Drainage Basins
Aims- to understand how water moves through a
drainage basin.
Aims- to understand how water moves through a
drainage basin.
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Precipitation falling on a watershed is first intercepted by vegetation
cover, trees, grass or crops- if present. Grass and trees hold raindrops on
their leaves. Interception is the percentage of rain or snow that plants
prevent from directly striking the ground. Some water evaporates directly
from plant surfaces. If the water is not evaporated then it drips onto the soil
(stemflow).
Once at the soil surface water follows several paths;
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Water may remain at the surface as puddles, soak into the soil or run off the
surface, where evaporation may take place.
 Water soaking into the soil is known as infiltration
 Surface flow across a slope is known as overland flow and is controlled by the
permeability of the soil.
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Groundwater, water entering the soil may flow through the soil virtually
parallel to the surface as throughflow or may penetrate deeper into the
underlying rocks by percolation.
Aims- to understand how water moves through a drainage
basin.
Rock type
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Impermeable rocks will have a slow infiltration rate so overland flow
will dominate.
Permeable rocks, water moves downward under gravity through the
rock, filling or saturating the pore spaces in the deepest sections,
which is known as the zone of saturation.
The upper surface of this zone is known as the water table.
The water which accumulates in the rock in the lower section is
known as groundwater. Flow from groundwater into streams and
rivers is known as base flow.
soil
rock
Water table
ROCK
Zone of saturation
If precipitation increases and more water is infiltrated into the soil and then
percolated into the rock then the amount of rock which is saturated increases
and the water table rises. If there is no precipitation then groundwater flow
increase and the water table falls.