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Lesson plan Grade 11 Science

Yarnbrook Farms' Salty Root Zone: A Salt Balance Lesson

A grade 11 lesson on why irrigation concentrates salt in soil, using a fictional farm water budget to find leaching fraction, drainage salinity, and salt load.

Objectives

  1. Students will explain why evapotranspiration raises the salt concentration of the water left in soil.
  2. Students will calculate leaching fraction, drainage salinity, and salt load from a farm water budget.
  3. Students will evaluate whether a switch to drip irrigation reduces harm to the crop and the river.

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.
Total dissolved solids (TDS)
Total dissolved solids is a measure of everything dissolved in water that you cannot see or filter out, like minerals and salts. Water with very high TDS can taste salty or bitter, even though it might look perfectly clear.

Also: leaching fraction, root zone, salt balance, osmotic potential

Teacher overview

Irrigation delivers both water and the salts dissolved in it, and a crop takes up almost pure water while leaving the salts behind. This lesson follows one year on the fictional Yarnbrook Farms and has students calculate what happens to those salts under two irrigation methods. The lesson is built around a case where cutting applied water lowers the total salt load reaching the river yet raises the salinity the roots must work against, so the more efficient method is not straightforwardly the better one. The closing task asks students to weigh those two results against each other.

Procedure

Hook (6 min): Pour about 20 mL of the prepared salt solution into each of two shallow dishes, weigh one, and set both under a desk lamp. Tell students the dishes will be revisited at the end of class. A day or two ahead, leave a third dish of the same solution to dry so its crystals can be shown at the close, since one period is too short for the class dishes to dry. Ask a single question and record answers on the board: when the water is gone, where will the salt be?

Direct instruction (10 min): Define evapotranspiration, root zone, and total dissolved solids. Establish the central idea, which is that plants and evaporation remove nearly pure water while the dissolved salts stay in the soil water that remains. Define the leaching fraction as the share of applied water that drains below the root zone, and state the salt balance rule this lesson uses: at steady state, essentially all of the salt applied leaves in the drainage water, so the drainage is concentrated by the reciprocal of the leaching fraction. Note that the root-zone estimate used here, the average of the applied and drainage concentrations, is a deliberate simplification.

Worked example (9 min): Model the calculation with numbers students will not reuse. On a fictional neighboring farm, 4.0 acre-feet per acre are applied and 3.0 acre-feet per acre are consumed by the crop, so 1.0 acre-foot drains and the leaching fraction is 0.25. If the applied water carries 600 mg/L of dissolved solids, the drainage carries 600 divided by 0.25, which is 2,400 mg/L. Walk through the units aloud at each step.

Guided practice (25 min): Distribute the handout. Students work items 1 through 10 in pairs and items 11 through 13 alone. Require units on every line. Watch for two common errors, which are dividing by the applied depth instead of the drainage depth when finding the concentration factor, and forgetting to convert acre-feet to million gallons before using the pounds relationship.

Formative check (5 min): Ask every student to write the answer to item 8 on a slip and hold it up. If more than a quarter of the class is off by more than ten percent, rework item 7 and item 8 on the board before continuing.

Close (5 min): Return to the dishes, weigh the one that was weighed at the start, and have students state what the mass change, and the crystals in the dish dried ahead, show. Then collect a one-sentence exit ticket: name the single number from the handout that would most worry a Yarnbrook agronomist, and say why.

Yarnbrook Farms' Salty Root Zone: A Salt Balance Lesson — Grade 11 Science
Name: ____________________________ Date: ______________

Student Handout

Yarnbrook Farms and the Cardon River are fictional, and every figure below is invented for this lesson. The farm grows alfalfa and irrigates from a canal. Show units on every line.

Use these values.

  • Applied irrigation water: 3.5 acre-feet per acre per year
  • Total dissolved solids in the applied water: 900 mg/L
  • Crop evapotranspiration: 2.8 acre-feet per acre per year
  • One acre-foot equals 325,851 gallons
  • Pounds per year equals concentration in mg/L times volume in million gallons times 8.34
  • Root-zone estimate: the average of the applied and drainage concentrations
  • In this scenario, alfalfa yield begins to fall when root-zone water passes 4,000 mg/L
  1. Calculate the depth of water draining below the root zone, in acre-feet per acre per year.
  2. Calculate the leaching fraction, which is drainage depth divided by applied depth.
  3. Assuming all applied salt leaves in the drainage, calculate the total dissolved solids of the drainage water in mg/L.
  4. Convert the 3.5 acre-feet of water applied per acre to million gallons.
  5. Calculate the mass of dissolved solids applied per acre in one year, in pounds.
  6. Estimate the average total dissolved solids of the water in the root zone, in mg/L.
  7. The farm proposes drip irrigation, which would cut applied water to 3.0 acre-feet per acre with evapotranspiration unchanged at 2.8 acre-feet per acre. Calculate the new leaching fraction.
  8. Calculate the total dissolved solids of the drainage water under drip irrigation, in mg/L.
  9. Estimate the average total dissolved solids of the root-zone water under drip irrigation, in mg/L.
  10. Calculate the mass of dissolved solids applied per acre in one year under drip irrigation, in pounds.
  11. Explain why the drainage water carries more dissolved solids than the water applied to the field.
  12. Explain why water enters roots less easily as the salinity of the root-zone water rises.
  13. Using your answers to items 9 and 10, state whether Yarnbrook should switch to drip irrigation and defend your position in three sentences.

