watershed

A watershed, also called a drainage basin or catchment, is the land area from which all precipitation and runoff drains toward a common outlet such as a stream, river, lake, or ocean. The boundary of a watershed is defined by topographic divides—high ridges and hills that separate one drainage network from another. Within a watershed, water follows paths through surface streams, wetlands, and lakes and also moves through the soil and bedrock as groundwater. The size of watersheds ranges from small, urban catchments to extensive basins that cover thousands of square kilometers.

Key processes include interception, infiltration, runoff, evapotranspiration, and groundwater flow. Precipitation that infiltrates recharges aquifers, while overland flow and channel flow convey surface water toward the outlet. Land cover, soils, geology, and relief shape how quickly water moves through a watershed and how much is stored or lost to evaporation.

Watershed management focuses on protecting and improving water quality, regulating flood risk, sustaining habitats, and ensuring reliable water supplies. Practices include land-use planning, restoration of riparian zones, pollution control, stormwater management, and stakeholder coordination. Hydrologists use tools such as rainfall-runoff models and geographic information systems to delineate watershed boundaries, estimate flows, and assess impacts of land-use changes.

Transboundary watersheds may require cooperation among jurisdictions. Climate change can alter precipitation patterns, affecting runoff, recharge, and streamflow, underscoring the importance of integrated watershed stewardship.