Greek Yδρoλoγια; from Ancient Greek ὕδωρ "water" + λoγoς "word, doctrine.

The science that studies the Earth's natural waters and the phenomena and processes that occur in them. Hydrology studies the patterns of water distribution on the surface and in the soil. It is subdivided into oceanology and land hydrology.

Hydrology studies all kinds of waters of hydrosphere in oceans, seas, rivers, lakes, reservoirs, swamps, soil and ground waters. Researches the water cycle in nature, the influence of human activity on it, and the management of the regime of water bodies and the water regime of individual territories. Analyzes hydrological elements for separate territories and the Earth as a whole. Assesses and forecasts the condition and rational use of water resources; uses methods used in geography, physics and other sciences.

Because of the specific features of the objects and methods of their study, hydrology is divided into three independent disciplines: oceanology (hydrology of the sea); land hydrology (studies water bodies of the land); and hydrogeology (hydrology of groundwater). 

Land hydrology is generally divided into river hydrology, limnology (lake hydrology), swamp hydrology, and glaciology (glacier hydrology). Depending on the focus of hydrological research, sometimes more specific sections are distinguished, such as soil hydrology, forest hydrology, agricultural hydrology, etc. As a result of the close interaction of hydrology with geophysics and geochemistry, new sciences have appeared: hydrophysics and hydrochemistry. 

The main field of hydrology studies is the water regime and water balance (hydrological cycle), the study of the water cycle in nature, spatial and temporal variations and changes in its elements under the influence of natural and anthropogenic factors. In practical application, hydrology is closely connected with water management and problems of rational use and protection of surface and groundwater from pollution and depletion, with development of methods of hydrological calculations and forecasts. In recent years, the ecological direction in hydrology has been increasingly developing.