Before break, we discussed watersheds and what constitutes a watershed. How do you think water moves through a watershed?

The Drainage The Drainage Basin Basin

as a systemas a system12/3/1312/3/13

A drainage basinA drainage basin

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

It’s boundary is marked by a ridge of high land called a watershed

Drainage basins have one main stream and many tributaries

Each tributary of main stream forms sub-drainage basin

What is the difference between a drainage basin and a watershed?

Watershed: A stretch of land that surrounds drainage basin

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 or the river ends

A drainage basin may be described as an open system.

It forms part of the hydrological cycle/ water cycle

Characteristics of a drainage Characteristics of a drainage basin are: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, etc. terms we will learn in

a moment

Important termsImportant terms Interception : precipitation lands on vegetation

Infiltration : Water that reaches ground surface will soak into soil. Overland flow : water moving over the ground.

Percolation : Some water sinks deep into the earth through openings in the soil.

percolate= gradually filter through the ground; trickle

Stemflow : water that flow down tree trunks or plant stems.

Input Storage Transfer Output

main input – precipitation in the form of: rain, snow, hail, sleet, 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 outputs, and the output from one

stage of the diagram will form the input for another

How are all drainage basins connected?

Water stored - on the surface, - in lakes and

channels or - in the

groundwater store.

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.

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

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