Hydrology is the study of the occurrence, distribution, and movement of water on, in, and above the earth. As such, it is an earth science. The cycle of movement of water between terrasphere (land), oceanosphere (ocean), and atmosphere (air) is termed the hydrologic cycle.
There are two broad sub-disciplines within the science of hydrology. The first is surface-water hydrology which focuses on water on and above the surface of the earth. Examples of applications of surface water hydrology are flooding and droughts. The second sub-discipline is ground-water hydrology or geohydrology, which focuses on the distribution and movement of water beneath the earth's surface, (i.e., groundwater). Ground-water hydrology is important for applications in water supply, irrigation and environmental engineering. Note that water in the oceans is the pervue of a separate discipline known as oceanography. And water in the atmosphere is mostly studied in meteorology.
Also included in hydrology is the study of motion of water and water-borne constituents — materials carried either as dissolved quantities or in separate phases. A related facet of hydrology is the determination of statistical flow prediction in rivers and streams. This information is essential to design and evaluation of natural and man-made channels, bridge openings and dams. Stream gage data has been collected and tabulated by the United States Geological Survey for many years and much of it is available online for analyisis.