The amount of water on the earth and within our atmosphere never increases or decreases. However, it is constantly being used and discarded by living things. It never stops moving around the planet and changing among its three states: solid, liquid and gas.

The hydrologic cycle is an unending and complex circuit that incorporates several different natural processes, including water evaporation, from the earth’s surface, particularly from larger bodies of water. As moist air is lifted, it cools and the water content of the air turns to liquid, condensing to form clouds. Moisture may be transported far from the water body of its source before it falls to the surface as precipitation. Once the water reaches the ground, some of the water may evaporate back into the atmosphere; some may be used by plants, which return water to the atmosphere through their leaves in a process called evapotranspiration; or the water may penetrate the surface and infiltrate the ground, becoming groundwater. Groundwater either follows the pull of gravity deeper into the ground, recharging the aquifer; seeps its way into oceans, rivers and streams; or is released back into the atmosphere through transpiration. The balance of water that remains on the earth's surface is runoff, which runs downhill toward the nearest body of water, where the cycle continues.