What is Hydrology? Water is an essential resource that is required by all life 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.
Studying these different aspects allow hydrologists to do many things such as calculate water budgets.  This process involves tracking where all the water goes in a watershed and creating an equation with inputs and outputs to understand water surplus and deficit.  Once completed this budget may be used by city planners to calculate drinking water availability, farmers to calculate irrigation needs and availability, industries to calculate if they can produce certain items, and mining companies to determine if excavation is cost-effective.  Studying floods is another thing that hydrologists do.  This can involve creating flood plain maps, modeling stream flow, and predicting what may happen under certain scenarios.  Hydrologists also study pollution by looking at the sources, transportation mechanisms, and the ultimate fate of the pollutant.  This involves looking at both point source pollution where the source is known and nonpoint source pollution where the source is not known.  Groundwater and surface water are investigated to see how the pollutant travels and how it reacts in nature. The observed data are used to determine where it will end up and if it is harmful to the aquatic environment.  Much of what hydrologists do involves field work, lab work, and modeling work.  This creates a more complete picture of the hydrologic cycle and aids policy makers in making their decisions involving water.