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Lesson plan Grade 8 Science

Basalt Hollow's Water Budget: Where a Year of Rain Actually Goes

A 50-minute grade 8 lesson where students calculate how a fictional town's yearly precipitation splits among evapotranspiration, runoff, and groundwater infiltration.

Objectives

  1. Students will calculate how a watershed's precipitation is divided among evapotranspiration, runoff, and infiltration using given percentages.
  2. Students will explain, using energy from the sun and gravity, why water follows each of the three pathways.
  3. Students will predict how an impervious surface such as pavement would change a watershed's water budget.

Vocabulary

Water budget
A water budget is a way of tracking all the water coming into an area, like rain and snowmelt, against all the water leaving it, like evaporation and streamflow. Scientists use water budgets to see whether a watershed or aquifer is gaining or losing water over time.
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.
Infiltration
Infiltration is when rain or melted snow soaks down into the soil instead of flowing away across the top of it. Water that infiltrates can feed plant roots or keep sinking to become groundwater.
Impervious surface
An impervious surface is any hard surface, like a road, roof, or parking lot, that rain cannot soak through. The more impervious surface a town has, the more stormwater rushes off it.

Teacher overview

Basalt Hollow (a fictional town used for this lesson) receives about 38 inches of precipitation each year, and students work out where all of that water goes. This lesson introduces the idea of a water budget: the same total precipitation split among evapotranspiration, surface runoff, and groundwater infiltration, three pathways driven by energy from the sun and the pull of gravity. A common misconception is that most precipitation ends up in streams or soaks into the ground; in many watersheds, including this one, the largest share returns to the atmosphere through evapotranspiration, and a land-use change such as new pavement can shift the split among the three pathways.

Procedure

Hook (5 min): Ask students, "If your town gets 38 inches of rain and snow in a year, where does all that water actually go?" Take a few guesses on the board without correcting them yet.

Direct instruction (12 min): Introduce the water-budget model for the fictional town of Basalt Hollow: of the 38 inches of precipitation each year, about 60 percent returns to the atmosphere through evaporation and plant transpiration (together called evapotranspiration), about 25 percent runs off across the surface into Basalt Creek, and about 15 percent infiltrates through the soil to recharge the aquifer beneath the town. Explain, using energy from the sun driving evaporation and gravity pulling water downward and across slopes, why water follows each of these three pathways. Model on the board how to convert one percentage of 38 inches into an actual number of inches.

Guided practice (15 min): In pairs, students calculate how many inches of Basalt Hollow's 38-inch precipitation total go to each of three pathways, then check that their three answers sum back to 38 inches.

  1. How many inches go to evapotranspiration?
  2. How many inches go to runoff?
  3. How many inches go to infiltration?

Independent practice (10 min): Each student answers two questions on a handout.

  1. If a dry year brings only 30 inches of precipitation instead of 38, and the same three percentages apply, how many inches would infiltrate to recharge the aquifer?
  2. Explain in two or three sentences why a dry year is a particular concern for a town like Basalt Hollow that depends on groundwater wells.

Closure / exit ticket (8 min): Students answer one question on an index card.

  1. Basalt Hollow's town council is considering paving a large parking lot on land that currently lets rain infiltrate into the ground. Using what you learned today, explain in two sentences how this might change the town's water budget. Collect and scan responses for understanding that impervious surfaces increase runoff and reduce infiltration.

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: 22.8 in.
    Guided practice, evapotranspiration: 0.60 × 38 inches = 22.8 inches.
  2. Answer: 9.5 in.
    Guided practice, runoff: 0.25 × 38 inches = 9.5 inches.
  3. Answer: 5.7 in.
    Guided practice, infiltration: 0.15 × 38 inches = 5.7 inches. Checking the sum: 22.8 + 9.5 + 5.7 = 38.0 inches, matching the year's total precipitation.
  4. Answer: 4.5 in.
    Independent practice, dry year: 0.15 × 30 inches = 4.5 inches of infiltration, assuming the same 15 percent share applies.
  5. Answer: A dry year lowers the amount recharging the aquifer.
    Independent practice, groundwater wells: any reasonable answer noting that a dry year reduces total precipitation, so 15 percent of a smaller total means less water infiltrates to recharge the aquifer, which could lower groundwater levels in wells if dry years continue.
  6. Answer: Paving increases runoff and decreases infiltration.
    Exit ticket: paving creates an impervious surface, so rain that used to soak into the ground runs off instead, which increases the runoff share of the water budget and decreases the amount recharging the aquifer that supplies the town's wells.

Differentiation

Support (below grade level)

Give students a partially completed table with the evapotranspiration amount already calculated (22.8 inches) so they only need to find the runoff and infiltration amounts, and allow calculators for every percentage calculation.

Extension (above grade level)

Ask students to confirm that Basalt Hollow's infiltration amount, 5.7 inches, is really 15 percent of the 38-inch total by dividing 5.7 by 38 and converting the result to a percentage (5.7 ÷ 38 = 0.15, or 15 percent), matching the figure given in the lesson.

ELL supports

Pre-teach "evapotranspiration," "runoff," and "infiltration" with a simple three-arrow diagram, one arrow up to a cloud, one arrow sideways to a creek, one arrow down into the ground, labeled in English and, where possible, students' home languages. Allow the exit ticket to be answered orally to the teacher instead of in writing.

Extensions

Have students find an approximate annual precipitation total for their own region, with teacher guidance, and discuss, using the same three-pathway idea, which pathway is probably largest where they live and why.