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The concept of drainage basins as open systems, focusing on the system processes and the influence of physical factors such as relief, geology, soil, vegetation cover, and climate. Students are encouraged to discuss the impact of each factor on the drainage basin system and complete related tasks.
Typology: Lecture notes
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ST: Using the image define what you think is meant^ by^ a^ drainage^ basin.
Watershed Drainage basins are local open systems. A drainage basin is an area of land drained by a river and its tributaries (river system). It includes water found in the water table and surface runoff. There is an imaginary line separating drainage
Thames basin Fig 1 EXT: What is significant about this line for flood prevention planners in Bath?
Interception Stemflow Soil Precipitation x 2 Infiltration x 2 Bedrock Watertable Groundwater flow Evaporation^ Surface^ runoff Channel catch Deep percolation Transpiration River Throughflow Fossil water Create a key and shade the input, fluxes, stores and output boxes on your diagram. Drainage basins are localised elements of the Global Hydrological Cycle which is a closed system. They are part of the process in which water returns to the sea under the influence of gravitational potential energy. Drainage basins however are 'open systems' which means that inputs are not governed by outputs. In short, inputs and outputs are not always in equilibrium. TASK: The Drainage Basin System
Precipitation Precipitation Interception Transpiration Infiltration Infiltration Soil Bedrock Watertable Fossil water Ground water Groundwater flow Throughflow Stem Flow Evaporation Channel catch Surface Runoff River
Relief What rainfall patterns can highland create? Impact on inputs? How quickly does steep/flat land drain? Impacts on flows/stores? Geology What differences does permeability of the rock have on stores, flows and outputs? Soil Sandy/peaty/clay? What differences would this make to flows, stores and outputs Vegetation cover How can this influence inputs? Does it have any seasonal variation? Is there a link with soil and climate? Impact on flows/outputs? Climate How does this influence inputs? Which climates promote a high/low level of input? How can temperature alter flows and outputs?
Physical factors within drainage basins determine the relative importance of inputs, flows and outputs. RELIEF VEGETATION SOIL TYPE/DEPTH (^) CLIMATE
Convectional rainfall Very common in areas where the ground is heated by the hot sun, such as the Tropics. This is why those areas experience heavy rainfalls most afternoons. The United Kingdom does experience some convectional rainfall during the summer, particularly in the South East of the country. Convectional rainfall occurs when: The surface of the earth is heated by the sun. The warm surface heats the air above it. Hot air always rises so this newly heated air does so. As it rises the aircools and begins to condensate. Further rising and cooling causes a large amount of condensation to occur and rain is formed. Convection tends to produce towering cumulonimbus clouds, which produce heavy rain and possible thunder and lightning.
Turbulence in the atmosphere and surface obstructions such as hills and mountains provide the initial upward push for the air. This rainfall occurs throughout the year near the equator daily in the afternoon. In middle latitudes, convectional rainfall occurs in early summer, in continental interiors. Cyclonic/Frontal rainfall: This type of precipitation associated with a cyclonic activity and occurs along the frontal zone (front is a narrow zone of transition, dividing two air masses of differing temperature and humidity characteristics, intersecting the earth's surface. Fronts are most clearly developed in areas where are masses converge) of convergence particularly at the ITCZ (inter tropical convergent zone) and at the polar fronts. In a zone of horizontal wind divergence, the warm air forced up over the colder air. In this slow ascend pressure decreases, air expands and cools, condensation and cooling produce a precipitation. These fronts and associated weather phenomenon are best developed in the middle latitudes, most of the winter precipitation of low lands in the middle latitudes is cyclonic or frontal is origin.
Condenses
Relief rainfall This is also called orographic rainfall, which is very common in the United Kingdom, especially on the West coast since the prevailing weather comes from that direction. Relief Rainfall occurs when: The prevailing winds pick up moisture from the sea as they travel across it, making the air moist. The moist air reaches the coast and is forced to rise over mountains and hills. This forces the air to cool and condense, forming clouds. The air continues to be forced over the mountains and so it drops its moisture as relief rain. Once over the top of the mountain the air will usually drop down the other side, warming as it does so. This means it has a greater ability to carry water moisture and so there is little rain on the far side of the mountain. This area is called the rain shadow.
