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Lesson plan Grade 8 Science

Amberfield Farm Irrigation: Comparing Water Use by Method

A 50-minute grade 8 science lesson where students model flood and drip irrigation as linear functions of acreage and compare the water each method uses.

Objectives

  1. Students will model flood and drip irrigation as linear functions of the acreage watered.
  2. Students will compare the two application rates and calculate the water drip irrigation saves.

Vocabulary

Key words: irrigation, application rate, efficiency, acre, slope

Teacher overview

Agriculture accounts for roughly 70 percent of the freshwater people withdraw worldwide, so how a farm applies water matters a great deal. This lesson compares two methods on the fictional Amberfield Farm: flood irrigation, which spreads water across a whole field, and drip irrigation, which delivers water slowly to each plant's roots. Students write each method as a linear function of the acreage watered and compare the slopes to see which uses less water. A common error this lesson addresses is comparing the per-acre rates alone; students learn to multiply each rate by the acreage to find and compare the season totals.

Procedure

Hook (5 min): Ask students to picture watering a garden two ways: flooding the whole bed, or dripping water only at each plant. Ask which they think wastes more water and why, and record a few predictions without confirming them.

Direct instruction (12 min): Introduce the fictional Amberfield Farm. Explain that an application rate is the gallons of water applied per acre. State the scenario rates: suppose flood irrigation applies 900,000 gallons per acre each season and drip applies 500,000 gallons per acre each season. Model writing flood irrigation as G = 900,000 × A, where A is acres and G is gallons, and identify 900,000 as the slope, or gallons per acre. Emphasize that comparing methods means comparing these slopes and then the totals.

Guided practice (13 min): As a class, build the drip function G = 500,000 × A and fill a small table of gallons for 10, 20, and 40 acres for both methods on chart paper. Have students describe, in a sentence, what the steeper slope means for the flood row.

Independent practice (15 min): Students complete the Student Handout, writing both functions, finding the season totals for a 40-acre field, and calculating the water drip saves. Circulate and check that students multiply the rate by the acreage rather than comparing rates alone.

Closure and exit ticket (5 min): As a formative check, students answer on an index card: "Which method has the larger slope, and does the larger slope mean more or less water per acre?" Collect and confirm that at least 80 percent identify flood as the larger slope and more water per acre before moving on.

Amberfield Farm Irrigation: Comparing Water Use by Method — Grade 8 Science
Name: ____________________________ Date: ______________

Student Handout

Suppose the fictional Amberfield Farm irrigates a 40-acre field. Flood irrigation applies 900,000 gallons per acre each season. Drip irrigation applies 500,000 gallons per acre each season.

  1. Write a linear function G = m × A for flood irrigation, where A is acres and G is gallons. What is m?
  2. Write the linear function for drip irrigation. What is m?
  3. Find the total gallons flood irrigation uses on the whole 40-acre field in one season.
  4. Find the total gallons drip irrigation uses on the whole 40-acre field in one season.
  5. How many gallons does drip irrigation save over flood irrigation on this field in one season?
  6. Compare the two slopes. Which method uses less water per acre, and by how many gallons per acre?
  7. Give one reason a farm might still choose flood irrigation even though drip uses less water per acre.

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: m = 900,000 gallons per acre.
    The flood function is G = 900,000 × A; the slope m is 900,000 gallons per acre.
  2. Answer: m = 500,000 gallons per acre.
    The drip function is G = 500,000 × A; the slope m is 500,000 gallons per acre.
  3. Answer: 36,000,000 gallons.
    900,000 × 40 = 36,000,000 gallons for the season.
  4. Answer: 20,000,000 gallons.
    500,000 × 40 = 20,000,000 gallons for the season.
  5. Answer: 16,000,000 gallons.
    36,000,000 − 20,000,000 = 16,000,000 gallons saved on this field in one season.
  6. Answer: Drip, by 400,000 gallons per acre.
    900,000 − 500,000 = 400,000, so drip uses 400,000 fewer gallons per acre because its slope is smaller.
  7. Answer: Cost or crop type.
    Accept any reasonable trade-off: drip systems cost more to install, some crops or soils suit flooding, or the farm may lack the equipment. The point is that lower water use is one factor among several.

Differentiation

Support

Provide the two functions already written and a partly completed table so students focus on multiplying the rate by 40 acres. Allow a calculator and let students say their answers aloud before writing them.

Extension

Ask students to find the field size at which drip irrigation would save exactly 8,000,000 gallons in a season, working backward from the 400,000-gallons-per-acre difference, and to explain their reasoning.

ELL supports

Pre-teach "irrigation," "application rate," and "slope" with a simple picture of a flooded field beside a drip line. Provide sentence frames for items 6 and 7, such as "Drip uses ___ fewer gallons per acre because ___."