John Hildebrand who has lived in the Artesian Valley, near Fowler, Kansas, since he was two years old, remembers why the valley has the name it does. "There were hundreds of natural springs in this valley. If you drilled a well for your house, the natural water pressure was enough to go through your hot-water system and out the shower head." There were marshes in Fowler in the 1920s, where cattle sank to their bellies in mud. And the early settlers went boating down Crooked Creek, in the shade of the cottonwoods, as far as Meade, twelve miles away.

Today the creek is dry, the bogs and the springs have gone, and the inhabitants of Fowler must dig deeper and deeper wells to bring up water. The reason is plain enough: seen from the air, the surrounding land is pockmarked with giant discs of green quarter-section pivot-irrigation systems water rich crops of corn, steadily depleting the underlying aquifer. Everybody in Fowler knows what is happening, but it is in nobody's interest to cut down his own consumption of water. That would just leave more for somebody else.

Five thousand miles to the east, near the Spanish city of Valencia, the waters of the River Turia are shared by some 15,000 farmers in an arrangement that dates back at least 550 years and probably longer. Each farmer, when his turn comes, takes as much water as he needs from the distributory canal and wastes none. He is discouraged from cheating watering out of turn merely by the watchful eyes of his neighbors above and below him on the canal. If they have a grievance, they can take it to the Tribunal de las Aguas, which meets on Thursday mornings outside the Apostles' door of the Cathedral of Valencia. Records dating back to the 1400s suggest that cheating is rare. The huerta of Valencia is a profitable region, growing at least two crops a year.

Two irrigation systems: one sustainable, equitable, and long-lived, the other a doomed free-for-all. Two case histories cited by political scientists who struggle to understand the persistent human failure to solve "common-pool resource problems." The only way to avoid abuse is self-restraint. And yet nobody knows how best to persuade the human race to exercise self-restraint.