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Lesson plan Grade 7 Science

Datchford Valley Menus: The Hidden Water Behind Our Food

A 50-minute grade 7 lesson where students compute the water footprint of foods, then build an evidence-based argument about how food choices add to water demand.

Objectives

  1. Students will calculate the water footprint of a food from its gallons-per-pound rate.
  2. Students will compare the water footprints of two menu choices using a ratio.
  3. Students will construct an evidence-based argument about food choices and water demand.

Vocabulary

Water footprint
All the water it takes to make the things we use and eat, counting water used far away. A hamburger's water footprint includes the water that grew the cattle's feed.

Also: irrigation, per-capita, freshwater withdrawal

Teacher overview

Most of the water a person "uses" never comes out of a tap; it is the water that grew their food. Farming accounts for roughly 70 percent of global freshwater withdrawals, so what a community eats shapes how much water it needs. This lesson centers on the fictional Datchford Valley, where a school cafeteria compares menu choices by their water footprint, the gallons of water it takes to produce them. A common misconception this lesson corrects is that food and water are separate topics; students often assume a meal's water use is only the water in the cup beside it, not the far larger amount used to grow the food.

Procedure

Hook (5 min): Ask students to guess how many gallons of water it takes to produce one quarter-pound beef patty. Write the guesses on the board, then tell them the answer will come from the day's math.

Direct instruction (12 min): Define water footprint as the total water used to produce a food. Give two rates from Water Footprint Network research: producing a pound of beef takes roughly 1,800 gallons of water, and a pound of wheat takes roughly 220 gallons. Note that farming is about 70 percent of global freshwater withdrawals, so these choices add up across a whole valley. Explain that much of a food's water footprint is rain that falls on pastures and feed fields, and that food is often grown far from where it is eaten, so these rates are global averages rather than water pumped from the valley's own rivers or wells.

Model the calculation (8 min): Work the first handout item as a class. A quarter-pound patty is 0.25 pound. At 1,800 gallons per pound, its footprint is 0.25 times 1,800, which is 450 gallons.

Guided and independent practice (15 min): Students complete the handout, computing the roll's footprint, the ratio between the two, and the week-long totals for 200 meals of each type.

Argument and share-out (8 min): Each student writes a two-sentence claim answering, "If Datchford Valley wants to lower the water demand of its school meals, which menu should it choose, and what evidence supports that?" Two or three students share; the teacher checks that each claim names a number from the handout as evidence.

Exit ticket (2 min): Students write the one food from the lesson with the largest water footprint. Collect and scan for "beef."

Datchford Valley Menus: The Hidden Water Behind Our Food — Grade 7 Science
Name: ____________________________ Date: ______________

Student Handout

Use these rates: a pound of beef takes about 1,800 gallons of water, and a pound of wheat takes about 220 gallons.

  1. A beef patty weighs a quarter pound (0.25 pound). What is its water footprint in gallons?
  2. A dinner roll uses about 0.1 pound of wheat. What is its water footprint in gallons?
  3. How many times larger is the patty's water footprint than the roll's? Round to the nearest tenth.
  4. In one week the cafeteria could serve 200 beef-patty meals or 200 wheat-roll meals. Find the total water footprint of each option, then find how many gallons the wheat-roll option would save.

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: 450 gallons.
    A quarter pound is 0.25 pound. 0.25 times 1,800 gallons per pound equals 450 gallons.
  2. Answer: 22 gallons.
    0.1 pound of wheat times 220 gallons per pound equals 22 gallons.
  3. Answer: About 20.5 times.
    450 gallons divided by 22 gallons equals about 20.45, which rounds to 20.5, so the patty's footprint is about 20.5 times larger.
  4. Answer: 85,600 gallons saved.
    200 patties at 450 gallons each is 90,000 gallons. 200 rolls at 22 gallons each is 4,400 gallons. 90,000 minus 4,400 equals 85,600 gallons the wheat-roll option would save.

Differentiation

Support

Provide the serving-size decimals (0.25 pound and 0.1 pound) already set up next to the correct rate, so students focus on the multiplication. Allow calculators and give a worked model for item 1.

Extension

Ask students to add a third menu item with its own gallons-per-pound rate they research from the class facts, then revise their argument to rank all three choices from lowest to highest water footprint with evidence.

ELL supports

Pre-teach "water footprint," "irrigation," and "per-capita" with a picture of a hamburger beside a large water tank and a roll beside a small cup. Provide a claim frame: "Datchford Valley should choose ___ because it uses ___ gallons less water."