Runoff is the movement of water along the earth’s surface as a result of precipitation. Runoff occurs when precipitation moves across the land surface eventually reaching streams or lakes.Or drainage or flowing off of precipitation from a catchment area through a surface channel.
The land area that contributes surface runoff to any point of interest is called a watershed. A large watershed can contain many smaller sub-watersheds. 
Drainage Basin
The tract of land (both surface and subsurface) drained by a river and its tributaries is called a drainage basin. A watershed area supplies surface runoff to a river or stream, whereas a drainage basin is the tract of land drained of both surface runoff and groundwater discharge. 
 Factors Affecting Runoff
  • Storm or Precipitation Characteristics
  • Shape and Size of Catchment 
  • Topography
  • Geological Characteristics 
  • Meteorological Characteristics 
  • Character of Catchment Surface 
  • Storage Characteristics
Storm or Precipitation Characteristics
◦Type or nature of storm and season
◦Intensity, Duration, and Frequency
◦Arial extent or distribution
◦Direction of storm movement
Shape and Size of Catchment
      High peak for wider catchment than for narrow catchment.
      More runoff for smooth, steep and windward side catchment.
Geological Characteristics 
      Greater runoff for impervious, rocky, surface and subsurface strata.
Meteorological Characteristics 
      Temperature, Humidity, Wind speed, Wind direction, and Pressure variation
Basin or Catchment’s Characteristics 
    More absorption for catchment with no natural drainage.Less runoff for cultivated, vegetated, and unsaturated surface.
Storage Characteristics

Depressions, Pools and ponds/lakes, Stream Channels, Check dams, Upstream reservoirs or tanks or weirs, Floodplain swamps, Groundwater storage in pervious deposits. All storages tend to reduce the peak flow.


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