1.1. THE DRAINAGE BASIN SYSTEM
Hydrology and Fluvial Geomorphology
Cambridge International AS & A Level
GEOGRAPHY
UNDERSTAND THE HYDROLOGICAL CYCLE
AS IT IS APPLIED TO DRAINAGE BASINS
WATER DOES NOT COME INTO OR LEAVE PLANET EARTH. WATER IS CONTINUOUSLY TRANSFERRED BETWEEN THE ATMOSPHERE AND THE OCEANS. THIS IS KNOWN AS THE GLOBAL HYDROLOGICAL CYCLE. THIS
SYSTEM IS A CLOSED SYSTEM. THERE ARE NO INPUTS OR OUTPUTS.
A drainage basin, also known as a watershed or catchment area, is an area of land where all
the water that falls in it, whether through precipitation or other means, drains into a
common outlet, usually a river, lake, or ocean.
The processes at work in a drainage basin are
complex and involve various components.
THE HYDROLOGICAL CYCLE
The hydrological cycle refers to the cycle of water between the atmosphere, lithosphere, and biosphere.
At a local scale – the drainage basin – the cycle has a single input, precipitation (PPT), and two major losses (outputs), Evapotranspiration (EVT) and runoff.
A third output, leakage, may also occur from the deeper subsurface to other basins. The drainage basin system is an open system as it allows the movement of energy and matter across its boundaries.
Water can be stored at several stages or levels within the cycle. These stores include vegetation, surface, soil moisture, groundwater, and water channels.
Human modifications are met at every scale. Relevant examples include large-scale changes in channel flow and storage, irrigation and land drainage, and large- scale abstraction of groundwater and surface water for domestic and industrial use.
UNDERSTAND THE DRAINAGE BASIN AS A SYSTEM WITH INPUTS, OUTPUTS,
STORES AND FLOWS
The drainage basin hydrological system is a local open system.
A drainage basin is an area of land drained by a river and its tributaries (river system).
It includes water found in the water table and surface run-off.
There is an imaginary line separating drainage basins called a watershed.
Usually, this is a ridge of highland.
The red line shows the watershed for a river basin.
Any precipitation that falls on the other side of the watershed will flow into a river in the adjacent river basin.
INPUTS
Inputs include precipitation (including rain and snow) and solar energy for evaporation.
INPUTS
Inputs – water coming into the system
Precipitation – all forms of moisture that reach
the Earth’s surface e.g. rain, snow, sleet and
hail.
STORAGES
Stores include puddles, rivers, lakes (surface
storage), glaciers, soil storage and groundwater storage along with water stored on vegetation (interception) following precipitation.
STORAGES
Storage – water stored in the system
Interception – refers to water that is caught and stored by vegetation, this storage is only temporary as it is often quickly evaporated (interception loss).
Throughfall – water that either falls through gaps in the vegetation or that drops from leaves or twigs
Stemflow – water that trickles along twigs and branches and finally down the main trunk
Soil Water – (soil moisture) is the subsurface water in the soil and subsurface layers above the water table
Surface storage – the total volume of water held on the Earth’s surface in lakes, ponds and puddles.
Groundwater storage – the storage of water underground in permeable rock strata.
Channel storage -the water held in a river or stream channel.
TRANSFERS/FLOWS
Transfers or flows include percolation,
overland flow, infiltration, stemflow,
throughflow and overland flow.
TRANSFERS OR FLOWS
Flows and Processes – water moving from one place to another
Channel flow – the movement of water within the river channel. This is also called a river’s discharge.
Infiltration – the downward movement of water into the soil surface.
Interflow – water flowing downhill through permeable rock above the water table.
Percolation – the gravity flow of water within the soil.
Stemflow – water running down a plant stem or tree trunk.
Surface Runoff – the movement of water over the surface of the land, usually when the ground is saturated or frozen or when precipitation is too intense from infiltration to occur.
Throughflow- the movement of water downslope within the soil layer. Throughflow is fast through pipes (cracks int he soil or animal burrows).
OUTPUTS
Outputs move moisture out of the drainage basin and include evaporation and transpiration from
vegetation (together known as evapotranspiration), run-off into the sea, and percolation of water to
underlying rock strata into underground stores.
OUTPUTS
Outputs – water leaving the system
Evaporation – the transformation of water droplets into water vapor by heating.
Transpiration – evaporation from plant leaves.
Evapotranspiration – the loss of water from a drainage basin into the atmosphere from the leaves of plants + loss from evaporation.
