What it shows
Growing food uses a lot of water. Collecting published figures and farm data for crops shows which foods use the most water and how a family’s diet affects its water footprint.
The science
Virtual water is the water used to grow and process a crop. Crops that need flooded fields, such as paddy, use more water per kilogram than millets or many vegetables. Comparing figures helps people choose foods and farming methods that use water wisely.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A notebook or spreadsheet
- Water-use figures for crops from an agriculture book or an official agriculture website
- A weekly menu of a family meal
- A calculator
- Interviews with a local farmer or gardener (with an adult)
- Chart paper and colours
Safety
For this project: Visit a farm only with an adult and permission. Follow the farmer’s instructions near water channels and machines.
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.
Look up typical water use in litres per kilogram for rice, wheat, pulses, millets and common vegetables.
Write the sources of each figure and note that values vary by region and method.
Interview a local farmer to find out how many irrigations his crop gets and how much water each takes.
List the crops in one family’s weekly meals with the mass of each.
Multiply mass by litres per kilogram to find the water in each food.
Add them up to get the family’s weekly virtual water use.
Draw a bar chart of water used per crop and per meal.
Suggest three changes to the diet or farming method that would cut water use.
What you should see
A table and chart show which crops are thirsty and which are frugal, and a weekly total for one household is compared against direct home water use.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is virtual water?
The hidden water used to produce food or goods.
2Which crop group usually uses the most?
Flood-irrigated rice and sugarcane.
3How could farmers use less water?
Drip irrigation, mulching and growing millets.
4Why do the numbers vary?
Climate, soil and method change the water used.
Take it further
Compare the virtual water in your meals with the water you use for washing and drinking.
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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