Writing prompt Grade 11 Science
Why Tanbury Springs Declined: Rainfall, Pumping, or Pines
Grade 11 students weigh three explanations for an 18-year decline in a fictional spring, then write an evidence-based argument with a counterclaim and a proposed test.
Objectives
- Students will compare three candidate causes of a spring decline against a long-term monitoring record.
- Students will use a rough water balance to test whether a proposed cause is large enough to explain an observed change.
- Students will write an argument with a claim, cited evidence, a counterclaim, and a rebuttal.
Vocabulary
- Baseflow
- Baseflow is the water that keeps a stream flowing even when it has not rained for a while. It comes from groundwater slowly seeping into the streambed.
- 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.
Also: recharge area, water balance, confounding variable, control site
Teacher overview
When a spring loses more than a third of its flow over two decades, three explanations usually compete: less rain, more pumping, or a change in what grows on the recharge area. This prompt gives students an 18-year monitoring record from a fictional town and asks them to argue which explanation the evidence supports. The record rewards anyone who checks whether the increase in pumping is large enough to account for the loss, because the rise in pumping is much smaller than the decline in discharge. A nearby control spring is included so that students can test the rainfall explanation rather than assume it.
Prompt
Background. Tanbury is a fictional town used for this assignment, and every figure below is invented for it. Tanbury Springs rises at the foot of Kellow Ridge and has supplied the town since it was founded. The ridge above the springs is the recharge area, about 26,000 acres of thin soil over fractured rock. Rain that soaks in there moves down through the rock and reappears at the springs, so the springs are fed by groundwater rather than by direct runoff.
Three things changed on the ridge during the record. The town and nearby farms drilled more wells and pumped more each year. Rainfall varied from year to year. Beginning around the start of the record, a timber company planted fast-growing pines on former pasture. Trees move far more water from the soil back to the atmosphere than pasture grass does, a process called evapotranspiration, and a pine stand uses more water as it matures.
| Period | Mean rainfall (in/yr) | Groundwater pumped (acre-ft/yr) | Pine plantation on the recharge area (acres) | Mean spring discharge (acre-ft/yr) | Depth to water, Well T-3 (ft) |
|---|---|---|---|---|---|
| 2007-2009 | 41.2 | 640 | 300 | 6,080 | 42 |
| 2010-2012 | 39.8 | 700 | 1,500 | 5,720 | 44 |
| 2013-2015 | 42.0 | 760 | 2,900 | 4,920 | 47 |
| 2016-2018 | 40.6 | 810 | 4,100 | 4,050 | 51 |
| 2019-2021 | 41.5 | 850 | 4,100 | 3,550 | 54 |
| 2022-2024 | 40.9 | 880 | 4,100 | 3,180 | 57 |
Two more pieces of evidence are available, both invented for this scenario. Corvel Spring lies seven miles north, on a recharge area with no plantation and almost no pumping. Its mean discharge went from 1,240 acre-feet per year in the first period to 1,190 acre-feet per year in the last. The fictional Kellow Ridge Forestry Cooperative measured water use under the mature pines and under the neighboring pasture, and reported that the pine stands returned about 6 inches per year more water to the atmosphere than the pasture did.
As the springs weakened, Tanbury drilled two municipal wells, moved its summer irrigation to night hours, and began paying farmers to fallow fields in dry years.
Writing task. Write an argument for the Tanbury Water Board that answers this question: which change on Kellow Ridge best explains the decline in spring discharge? Your argument must state a clear claim, support it with specific values from the table, and include at least one rough calculation showing whether a proposed cause is large enough to explain the loss. It must state one rival explanation fairly and then rebut it with evidence. It must explain how the shrinking supply changed what people in Tanbury do. Finally, it must name one additional measurement that would test your claim, and say what result would count against you.
Planning Organizer
Complete every row before drafting. The rows match the criteria your writing will be scored on.
| Section of your argument | Notes and figures you will use |
|---|---|
| My claim about the dominant cause | |
| Evidence from the discharge column | |
| Evidence from the rainfall column and the control spring | |
| Evidence from the pumping column | |
| Evidence from the plantation column | |
| My rough calculation and what it shows | |
| The rival explanation, stated fairly | |
| My rebuttal to that rival | |
| How the decline changed decisions in Tanbury | |
| One measurement that would test my claim | |
| A result that would count against my claim |
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Exemplar & Rubric
Exemplar response
The growth of the pine plantation is the best explanation for the decline of Tanbury Springs. Discharge fell from 6,080 acre-feet per year in 2007 through 2009 to 3,180 acre-feet per year in 2022 through 2024, a loss of 2,900 acre-feet per year. Over the same period, the planted area on the recharge ridge grew from 300 acres to 4,100 acres, an increase of 3,800 acres. If the mature pines return about 6 inches per year more water to the atmosphere than the pasture they replaced, then the new plantation intercepts roughly 3,800 acres times half a foot, or about 1,900 acre-feet per year. That single change accounts for about two-thirds of the observed loss, and no other change on the ridge is close to that size.
