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Lesson plan Grade 11 Science

Verrin Valley Irrigation: Crop Water Demand and Efficiency

A 50-minute grade 11 lesson computing a fictional farm's crop water need and comparing flood and drip irrigation efficiency to find the water saved.

Objectives

  1. Students will calculate a field's seasonal crop water requirement in acre-feet.
  2. Students will use irrigation efficiency to find how much water must be applied under two systems.
  3. Students will explain why efficiency gains in agriculture affect total water use so strongly.

Vocabulary

Evapotranspiration
Evapotranspiration is the total amount of water that leaves the land and goes into the air, both from wet surfaces and from plants. Scientists combine evaporation and transpiration into one word.
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.
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.
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

A crop needs a certain depth of water over a season to meet evapotranspiration, but a farm always has to withdraw more than that, because no irrigation system delivers every drop to the roots. This lesson uses the fictional Verrin Valley farm to make that gap concrete: students compute a field's crop water requirement, then find how much water a flood system and a drip system each have to apply to satisfy it. A common student error is to assume applied water equals crop need, which ignores efficiency and badly underestimates withdrawals; the handout is built so students first find the requirement, then divide by efficiency to see the difference. Because agriculture accounts for roughly 70 percent of global freshwater withdrawals, small efficiency gains scale into large water savings.

Procedure

Hook (5 min): Show the fictional Verrin Valley farm's problem: a 40-acre field, a dry summer, and a choice between the flood irrigation the farm has always used and a new drip system. Ask students to guess how much water the switch might save, and record guesses.

Direct instruction (12 min): Define evapotranspiration as the water a crop consumes through transpiration and soil evaporation, expressed as a depth over the season. Introduce the acre-foot (one acre covered one foot deep) and show that a depth in inches converts to acre-feet by dividing by 12 and multiplying by the acreage. Then define irrigation efficiency as the crop requirement divided by the water actually applied, and explain that applied water = requirement divided by efficiency. Note that flood systems commonly run near 60 percent efficient while well-managed drip can reach about 90 percent.

Guided practice (10 min): With the class, convert a 12-inch seasonal requirement on a 10-acre field into acre-feet (10 acre-feet), then find the applied water at 60 percent efficiency (about 16.7 acre-feet). Have students restate in their own words why the applied figure is larger than the requirement.

Independent practice (18 min): Students complete the Student Handout, computing the Verrin Valley field's requirement, the applied water for both systems, the seasonal saving, and its value in gallons, then reasoning about where the lost water goes and why efficiency matters at scale.

Closure and exit ticket (5 min): On an index card, students answer: "A field needs 30 acre-feet of water. At 75 percent efficiency, how much must be applied?" (40 acre-feet). Collect and check that students divided by efficiency rather than multiplied.

Verrin Valley Irrigation: Crop Water Demand and Efficiency — Grade 11 Science
Name: ____________________________ Date: ______________

Student Handout

The fictional Verrin Valley farm irrigates a 40-acre field. Over the growing season the crop needs a water depth of 24 inches (its evapotranspiration). Use these rules: a depth in inches becomes acre-feet by dividing by 12 and multiplying by the acreage; applied water = crop requirement divided by efficiency; and 1 acre-foot = 325,851 gallons.

  1. Calculate the field's seasonal crop water requirement in acre-feet.
  2. Flood irrigation on this field is 60 percent efficient. How many acre-feet must be applied to meet the requirement?
  3. A drip system is 90 percent efficient. How many acre-feet must be applied to meet the same requirement?
  4. How many acre-feet would switching from flood to drip save over the season?
  5. Convert that seasonal saving to gallons.
  6. Explain where the "extra" water applied under flood irrigation goes, and why drip loses less of it.
  7. Agriculture accounts for roughly 70 percent of global freshwater withdrawals. Explain why improving irrigation efficiency has such a large effect on total water use.
  8. Drip equipment costs more to install than flood irrigation. Using the water saved, give one reason the switch could still pay off during a multi-year drought.

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: 80 acre-feet.
    24 inches divided by 12 = 2 feet of depth; 2 feet x 40 acres = 80 acre-feet.
  2. Answer: 133.3 acre-feet.
    80 acre-feet divided by 0.60 = 133.3 acre-feet applied.
  3. Answer: 88.9 acre-feet.
    80 acre-feet divided by 0.90 = 88.9 acre-feet applied.
  4. Answer: 44.4 acre-feet.
    133.3 minus 88.9 = 44.4 acre-feet saved each season.
  5. Answer: about 14.5 million gallons.
    44.4 acre-feet x 325,851 gallons per acre-foot = 14,467,784 gallons, about 14.5 million gallons.
  6. Answer: It is lost to deep percolation, runoff, and evaporation.
    Under flood irrigation, much of the applied water sinks below the root zone, runs off the field's edges, or evaporates from the wide wet surface; drip delivers water slowly at the roots, so far less is lost to those paths.
  7. Answer: Agriculture is the largest single water user.
    Since farming withdraws about 70 percent of the world's fresh water, a percentage improvement in how efficiently crops are watered saves a very large absolute volume, more than a similar improvement in a smaller-using sector could.
  8. Answer: The saved water carries the farm and the aquifer through dry years.
    Accept any sound reason, such as the saved water letting the farm keep irrigating when supplies are curtailed, reducing groundwater overdraft, or lowering pumping energy costs; over several drought years the value of that reliability can exceed the equipment cost.

Differentiation

Support

Provide the acre-foot conversion worked as a labeled example (a different field) before students start, and pre-fill the crop requirement of 80 acre-feet so students can focus on dividing by efficiency in items 2 and 3. Allow calculators throughout.

Extension

Have students determine what efficiency a system would need to reach so the field could be irrigated with only 100 acre-feet of applied water, working backward from the 80-acre-foot requirement.

ELL supports

Pre-teach "efficiency," "applied water," and "acre-foot" with a diagram showing a full delivery, the part the crop uses, and the part lost. Provide a sentence frame for item 6: "Flood irrigation loses water to ___, but drip loses less because ___."