Rivers Revision Notes

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Rivers
The drainage basin hydrological system: the water balance
Key terms
Vocab Definition
Transpiration The process by which water is lost by a plant on the underside of its leaf
(through the stomata), an output.
Evaporation The process by which water is changed into water vapour and lost directly
into the atmosphere (an output) from water surfaces including vegetation. A
large amount of energy is required, usually from the sun or wind movement.
Evapotranspiratio The total amount of water that is lost from a vegetated land surface into the
n atmosphere through the processes of evaporation and transpiration.
Precipitation Major input; all forms of moisture that fall from the atmosphere to the
surface of the earth; sleet/hail/snow/rain.
Interception The process by which precipitation is caught by vegetation and therefore
stored there so prevented from reaching the soil surface; some of this
moisture is removed by evaporation.
Throughfall This is a transfer of water; it is water falling through the leaves and occurs
when there is more precipitation falling onto the interception layer than can
remain on the leaves.
Stemflow This is the vertical transfer of water along the branches or stems (or trunks)
of vegetation to reach the ground.
Surface storage This is a store of water on the surface of the soil, often due to it being hard
from a lack of rain in summer months of due to saturation ­ water will then
be transferred once the soil has softened enough to allow it to soak through
into the soil.
Surface run off This is the horizontal transfer of water along the surface due to the ground
being saturated or due to initial heavy rainfall.
Infiltration This is the vertical transfer of water into and through soil. Its rates are
affected by the nature of the soil. It is the maximum rate at which water can
pass through the soil.
Soil moisture This is when water is stored in the soil, sometimes due to impermeable rock
storage laying beneath the soil
Vegetation This is moisture stored in vegetation
storage
Throughflow This is the lateral transfer of water through soil. It moves downslope through
the sub soil by its gravity ­ particularly effective when underlying
impermeable rock preventing percolation.
Channel storage This is the water as it is stored in the river.
Channel flow Most water reaches the river through three transfers: surface run off,
throughflow and groundwater flow ­ the water then flows towards the sea
and is lost from the drainage basin.
Percolation The vertical transfer of water into and through the permeable rock beneath
the soil surface, it is a constant slow movement of water through rock.
Groundwater The constant percolation creates groundwater storage as the water is kept in
storage the permeable rock.
Groundwater This is water transferring laterally along permeable rock, it occurs as water
flow eventually collects above an impermeable rock layer, this saturates the rock
and so water moves laterally through it to the river channel.

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The drainage basin is an area of land drained by a river and its tributaries. The hydrological cycle is
the process by which water is moved within the drainage basin. The drainage basin hydrological
cycle is an open system as water is gained and lost.…read more

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The water budget:
The water budget refers to the balance between inputs and outputs. Basically, what goes in (in the
form of precipitation) must either come out (as channel flow or evapotranspiration) or must be
stored.
The water budget is in dynamic equilibrium, this means it changes but each change is balanced
elsewhere in the system.…read more

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Ghana's water budget:
Flood hydrographs
Key terms
Vocab Definition
Hydrograph A graph showing variations in river discharge, in cumecs, over a period of
time. A graphical representation of stream discharge during a storm or flood.
There are two main types: flashy and steady. They show how a river
responds to a rainfall event.
Baseflow Groundwater flow/throughflow; water that reaches the channel largely
through slow throughflow and from permeable rock below the water table.…read more

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Factors affecting river discharge/hydrographs:
Factor Flashy hydrograph: responds quickly to a Steady hydrograph: responds slowly to a
affecting rainfall event rainfall event
Hydrograph
Precipitatio Prolonged rainfall: soil becomes If rain falls gently over a long
n saturated so there is less period of time it encourages
infiltration and more surface run infiltration so more soil moisture
off, short lag time and more storage
flashiness If snow falls it reduces the volume
Intense storms: rainfall exceeds of water reaching the river
infiltration capacity (e.g.…read more

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Relief Steep slopes encourage rapid Gentle slopes encourage
surface run off therefore a infiltration and underground
decrease in lag time and increase in storage; increasing the lag time
peak discharge and decreasing peak discharge
Soil This hydrograph may have clay soils This soil may be deep therefore
which encourage surface run off increasing its soil moisture storage
due to their platy structure; they capacity
are difficult to infiltrate so little Thus there is a longer lag time as
water does water infiltrates and is stored…read more

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