The Hydrologic Cycle
The vast and complex circulation of water between the earth and the atmosphere is called the hydrologic cycle. The cycle works this way: Precipitation falls from the atmosphere onto the land or into rivers, streams, lakes, and oceans. Most of this water returns directly to the atmosphere by evaporation, the process by which water is changed into vapor. Water also returns to the atmosphere by transpiration, the water taken up by the plants from the soil through their roots and released through their leaves as water vapor. Some precipitation flows across the land to streams and rivers as surface runoff. The remainder percolates downward through the ground to the saturated zone where all available openings in the earth materials are filled with water. Groundwater flows under the influence of pressure and gravity and eventually discharges at the land surface as springs or as seepage into streams, rivers, lakes, or wetlands. Once on the surface, the water can evaporate. When water vapor cools, it condenses into clouds from which precipitation falls to the earth, completing the cycle.