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The drainage basin as a system   lesson 2
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The drainage basin as a system lesson 2

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  • 1. The Drainage Basin as a system
  • 2. A drainage basin An area of land drained by a river and its tributaries where any precipitation is collected or drained into it. Its boundary is marked by a ridge of high land called watershed Drainage basins have one main stream and many tributaries
  • 3. Watershed: A stretch of land that surrounds drainage basin (a.k.a. boundary)Drainage Basin: A river system that collects rainwater and comprises of streams and riverTributaries: Streams that flow in the river which is mainly found in the upper course.Distributaries: Branch out from the river which is mainly found in the river delta.River Source: Point where the river startsRiver Mouth: Place where the river meets the sea / ocean (River ends)
  • 4.  A drainage basin may be described as an open system. It forms part of the hydrological cycle
  • 5. Characteristics of a drainage basin are: Inputs : - in the form of precipitation Outputs:- water is lost through evapotranspiration Within the system, some of the water:  Is stored in water storage such as lakes or soil  Passes through a series of transfers or flow e.g. infiltration, percolation and throughflow
  • 6. Important terms.. Interception : precipitation lands on vegetation. Infiltration : Water that reaches ground surface will soak into soil. Overland flow : water moving over the ground. Throughfall : precipitation passing through the plant canopy Percolation : Some water sinks deep into the earth (percolates) through openings in the soil. Stemflow : water that flow down tree trunks or plant stems. Groundwater : water transfer through permeable rocks from the soil above. Baseflow : water flows from groundwater stores to the stream Throughflow : water moving through the soil. Surface run-off : water that flows over the land surface.
  • 7.  main input - precipitation. The type: rain or snow, hail, etc. Factors affecting the amount of water in the system:  the intensity,  the duration and  frequency. Each subsystem of the drainage basin system will also have  inputs and ouputs, and  the output from one stage of the diagram will form the input for another.
  • 8.  Water stored - on the surface, - in lakes and channels or - in the groundwater store. Reach groundwater via infiltration and percolation. some water will be stored in the soil and rock. The amount of water stored will vary depending on:  the porosity of the soil and  the permeability of the rock. Water can be temporarily stored via interception. – in the storage of water on leaf and plant stems.
  • 9.  all the water flowing over the drainage basin’s surface is called runoff. made up of streamflow - flow through permanent river channels and overland flow or surface runoff. Overland flow - transfers water through the basin either as:  sheetwash, across the surface, or  in tiny channels called rills. Beneath the surface, water is transferred via throughflow, which is the movement of water through the lower soil towards rivers, and groundwater flow (base flow). Water that has been intercepted by foliage may also be transferred, either directly as throughfall, or by running down branches and stems via stemflow.
  • 10.  final release of the water in a drainage basin flow into the sea will be the main output of a drainage basin. Some water will also be lost via evapotranspiration.  Evaporation from water bodies and soils  Transpiration from plants