Blue vs Green Water

Rain fed agriculture covers 80% of the world's cultivated land, and is responsible for about 60% of crop production. In rain fed agriculture the soil stores the rain and releases it slowly to the plants. This rainwater used in agriculture is part of what is called 'green water’ and is a characteristic of the land on which it falls. It is usually not subject to competition from other sectors.

Yields in rain fed agriculture are still far from their potential. In 2005, cereal yields were about 1.1.5 tonnes per hectare in Sub-Saharan Africa, compared with 5 tonnes per hectare in Europe. ‘Green water’ or rain fed agriculture has an important role to play in sustaining populations and securing future food supplies.

By contrast, in irrigation, water is extracted from rivers, lakes and aquifers (which is referred to as ‘blue water'). This ‘blue water’ is applied on land, where most of is consumed by evapo transportation. Irrigation competes with other sectors (including the environment) for ‘blue water.’ Today irrigated agriculture covers about 20% of cultivated land and accounts for 40% of global food production. 'Grey water' is polluted or waste water, which can be recycled and purified. Grey water will play an important role in securing future global water supplies.