Water stress is a term much used in recent books on the world water situation. Coined by Swedish hydrologist Malin Falkenmark, the term was intended as a rough indicator of national water scarcity. Any country with available supplies in the 1,000-2,000 m3/capita ranged are designated as water stressed, when the figure drops below 1,000 m3/yr the country is considered to be in a state of water 'scarcity'.

The problem with these definitions is that they involve the most deadly statistical sin - averaging across non-homogenous populations. The best known example of this is that of a person whose feet are in the oven and head is in the refrigerator, but who is 'on the average' at a comfortable temperature!

For purposes of comparing widely levels of water scarcity varying across regions of very different climate and affluence, this index is, on the average, seriously misleading.

In many rural communities in Africa and the Middle East, a family of ten, supplied with 274 litres, or 27.4 litres per person per day, would consider itself blessed. They would be surprised to hear themselves described as short of water if that had 100 times this much.

The fact is that the annual quantity that might be considered scarce depends totally on the climate, and the level of income and development. Actual water use in the year 1995 varied widely. The 36 most humid and lowest income countries used about 27 m3/capita/year out of a renewable resource availability of over 5,000. At the other end of the spectrum, the 7 driest and highest income countries used about 1,400 m3/cap/year out of a renewable resource of about 3,200.