Water is one of our most precious natural resources. Without it, there would be no life on earth. Hydrology has evolved as a science in response to the need to understand the complex water system of the earth and help solve water problems. This hydrology primer gives you information about water on Earth .Studying the movement, availability, and quality of water are the jobs of a hydrologist.  More specifically hydrologists study the chemical properties, biological interactions, and the physical processes that govern the water cycle. 
The water cycle or hydrologic cycle is a process by which water is continuously cycled around the earth.  This happens through different pathways and at different rates but the central concepts remain the same.   Water evaporates from the ocean, condenses as clouds, moves over land, and precipitates.  From there it can enter ground water, evaporate again, or enter a stream or lake.  It will eventually find its way back to the ocean either by falling as precipitation, flowing with a river, or by moving ever so slowly with ground water.  The hydrologic cycle is also a process that transfers heat energy.  Heat is transported pole ward by water being evaporated and then condensing which releases heat.  Without the water cycle the climate would be much more frigid and areas away from the equator would be much less habitable.