The hydrologic cycle is the movement and storage of water between land, streams, oceans, and the atmosphere, driven by the energy of the sun. The sun’s energy drives evaporation, the conversion of liquid water in lakes, streams, puddles, and within the soil to vapor. The sun also drives plants to release water vapor while photosynthesizing; this process is termed evapotranspiration as it is the combination of evaporation and plant transpiration (essentially plant gas exchange). The water vapor rises into the atmosphere and eventually forms clouds. Atmospheric processes promote condensation, where moisture in clouds becomes liquid water once again and falls to the earth as rain or snow.
Water falling to the earth as precipitation may be stored in snowpack, lakes, oceans or rivers; may infiltrate the soil; or may flow along the ground surface as runoff. Water that infiltrates the soil may be taken up by plant roots or may eventually reach the water table. Surface runoff can be undesirable, as large quantities of soil and dissolved chemicals can be transported into waterways through this process. Through processes of infiltration and runoff, much precipitation eventually reaches larger water bodies, from which evaporation occurs.
An interactive tool depicting the hydrologic cycle.