What it shows
Water leaves open surfaces at a rate set by heat, wind and humidity. Daily readings from identical pans show how much water a field or a tank loses and why crops need irrigation in summer.
The science
Evaporation is the escape of the fastest water molecules from the surface. It speeds up with higher temperature because more molecules have enough energy, with wind because moist air above the surface is carried away, and with lower humidity because the air can take up more vapour. The loss is usually given in millimetres of depth per day. In hot, dry summers in much of India, open pans lose 6 to 12 mm a day, and a crop uses a similar order of water, which is why mulching and drip irrigation save water.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- 4 identical shallow steel or plastic pans, 25 cm wide
- A ruler marked in millimetres or a long nail with a scale
- A measuring jug
- Water
- A thermometer
- A stand to keep pans at the same height
- Mesh to stop birds drinking and a notebook
Safety
For this project: Place the pans where they will not be tipped over, away from young children, and empty them after the test to prevent mosquito breeding.
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.
Fill four pans to a depth of exactly 30 mm at the same time of day, using the jug and checking with the ruler.
Place the first pan in full sun, the second in the shade of a wall, the third in the sun with a fan or a windy spot, and the fourth in the sun with a lid of mesh or a floating layer of oil, as the control.
Record the air temperature and the weather at the same time every day.
Measure the depth of water in each pan at the same hour every day, reading the ruler at eye level, and top up to 30 mm after measuring. Note the amount added.
Write the loss each day in millimetres and plot a line graph for each pan over two weeks.
Find the mean daily loss for each pan and compare them in a bar chart.
Work out the loss for a 10 m by 10 m pond or tank in one day: area in square metres times loss in metres, and change to litres.
Write which of sun, shade, wind and cover saves most water, and say how a farmer could use this.
What you should see
A graph showing the sun and wind pans lose the most, the shaded pan much less and the covered pan least, with daily losses recorded in millimetres and a pond water loss estimate.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why does wind increase evaporation?
It removes the moist air above the surface so more vapour can escape.
2What makes evaporation faster at high temperature?
More molecules have enough energy to leave the liquid.
3How does mulching help a field?
It shades the soil and slows evaporation.
4Why does a thin layer of oil reduce evaporation?
It forms a barrier that traps water molecules below.
Take it further
Compare your pan data with the rainfall record for the same weeks and decide whether rain or evaporation is bigger in your area.
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.
Free ebooks for this project
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