All water is part of the hydrologic system. Energy from the sun causes evaporation of water from the land, lakes, and oceans, which eventually falls as rain or snow. Once it reaches land, some water may move below ground, but eventually, virtually all of it will end up back at the surface. The figure below shows how water may move beneath the surface at different depths and travel at different speeds. Once water infiltrates below a depth where evaporation or plant roots can't reach, it may remain as groundwater for months, years, decades or even longer.
Have you ever wondered how rivers continue to flow even if it hasn't rained for weeks? The answer is groundwater, stored in rock formations, continues to supply water through banks and beds of rivers and streams. Once in the river, groundwater becomes surface water! In some cases, particularly in dry areas, rivers may lose water that soaks through the riverbed to reach groundwater. In these cases, surface water becomes groundwater!