How much rain does the roof catch: from gauge depth to tank litres

The idea

Rainfall depth times catchment area gives a volume, so a school roof and a rain gauge reading predict the litres a tank receives from one storm.

Safety card

What you need

• School site plan with a scale bar (or the aerial image at a known scale) to measure one building's roof plan area
• The school rain gauge (see the Stage 1 entry) or the nearest Bureau station's daily total
• Tape to measure one wall of the same building and the eaves overhang at each end, as a check on the plan
• A rainwater tank on site with its nameplate capacity, and a way to read its level (sight tube or dipstick)

How to do it

1. Find which roof sections have downpipes that feed the tank. Measure that part of the roof plan outline on the site plan and convert to m² with the scale (a 15 cm × 8 cm rectangle at 1:200 is 30 m × 16 m = 480 m²). Check one side with the tape: measure the wall on the ground and add the eaves overhang at each end (wall to gutter line), because the roof's plan outline includes its eaves; for the 30 m × 16 m roof above with 600 mm eaves the walls measure 28.8 m × 14.8 m, so a wall footprint of 426.2 m² would read 11 % below the 480 m² roof plan.
2. Read the rain gauge after a storm (mm).
3. Volume caught = depth (mm) × area (m²) litres, because 1 mm over 1 m² is 1 L. Apply a runoff coefficient of 0.9 for a metal roof (water that wets the roof and gutters and never runs off) and state it as an assumption; a first-flush diverter is not part of the 0.9, so subtract its volume, read from its label or measured, separately.
4. If the tank is connected, read the tank level before and after the storm and convert the level change to litres from the tank dimensions (cylinder: π r² × Δh).
5. Compare the predicted and measured litres and explain the difference (overflow, first-flush diverter, leaks).

What you should see

A predicted volume such as 25 mm × 200 m² × 0.9 = 4500 L for a small building, and, where a tank exists, a measured change. The measured change is expected to be below the prediction once the tank overflows, and below it by the first-flush volume where a diverter is fitted; the gap that remains once those two are taken off shows how well the 0.9 assumption fits this roof. It worked if the roof area was checked with the tape, only roof draining to the tank was counted, and the gauge reading is from the same storm.

What changes

This activity lists no variables to change, measure and keep the same.

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

• A few millimetres of rain is too little to fill anything.
• Tank size, not roof size, sets how much water a storm delivers.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

• Geography K–10 Syllabus (2015), NESA; current and taught in 2026, replaced from 2027. The outcome codes are not printed on the NESA landing page for this syllabus, which only links the document, so the url given is the syllabus document itself (geography-k-10-syllabus-2015.docx); the code and its outcome text were read in that document on 23 September 2026.GE4-5GE4-7
• Geography 7–10 Syllabus (2024), NESA; implementation from 2027, not yet taught; code read on this page on 22 September 2026GE4-MAN-01GE4-TAP-01
• Australian Curriculum v9AC9HG7K02AC9HG7S03AC9HG7S05

Sources

The pages the author read to write this activity.

1. www.metlink.org/fieldwork-resource/a-weather-station-for-your-school
2. www.ga.gov.au/education/classroom-resources/map-reading-guide
3. curriculum.nsw.edu.au/learning-areas/hsie/geography-7-10-2024/outcomes