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Lesson plan Grade 12 Science

Irrigation Efficiency and Water Savings in Verrand Valley

A 50-minute grade 12 science lesson where students convert irrigation depth to acre-feet and gallons and compare the water a fictional farm saves by switching to drip.

Objectives

  1. Students will convert an irrigation depth over a field area into acre-feet and gallons.
  2. Students will calculate how much water must be diverted from a river at a given irrigation efficiency.
  3. Students will evaluate the water savings and trade-offs of switching irrigation methods.

Vocabulary

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.
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.
Virtual water
The hidden water it took to make something. A hamburger, a cotton shirt, and a sheet of paper each carry a large invisible water cost.
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.

Also: acre-inch

Teacher overview

Agriculture accounts for roughly 70 percent of the world's freshwater withdrawals, so how efficiently a farm applies water has a large effect on a whole river basin. This lesson uses a fictional 120-acre field in the Verrand Valley irrigation district to move from an irrigation depth to a diversion volume, then to compare flood irrigation with drip. Students see that the water a crop needs and the water a farm must divert from the river are not the same number, because some of every diversion never reaches the crop, though part of that water returns to the river or aquifer.

A common misconception is that the diverted water equals the crop's need. At 70 percent efficiency, only 70 percent of the diverted water reaches the crop, so the diversion must be larger than the need. The handout is fully self-contained, so a student who has only the handout can reproduce every answer.

Procedure

Hook (5 min): Ask students to estimate how much water it takes to grow the food on a lunch tray. Offer one anchor from the water-footprint idea, that a pound of beef takes far more water than a pound of wheat, and record a few guesses without confirming them yet.

Direct instruction (12 min): Define irrigation depth in inches, the acre-inch and the acre-foot, and irrigation efficiency as the fraction of diverted water that reaches the crop. Explain that 12 acre-inches equal 1 acre-foot and that 1 acre-foot equals 325,851 gallons. Note that agriculture uses about 70 percent of global freshwater withdrawals, which is why efficiency gains matter at a basin scale.

Worked example (10 min): Model the method with numbers different from the handout. Suppose a 60-acre field in the fictional district needs 18 inches over the season. Show 18 x 60 = 1,080 acre-inches, then 1,080 / 12 = 90 acre-feet, and, at 80 percent efficiency, a diversion of 90 / 0.80 = 112.5 acre-feet.

Guided practice (8 min): Students complete handout items 1 through 3 with a partner while you circulate, checking that they convert acre-inches to acre-feet before converting to gallons.

Independent practice (10 min): Students finish handout items 4 through 10, including the flood-versus-drip comparison and the short evaluation items.

Closure / exit ticket (5 min): On an index card, each student answers: "A field needs 100 acre-feet delivered. At 50 percent efficiency, how many acre-feet must be diverted?" Collect and check for the method (100 / 0.50 = 200 acre-feet). If fewer than four of five cards divide by the efficiency, reteach that step next session.

Irrigation Efficiency and Water Savings in Verrand Valley — Grade 12 Science
Name: ____________________________ Date: ______________

Student Handout

Suppose a 120-acre alfalfa field in the fictional Verrand Valley irrigation district needs 24 inches of water over the season. Use these facts: 12 acre-inches equal 1 acre-foot, and 1 acre-foot equals 325,851 gallons. Irrigation efficiency is the fraction of diverted water that reaches the crop.

  1. Calculate the water the crop needs in acre-inches: the depth of 24 inches over 120 acres.

  2. Convert your answer from item 1 to acre-feet.

  3. Convert the acre-feet from item 2 to gallons, using 1 acre-foot = 325,851 gallons.

  4. Flood irrigation is 70 percent efficient. How many acre-feet must be diverted from the river to deliver the crop's need from item 2?

  5. Drip irrigation is 90 percent efficient. How many acre-feet must be diverted to deliver the same crop need?

  6. How many acre-feet per season are saved by switching from flood irrigation to drip?

  7. Convert the seasonal savings from item 6 to gallons, using 1 acre-foot = 325,851 gallons.

  8. Agriculture uses about 70 percent of global freshwater withdrawals. Explain why improving irrigation efficiency has a large effect on total water use.

  9. Producing food takes water too: about 1,800 gallons per pound of beef and about 220 gallons per pound of wheat. Explain what "virtual water" means using these numbers.

  10. Give one benefit and one trade-off a farmer weighs when deciding whether to switch from flood to drip 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: 2,880 acre-inches.
    24 inches x 120 acres = 2,880 acre-inches.

  2. Answer: 240 acre-feet.
    2,880 acre-inches / 12 = 240 acre-feet.

  3. Answer: 78,204,240 gallons.
    240 acre-feet x 325,851 gallons per acre-foot = 78,204,240 gallons.

  4. Answer: 342.9 acre-feet.
    Diversion = need / efficiency = 240 / 0.70 = 342.857, which rounds to 342.9 acre-feet.

  5. Answer: 266.7 acre-feet.
    Diversion = 240 / 0.90 = 266.667, which rounds to 266.7 acre-feet.

  6. Answer: 76.2 acre-feet.
    Savings = 342.857 - 266.667 = 76.19, which rounds to 76.2 acre-feet.

  7. Answer: about 24,827,000 gallons.
    76.19 acre-feet x 325,851 gallons per acre-foot = about 24,827,000 gallons, roughly 24.8 million gallons saved per season.

  8. Answer: A small percentage change applies to an enormous total.
    Because farming uses about 70 percent of freshwater withdrawals, even a modest efficiency gain on that large share can free up as much water as much larger percentage cuts in smaller uses.

  9. Answer: Virtual water is the water used to produce a good.
    The 1,800 gallons behind a pound of beef and the 220 gallons behind a pound of wheat are "virtual" because the water is not in the food itself but was needed to grow it, so buying food is indirectly using water.

  10. Answer: Accept any one benefit paired with one trade-off.
    Benefits include using less water and losing less to runoff and evaporation. Trade-offs include the cost of installing drip lines, the energy or maintenance they require, and the risk of clogged emitters.

Differentiation

Support

Provide the conversion chain (acre-inches to acre-feet to gallons) as a labeled flowchart, and complete item 1 and item 2 together before students continue. Give the efficiency items with the division already set up.

Extension

Have students find the irrigation efficiency that would let the farm deliver the crop's 240 acre-feet need with a diversion of no more than 300 acre-feet, and describe what practices might reach it.

ELL supports

Pre-teach "diversion," "efficiency," and "virtual water" with a diagram of a river feeding a canal that loses some water before reaching a field. Provide sentence frames for items 8 through 10.

Extensions

Have students estimate the virtual water in a simple meal using the beef and wheat figures, then discuss how food choices connect to farm water use. A short written estimate links the Verrand Valley math to decisions students make every day.