The drainage basin: a component of the water cycle

A drainage basin is an area of land where water from precipitation collects and flows into a common outlet, such as a river or a lake. Drainage basins are essential components of the Earth’s water cycle and play a crucial role in maintaining the balance of water resources. In this section, we will explore the various elements of the drainage basin system and their interrelationships.

Drainage basin hydrological cycle

Inputs

Precipitation is the primary input of water into a drainage basin. It includes various forms of moisture, such as rain, snow, hail, and sleet. The amount and type of precipitation vary depending on geographic location, climate, and season. Precipitation provides water for various processes within the drainage basin system.

Stores

Vegetation within a drainage basin serves as a store for precipitation. When rain falls, some of it is intercepted by leaves, branches, and other plant surfaces, preventing it from reaching the ground directly. This intercepted water is either evaporated back into the atmosphere, absorbed by the plants or later transpired.

Transfers

Transfers refer to water movement within the drainage basin from one location to another. There are several processes involved in water transfers:

Surface runoff/Overland flow
This occurs when the ground becomes saturated, and excess water flows over the land surface toward lower elevations.
Infiltration
The process by which water seeps into the soil and becomes part of the soil moisture.
Throughflow
The horizontal movement of water within the soil, typically following natural pathways such as soil layers and root systems.
Percolation
The downward movement of water through the soil and into the underlying rock layers, eventually reaching the water table.
Groundwater flow
The slow movement of water through underground aquifers towards rivers, lakes, and oceans.

Outputs

Outputs refer to how water leaves the drainage basin system. The two main outputs are:

River discharge
The flow of water in rivers and streams ultimately reaches the ocean or other bodies of water. The discharge is influenced by factors such as precipitation, the size of the drainage basin, and human interventions like dams and reservoirs.
Evapotranspiration
The combined process of evaporation and transpiration. Evaporation occurs when water on the Earth’s surface, such as on plant leaves or soil, changes into water vapour and returns to the atmosphere. Transpiration is the release of water vapour from plants through tiny openings called stomata. Evapotranspiration is a key output of water from the drainage basin system.