Yes, we are in the post-industrial age, and the production of autos, houses, airliners and other "goods' can be taken for granted. But Sandra Postel in Pillar of Sand warns that there is no such thing as a "post-agricultural age." Because irrigated agriculture provides 40 percent of the globe's food today, and because in the past, civilizations based on irrigation have often failed, we had better pay attention.

The problem is, we don't know what exactly destroyed ancient Mesopotamia or the civilization built by the Hohokam in the American Southwest. So we are left to ponder our present imponderables: climate change, the salting of the earth's soils, the silting of reservoirs, the calcifying of massive bureaucracies such as the U.S. Bureau of Reclamation. We can put a ruler to any one of these, but how do they interact, with what speed do they advance, and can our society, which many think is too ingenious for its own good, figure ways around them?

Conventionally, which means Malthusian reasoning, food production is in a race against population production. At the moment, it is a race run at a slowing rate. In the 1990s, grain production dropped from the 2 percent annual growth rate of 1950 to 1990, to this decade's 1 percent growth rate. But population growth has also slowed, from 2.2 percent in 1962 to 1.4 percent in 1998. Is the glass half full and filling? Or half empty and getting emptier?

On balance, Sandra Postel's glass is half full. She understands the salinization and other dangers that could wipe out millions of hectares of now productive irrigated land. And she knows that even a 1.4 percent population growth rate adds 80 million people a year to the earth.

But she traveled widely in writing this book, and she found many innovations that could buy us time. Most surprising, perhaps, she became a fan of marketing - especially the marketing of cheap, efficient, foot-powered irrigation pumps that allow people with small plots of land in Third World countries to reach groundwater lying only a few feet beneath the surface. This water, when spread over the land, gives them the food and cash not just to survive, but also to create the base of a vital economy.

Overall, this book is that rarest of works: a calm approach to the intertwined questions of population and food. Instead of going into the research with all the answers, the writer went in with good questions and found interesting answers. As a bonus, it's filled with engaging anecdotes (case studies, if you prefer), and is a well-written, quick read.

* Ed Marston