Most of the rain that falls onto the earth's surface evaporates or roars down to the ocean in spring floods. Around 12,500 cubic kilometers of runoff is accessible in the slower flows of rivers and lakes. This would be enough for current demand if it were well distributed around the globe. Since it is not, and since water consumption has risen seven times since 1900 and water conservation is inadequate, many rivers are being depleted at an alarming rate.