What it shows
Soil takes in water at a certain speed, called its infiltration rate. Measuring it in the playground, a field and a forest floor shows why some ground floods while other ground soaks up the monsoon.
The science
Infiltration rate is the depth of water that enters the soil in unit time, in millimetres per hour. It depends on the size of the gaps between soil particles: sand has large gaps and a fast rate, clay has tiny gaps and a slow rate. Roots and worms make channels that increase it, while compaction and paved surfaces cut it down. When rain falls faster than the soil can take it in, the extra runs off the surface, causing floods and soil erosion. The ring keeps water from spreading sideways so that the water moves downward.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- 3 tin cans with both ends removed (an adult must cut and file edges), 7 to 10 cm wide
- A wooden block and a hammer or mallet
- A measuring cup
- A 2 litre bottle of water per test
- A stopwatch
- A ruler
- A notebook
Safety
For this project: An adult must remove the can ends and file the sharp edges or cover them with tape. Wear gloves when pushing the ring into the soil and keep fingers clear of the hammer.
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.
Step 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.
Choose three sites such as a trampled playground, a lawn or field and the soil under a tree. Clear any leaves and stones.
Place the block on top of the ring and hammer it 5 cm into the soil. Press the soil around the ring to stop leaks.
Pour 100 ml of water slowly to wet the soil, and wait until it has gone. This fills the pores so that your test shows a steady rate.
Pour a measured amount of water, such as 200 ml, into the ring and start the stopwatch at once.
Stop the watch when the water surface just disappears into the soil and write the time. A common mistake is to stop when the surface turns dull instead of when no water is left.
Work out the depth of the water: volume in cm3 divided by the area of the ring in cm2. Then divide by the time to get the rate in cm per hour and change it to mm per hour.
Repeat each test three times and take the average. Plot a bar chart for the three soils.
Compare the infiltration rates with local monsoon rainfall of 20 to 50 mm per hour in heavy storms and say which sites would give surface runoff.
What you should see
A bar chart showing that the compacted playground takes in water slowly, the lawn faster and the soil under the tree fastest. Some sites have rates below heavy rain and would flood.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
01What is infiltration rate?
The speed at which water enters the soil, usually in mm per hour.
02Why is clay slow?
Its pores are very tiny, so water moves through them slowly.
03Why does the soil under a tree take in water well?
Roots, worms and leaf litter keep it loose and full of channels.
04Why does compaction cause floods?
Compressed soil has fewer gaps, so more rain runs off over the surface.
Take it further
Measure infiltration before and after a day of rain and see how wet soil slows the rate.
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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