Hydrology is the scientific study of the movement, distribution, and management of water on Earth and other planets, including the water cycle, water resources, and environmental watershed sustainability. A practitioner of hydrology is called a hydrologist. Hydrologists are scientists studying earth or environmental science, civil or environmental engineering, and physical geography. Using various analytical methods and scientific techniques, they collect and analyze data to help solve water related problems such as environmental preservation, natural disasters, and water management.

Hydrology subdivides into surface water hydrology, groundwater hydrology (hydrogeology), and marine hydrology. Domains of hydrology include hydrometeorology, surface hydrology, hydrogeology, drainage-basin management, and water quality, where water plays the central role.

• Chemical hydrology is the study of the chemical characteristics of water.
• Ecohydrology is the study of interactions between organisms and the hydrologic cycle.
• Hydrogeology is the study of the presence and movement of groundwater.
• Hydro geochemistry is the study of how terrestrial water dissolves minerals weathering and this effect on water chemistry.
• Hydro informatics is the adaptation of information technology to hydrology and water resources applications.
• Hydrometeorology is the study of the transfer of water and energy between land and water body surfaces and the lower atmosphere.
• Isotope hydrology is the study of the isotopic signatures of water.
• Surface hydrology is the study of hydrologic processes that operate at or near Earth's surface.
• Drainage basin management covers water storage, in the form of reservoirs, and floods protection.
• Water quality includes the chemistry of water in rivers and lakes, both of pollutants and natural solutes.