dEFINITION
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The drainage basin hydrological cycle may be defined as a single river basin bounded by its own watershed and the sea. It has 4 mechanisms
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- Inputs - precipitation including rain and snow, and solar energy for evaporation.
- Outputs - evaporation and transpiration from plants (evapotranspiration), runoff into the sea, percolation of water into underlying rock strata.
- Stores - puddles, rivers, lakes (surface storage), soil, groundwater storage and water stored in vegetation.
- Transfers or Flows - infiltration, Percolation, overland flow, throughflow, groundwater flow.
KEY TERMS
- Dynamic Equilibrium - rivers are constantly changing over time to reach a balance with the processes that determine their form. As the flows of energy and materials passing through the river change over time, so to does the river towards this equilibrium.
- Evaporation - Water warmed by the sun that turns into a gas.
- Evapotranspiration - The loss of water from a drainage basin to the atmosphere from the leaves of plants.
- Groundwater flow - The movement of water through underlying rock strata.
- Groundwater storage - water stored underground in permeable rock strata.
- Infiltration - the movement of water downward into the soil surface.
- Overland Flow - When water flows over the surface of the ground. This can happen because the soil can be saturated, the ground might be frozen or the underlying rock may be impermeable.
- Percolation - The downward movement of water through underlying porous rock, collecting as groundwater
- Precipitation - water falling in any form.
- Surface Storage - the total water held on the Earth's surface.
- Throughflow - The movement of water within the soil sideways towards the river.
- Water table - The top of the zone of saturation.
- Watershed - The boundary of the drainage basin.
Systems Theory
An aspect of systems theory can be used to explain the hydrological cycle. Systems can be classified into three types:
When inputs and outputs of the system are balanced, they are said to be in dynamic equilibrium, because in reality most basins are continuously changing to reach, but rarely achieving balance.
Whilst the river shows the flow of water over land, there are two other key features of a drainage basin.
An aspect of systems theory can be used to explain the hydrological cycle. Systems can be classified into three types:
- Isolated: no input or output of energy, the universe is an example of this
- Closed: there is input, output and transfer of energy but not of matter or mass - a planet for example
- Open: inputs and outputs of both energy and matter. Most environmental systems are examples of open systems.
When inputs and outputs of the system are balanced, they are said to be in dynamic equilibrium, because in reality most basins are continuously changing to reach, but rarely achieving balance.
Whilst the river shows the flow of water over land, there are two other key features of a drainage basin.
- Infiltration - which is affected by the following: The nature of the precipitation (duration, total,area), vegetation interception, depression storage, Evapotranspiration, slope length and angle and the nature of the soil.
- Groundwater - water collects in permeable rocks, known as aquifers. Variations in the level of the water table reflect surface topography. It is high in the valley and deep in the hills. The surface topography affects the flow of the groundwater, and this is known as the Hydraulic Gradient. Velocity ranges between 1mm/year and 1mm/day. Most rivers gain their water and flow from a combination of surface run-off and groundwater flow. Groundwater having a bigger influence in summer and autumn, run-off in winter and spring.
The balance between water inputs and outputs of a drainage basin may be shown as a water budget graph:
Five Key Points
- A drainage basin is a complex open system that transfers energy and matter as part of the water cycle.
- Water is continually entering and exiting a drainage basin creating dynamic equilibrium.
- Along with the river as the most obviously flow of water out of a basin, water flows out by infiltration and groundwater flow.
- A drainage basin has 4 mechanisms - inputs, outputs, stores and transfers and flows
- A drainage Basin operates as part of the hydrological cycle.