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AQA AS Core section: Rivers
1- The drainage basin hydrological cycle TB page 2
Where slopes meet at the bottom of a valley, water is concentrated and forms a river channel.
A drainage basin is an area of land drained by a river and its tributaries. Drainage basins are separated
from each other by ridges of higher land called watershed.
A drainage basin is an open system- a system where there are inputs and/or outputs of energy and
matter- with inputs (water and energy) and outputs of water. The water enters the system as
precipitation and leaves as stream flow or as evaporation or transpiration.
Inputs: energy from the sun/precipitation
Stores such as glaciers, rivers, lakes and vegetation
Transfers and flows move water through the system, such as infiltration.
Outputs: moisture lost by evapotranspiration + runoff into the sea
Key terms:
1- Precipitation (input): All forms of rainfall, snow, frost, hail and dew. It is the transfer of moisture in
the atmosphere to the land.
2- Evaporation + Transpiration=Evapotranspiration (output)
Evaporation: The change of state from liquid to gas from exposed water surfaces
Transpiration: The loss of water from the leaves of plants
Evapotranspiration: the combined losses
3- Surface stores such as glaciers and lakes
4- Interception (Storage): The process by which precipitation is trapped on the surface of vegetation.
Interception loss (output): The proportion of water which evaporates from the surface of vegetation.
After interception…transfers
5- Stemflow : Intercepted precipitation which runs down the stems of plants to reach the ground.
6- Throughfall: The precipitation which falls straight through the vegetation (or drips off leaves) and
reaches the ground.
7- Overland flow/ surface runoff (transfers): Water that moves over the ground surface.
Water moving as overland flow reaches the river channel much more quickly than water flowing
through soil and rocks. The more overland flow that takes place, the faster the river level rises and the
more likely flooding is to occur.
Overland flow happens from a combination of the following reasons:
- The ground surface is relatively impermeable and the water cannot infiltrate as a result of, foe
example, urban development.
- Vegetation cover has been removed, reducing the infiltration rates and the interception loss. The extra
water runs over the ground surface.
- When the soil store is already full of water, the term used is saturated overland flow.
- The rainfall comes down faster than it can infiltrate the soil. This can occur with intense summer
storms.
8- Infiltration (Transfer): process by which water soaks into or is absorbed by the soil.
It is a very important process, because water entering the soil will move slowly through the drainage
basin to the river channel, whereas water which does not infiltrate will flow relatively quickly over the
surface to a river as overland flow.
Infiltration rate: refers to how much water IS passing through in a certain time.
Infiltration capacity: is the maximum rate at which water CAN enter the soil in a particular case
[measured in cubic millimetres per hour].
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9- Throughflow (Transfer): The downslope flow of water within the soil.
10- Percolation (transfer): As water reaches the underlying soil or rock layers, which tend to be more
compact, its progress is slowed. This slow movement , called percolation, creates groundwater storage.
11- Groundwater flow/baseflow (transfer): Groundwater refers to subsurface water. The upper layer
of the permanently saturated zone is known as the water table and it varies seasonally. Groundwater
flow is the slow movement of water through the underlying geology via cracks and joints below the
water table and it varies seasonally. The slow flow through rock produces a steady input of water which
varies relatively little in comparison with the discharge of the whole river. This explains why rivers can
continue to flow even if it has not rained for weeks.