← Back to Simulation Teacher Notes
AP Environmental Science Teacher Reference For Instructor Use

Teacher Notes:
Green Roof Runoff Simulation

Scientific background, model assumptions, classroom strategies, and discussion guidance for the McKinley High School stormwater simulator.

🎯

Purpose of the Simulation

This simulation allows students to explore how converting conventional rooftops to green roofs can influence stormwater runoff and water quality in urban environments. The model helps students examine trade-offs between different pollutants and understand how green infrastructure can influence watershed health.

🌿 Core Goal
The goal is conceptual understanding, not engineering prediction. Students should leave understanding the mechanisms, trade-offs, and limitations — not memorizing specific numbers.

Students can investigate questions such as:

  • How does increasing green roof coverage affect pollutant loads?
  • Why might some pollutants decrease while others increase?
  • What are the trade-offs involved in green infrastructure design?
  • How might stormwater management decisions influence urban streams?

The simulation is particularly useful when discussing urban watersheds, stormwater pollution, and green infrastructure solutions.

🔬

Scientific Background

Urban Stormwater and Roof Runoff

In cities, large areas of impervious surfaces — such as roofs and pavement — prevent rainfall from infiltrating into soil. Instead, water runs off quickly into storm drains and nearby streams. As stormwater moves across surfaces, it picks up pollutants including:

  • Atmospheric nitrogen deposition
  • Dust and particulate matter
  • Metals and trace elements
  • Nutrients from organic debris

This runoff can contribute to water quality problems such as nutrient pollution and eutrophication in downstream waterways — including Scajaquada Creek and, ultimately, the Niagara River.

How Green Roofs Work

Green roofs replace traditional roofing materials with layered systems that typically include a waterproof membrane, drainage layer, lightweight growing media, and vegetation. These layers capture, store, and absorb rainfall through plant uptake, reducing the volume and velocity of stormwater leaving the roof. Environmental benefits include:

  • Reduced stormwater runoff volume
  • Delayed peak discharge during storms
  • Urban heat island mitigation
  • Habitat for pollinators and insects
  • Improved building energy efficiency
  • Aesthetic and psychological benefits

However, green roofs also alter the chemistry of the runoff water that does leave the roof — which is the central focus of this simulation.

Nutrient Dynamics in Green Roofs

Green roof substrates contain organic matter and mineral components that support plant growth. These materials influence runoff chemistry in complex ways. Research findings across multiple studies show consistent patterns:

✅ Typically Decrease
  • Nitrate (NO₃⁻) — plant/microbial uptake
  • Ammonium (NH₄⁺) — nitrification + uptake
⚠️ Typically Increase or Release
  • Phosphorus (PO₄³⁻) — leaches from media
  • Magnesium — mineral dissolution
  • Boron — substrate amendments

Nitrogen decreases because plants and soil microbes actively take up nitrogen during growth. Phosphorus increases because it leaches from the growing media — particularly in younger roofs before the substrate weathers and equilibrates. The magnitude of these effects varies substantially depending on substrate composition, roof age, fertilizer use, vegetation type, rainfall intensity, and maintenance practices.

💡 Teaching Tip — The Trade-off
The phosphorus increase is the most discussion-rich element of the lesson. Students often assume "green = clean across the board." The phosphorus data disrupts that assumption and creates a genuine scientific controversy worth debating: if a green roof reduces nitrogen but increases phosphorus, is it a net benefit for water quality? The answer depends on which nutrient is limiting algal growth in the receiving water body.
⚙️

Model Assumptions & Limitations

To keep the simulation understandable for high school students, the model simplifies several aspects of real green roof behavior. Students who recognize these assumptions are practicing authentic scientific thinking.

