Lesson plan Grade 9 Science
Tamarack Basin Water Budget: Inflow, Storage, and Loss
A 50-minute grade 9 Earth-science lesson where students build a one-year reservoir water budget for a fictional basin and calculate how storage and residence time change.
Objectives
- Students will build a one-year water budget balancing inflow against withdrawals and evaporation.
- Students will calculate the change in reservoir storage and predict the ending storage.
- Students will calculate a reservoir's residence time and explain what it means.
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.
- 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.
- Residence time
- Residence time is how long a drop of water typically stays in one part of the water cycle before moving on. Water passes through the air quickly but can stay underground for a very long time.
Also: inflow, storage
Teacher overview
A reservoir gains water from rain and streamflow and loses it to withdrawals and evaporation. Over a year, the change in storage is simply what came in minus what went out, which is the idea behind a water budget. This lesson uses the fictional Tamarack basin so students can build that budget with clear numbers and then find the reservoir's residence time. A common misconception is that a reservoir stays full as long as some water keeps flowing in; the budget shows that storage falls whenever total outflow is larger than inflow, even in a year with real inflow.
Procedure
Hook (5 min): Project a simple picture of a reservoir with arrows for rain and streamflow coming in and arrows for withdrawals and evaporation going out. Ask students to predict what happens to the water level over a year if more water leaves than arrives. Record two or three predictions on the board without judging them.
Direct instruction with worked example (15 min): Introduce the water budget: change in storage = inflow − (withdrawals + evaporation). Work the fictional Tamarack basin for one year on the board. The reservoir starts the year holding 40,000 acre-feet. Inflow (rain plus streamflow) is 25,000 acre-feet. Withdrawals are 22,000 acre-feet and evaporation is 6,000 acre-feet. Show that total outflow is 22,000 + 6,000 = 28,000 acre-feet, so the change in storage is 25,000 − 28,000 = −3,000 acre-feet, and the ending storage is 40,000 − 3,000 = 37,000 acre-feet. Then introduce residence time = storage ÷ inflow, and compute 40,000 ÷ 25,000 = 1.6 years, explaining that this is roughly how long a drop of water stays in the reservoir.
Guided practice (15 min): Hand out the printed water-budget page and work item 1 together so students see how to set up the outflow total. Then have pairs complete items 2 through 6, which continue the Tamarack basin into a drier second year. Circulate and check that pairs are subtracting total outflow, not just withdrawals.
Independent practice (10 min): Students finish any remaining handout items on their own and check their ending-storage answer against a partner's.
Closure and exit ticket (5 min): Formative check. On an index card, each student answers: "A basin holds 30,000 acre-feet, gains 12,000 acre-feet of inflow, and loses 15,000 acre-feet to withdrawals and evaporation combined. What is the change in storage?" Collect the cards and scan for the correct method (12,000 − 15,000 = −3,000 acre-feet). If more than a few students add instead of subtract, reteach the budget setup at the start of the next class.
Student Handout
Suppose the fictional Tamarack basin enters a drier second year. It starts the year holding 37,000 acre-feet. Inflow drops to 19,000 acre-feet, withdrawals stay at 22,000 acre-feet, and evaporation rises to 6,500 acre-feet. Use change in storage = inflow − (withdrawals + evaporation).
- Find the total outflow for the year (withdrawals plus evaporation).
- Find the change in storage for the year.
- Find the reservoir's ending storage at the end of the second year.
- The town decides to cut withdrawals to 17,000 acre-feet. With inflow still 19,000 and evaporation still 6,500 acre-feet, find the new change in storage.
- Find the new ending storage if the town makes that cut.
- Find the residence time at the start of the second year, using storage ÷ inflow with the 37,000 acre-feet starting storage and the 19,000 acre-feet inflow. Round to the nearest tenth of a year.
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: 28,500 acre-feet. 22,000 + 6,500 = 28,500 acre-feet of total outflow.
- Answer: −9,500 acre-feet. 19,000 − 28,500 = −9,500 acre-feet.
- Answer: 27,500 acre-feet. 37,000 − 9,500 = 27,500 acre-feet remaining.
- Answer: −4,500 acre-feet. New outflow is 17,000 + 6,500 = 23,500 acre-feet, so 19,000 − 23,500 = −4,500 acre-feet.
- Answer: 32,500 acre-feet. 37,000 − 4,500 = 32,500 acre-feet remaining.
- Answer: 1.9 years. 37,000 ÷ 19,000 = 1.947, which rounds to 1.9 years.
Differentiation
Support
Give students a partly filled budget table with the outflow total already computed for item 1, and provide a number line so students can see that inflow smaller than outflow produces a negative change.
Extension
Have students find the inflow the second year would have needed in order to keep storage exactly steady (a change of zero), and explain what that inflow tells them about how close the basin was to balance.
ELL supports
Pre-teach "inflow," "storage," "evaporation," and "residence time" with a labeled reservoir diagram using arrows for water in and water out. Provide a sentence frame for the exit ticket: "The change in storage is ___ acre-feet because inflow was ___ than total outflow."