Most people worldwide are unaware of how much water they use each day, although the figures are becoming more known, especially where smart meters are installed. Direct use varies from about 70 litres per person per day (l/h/d) in the Far East, to 150 l/h/d in Western Europe, to 300 l/h/d in the US. However this is just direct use, i.e. that used 'from the tap' for clothes washing, cooking, hand washing, shower and bath, toilet and outdoor watering). This direct use is a fraction of the water that is utilised per person in order to meet their daily needs. Water is required to grow food, manufacture goods, and produce energy. Such water use is often referred to as 'virtual water' and the measurement of how much water is used by an individual or a country is termed its 'water footprint'.
Water footprints are sub-divided into three types, green, blue and grey. In simple terms the green water footprint measures the water flowing in aquifers and rivers used for irrigation, washing, processing and cooling, etc. The blue water footprint measures the volume of groundwater and surface water consumed, i.e. withdrawn and then evaporated. The grey water footprint measures the volume of water flow in aquifers and rivers polluted by humans (see http://www.waterfootprint.org for more details and figures).
The water footprint of nations varies greatly, with in general terms that of the developed world being far higher than the developing. Whilst the average is about 1,400m3 per person per year, in India this is about 1,000m3 per person per year, in the UK 1,700 m3 per person per year and the US 2,800 m3 per person per year. However this is not the whole picture as, whilst 20% of the water used in the US is from product sourced from other countries, in the UK it is about 70%. Water footprint is thus a key issue with regard to global water security.
In the next few blogs I will be looking at the effects of these footprints, and how we can both reduce our footprint and manage water sustainably.