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Lesson plan Grade 9 Science

Halesworth Farms: Crop Water Footprints and Irrigation Efficiency

A 50-minute grade 9 lesson where students compare the water footprint of crops and the efficiency of flood versus drip irrigation on a fictional valley farm.

Objectives

  1. Students will calculate the water footprint of different crops and compare them using stated gallons-per-pound values.
  2. Students will compare flood and drip irrigation efficiency and explain how efficiency affects a farm's water use and sustainability.

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.
Irrigation efficiency
The share of irrigation water that crops actually use compared with the amount taken from the river or well. Higher efficiency means less waste.
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.
Consumptive use
The share of water that is truly used up, by evaporating or being taken up by plants, so it cannot return to the river for others. The rest flows back and can be used again downstream.

Teacher overview

Agriculture is the largest user of fresh water, accounting for roughly 70 percent of the world's freshwater withdrawals, so how farmers choose crops and irrigation methods shapes how much water a region has left for everything else. This lesson centers on Halesworth Farms, a fictional operation in the Coldwater Valley, and asks students to compare the water footprint of crops and the efficiency of two irrigation methods. A common misconception is that all crops and all watering methods use about the same amount of water; the numbers in this lesson show differences of many times over, which the procedure is built to make concrete.

Procedure

Hook (5 min): Ask students to predict which takes more water to produce, one pound of beef or one pound of wheat, and roughly how many times more. Take a quick show of hands without revealing answers yet.

Direct instruction (12 min): Introduce the idea of a water footprint, the total water used to produce something. Share the widely cited estimates that producing a pound of beef takes roughly 1,800 gallons of water while a pound of wheat takes roughly 220 gallons (Water Footprint Network global averages). Point out that most of the beef figure is rainwater used by the pasture and feed crops cattle eat, not irrigation or tap water. Note also that agriculture uses about 70 percent of the world's fresh water. Then define irrigation efficiency as the share of pumped water that actually reaches the crop's roots, and contrast flood irrigation, where much water is lost to evaporation and runoff, with drip irrigation, which delivers water straight to the roots.

Guided practice (10 min): Work item 1 of the handout together, showing how to multiply pounds by gallons per pound. Then set up item 4 as a class, emphasizing that with 60 percent efficiency the farm must pump more than the crop actually needs, so the applied amount is the needed amount divided by the efficiency.

Independent practice (15 min): Students complete the remaining handout items on their own or in pairs, comparing crop water footprints and the flood-versus-drip pumping totals for one field.

Closure / exit ticket (8 min): On an index card, each student answers: "Halesworth Farms wants to lower its water use during a dry year. Name one change to crops or irrigation and explain how it helps." Collect cards and confirm at least 80 percent name a specific change with a valid reason before moving on.

Halesworth Farms: Crop Water Footprints and Irrigation Efficiency — Grade 9 Science
Name: ____________________________ Date: ______________

Student Handout

Halesworth Farms is a fictional farm. Use these facts for every item, and show your work.

  • A pound of beef has a water footprint of about 1,800 gallons.
  • A pound of wheat has a water footprint of about 220 gallons.
  • Flood irrigation on this farm is 60 percent efficient; drip irrigation is 90 percent efficient.
  • One field needs 360,000 gallons of water to reach the crop's roots in a season.
  1. A rancher at Halesworth Farms produces 500 pounds of beef. Using 1,800 gallons per pound, how many gallons of water does that beef represent?
  2. A neighbor grows 500 pounds of wheat. Using 220 gallons per pound, how many gallons of water does that wheat represent?
  3. How many times more water does the 500 pounds of beef take than the 500 pounds of wheat? Round to the nearest tenth.
  4. The field needs 360,000 gallons at the roots. With flood irrigation at 60 percent efficiency, how many gallons must be pumped from the river? (Pumped = needed / efficiency.)
  5. With drip irrigation at 90 percent efficiency, how many gallons must be pumped to deliver the same 360,000 gallons to the roots?
  6. How many gallons does the farm save on this one field by switching from flood to drip irrigation?
  7. Explain why drip irrigation lets Halesworth Farms grow the same crop while pumping less water from the river, and how that helps the valley's water supply during a dry year.

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: 900,000 gallons. 500 pounds x 1,800 gallons per pound = 900,000 gallons.
  2. Answer: 110,000 gallons. 500 pounds x 220 gallons per pound = 110,000 gallons.
  3. Answer: About 8.2 times. 900,000 / 110,000 = 8.18, which rounds to 8.2 and matches the ratio of the footprints, 1,800 / 220 = 8.18.
  4. Answer: 600,000 gallons. 360,000 gallons / 0.60 = 600,000 gallons pumped, because only 60 percent of the pumped water reaches the roots.
  5. Answer: 400,000 gallons. 360,000 gallons / 0.90 = 400,000 gallons pumped, since 90 percent reaches the roots.
  6. Answer: 200,000 gallons. 600,000 - 400,000 = 200,000 gallons saved on this field.
  7. Answer: Drip irrigation delivers water directly to the roots, so far less is lost to evaporation and runoff. The crop still gets the 360,000 gallons it needs, but the farm pumps only 400,000 gallons instead of 600,000. Pumping less leaves more water in the river and aquifer, which matters most in a dry year when supplies are already short. Accept any answer that connects higher efficiency to less water pumped and more water left in the valley.

Differentiation

Support

Give students a partially completed handout with items 1 and 4 already worked, so they can follow the pattern for the parallel items 2 and 5. Provide a calculator and a note reminding them that dividing by an efficiency less than 1 makes the pumped total larger than the amount the crop needs.

Extension

Ask students to calculate how many acres of wheat, at a stated yield, Halesworth Farms could water with the 200,000 gallons saved in item 6, and to argue whether switching one crop from beef feed to wheat would meaningfully change the valley's total water use.

ELL supports

Pre-teach "efficiency" (the useful share) and "footprint" (total amount used) with simple visuals, such as a leaky bucket for flood irrigation and a straw to the roots for drip. Provide the sentence frame "Drip irrigation pumps ___ water than flood irrigation because ___" for item 7 and the exit ticket.

Extensions

Have students research the main irrigated crops grown in their own state, look up a published water-footprint estimate for one of them, and prepare a short recommendation to a fictional water board about which crop and irrigation method they would encourage during a multi-year drought.