Worksheet Grade 10 Social Studies & Civics
Oakhanger County's Crop Shift: Virtual Water and Land Use
A grade 10 worksheet using a fictional county's twenty-year crop table to analyze how a shift from hay to orchards moved water into distant trade.
Objectives
- Students will calculate total irrigation water applied under two different crop mixes on the same acreage.
- Students will explain how a per-acre return difference created an incentive that reshaped county land use.
- Students will describe how crop choice moves the benefit of local groundwater to buyers outside the region.
Vocabulary
- 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.
Also: crop mix, incentive, land use
Teacher overview
A county's water use can change without any change in the number of irrigated acres, because different crops draw different amounts of water. This worksheet gives students a twenty-year crop table for the fictional Oakhanger County, where irrigated acreage held steady while hay gave way to orchards. Students calculate the water applied under each crop mix, work out the per-acre return that drove the change, and then explain how a local land-use decision sends the benefit of county groundwater to buyers far away. One misconception worth naming before students begin is the belief that a drop in acres planted to a thirsty crop automatically lowers a region's water use; here the alfalfa acreage falls by more than half and the county's total water applied barely moves.
Student Worksheet
Oakhanger County is a fictional farming county that irrigates entirely from one aquifer. Its total irrigated acreage has not changed since 2005, but its crop mix has. Use the table below to answer the questions, and show your work.
| Crop | 2005 acres | 2025 acres | Water applied per acre each year (acre-feet) |
|---|---|---|---|
| Alfalfa hay | 42,000 | 18,000 | 4.0 |
| Winter wheat | 30,000 | 26,000 | 1.5 |
| Almond orchard | 6,000 | 34,000 | 3.5 |
Two figures come from the county's own records. In 2005 an acre of alfalfa returned about $900 and an acre of almonds about $4,200. The county also ships about 2,000,000 pounds of wheat to buyers outside the region each year.
- Multiply acres by water applied per acre for each 2005 crop, then add the three products. What was the total irrigation water applied in 2005, in acre-feet?
- Do the same for the 2025 column. What was the total irrigation water applied in 2025, in acre-feet?
- Using your answers to items 1 and 2, what was the percent change in total water applied between 2005 and 2025? Round to the nearest tenth of a percent and say whether it is an increase or a decrease.
- What was the percent change in almond acres between 2005 and 2025? Round to the nearest tenth of a percent.
- What was the percent change in alfalfa acres between 2005 and 2025? Round to the nearest tenth of a percent.
- Using the two return figures given above, how much more did an acre of almonds return than an acre of alfalfa?
- Growers chose the crop mix shown in the 2025 column. Using your answer to item 6, explain in two or three sentences what made that choice attractive to a grower.
- Commonly cited estimates from the Water Footprint Network put the water needed to produce a pound of beef at roughly 1,800 gallons and a pound of wheat at roughly 220 gallons. How many times more water does a pound of beef take than a pound of wheat? Round to the nearest tenth.
- Using the wheat estimate in item 8, how many gallons of water are represented by the 2,000,000 pounds of wheat the county ships out each year?
- Total irrigated acres were the same in both years. Using your answers to items 1 through 3, explain what changed in Oakhanger County and what did not, and why a person who looked only at the drop in alfalfa acres would misjudge the county's water use.
- Almonds from Oakhanger are sold far outside the region, while the alfalfa they replaced was fed to cattle inside the county. Explain how that change moved the benefit of Oakhanger's groundwater, and say who receives that benefit now.
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Answer Key
- Answer: 234,000 acre-feet. Alfalfa 42,000 by 4.0 is 168,000; wheat 30,000 by 1.5 is 45,000; almonds 6,000 by 3.5 is 21,000. The three products add to 234,000.
- Answer: 230,000 acre-feet. Alfalfa 18,000 by 4.0 is 72,000; wheat 26,000 by 1.5 is 39,000; almonds 34,000 by 3.5 is 119,000. The three products add to 230,000.
- Answer: 1.7 percent decrease. The change is 230,000 minus 234,000, or negative 4,000. Dividing by 234,000 gives negative 0.0171, which is a decrease of about 1.7 percent.
- Answer: 466.7 percent increase. The change is 34,000 minus 6,000, or 28,000. Dividing by 6,000 gives 4.6667, or about 466.7 percent.
- Answer: 57.1 percent decrease. The change is 18,000 minus 42,000, or negative 24,000. Dividing by 42,000 gives negative 0.5714, or a decrease of about 57.1 percent.
- Answer: $3,300 per acre. Subtracting 900 from 4,200 leaves 3,300.
- Answer: Accept any answer that ties the choice to the return difference from item 6. A strong response notes that an acre planted to almonds returned several times what an acre of hay returned, so a grower who could raise the capital had a clear reason to replace hay even though orchards take years to bear and cannot be fallowed in a dry year.
- Answer: About 8.2 times. Dividing 1,800 gallons by 220 gallons gives 8.18, which rounds to 8.2.
- Answer: 440,000,000 gallons. Multiplying 2,000,000 pounds by 220 gallons per pound gives 440 million gallons.
- Answer: Accept any response that separates acreage from water applied. What changed is the crop mix, and with it the per-acre demand; what did not change is the total irrigated footprint or, as item 3 shows, the total water applied, which fell only about 1.7 percent. Alfalfa acres fell by more than half, yet total water fell only about 1.7 percent, because the almond acres that replaced most of the alfalfa use nearly as much water per acre, 3.5 acre-feet against 4.0.
- Answer: Accept any response that describes an outward transfer. The alfalfa fed cattle inside the county, so the water stayed in the local economy as feed and livestock, while the almonds are shipped to buyers elsewhere, so the water leaves as an embedded, or virtual, quantity in the crop. The benefit now goes largely to distant buyers and to the growers and shippers who serve them, and the aquifer that supports it is still local.
Differentiation
Support
Give students a prepared calculation grid for items 1 and 2 with the six multiplications already labeled by crop and year, and allow calculators throughout. Work the percent-change formula once on the board using the wheat row, which appears in neither item 4 nor item 5, so the modeled example does not give away an answer.
Extension
Have students propose a crop mix for 2035 that holds total water applied at or below 200,000 acre-feet while keeping all 78,000 acres irrigated, then state which growers would bear the cost of that mix and what policy would be needed to bring it about.
ELL supports
Pre-teach "acre-foot," "crop mix," and "virtual water" with a labeled sketch showing one acre covered a foot deep and a crate of almonds with the water it took to grow. Allow items 7, 10, and 11 to be answered first in a short bulleted list and then expanded into sentences with a partner.
Extensions
Ask students to find which crops their own state exports in the largest volumes and to describe, in a paragraph, where the water used to grow them ultimately goes. A short class discussion can compare the fictional Oakhanger pattern with what students find, including whether their state's exports draw on groundwater, surface water, or rainfall alone.