Water for Schools
Menu

Worksheet Grade 7 Science

Oster Grove Paves a Field: Stormwater Runoff and Percent Change

A grade 7 science worksheet where students compare stormwater runoff from a grass field and a parking lot, using the runoff formula and percent change.

Objectives

  1. Students will calculate stormwater runoff volume from area, rainfall, and a runoff fraction.
  2. Students will compare runoff from a grass surface and a paved surface.
  3. Students will find the percent increase in runoff caused by paving a field.

Vocabulary

Stormwater
Stormwater is rain or melted snow that flows off roofs, streets, and parking lots. In most towns it goes into storm drains and flows to a stream without being treated, carrying whatever it picked up.

Also: runoff, impervious, percent increase

Teacher overview

When rain falls on grass, most of it soaks into the ground; when it falls on pavement, most of it runs off into storm drains. This worksheet has students compare the two using one plot of land in a town that wants to pave a field for parking. The task builds percent-change practice on a real question that many towns face, and it lets students see how a land-use choice changes where the water goes.

Oster Grove Paves a Field: Stormwater Runoff and Percent Change — Grade 7 Science
Name: ____________________________ Date: ______________

Student Worksheet

The town of Oster Grove is deciding whether to pave a grassy field into a parking lot. Use these values and the runoff formula:

  • The field is 20,000 square feet.
  • A storm drops 2 inches of rain.
  • Runoff formula: rain that falls (gallons) = area (sq ft) x rainfall (inches) x 0.623. Runoff = rain that falls x the fraction that runs off.
  • As grass, about 20% (0.20) of the rain runs off. As pavement, about 90% (0.90) runs off.
  1. First find the total rain that falls on the field. Using the formula, how many gallons of rain land on the 20,000 square foot field during the 2-inch storm?
  2. As a grass field, only 0.20 of that rain runs off. How many gallons run off the grass field?
  3. As a parking lot, 0.90 of that rain runs off. How many gallons run off the parking lot?
  4. How many more gallons run off as pavement than as grass?
  5. Compared with the grass runoff in item 2, what is the percent increase in runoff after paving? Round to the nearest whole percent.
  6. As grass, the rain that does not run off soaks into the ground. How many gallons soak into the grass field?
  7. Oster Grove considers a compromise: keep half the field as grass (10,000 sq ft) and pave the other half (10,000 sq ft). Find the total runoff from this half-and-half plan, then state how many fewer gallons run off than with full pavement.
  8. Explain why more runoff from the parking lot could cause problems downstream, such as flooding or pollution reaching a creek.
  9. Name one design that would let a parking lot produce less runoff, and explain how it works.

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: 24,920 gallons. 20,000 sq ft x 2 in x 0.623 = 24,920 gallons of rain fall on the field.
  2. Answer: 4,984 gallons. 24,920 x 0.20 = 4,984 gallons run off the grass field.
  3. Answer: 22,428 gallons. 24,920 x 0.90 = 22,428 gallons run off the parking lot.
  4. Answer: 17,444 gallons. 22,428 - 4,984 = 17,444 more gallons run off as pavement.
  5. Answer: 350 percent. The increase is 17,444 gallons; 17,444 / 4,984 = 3.5, which is a 350% increase.
  6. Answer: 19,936 gallons. The rain that soaks in is 0.80 of the total; 24,920 x 0.80 = 19,936 gallons.
  7. Answer: 13,706 gallons, which is 8,722 fewer than full pavement. Each half receives 10,000 x 2 x 0.623 = 12,460 gallons; grass half runoff is 12,460 x 0.20 = 2,492 and pavement half is 12,460 x 0.90 = 11,214, totaling 13,706 gallons. Full pavement runs off 22,428, so the plan saves 22,428 - 13,706 = 8,722 gallons.
  8. Answer: Pavement sends far more water to storm drains at once, which can overflow drains and flood streets, and the fast-moving water carries oil, litter, and other pollutants from the lot into nearby creeks.
  9. Answer: Accept any reasonable answer, such as permeable pavement that lets water soak through, a rain garden that collects runoff, or a detention basin that holds water and releases it slowly, with a short explanation of how it reduces runoff.

Differentiation

Support

Provide the value from item 1 (24,920 gallons) already computed so students can begin at item 2, and give a worked model of finding a percent increase before item 5. Allow a calculator throughout and accept answers rounded to the nearest hundred gallons.

Extension

Ask students to find how large a rain garden, in gallons of storage, the town would need to hold all the extra runoff from item 4 during one storm, and to explain any assumption they make.

ELL supports

Pre-teach "runoff," "soak in," "pavement," and "percent increase" with a quick demonstration pouring water on a sponge and on a plastic lid. Provide a labeled diagram of rain hitting grass versus pavement, and allow numeric answers with short phrases.