hydrology

Hydrology is the science concerned with the occurrence, distribution, movement, and properties of Earth's water and its interactions with the environment. It encompasses the atmosphere, surface waters, soils, groundwater, and the cryosphere, integrating physical, chemical, and biological processes to understand how water enters, flows through, and leaves landscapes.

The core concept is the hydrological cycle: precipitation, evapotranspiration, infiltration, runoff, storage in soils and bodies of water, and groundwater movement. Key processes include rainfall, snowmelt, evaporation, condensation, infiltration, percolation, river discharge, and aquifer exchange. Water quality and quantity are central concerns, affected by natural variability and human activities.

Branches of hydrology include surface hydrology, which studies streams, rivers, lakes, and wetlands; hydrogeology, which focuses on groundwater and aquifers; and atmospheric or climatological hydrology, which examines precipitation patterns and climate links. Applied fields such as hydroinformatics, ecohydrology, and water resources engineering emphasize forecasting, management, and infrastructure design. Measurements rely on hydrometric networks (stream gauges, rainfall observatories), groundwater monitoring, remote sensing, and modeling.

Hydrology supports water-resource planning, flood forecasting and risk assessment, drought analysis, irrigation, environmental management, and ecosystem health. It provides essential inputs for climatology, geology, and environmental science by characterizing watershed behavior under changing climate and land use.

Historically, modern hydrology developed through systematic measurement, theoretical advances, and institutional organization in the 19th and 20th centuries, enabling quantitative analysis of water resources and their management.