The Concept of a Drainage Basin

• A drainage basin is an area of land where all surface water from rain or melting snow converges to a single point, usually the exit of the basin or into a larger body of water like a river, lake or sea.
• Also known as a catchment area, each basin is separated topographically from adjacent basins by a ridge, hill, or mountain, which is known as a water divide.
• The drainage basin includes all the terrestrial surfaces and contexts where water moves under gravity towards the outlet.

Inputs, Outputs, and Storage in a Drainage Basin

• The main input into a drainage basin is precipitation in the form of snow or rainfall.
• Outputs from the basin include evapotranspiration (combination of evaporation and plant transpiration) and river discharge, which is the amount of water flowing out of the basin.
• The store component includes processes of water storage like interception by vegetation, soil moisture storage, ground water storage and channel storage.

The Drainage Basin as a System

• The drainage basin operates as a system because it involves a series of interrelated physical processes - inputs, outputs, storage, and transfers (or flow).
• This whole process of interaction between precipitation, run-off, evaporation and storage can also be referred to as the water or hydrological cycle operating at a drainage basin level.

Flow of Water within the Drainage Basin

• Overland flow (or surface run-off) is the primary method of transportation for water into river channels, usually following a path of least resistance.
• In contrast, throughflow refers to water that infiltrates the soil surface and moves laterally through the upper soil layers towards the river channel.
• Baseflow or groundwater flow is the part of a river's discharge that is provided by groundwater seeping into the river channel, which is a slower transfer process.

Factors Affecting the Behaviour of the Drainage Basin System

• The behavior and rate of water movement in a drainage basin is influenced by several factors including the type and amount of precipitation, vegetation cover, type of soil, and the shape and size of the basin.
• Human activities such as land use changes, deforestation, urbanisation and agricultural practices can also significantly affect the way a drainage basin system operates.

Understanding The Drainage Basin through Hydrographs

• A hydrograph is a useful tool in Geography to illustrate the relationship between rainfall and the discharge of a river over a specified period.
• It helps to visualize how a river responds to a period of rainfall and can therefore help in flood prediction and management.