The Drainage Basin

as a system

What is a drainage basin?

A drainage basin

An area of land drained by a river and its tributaries where any precipitation is collected or drained into it.

Its boundary is marked by a ridge of high land called watershed

Drainage basins have one main stream and many tributaries

Each tributary of main stream forms sub-drainage basin

Watershed: A stretch of land that surrounds drainage basin (a.k.a. boundary)

Drainage Basin: A river system that collects rainwater and comprises of streams and river

Tributaries: Streams that flow in the river which is mainly found in the upper course.

Distributaries: Branch out from the river which is mainly found in the river delta.

River Source: Point where the river startsRiver Mouth: Place where the river meets the sea / ocean (River ends)

A drainage basin may be described as an open system.

It forms part of the hydrological cycle

Characteristics of a drainage basin are:

Inputs : - in the form of precipitation Outputs:- water is lost through

evapotranspiration

Within the system, some of the water:

Is stored in water storage such as lakes or soil Passes through a series of transfers or flow e.g.

infiltration, percolation and throughflow

Important terms.. Interception : precipitation lands on vegetation. Infiltration : Water that reaches ground surface will soak into soil. Overland flow : water moving over the ground. Throughfall : precipitation passing through the plant canopy Percolation : Some water sinks deep into the earth (percolates)

through openings in the soil. Stemflow : water that flow down tree trunks or plant stems. Groundwater : water transfer through permeable rocks from the soil

above. Baseflow : water flows from groundwater stores to the stream Throughflow : water moving through the soil. Surface run-off : water

that flows over the land surface.

Input main input - precipitation.

The type: rain or snow, hail, etc.

Factors affecting the amount of water in the system:

the intensity, the duration and frequency.

Each subsystem of the drainage basin system will also have

inputs and ouputs, and the output from one stage

of the diagram will form the input for another.

Storage Water stored - on the surface, - in lakes and channels

or - in the groundwater

store.

Reach groundwater via infiltration and percolation.

some water will be stored in the soil and rock.

The amount of water stored will vary depending on:

the porosity of the soil and the permeability of the rock.

Water can be temporarily stored via interception. – in the storage of water on leaf and plant stems.

Transfer all the water flowing over the drainage

basin’s surface is called runoff.

made up of streamflow - flow through permanent river channels and

overland flow or surface runoff.

Overland flow - transfers water through the basin either as:

sheetwash, across the surface, or in tiny channels called rills.

Beneath the surface, water is transferred via throughflow, which is the movement of water through the lower soil towards rivers, and groundwater flow (base flow).

Water that has been intercepted by foliage may also be transferred, either directly as throughfall, or by running down branches and stems via stemflow.

Output

final release of the water in a drainage basin

flow into the sea will be the main output of a drainage basin.

Some water will also be lost via evapotranspiration.

Evaporation from water bodies and soils Transpiration from plants