What it shows
The water a roof can harvest equals roof area times rainfall times a runoff factor. Measuring a real roof and using the local annual rainfall shows that even a small house in a 800 millimetre rainfall town can collect tens of thousands of litres a year, often more than the family drinks.
The science
Rainfall is measured as a depth in millimetres, the height water would reach on a flat surface. A roof of area A square metres receiving R millimetres of rain collects A times R litres, because one millimetre over one square metre is one litre. Some water evaporates, splashes or wets the roof first, so a runoff coefficient of about 0.8 for a tiled or concrete roof gives the usable amount. Comparing the monthly yield with monthly household use shows how big a storage tank is needed and when it would run dry.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A measuring tape
- Annual and monthly rainfall figures for your town from the weather office website or a textbook
- A calculator or a spreadsheet
- Graph paper
- A sketch of the house roof
- A rain gauge if one is available
Safety
This project uses safe, everyday materials. Still, read the rules below with a parent before you start.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Measure the length and width of the roof from the ground plan, or pace it if measuring is unsafe, and work out the area in square metres.
Write down the monthly rainfall for your town for all twelve months and the annual total.
Calculate the monthly harvest: area times rainfall in millimetres times 0.8, in litres.
Estimate the family's daily water use for drinking and cooking, about 10 litres a person, and multiply by the days in the month.
Plot harvest and use month by month on one graph.
Find the months where harvest exceeds use and the longest dry gap between rainy months.
Work out the tank size needed to carry drinking water through the dry gap.
If a rain gauge is available, check one month's actual rainfall against the average and recalculate.
What you should see
A graph showing that in most Indian towns four monsoon months supply most of the year's harvest, a yearly total in the tens of thousands of litres for a 50 square metre roof, and a recommended tank size in litres.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1How many litres fall on one square metre in 1 millimetre of rain?
One litre.
2Why use a runoff coefficient?
Some rain evaporates, splashes off or soaks the roof before any runs off.
3Why is a tank needed even with heavy rain?
Most rain falls in a few months and must be stored for the dry months.
4What is the first flush and why discard it?
The first few minutes of rain, which carry dust and bird droppings off the roof.
Take it further
Cost a tank of the size you calculated and compare it with buying tanker water for the dry months.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
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