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The water cycle is also known as the hydrological cycle.

Seas and oceans contain 97% of the world's water, and ice holds 2%.

That leaves just 1% of the world's water as fresh water on land or in the air.

This water is recycled again and again through the processes of evaporation, condensation and water transfers such as surface runoff.

Key terms:

• Infiltration - The downward movement of water from the surface into the soil.

• Surface runoff - Water flows across the surface of the earth becoming a stream, tributary or river.

• Precipitation - Rain, hail, sleet or snow which is input to the drainage basin system.

• Evapotranspiration - A combination of water evaporated from land and water surfaces, including vegetation, and water released through plant leaves into the atmosphere.

• Throughflow - Water flows downhill within the soil.

• Groundwater flow - Water flowing slowly through rocks towards the sea.

• Percolation - The downward movement of water from the soil into the rock beneath.

A major component of the water cycle is the drainage basin system.

There are many inter-relationships within this system:

• Inputs: How water is introduced into the drainage basin system. This is known as precipitation.

• Stores: How water is stored or held for a period of time within the drainage basin system - interception (by vegetation), soil moisture, surface storage (lakes), groundwater.

• Transfers: A process or flow of water from one place to another in the drainage basin system - surface run-off/overland flow,infiltration, percolation, through-flow, groundwater flow.

• Outputs: How the water is released either back to the sea or back into the atmosphere - river discharge, evapotranspiration.

• Drainage basin - the area of land drained by a river.

• Channel - the bed and banks that form the route of the river.

• Watershed - the edge of highland surrounding a drainage basin which marks the boundary between two drainage basins.

• Source - the beginning or start of a river.

• Confluence - the point at which two rivers or streams join.

• Tributary - a stream or smaller river which joins a larger stream or river.

• Mouth - the point where the river comes to the end, usually when entering a sea.

A river changes shape as it flows from its source to its mouth. A section of the course of a river drawn from source to mouth is known as a long profile.

The long profile is divided into three stages. The upper course, the middle course and the lower course, each of which have distinctive characteristics.

The source of a river is often, but not always, in an upland area. Near the source, a river flows over steep slopes with uneven surfaces.

It often flows over a series of waterfalls and rapids.

As a river flows down steep slopes, the water performs vertical erosion. This form of erosion cuts down towards the river bed and carves out steep-sided V-shaped valleys.

As the river flows towards the mouth, the gradient of the slope becomes less steep.

Eventually the river will flow over flat land as it approaches the sea.

As the river moves from the source to the mouth – both the depth of the river and the width of the river will both increase.

The load of a river will also change as it is transported and eroded along the river's profile.

As a result, the size and shape of stones will become smaller and more rounded as they journey through the river profile.

The discharge will increase as the river approaches the sea.

The discharge is usually calculated as the cross sectional area (depth x width) multiplied by the velocity or speed of the water.

A cross profile shows a cross-section of a river’s channel and valley at a certain point along the river’s course.

Near the source of a river - A - there is more vertical erosion as the river flows downhill, using its energy to overcome friction.

As a result, the channels are narrow and shallow and may contain large boulders and angular fragments eroded and weathered from the steep valley sides.

The sediment in the river creates turbulence and friction.

As the river approaches the mouth, velocity and energy increase due to increased discharge related to the wider and deeper channel, and less friction to the smaller load and reduced contact of the water with the banks and bed.

The river performs more lateral erosion making the channel wider, and smoother - B and C.

As a result there is less turbulence and friction, making the flow of water more efficient.