,200,000), 5,000 times $1,100 ($5,500,000), 7,000 times $1,900 ($13,300,000), and 2,000 times $2,800 ($5,600,000), which sum to $28,600,000.
- Answer: $1,430 per acre-foot average. $28,600,000 divided by 20,000 acre-feet equals $1,430 per acre-foot.
- Answer: $2,800 per acre-foot marginal cost. The last and most expensive source tapped is desalination, so the final acre-foot costs $2,800, well above the $1,430 average.
- Answer: The marginal cost exceeds the average because the cheap options run out; once conservation, leak repair, and reuse are exhausted, the target can only be met with costlier desalination, which pulls the last unit's cost above the blended average.
- Answer: Council-round result: replacing 3,000 lost conservation acre-feet with desalination raises desalination use to 5,000 acre-feet and the total to about $34,900,000. Reasonable non-cost factors include drought reliability, energy use and brine disposal for desalination, public acceptance of reuse, and how the cost is shared among ratepayers.
Differentiation
Support
Give teams a partially filled recording sheet with the conservation and leak-repair rows already computed, so they only complete the reuse and desalination rows and the totals. Provide a worked marginal-versus-average example before step 3.
Extension
Ask teams to add a fifth option they research using only the handout's logic, such as a higher-cost but drought-proof source, assign it a plausible yield and cost, and explain how adding it changes the least-cost portfolio and the marginal cost.
ELL supports
Pre-teach "marginal cost," "average cost," "acre-foot," "reuse," and "desalination" with a simple ladder diagram showing each source stacked from cheapest to most expensive. Allow teams to present their plan using the recording sheet and short phrases rather than a full spoken explanation.