>> Water is essential to life. All living organisms are composed of water, but about 97 percent of water is locked in the oceans. The atmosphere holds much less than the oceans, only about 13,000 cubic kilometers of water per year. So how do we get drinkable water? To understand this, we need to learn how water cycles from the ocean to the atmosphere to the land and then back to the ocean. Let's start with the sun. As it drives the water cycle by heating water in the ocean, soil, streams, rivers, and lakes, causing it to evaporate into the atmosphere as water vapor. In fact, evaporation returns about 85 percent of the water vapor to the atmosphere. In addition to evaporation, there is another way water can enter the atmosphere. Plants lose a significant amount of water vapor through their leaves during the process of transpiration. Evaporation and transpiration together is called evapotranspiration. After water vapor enters the atmosphere by evaporation and transpiration, it cools and condenses to droplets forming our clouds. As clouds form, wind is continuously moving these air masses over the land. In fact, wind moves about 40,000 cubic kilometers of the water in air masses over the land. But what happens when the clouds can't hold moisture any longer? Water is released from the clouds in the form of precipitation, which can be rain, sleet, snow, or hail. A total of about 496,000 cubic kilometers of precipitation falls to the land and ocean. So where does water go once it falls on the land? It may re-enter the atmosphere by evaporation, or it may infiltrate and percolate, which means seep into the soil to become ground water, water in soil and aquifers. Aquifers are permeable rock that holds water. Ground water is necessary for all living organisms and also recharges all bodies of water. What if precipitation happens faster than water can enter the ground? Water then remains on the surface as runoff and flows into streams, rivers, lakes, and, eventually, the ocean, completing the cycle. The hydrologic cycle continues to move about one hundred million billion gallons of water a year. Without this process, life on Earth would be impossible.