The water cycle, or hydrologic cycle, describes the continuous movement of water above, on, and below the surface of the Earth. Water in the atmosphere occurs as precipitation and as water vapor, which can be seen as clouds. Water on the Earth's surface occurs as streams, lakes, wetlands, bays and oceans, as well as the solid forms of snow and ice. Water below the surface of the Earth is groundwater.
The hydrologic cycle is commonly portrayed by a simplified diagram showing only major transfers of water between the atmosphere, land, water bodies and ground. However, for understanding hydrologic processes and managing water resources, the hydrologic cycle needs to be viewed at a wide range of scales and as having a great deal of variability in time and space.
USGS Circular 1139, Ground Water and Surface Water, A Single Resource, includes a more detailed description of the hydrologic cycle in a section titled: The hydrologic cycle and interactions of ground water and surface water.