Rainfall can be ruled out. The period means stayed between 39.8 and 42.0 inches per year, with the wettest period, 2013 through 2015, falling in the middle of the decline rather than at its start. Corvel Spring provides a control. It draws from a recharge area with no plantation and almost no pumping, and it receives the same regional weather, yet its discharge moved only from 1,240 to 1,190 acre-feet per year. If a regional rainfall deficit were driving the loss at Tanbury, Corvel Spring should have dropped as well, and it did not.
Some board members will argue that pumping is the true cause, and the argument deserves a hearing. Pumping did rise steadily through the whole record, and the water level in Well T-3 fell from 42 to 57 feet below the surface, which shows that groundwater storage under the ridge is being drawn down. Pumping also removes water that would otherwise reach the springs, so some of the decline belongs to it. The problem is one of size. Pumping rose by only 240 acre-feet per year across the whole record, which is less than a tenth of the 2,900 acre-foot decline in discharge. Even if every additional gallon pumped came directly out of spring flow, pumping could account for only a small share of the loss. The falling water level in Well T-3 is also consistent with the plantation explanation, because trees that intercept recharge lower the water table just as pumping does. That makes the well record a confounding variable rather than proof of overpumping.
The decline changed how Tanbury uses water. The town drilled two municipal wells to replace flow it once took by gravity from the springs, shifted irrigation to night hours to cut evaporation losses, and began paying farmers to fallow fields in dry years. Each of those decisions costs money, while the springs once supplied the same water for free.
One measurement would test my claim directly. The forestry cooperative could install paired soil moisture and sap flow sensors in a planted stand and in the remaining pasture, and report recharge below the root zone in both for three years. If recharge under the pines proved similar to recharge under the pasture, my estimate of 1,900 acre-feet per year would collapse and the pumping explanation would gain strength. Until that measurement exists, the size of the plantation effect makes it the most likely driver.
Rubric
| Criterion | Emerging | Developing | Proficient | Advanced |
|---|---|---|---|---|
| Claim | No clear claim, or the claim shifts | Names a cause but does not commit to it as dominant | States one clear claim about the dominant cause | States the claim and qualifies it appropriately given the remaining uncertainty |
| Evidence from the record | Cites no values | Cites values from one column | Cites values from at least three columns and links each to the claim | Uses all five columns plus the control spring, and explains what each rules in or out |
| Quantitative reasoning | No calculation attempted | Attempts a calculation with an error that is not caught | Completes a correct rough calculation and compares it with the observed decline | Compares magnitudes for two candidate causes and states what the residual gap implies |
| Counterclaim and rebuttal | No rival explanation mentioned | Mentions a rival without a rebuttal | States a rival fairly and rebuts it with cited evidence | Concedes the valid part of the rival, then rebuts on magnitude and on the confounded well record |
| Human response and testing | Neither addressed | One addressed briefly | Both addressed with specifics from the scenario | Both addressed, and the proposed test names a result that would falsify the claim |
Differentiation
Support
Give these students the table with the rainfall column already summarized in one sentence, and provide the rough calculation set up as a fill-in equation with the acre and foot figures supplied. Allow them to write four labeled sections rather than connected paragraphs, and pair them with a partner for the counterclaim section.
Extension
Ask these students to write a second, shorter argument taking the opposite position, then compare the two and explain in a paragraph which evidence they found hardest to explain away. They can also propose how the timber company and the town might share the cost of the monitoring study.
ELL supports
Pre-teach recharge area, evapotranspiration, discharge, and control site with a labeled ridge cross-section drawing. Provide the frames "The data show ___, which means ___" and "Some would argue ___, but the record shows ___." Allow the Planning Organizer to be filled in with phrases and numbers before drafting.
Extensions
Have students look up how their own state reports spring or streamflow data and identify one gaged site with a long record, then describe in a paragraph what additional land-cover information they would need before drawing a conclusion about a trend they see there.