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Lesson plan Grade 7 Science

Tarlton Farm Irrigation Efficiency: A Grade 7 Water-Use Lesson

A 50-minute grade 7 lesson comparing flood and drip irrigation and crop water footprints to show how farms can withdraw less water from rivers and aquifers.

Objectives

  1. Students will calculate the water used to grow crops using water-footprint values.
  2. Students will compare how much water reaches crop roots under flood and drip irrigation.
  3. Students will explain how efficient irrigation reduces water withdrawn from rivers and aquifers.

Vocabulary

Drip irrigation
Watering plants through tubes that release water slowly, drop by drop, right at the roots. Very little water is wasted along the way.
Flood irrigation
Watering crops by letting water flow across the field or down furrows between the rows. It is one of the oldest irrigation methods, and it can lose a lot of water.
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, efficiency

Teacher overview

Growing food takes a great deal of water, and farms account for about 70 percent of the fresh water people withdraw worldwide. How that water is applied makes a large difference: flood irrigation spreads water across a whole field, while drip irrigation delivers it right to the roots. This lesson uses the fictional Tarlton Farm to have students compare the two methods and the water footprints of different foods. A common misconception is that the choice of irrigation method is a minor detail; the numbers in this lesson show that switching methods can change how much water reaches the crop by thousands of gallons.

Procedure

Hook (5 min): Ask students to guess how much water it takes to grow the food for one meal. Record a few guesses, then tell them a single pound of beef can take about 1,800 gallons and a pound of wheat about 220 gallons. Let the size of those numbers land before moving on. Point out that most of the beef figure is rainwater used by the pasture and feed crops cattle eat, not irrigation or tap water.

Direct instruction (12 min): Define water footprint, flood irrigation, and drip irrigation. Explain that not all applied water reaches the crop: some evaporates or drains away. Introduce the idea of efficiency as the share of applied water that reaches the roots, and note that more efficient irrigation lets a farm withdraw less water from rivers and aquifers, which is the human impact students are learning to reduce.

Guided practice (13 min): Work handout items 1 through 4 together, modeling how to multiply a per-pound water footprint by a harvest size and how to find a percent of a quantity. Have students explain each step to a partner.

Independent practice (12 min): Students complete handout items 5 and 6 on their own, comparing how much water reaches the roots under flood and drip irrigation, then check with a partner.

Closure and exit ticket (8 min): Each student answers one question on a card: "Name one reason drip irrigation withdraws less water from a river than flood irrigation." Collect and scan the cards; if most students connect efficiency to less withdrawal, move on, and if not, reteach the efficiency idea next session.

Tarlton Farm Irrigation Efficiency: A Grade 7 Water-Use Lesson — Grade 7 Science
Name: ____________________________ Date: ______________

Student Handout

Tarlton Farm is a fictional farm. Use the water-footprint values and irrigation numbers given in each item.

  1. Agriculture accounts for about 70 percent of the fresh water withdrawn worldwide. A scenario county withdraws 40 million gallons of fresh water each day. How many million gallons go to farms at that 70 percent share?
  2. Producing 1 pound of wheat takes about 220 gallons of water. How much water does a 250-pound wheat harvest represent?
  3. Producing 1 pound of beef takes about 1,800 gallons of water. How much water does 250 pounds of beef represent?
  4. Using your answers to items 2 and 3, how many times more water does a pound of beef take than a pound of wheat?
  5. Tarlton Farm's flood irrigation puts 20,000 gallons on a field, but only 55 percent of it reaches the crop roots. How many gallons reach the roots?
  6. A drip system puts the same 20,000 gallons on the field, with 90 percent reaching the roots. How many more gallons reach the roots with drip than with flood irrigation?

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: 28 million gallons.
    0.70 times 40 million gallons equals 28 million gallons going to farms.
  2. Answer: 55,000 gallons.
    250 pounds times 220 gallons per pound equals 55,000 gallons.
  3. Answer: 450,000 gallons.
    250 pounds times 1,800 gallons per pound equals 450,000 gallons.
  4. Answer: About 8 times.
    450,000 gallons divided by 55,000 gallons is about 8.2, the same as 1,800 divided by 220, so a pound of beef takes about 8 times as much water as a pound of wheat.
  5. Answer: 11,000 gallons.
    55 percent of 20,000 gallons is 0.55 times 20,000, which equals 11,000 gallons reaching the roots.
  6. Answer: 7,000 gallons more.
    Drip delivers 0.90 times 20,000, which is 18,000 gallons to the roots. Then 18,000 minus 11,000 equals 7,000 more gallons than flood irrigation.

Differentiation

Support

Provide the multiplication set up for items 2 and 3 (for example, "250 times 220") and a calculator. Give a simple bar picture showing "water applied" and "water that reaches the roots" so students can see what the percent represents.

Extension

Ask students to find what percent of the applied water is lost under flood irrigation, then propose one change Tarlton Farm could make to lose less, and explain how it would lower the farm's water withdrawal.

ELL supports

Pre-teach "irrigation," "roots," and "reach" with a quick sketch of a plant with water dripping to its roots. Provide a sentence frame for the exit ticket: "Drip irrigation withdraws less water because ___."