Lesson plan Grade 12 Science
Tamarack Basin Drought Model: A Reservoir Water-Budget Lesson
A 55-minute grade 12 lesson where students build a spreadsheet water-budget model of a fictional reservoir and test how conservation extends the days of supply.
Objectives
- Students will build a spreadsheet water-budget model that tracks reservoir storage against supply and demand.
- Students will calculate days of supply under a baseline and a conservation scenario using unit conversions.
- Students will explain two limitations of a simplified water-budget simulation.
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.
- 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.
Also: days of supply, net draw, conservation
Teacher overview
The fictional city of Tamarack draws its water from a reservoir in the Tamarack Basin, and a multi-year drought has cut the inflow that refills it. This lesson has students build a simple spreadsheet water budget that tracks storage against demand and inflow, then test how a conservation order stretches the remaining supply. A common misconception is that storage drains at the full demand rate. Because inflow still adds some water each day, storage falls only at the net draw, which is demand minus inflow, so students must subtract before they divide.
Procedure
Framing the drought (7 min): Present the fictional Tamarack Basin reservoir: 9,000 acre-feet of usable storage, currently 70 percent full, serving a city that uses 4.0 MGD while drought inflow has dropped to 1.2 MGD. Ask students to predict, without calculating, how long the water will last, and record a few guesses on the board to revisit at the end.
Modeling the budget (13 min): Demonstrate the spreadsheet structure with a different, smaller worked example so students see the method rather than the handout answers. Model a reservoir of 1,000 acre-feet at a 2.0 MGD demand and 0.5 MGD inflow: convert acre-feet to gallons, find the net draw of 1.5 MGD, and divide to get days of supply. Set up columns for storage, inflow, demand, net draw, and days remaining, and show how changing one input cell updates the result.
Guided practice (15 min): In pairs, students build their own version of the model in a spreadsheet or on graph paper for the Tamarack values, filling the storage, inflow, demand, and net-draw cells. Circulate and check that each pair has subtracted inflow from demand before dividing, the step the misconception targets.
Independent handout (15 min): Students complete the handout questions individually, computing the baseline days of supply, then the days of supply under a 25 percent conservation cut, and the difference between them.
Formative check and share-out (5 min): Each pair reports the number of extra days conservation buys. The check for understanding is met when at least 80 percent of pairs report about 407 days and can name the net-draw step as the reason storage lasts longer. Return to the opening guesses and compare.
Student Handout
Use these values for the fictional Tamarack Basin reservoir and the city of Tamarack. Show your work.
- Usable storage: 9,000 acre-feet, currently 70 percent full
- City demand: 4.0 MGD
- Drought inflow: 1.2 MGD
- Conversion: 1 acre-foot = 325,851 gallons
Convert the reservoir's usable storage of 9,000 acre-feet to gallons.
The reservoir is 70 percent full. Calculate the current storage in gallons (70 percent of your answer to question 1).
Calculate the net daily draw on storage, in MGD, as the city demand minus the drought inflow.
Calculate the baseline days of supply: current storage in gallons (question 2) divided by the net daily draw in gallons per day (question 3). Round to the nearest whole day.
A 25 percent conservation order cuts demand from 4.0 MGD to 3.0 MGD. Calculate the new net daily draw, in MGD.
Calculate the days of supply under the conservation order, using the reduced net draw from question 5. Round to the nearest whole day.
How many additional days of supply did the conservation order buy, compared with the baseline in question 4?
In your spreadsheet, which input cell would you change to model a deeper drought, and which cell stays fixed no matter how bad the drought gets? Explain.
Name and explain one real process this simple water budget leaves out.
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: 2,932,659,000 gallons.
9,000 acre-feet x 325,851 gallons per acre-foot = 2,932,659,000 gallons, about 2.93 billion gallons.Answer: 2,052,861,300 gallons.
70 percent of 2,932,659,000 = 0.70 x 2,932,659,000 = 2,052,861,300 gallons, about 2.05 billion gallons.Answer: 2.8 MGD.
Net draw = demand minus inflow = 4.0 MGD - 1.2 MGD = 2.8 MGD.Answer: 733 days.
2,052,861,300 gallons / 2,800,000 gallons per day = 733.2, which rounds to 733 days.Answer: 1.8 MGD.
Conservation cuts demand to 3.0 MGD, so net draw = 3.0 MGD - 1.2 MGD = 1.8 MGD.Answer: 1,140 days.
2,052,861,300 gallons / 1,800,000 gallons per day = 1,140.48, which rounds to 1,140 days.Answer: 407 days.
1,140 days minus 733 days = 407 additional days bought by the conservation order.Answer: Lower the inflow cell; the usable storage capacity stays fixed.
A deeper drought is modeled by reducing the inflow input, which raises the net draw and shortens the days of supply. The reservoir's usable storage capacity is a physical limit that does not change with drought severity, so that cell stays fixed.Answer: Accept any one real process with a clear explanation.
Reasonable answers include evaporation from the reservoir surface, which removes water the model ignores; demand that rises in hot weather rather than staying constant; inflow that varies day to day; or population growth that raises demand over time. Each would change the days of supply the simple budget predicts.
Differentiation
Support
Provide a pre-built spreadsheet with the column headers and the acre-foot conversion already entered, so students fill only the storage, inflow, and demand cells. Give question 3's subtraction set up as demand minus inflow.
Extension
Ask students to add an evaporation row that removes a small daily volume they choose from storage, then recompute the baseline days of supply and describe how sensitive the result is to that added loss.
ELL supports
Pre-teach "storage," "inflow," "demand," and "net draw" with a labeled diagram of a reservoir with an inflow arrow and an outflow arrow. Provide sentence frames for questions 8 and 9 and allow spoken responses.
Extensions
Have students find the usable storage and service population of a real nearby reservoir from the utility's website, then rerun the model with those figures to estimate how many days of supply their own community would have under a similar drought.