Groundwater Information Center
Groundwater is a component of the hydrologic cycle. In simple terms, water or one of its forms-water vapor and ice-can be found at the earth's surface, in the atmosphere, or beneath the earth's surface. The hydrologic cycle has no beginning or ending location; however, it is often thought of as beginning in the oceans. Water evaporates from a surface water source such as an ocean, lake, or through transpiration from plants. The water vapor may move over the land and condense to form clouds, allowing the water to return to the earth's surface as precipitation (rain or snow). Some of the snow will end up in polar ice caps or in glaciers. Most of the rain and snowmelt will either become overland flow in channels or will infiltrate into the subsurface. Some of the infiltrated water will be transpired by plants and returned to the atmosphere, while some will cling to particles surrounding the pore spaces in the subsurface, remaining in the unsaturated zone. The rest of the infiltrated water will move gradually, driven by gravity, into the saturated zone of the subsurface, becoming groundwater. From here, groundwater will flow toward points of discharge such as rivers, lakes, or the ocean to begin the cycle anew.