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Water cycle basics
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.

Groundwater is water beneath the earth’s surface that fills in open spaces (or pores) in soil or rock, similar to the way water fills a sponge. Contrary to popular belief, groundwater is typically not found as underground rivers or lakes. Groundwater is commonly found in geologic, water-bearing formations called aquifers. Groundwater comes from rain or snow that falls to the earth and infiltrates through the ground. Water seeping down below the ground’s surface adds to the water already there and is said to be recharging (replenishing) the groundwater. The amount of rain or snow that falls in an area—and the rate at which it falls—can influence groundwater levels.

Surface Water
Surface water is water in lakes, streams, rivers and oceans—water that is found on the earth’s surface. Groundwater and surface water are often interconnected. Groundwater can move through the ground and into a lake or river. Or, water in a lake can seep down into the ground and become groundwater. In the image below, notice how the surface of the river is at the same level as the water table.