HYDRODYNAMICS!!! 

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It's true, the water we use today has been around for hundreds of millions of years, and probably the amount of the water hasn't changed very much. Water moves around the world, changes forms, is taken in by plants and animals, but never really disappears. It "travels" in a large, continuous cycle. We call this the Hydrologic Cycle. ("hydro" means water)

Imagine a bucket with a hole in the bottom. The bucket holds all of the water on the Earth; surface water, water in the ground, or water in the atmosphere. But the water can escape. If it left undisturbed, the bucket would soon be empty. In order to keep the water level in bucket constant, the same amount of water that leaves must return.

Many processes work together to keep Earth's water moving in a cycle. There are five processes at work in the hydrologic cycle: condensation, precipitation, infiltration, runoff, and evapotranspiration. These occur simultaneously and, except for precipitation, continuously.

Together, these five processes - condensation, precipitation, infiltration, runoff, and evapotranspiration- make up the Hydrologic Cycle. Water vapor condenses to form clouds, which result in precipitation when the conditions are suitable. Precipitation falls to the surface and infiltrates the soil or flows to the ocean as runoff. Surface water (e.g., lakes, streams, oceans, etc.), evaporates, returning moisture to the atmosphere, while plants return water to the atmosphere by transpiration.