The answer that is most suitable is earth's hydrologic cycle.
Water does not remain locked up in the oceans, icecaps, groundwater systems , or the atmosphere. Instead water is continually moving from one reservoir to another. This movement of water is called the hydrologic cycle.
Although water in the hydrologic cycle is constantly in motion, it never leaves the earth. The earth is nearly a "closed system" like a terrarium. This means the earth neither gains nor loses much matter, including water.
