FORM FIVE SELECTIONS 2021** FORM 5 JOINING INSTRUCTIONS
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DRAINAGE BASIN
Background
information
The drainage basin is the
collecting ground and storage container for precipitation, the system of routes by which water and sediments
are transported to the ocean and lakes. It is an area of land
drained by a river and its tributaries. Each tributary has its own basin
area. Its boundary, known as the watershed, is a ridge of high land, beyond
which precipitation will drain into adjacent basins. It is separated from one
another with drainage divides. Water falling on the earth’s surface tends to be
organized within the drainage basin. The
basin forms the natural unit which most fluvial geomorphological processes
operate. The surface of the drainage
basin is the zone of interaction between atmosphere, hydrosphere, lithosphere,
pedosphere and biosphere. It is treated as an open
system, forming part of the water cycle.
According to Summerfield (1996), it has both
inputs and outputs. Inputs in the drainage basin are precipitation and snow.
The precipitation forms the major input into
the system varying over time and space. Outputs in the drainage basin on the
other side occur when the system looses water, dissolved solutes and sediments.
Water is lost either by stream/river carrying the water out to the sea and through evaporation and transpiration (evapotranspiration).
Water
storage in the drainage basin
In
the drainage basin, water is stored in a number of ways. These are as follows:
Interception
storage: This is the first store of
the drainage basin. The first raindrops of a storm will fall on trees or
plants, which shelter the underlying ground. This is called interception
storage, and naturally will be greater in a woodland area than over
grassland. If the precipitation is light and of short duration, much of the
water may never reach the ground and may be quickly lost to the system through
evaporation. It is estimated that in a woodland area up to 30 per cent of the
precipitation may be lost because of interception, which helps to account for
reduced soil erosion in forests (Summerfield, 1996). In an area of deciduous
trees, both interception and evapotranspiration rates will be higher in summer.
Surface
storage: After a warm and a
dry spell the ground may be hard. So at the start of a rainfall event water
will lie on the surface until the upper layers get moistened sufficiently to
allow the water to soak downwards. A greater amount of water is stored on the
surface as swamps or as ice.
Aeration
zone storage: It includes water
stored as soil moisture within the pore space above the water table
Ground
water storage: As the excess water
reaches the underlying soil or rock layers, which tend to be more compact, its
progress is slowed. This constant movement or transfer, called percolation,
creates groundwater storage. Water eventually collects above an
impermeable rock or soil creating a zone of saturation. The upper level
of saturated material, i.e. the upper surface of the groundwater layer, is known
as the water table.
Channel
storage: It includes all water
within the stream/river channel.
Movement
water in the drainage basin
There
are a number of ways water move from one place to another in the drainage
basin. Summerfield (1996) classified these movements into five categories.
These categories are as follows:
Stemflow
and dripping
These
occur in area covered with vegetation. Intercepted water on the leaves are
transferred to the ground by two possible routes by dropping-off the leaves, dripping
and or by flowing down the trunk and along branches, stemflow.
Overland
flow/ surface run-off
This
is a major water transfer in the drainage basin. When the soil layers are fully
saturated with water, then water is forced to travel over its surface. It
usually takes the form of sheetwash, but the water may also run
down slope in small rivulets. Runoff is generated when rainfall intensity
exceeds the infiltration capacity of the soil, leading to the build-up of a
surface layer of water. If precipitation is very heavy at the beginning of the
storm then the ground may be incapable of absorbing all of the rain. As a
result, excess water flows away over the surface, a transfer known as surface
runoff or overland flow .The amount of runoff is affected by the vegetation
cover, surface roughness and the gradient and length of the slope.
Infiltration
(through and interflow)
Infiltration
is the vertical passage of water from the soils surface through its different
layers. The rate of infiltration will depend on the porosity and permeability
of the ground. The greater the permeability and porosity of the soil, the
faster is the rate of water infiltration into the ground. The rate of
infiltration of water changes with time. Shortly after rain begins to fall, the
rate will be relatively high since there will be more available air spaces
between the soil particles. As they become filled, the infiltration rate will
be reduced, but it will fall to a constant level as eventually a balance is
reached between water infiltrating into and draining from the soil. During
drier periods, some water may be drawn up towards the surface by capillary
action while at all times plant roots are likely to take up moisture from
the soil (vegetation storage) which may later be lost from the system by
transpiration.
Base
flow
This
is slow transfer of water laterally below water table toward the lowland area.
Groundwater levels usually respond slowly to surface storms or droughts.
During a lengthy dry period some of the groundwater store will be utilised as
river levels fall. In a subsequent wetter period, groundwater is replaced
before the level of the river can rise appreciably. If the water table reaches
the surface it means that the ground will be saturated and excess water forms a
marsh where the land is flat or becomes surface runoff if the ground is
sloping.
Channel
flow
Although
some rain does fall directly into the channel of a river, most water reaches it
by surface runoff (overland flow), infiltration (lateral flow through the
soil/rock) and or base flow. Hence, once water is in the river as
channel storage, water flows towards the sea or lake, where it is lost to the
drainage basin system together with sediments and dissolved matters. The
transport routes for these outputs are provided with the river channels under
the influence of the drainage basin slopes.
Important
aspects of the drainage basin
a) Basin
forms and channel pattern
The
basin forms take under consideration parameters such as the area, length, shape
and the relief of the basin. The size of a drainage basin influences the amount
of water yield (Summerfield, 1996). The length, shape and relief affect the
rate at which water is discharged from the basin and the total yield of
sediment. The length and character of the stream channels affect the
availability of sediment for stream transport and the rate at which water and
sediment are discharged.
Drainage
patterns
There are a number of
drainage patterns. The most common are as follows:
Dendritic patterns: Formed on horizontally bedded and uniform sediment or
on uniformly resistant crystalline rocks.
These are spreading branches type of patterns (Fig. 6.1).
Parallel drainage: Develops on moderate to steep regional slopes. Also
where there are bands of elongated and parallel outcropping resistant rocks
(Fig 6.1).
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