Surface water is the kind we are all most familiar with; it is what we see all the time in rivers, lakes and ponds. This is the most common sourcer; about 70% of the world's people use only surface water.

There is a lot of surface water about; nearly 360,000 km3 stored in the world's lakes and rivers. Of this about 10%, or 40,000 km3 is replenished each year by runoff from surrounding watersheds and inflow from adjacent aquifers.

This 40,000 km3 figure is technically the 'renewable surface water supply', but about two thirds of it runs off in floods, leaving only about 14,000 km3 as a relatively stable supply. This would still would be enough for more than 6,000 litres/day each, plenty for everybody, even with 6 billion people on earth. The trouble is that most of this water is not in the right place at the right time for everyone to get a fair share.

To control the flow of water toward the sea at times when we do not need it, reduce the quantity discharged to the sea during floods, and store it for when it is needed, much of the water development of the 20th century has involved dam construction. This major engineering activity now appears to be coming to an end.

Surface waters are also where we dump most of our municipal and industrial wastewater, and the resulting pollution is one of the main reasons why it is so hard to find a source of good water to drink these days.