Model Assumption What It Simplifies Status
Fixed pollutant concentrations (mg/L) for each roof type Real concentrations vary storm-to-storm, season-to-season, and by roof age Simplified
Linear scaling of pollutant load with roof area Real systems are non-linear; larger areas may have different detention times Simplified
Uniform rainfall across the entire roof Real storms have spatial gradients; some roof areas receive more water Simplified
Identical green roof performance at all coverages Performance depends on substrate age, vegetation, and design Variable in reality
No evapotranspiration, detention, or first-flush effects Green roofs retain some water; first flush carries higher loads Not modeled
No seasonal variation in plant uptake Nitrogen uptake is higher in summer growing season Not modeled
No antecedent dry period effects Longer dry periods accumulate more dry deposition before a storm Not modeled
⚠️ Important for AP Exam Alignment
AP Environmental Science asks students to evaluate model limitations. Asking students to identify two or three assumptions in this model and explain how they might affect results is an excellent exam-prep activity that aligns with Science Practice 3 (quantitative reasoning) and Science Practice 5 (CER).
🏫

Suggested Classroom Uses

Modeling Practice (NGSS / Science and Engineering Practices)

Students can use the simulation to analyze how changes in system inputs influence outputs, identify model limitations and assumptions, and propose improvements to the model.

"What variables might be missing from this model that could affect real green roof runoff? How would you change the model to account for one of them?"

Systems Thinking

The simulation is well-suited for discussions about trade-offs between pollutants and competing environmental goals.

"If green roofs reduce nitrogen but increase phosphorus, how should cities evaluate whether they are a net benefit? What additional information would you need to answer that question?"

Local Watershed Applications

Connect the simulation explicitly to local urban streams and stormwater systems. Students can investigate sources of nutrient pollution in their watershed, how green infrastructure could reduce runoff, and how other solutions compare — such as rain gardens, bioswales, or permeable pavement.

🌊 Buffalo Connection
Scajaquada Creek has been listed as an impaired waterway by the NYS DEC, with documented problems including nutrient enrichment, low dissolved oxygen, and degraded habitat. The Buffalo Sewer Authority has active green infrastructure programs. This simulation connects directly to real decisions being made in the community students live in.

Extension Activities

  • Have students design their own improved version of the simulation, specifying which variables they would add and why
  • Compare green roofs with other stormwater management strategies (rain gardens, permeable pavement, bioswales) using cost-benefit analysis
  • Investigate real green roof monitoring data — several universities publish long-term green roof runoff data
  • Calculate watershed-scale runoff reductions if all Buffalo schools adopted green roofs
  • Debate: should the Buffalo City School District prioritize green roof expansion at McKinley? Use simulation data plus economic and equity considerations
📐 APES Standards

ERT-3.A Nonpoint source stormwater pollution

ERT-3.B Eutrophication & nutrient loading

ERT-3.C Green infrastructure solutions

EIN-1.B Nitrogen & phosphorus cycles

ENG-1.A Environmental trade-offs

SP3 Quantitative data analysis

SP5 Claim–Evidence–Reasoning

📋 Worksheet Parts
  • Part 1 — Reading comprehension (wet/dry deposition, first flush)
  • Part 2 — Data collection tables (coverage vs. rainfall)
  • Part 3 — Analysis questions (patterns & trade-offs)
  • Part 4 — Scenario explorer (five coverage levels)
  • Part 5 — CER written response (summative)
⏱ Pacing Guide
  • 5 min — Hook & predictions
  • 8 min — Reading + TTS (Part 1)
  • 5 min — Guided demo of controls
  • 18 min — Data collection (Parts 2–3)
  • 7 min — Scenario explorer (Part 4)
  • 12 min — CER exit task (Part 5)
  • 5 min — Debrief & predictions revisit
🔢 Concentration Data

Source: Berndtsson (2010), Ecol. Engineering 36(4)

  • Nitrate: green 0.27 · conv 0.87 mg/L
  • Ammonium: green 0.86 · conv 1.47 mg/L
  • Phosphorus: green 0.47 · conv 0.25 mg/L
  • Magnesium: green 2.92 · conv 0.25 mg/L
  • Boron: green 0.58 · conv 0.03 mg/L

Formula: vol (gal) = area × rainfall × 0.623; × 3.785 = liters; × conc ÷ 1000 = grams

💬 Discussion Questions
  • "Is a green roof always better for water quality?"
  • "Which nutrient matters more in Scajaquada Creek — nitrogen or phosphorus? How would you find out?"
  • "Why might a 10-year-old green roof perform differently than a brand-new one?"
  • "What would make this simulation more useful for a city engineer?"
🌿 Key Takeaway for Students Green roofs can play an important role in urban stormwater management, but their environmental effects depend on design, context, and management. Models like this one help scientists and planners explore potential outcomes — but real-world systems are always more complex.