The amount of water on earth is immense: an estimated 1,360 million cubic kilometers (326 million cubic miles). Of this total, the vast bulk - 97.2 percent - is part of the world ocean. Ice caps and glaciers account for another 2.15 percent, leaving 0.65 percent to be divided among the world's lakes, streams, subsurface water and the atmosphere.

The unending circulation of the earth's water supply between the oceans, the atmosphere, and the continents has come to be called the hydrologic or water cycle.

Understanding the hydrologic cycle is important to understanding hydropower.

Figure 1 Water distribution on earth