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: 0.7 acre-feet per acre per year. Applied water of 3.5 acre-feet minus crop use of 2.8 acre-feet leaves 0.7 acre-feet to drain below the root zone.
  2. Answer: 0.20. Divide the drainage depth of 0.7 acre-feet by the applied depth of 3.5 acre-feet.
  3. Answer: 4,500 mg/L. All of the salt applied in 3.5 acre-feet leaves in 0.7 acre-feet, so the concentration rises by a factor of 1 divided by 0.20, which is 5, and 900 times 5 is 4,500 mg/L.
  4. Answer: 1.14 million gallons. Multiply 3.5 acre-feet by 325,851 gallons per acre-foot to get 1,140,479 gallons, which is 1.14 million gallons.
  5. Answer: 8,560 pounds. Multiply 900 mg/L by 1.14 million gallons by 8.34 pounds per gallon, which gives about 8,560 pounds of dissolved solids applied per acre each year.
  6. Answer: 2,700 mg/L. The average of the applied 900 mg/L and the drainage 4,500 mg/L is 2,700 mg/L, which is below the yield threshold in this scenario.
  7. Answer: 0.067. Drainage falls to 3.0 minus 2.8, or 0.2 acre-feet, and 0.2 divided by 3.0 is 0.0667.
  8. Answer: 13,500 mg/L. The concentration factor is 1 divided by 0.0667, which is 15, and 900 times 15 is 13,500 mg/L.
  9. Answer: 7,200 mg/L. The average of 900 mg/L and 13,500 mg/L is 7,200 mg/L, which is well above the 4,000 mg/L point where yield begins to fall in this scenario.
  10. Answer: 7,340 pounds. Three acre-feet is 977,553 gallons, or 0.978 million gallons, and 900 times 0.978 times 8.34 gives about 7,340 pounds per acre each year.
  11. Answer: The crop removes water but not salt. Evapotranspiration moves nearly pure water from the soil to the atmosphere, and the dissolved solids stay behind in the water that remains. The same mass of salt is then carried by a much smaller volume of drainage water, so its concentration rises by the reciprocal of the leaching fraction.
  12. Answer: Salt lowers the osmotic potential of the soil water. Water moves into a root when the water potential inside the root is lower than the water potential of the soil water. Adding dissolved solids to the soil water lowers its potential toward the value inside the root, so the difference driving uptake shrinks. The plant must spend more energy to take up the same water, and above a threshold it cannot keep up, so growth slows even though the soil is wet.
  13. Answer: Accept either position when it is defended with both figures. A well-argued answer notes that drip cuts the salt applied from about 8,560 to about 7,340 pounds per acre, which reduces the load that eventually reaches the Cardon River, while pushing estimated root-zone salinity from 2,700 mg/L to 7,200 mg/L, far past the point where alfalfa yield falls. The strongest answers propose drip combined with a deliberate leaching irrigation, or a switch to a more salt-tolerant crop, and note that the river benefit is real but modest compared with the harm to the root zone.

Differentiation

Support

Give these students a handout with the salt balance drawn as a labeled box showing water in, water out through the crop, and water out through drainage, with arrows for where the salt goes. Supply the concentration factor as a completed fraction for item 3, and allow a partner check after every second item.

Extension

Ask these students to find the leaching fraction that would hold the estimated root-zone salinity at the yield threshold in this scenario, and to state how much water per acre that would require. They can also explain why a field with uneven water distribution needs a larger leaching fraction than a uniform field to protect the same crop.

ELL supports

Pre-teach evapotranspiration, leaching, root zone, and dissolved solids with a labeled soil profile drawing that shows water leaving upward as vapor and downward as drainage. Provide the frames "The salt stays because ___" and "When the leaching fraction falls, the drainage becomes ___." Allow the salt balance to be explained orally before it is written.

Extensions

Have students research how salinity is measured in the field with a conductivity meter and write a paragraph explaining why a farmer would prefer that instrument to sending samples to a laboratory. Students can also compare the drainage from this fictional farm with the way a river gains salinity between its headwaters and its mouth, and explain what irrigated agriculture contributes.