The second of the world's big water problems (some may argue it is the first) is population. When there were fewer than 1 billion people on earth (which was, remarkably, only about 200 years ago) everybody lived quite near a water source of some sort, because life wasn't possible any other way. And if the source dried up and they could not fetch some from somewhere nearby or move on to a better-endowed spot, they just died.

This represented an ecological system more or less in balance, with the population sustained by the available resource. Slowly we learned to transport the water further from the source, and to store it from one season to the next, so that a larger population could be supported.

What really upset the balance, however, was the advent of modern medicine, in particular the public health programs that permitted world population to jump from 2 billion in 1930 to 5 billion in the next 70 years.

A high fraction of this increased population lives in countries that are chronically poor, so that development of new water sources (even where these still exist) and distribution systems has been unable to keep up.