Roof Runoff Pollution and Rainfall Intensity

ISEF Category: Earth and Environmental Sciences

Subcategory: Water Science  ·  Difficulty: Intermediate  ·  Setup: Home Setup  ·  Time: 1 to 2 Months

The Hook

The first rain after a dry spell can carry a bigger pollution punch than later rain. Your roof acts like a giant collection tray, so dirt, dust, and bird droppings can wash into the first runoff fast. That makes roof runoff a real water quality problem, even in a backyard. You can measure it with simple field tests and a model that predicts how the flush changes.

What Is It?

This project looks at the first-flush effect on a roof. First flush means the first part of runoff after rain starts. That early water often carries more debris and dissolved material than water that comes later. Think of a sponge with dust on top. The first water that passes through picks up the most grime.

You will test how rainfall intensity changes runoff quality. Rainfall intensity means how hard the rain falls. Faster rain can move water across the roof more quickly, which changes how long pollutants sit in contact with flowing water. You can measure total dissolved solids, or TDS, turbidity, which is how cloudy the water looks, and nitrate with test strips. Then you can compare those results to a simple kinematic-wave roof-runoff model, which is a math model that describes how water moves over a sloped surface.

Why This Is a Good Topic

This is a strong science fair topic because you can measure a real environmental problem with tools you can access. The question is clear, the variables are measurable, and you can compare storm conditions or simulated rainfall conditions across trials. You will learn sampling, water chemistry, data analysis, and basic modeling. Those skills make the project useful even if the final trend is messy.

Research Questions

• How does rainfall intensity affect the concentration of TDS in the first runoff from a roof?
• How does rainfall intensity affect turbidity in the first runoff from a roof?
• How does rainfall intensity affect nitrate levels measured with test strips in roof runoff?
• What is the effect of roof material on the first-flush pollutant load under similar rainfall intensity?
• To what extent does runoff quality change between the first runoff sample and later runoff samples?
• Which rainfall intensity best matches the peak pollutant load predicted by a kinematic-wave roof-runoff model?

Basic Materials

• Clean collection containers or wide-mouth bottles.
• Funnel and tubing or a simple gutter collection setup.
• Digital kitchen scale with 0.1 g accuracy.
• Graduated cylinder or measuring jug.
• TDS meter.
• Turbidity tube or homemade turbidity test setup.
• Nitrate test strips.
• Thermometer.
• Stopwatch or phone timer.
• Notebook or spreadsheet for data logging.
• Rain gauge or local weather app data.
• Latex-free gloves and safety goggles.

Advanced Materials

• Portable turbidity meter.
• Handheld conductivity meter for comparison with TDS readings.
• Automated rain gauge with data logging.
• Flow meter or marked collection tank for runoff volume estimation.
• Inclined test roof panel or controllable roof runoff simulator.
• Spectrophotometer with nitrate assay, if available.
• Sampling bottles for replicate storm fractions.
• GIS or roof-slope measurement tools for drainage mapping.
• Python or R for model fitting and uncertainty analysis.

Software & Tools

• Google Sheets: Organizes runoff data, calculates averages, and makes simple graphs.
• Excel: Helps you compare sample fractions, build charts, and track storm-by-storm trends.
• Python: Fits your runoff model and checks how well predicted pollutant load matches measured data.
• ImageJ: Estimates turbidity from standardized photos if you build a consistent imaging setup.
• R: Runs statistical tests and visualizes differences between rainfall intensity groups.

Experiment Steps

1. Define the roof area, slope, and drainage path you will study, then decide how you will mark the first-flush fraction.
2. Choose one rainfall variable to compare first, such as storm intensity, while keeping roof type, collection point, and sampling method fixed.
3. Plan your sampling sequence so you can separate early runoff from later runoff and compare the changing pollutant load over the storm.
4. Build a measurement plan that links each sample to TDS, turbidity, nitrate, and runoff volume in the same dataset.
5. Fit a simple kinematic-wave model to your roof geometry and use it to predict when the first flush should appear.
6. Decide how you will test model error, compare replicate storms, and show whether intensity changes the strength of the first flush.

Common Pitfalls

• Mixing up rainfall intensity and total rainfall, which makes storm comparisons meaningless.
• Collecting runoff from a roof section that shares drainage with another surface, which contaminates the sample source.
• Using unstandardized lighting or muddy containers when reading turbidity, which adds noise to the measurements.
• Testing nitrate strips outside their reliable range, which flattens real differences between samples.
• Forgetting to pair each sample with runoff volume, which prevents you from calculating pollutant load instead of only concentration.

What Makes This Competitive

A stronger project does more than compare a few water samples. You can stand out by pairing water chemistry with a real runoff model and by calculating pollutant load, not just concentration. Good replication matters too, especially across different storms or intensity ranges. If you test how well the model predicts the timing and size of the first flush, your project starts to look like environmental engineering, not just sample testing.

Project Variations

• Compare roof runoff from asphalt shingles, metal, and tile to see how surface material changes first-flush pollutant load.
• Swap nitrate strips for phosphate strips or conductivity readings to test whether the pollutant pattern changes across indicators.
• Analyze the same runoff data with a simple event mean concentration model instead of a kinematic-wave model to compare which predicts first flush better.

Learn More

• USGS Water Science School: Explains runoff, water quality, and stormwater basics, and you can find it by searching the USGS site.
• NOAA National Weather Service: Offers rainfall and storm data that can help you match your runoff samples to real weather conditions, and you can find it on NOAA's site.
• EPA Stormwater resources: Gives plain-language background on roof runoff and pollutant transport, and you can find it by searching the EPA site for stormwater basics.
• PubMed: Search for review articles on first-flush roof runoff, rainfall intensity, and urban water quality to see how researchers frame the problem.
• MIT OpenCourseWare: Search for hydrology or environmental engineering course notes to review runoff concepts and simple flow models.
• NIH PubChem: Look up nitrate and related compounds to review chemical properties and safety background.