The amount of fresh water on earth is only 2.5% and the rest is salt water. While the amount of water on earth does not change much, it does change its form, from vapor to rain, snow, or ice. This continuous movement of water between earth and the atmosphere is called the hydrologic cycle.
HOW THE CYCLE WORKS:
When precipitation, in the form of rain or snow, reaches land, it percolates down through the soil to recharge our aquifers (underground reservoirs). Some of this precipitation first gathers in the form of ponds, lakes, streams or rivers before soaking into the ground or flowing out into the sea.
Much of the water that does not soak in evaporates, condensing in the atmosphere in the form of clouds, then falls to the earth again as precipitation. Plants and trees take up groundwater through their root system, using it to survive and releases it back into the atmosphere, which is referred to as evapotranspiration.
Besides providing water to animals, plants, and people, this cycle also helps transport nutrients, sediment, and other substances into and out of aquatic ecosystems.