Drainage basin features and the hydrological cycle

A drainage basin is the area of land drained by a river and its tributaries. Understanding the vocabulary of a drainage basin and the hydrological cycle is the foundation for every other river topic in CCEA Unit 1.

Key drainage basin vocabulary

Source: the starting point of a river, usually in upland areas. Often a spring (where groundwater reaches the surface) or a boggy moorland.

Mouth: where the river meets the sea, a lake or a larger river. At the mouth, discharge is greatest and sediment load heaviest.

Tributary: a smaller river or stream that flows into a larger one, adding to its discharge.

Confluence: the point where two rivers meet. The River Bann and the River Six Mile Water confluence near Toome is a recognisable NI example.

Watershed: the imaginary boundary — usually following the line of hills — that separates one drainage basin from the next. Rain falling on one side of the watershed flows into one river system; rain on the other side flows into a different system.

Channel: the course taken by the river between its banks.

Floodplain: the flat, low-lying land adjacent to the river that is flooded periodically.

Estuary: the wide tidal mouth of a river where fresh water mixes with salt water. The Lower Bann estuary near Coleraine is a classic NI example.

NI River Bann context: the Upper Bann drains the Mourne Mountains and flows into Lough Neagh (the largest freshwater lake in the British Isles at 388 km²); the Lower Bann drains northward from Lough Neagh to the Atlantic Ocean near Coleraine.

The hydrological cycle (water cycle)

The hydrological cycle is the continuous movement of water through the atmosphere, land surface, and underground. It is a closed system at the global scale — water is neither created nor destroyed, only transferred.

Key inputs and outputs in a drainage basin:

Inputs:

• Precipitation (rain, snow, sleet, hail): the main input of water to a drainage basin.

Stores (where water is held):

• Interception storage: water trapped on leaves and vegetation before reaching the ground.
• Surface storage: lakes, ponds, puddles.
• Soil moisture storage: water held in the upper layer of soil.
• Groundwater storage: water in permeable rock below the soil (the water table).
• Channel storage: water in the river itself.

Transfers (how water moves between stores):

• Throughfall / stemflow: water drips from vegetation to the ground or runs down plant stems.
• Infiltration: water soaks into the soil.
• Percolation: water moves down from soil into groundwater rock.
• Throughflow: water moves laterally through the soil towards a river.
• Groundwater flow (baseflow): very slow movement of water through permeable rock to the river.
• Surface runoff (overland flow): water moves across the surface when the ground is impermeable or saturated.
• Evapotranspiration: water evaporates from surfaces and transpires from plant leaves — the main output from a drainage basin.

Output:

• River discharge: water eventually flows out to sea.
• Evapotranspiration: the main output of water from the drainage basin to the atmosphere.

Storm hydrograph

A storm hydrograph shows the change in river discharge over time following a rainfall event. Key features:

• Rising limb: discharge increases as rainfall reaches the river.
• Peak discharge: the highest flow.
• Lag time: the delay between peak rainfall and peak discharge. Short lag times indicate fast-responding catchments (urban, impermeable rock, little vegetation).
• Falling limb (recession): discharge decreases as the storm water drains away.

Factors making lag time shorter (flashier hydrograph): urbanisation, steep gradients, impermeable rock, little vegetation, saturated soils. Factors making lag time longer (flatter hydrograph): forests, permeable rock (limestone, chalk), gentle gradients, dry soils with high infiltration capacity.