A water footprint is a value of the water that is actually used in making a product or carrying out an activity. It can be estimated for a house, a town, a business, a crop, a country or a meal – anything that uses water. The water footprint is useful as a means of comparing how much water it takes to grow different crops or make different types of food.
The water footprint is often separated into green, blue and grey water (though these names do not refer to the colour of the water).
[Please note - this page is in progress and incomplete at 5 February 2012]
Green water originates from rain that falls on the soil, gets held by the soil and is then taken up by plants to enable them to survive and grow. As they grow, water is lost by a process of transpiration from the leaves to the air.
Each kilogram produced by a crop needs around 1000 kilogram of water (very approximately – the actual value depends on many other things). So even before we consider blue and grey water, the amount of green water is pretty big. Think of a kilo bag of flour or sugar – then think of a thousand one-litre bottles of water. That’s about what it takes.
Blue water is that stored in ponds, rivers and reservoirs on the earth’s surface and deep underground in aquifers. Blue water originates from rain, just like green water, but as soon as it runs off the field into a ditch or stream or else percolates down through the soil to the rock underneath, it becomes blue water.
The farms at the Institute grow crops on both green and blue water. The green water originates as snow or rainfall, which is then taken up by plants from the soil and transpired through leaves; the blue water is that sucked up from deep below the earth through a bore hole into a water storage tank, from where it is put onto the soil as irrigation to help crops like potatoes when it gets dry.
Grey water is usually defined as the amount of unpolluted water that would be needed to dilute the pollution in water to the point where it is no longer harmful. Today, our polluted water is purified in sewage treatment works rather than diluted, but the purification needs a lot of machines, energy and people, all of which use their own water.
Grey water provides most of the needs of people in many parts of the world. In some regions of the UK, not supplied directly from blue water reservoirs, the water coming out of the taps has been recycled from rivers, and so has a grey water footprint. In many countries of the world, people have no choice but to use polluted water to drink and add to their food.
People eating a meal in northern Europe will likely use green, blue and grey water. The crops that went to make the meal used green water from the soil in their country of origin. If the crops were irrigated, it would be with blue water from a pond or river or grey water that had been treated (we hope). The food materials would be washed and processed in a factory using blue or grey water. Water added to the ingredients to make the meal and to wash up afterwards would be blue or grey water from the tap. The washing up water will go down the sink as grey water. After a few hours and the food is digested, they will add to the world’s grey water when they go to the toilet and wash their hands.
You perhaps wouldn’t think that preparing and eating a simple meal involved so many different types of water, some of it thousands of miles away. When added to the green water used to grow the crops, the blue and grey water raise the total water used to make a kilogram of food to thousands and ten-thousands of kilograms of water. It is difficult to estimate accurately the total water footprint of food, but the reports and other articles under Further information will explain how it is done.
Anyone wishing to know more or make a detailed study of water footprint will find information at the website of the Water Footprint Network.
The history, concepts and value of water footprint and green, blue and grey water are explained in The water footprint assessment manual: setting the global standard by Arjen Y. Hoekstra, Ashok K. Chapagain, Maite M. Aldaya, Mesfin M. Mekonnen. Publisher: Earthscan. 2011, ISBN: 978-1-84971-279-8.
A reference list will also be placed here containing links to articles on water footprint.