The cycling of water through the atmosphere is essential to renew the fresh water resources on which all life depends. In an average year the sun’s energy lifts 86% of all evaporated water from the oceans while 14% is evaporated from land. When water evaporates the salt is left behind so water in the atmosphere and rainwater are fresh. When it rains 22% of rainwater falls over land and 78% falls into the sea. Thus every year there is a net transference of fresh water from the oceans to land. Although fresh water is a 'renewable resource', the total amount of water on the planet, and the total amount of fresh water available at any particular time is finite.

At today’s population size the world’s annual, renewable fresh water supply provides several times what is necessary for each individual to live comfortably. But global rainfall patterns are not evenly distributed, rather they vary greatly from region to region and with season. Most precipitation occurs in the tropical regions while the driest regions are in the subtropics (between 15o and 30o north and south of the equator). This is where the majority of the world’s deserts are located.