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Lesson plan Grade 6 Math

Wickholt Nursery's Pond: Evaporation Rates and Percent Lost

A 45-minute grade 6 lesson in which students turn a holding pond's 1.4-inch weekly drop into gallons and find what percent of the pond evaporated.

Objectives

  1. Students will find a daily evaporation rate in inches per day from a multi-day change in water level.
  2. Students will convert a change in water depth on a known surface area into gallons.
  3. Students will express a week of evaporation loss as a percent of a pond's full volume.

Vocabulary

Evaporation
Evaporation happens when liquid water, often warmed by the sun, turns into an invisible gas called water vapor that rises into the air. It is how water leaves oceans, lakes, and puddles.

Also: unit rate, surface area, percent

Teacher overview

Evaporation is measured as a change in depth, because a gauge board standing in a pond shows the drop in inches even though the water lost is counted in gallons. This lesson uses the fictional Wickholt Nursery, whose outdoor holding pond dropped 1.4 inches during a week with no rain and no pumping, so students first find a unit rate in inches per day and then convert that depth into gallons. The misconception this lesson addresses is that a drop of an inch or two must be a small loss because the number itself is small, when in fact the pond's surface area turns every inch of depth into thousands of gallons.

Procedure

Hook (5 min): Ask students how they could tell whether a pond is losing water when nobody is pumping any out of it. Collect answers and steer the discussion toward measuring the water level against a fixed mark. Explain that the fictional Wickholt Nursery keeps a gauge board on the side of its holding pond, and read the week's two entries aloud: the level stood 1.4 inches lower after seven days with no rain.

Direct instruction (12 min): Define evaporation as water leaving the pond surface as vapor, and point out that the gauge reports the loss as a depth rather than as a volume. Model the unit rate on chart paper by dividing 1.4 inches by 7 days, which gives 0.2 inch each day. Then model the conversion to gallons using two stated figures for the fictional pond: its surface covers 20,000 square feet, and one inch of water standing on one square foot is 0.623 gallons. Work the product 20,000 times 0.623 aloud, which gives 12,460 gallons for every inch of depth.

Guided practice (12 min): Have pairs use the 12,460-gallon figure to find the gallons behind the full 1.4-inch drop, showing the multiplication rather than only the result. Circulate and press any pair that multiplies 1.4 by 0.623 alone to say what unit that answer would carry. Pause for a formative check: ask for thumbs up or thumbs down on the statement that a deeper pond loses more water to evaporation each day than a shallow pond of the same surface area. If more than five students put thumbs up, re-teach with two paper rectangles of equal area to show that evaporation depends on the surface, not on the depth below it.

Independent practice (11 min): Students work items 1 through 7 of the handout alone, then compare answers with a partner and mark any item where they disagree for the closing discussion.

Closure (5 min): Students complete item 8 as an exit ticket and hand it in on the way out. Scan for students who multiply by 0.623 but leave out the surface area, because that error shows up as an answer near 1 gallon rather than near 6,000 gallons.

Wickholt Nursery's Pond: Evaporation Rates and Percent Lost — Grade 6 Math
Name: ____________________________ Date: ______________

Student Handout

The fictional Wickholt Nursery keeps its irrigation water in an outdoor holding pond. Use these facts for every item.

  • The pond's surface covers 20,000 square feet.
  • One inch of water on one square foot is 0.623 gallons.
  • The pond holds 200,000 gallons when it is full.
  • During one week with no rain, the gauge board showed the level drop 1.4 inches in 7 days.
  1. The level dropped 1.4 inches in 7 days. What was the evaporation rate in inches per day?
  2. At that same daily rate, how many inches would evaporate in 30 days?
  3. The pond covers 20,000 square feet, and one inch on one square foot is 0.623 gallons. How many gallons leave the pond when the level drops one inch?
  4. How many gallons evaporated during the week when the level dropped 1.4 inches?
  5. The pond holds 200,000 gallons when full. What percent of a full pond was lost that week? Round to the nearest tenth of a percent.
  6. How many gallons evaporate on a single day, when the level drops 0.2 inch?
  7. When the nursery runs its well, the well adds 2,500 gallons to the pond each day. How many more gallons would the well add each day than evaporation removes?
  8. A second holding pond at the same nursery covers 10,000 square feet. How many gallons does it lose when its level drops one inch?

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: 0.2 inch per day.
    Divide the drop by the number of days. 1.4 divided by 7 is 0.2, so the pond lost 0.2 inch each day.
  2. Answer: 6 inches.
    Multiply the daily rate by the number of days: 0.2 times 30 is 6 inches.
  3. Answer: 12,460 gallons.
    Multiply the surface area by the gallons in one inch on one square foot: 20,000 times 0.623 is 12,460 gallons.
  4. Answer: 17,444 gallons.
    One inch is 12,460 gallons, so 1.4 inches is 1.4 times 12,460, which is 17,444 gallons.
  5. Answer: 8.7 percent.
    Divide the loss by the full volume: 17,444 divided by 200,000 is 0.08722, which rounds to 8.7 percent.
  6. Answer: 2,492 gallons.
    A day's drop is 0.2 inch, so multiply 0.2 by 12,460 gallons per inch, which gives 2,492 gallons.
  7. Answer: 8 gallons.
    The well adds 2,500 gallons while evaporation removes 2,492 gallons, and 2,500 minus 2,492 is 8 gallons.
  8. Answer: 6,230 gallons.
    Multiply the smaller pond's area by the same figure: 10,000 times 0.623 is 6,230 gallons, which is half the first pond's answer because its surface area is half as large.

Differentiation

Support

Give these students a conversion strip for the fictional pond that reads inches divided by days equals inches per day, and area times 0.623 equals gallons per inch. Assign items 1, 2, 3, 4, and 8, and provide the multiplication in item 3 already set up as 20,000 times 0.623 so the work begins with the arithmetic rather than with choosing an operation.

Extension

Ask these students to find how many days the fictional nursery could go without running its well before the pond drops below half full, assuming evaporation continues at 0.2 inch each day and the pond starts full. They should show the daily loss in gallons, state the number of days, and explain why the answer would change if the pond held the same water in a narrower, deeper basin.

ELL supports

Pre-teach evaporation, surface area, and gauge board with a sketch of a pond seen from above beside the same pond seen from the side, labeling which view shows area and which shows depth. Provide these sentence frames: "The pond lost ___ inches in ___ days, so the rate is ___ inch each day." and "One inch of depth is ___ gallons, so ___ inches is ___ gallons."

Extensions

Set two wide pans and one narrow jar, all filled to the same depth, on a sunny windowsill and have students measure the depth in each container every day for a week with a ruler. Comparing the daily drop in the wide pans with the drop in the jar gives the class its own version of the rate the fictional nursery measured. Then have students multiply each container's weekly drop by its surface area to compare the volume of water each one lost. The drop in depth is usually similar in every container, while the wide pans lose far more water, which matches the pond calculation; a tall jar's walls can also shelter the water and slow its drop slightly.