5.2 The Drainage Basin Flashcards
what is precipitation a product of?
condensation of atmospheric water vapour that falls under gravity
3 types of rainfall
orographic
convectional
frontal
orographic rainfall
warm air rises over high areas, it cools and condenses forming clouds, these descend on the other side of the hill, forming a rain shadow
-depends on relief and location of the land immediate after the sea
convectional rainfall
the sun heats the land and air daily, causing it to rise, it then cools and the moisture condenses, forming rainfall
-this is common in tropical climates, for example in tropical rainforests where rain is common mid-morning before temperatures rise too high for condensation to occur in the area
frontal rainfall
this is where two air masses meet- causing a depression to occur
at the front, warm moist air is forced to rise above the cold air mass, causing moisture to cool and condense, leading to heavy rainfall along the front
-around 100 depressions hit the UK each year
what rainfall is common in the UK?
in the western and northern parts of the UK, orographic rainfall is common as warm air from the tropics travels over the Atlantic Ocean, it rises at the western up-land areas leading to heavy rainfall
how can precipitation vary?
amount, intensity, seasonality and distribution
how does precipitation vary in amount?
drainage basins are different sizes, therefore larger basins can hold more water
how does precipitation vary in intensity?
high intensities maybe due to storms or extremely high rainfall can result in flooding
how does precipitation vary in seasonality?
warm climates and dry summers can lead to drainage basins becoming dried up, altering flows
how does precipitation vary in distribution?
large basins vary with tributaries starting in a variety of locations with different climate zones e.g. the river nile or the ganges
urban areas and flooding
often have high flood risk due to impermeable surfaces increasing surface run-off, reducing lag time. however, they often have drainage systems to reduce this, such as drains, sewers, floodplains etc
deserts and precipitation
often little to no rainfall, low rates of interception means when rainfall occurs evaporation is high, but also infiltration is high in sand
meadows and precipitation
they hold lots of water, infiltration and interception occur
physical factors impacting the drainage basin
climate, soils, vegetation, geology and relief
how does climate impact the drainage basin?
as temp increases, evapo-trans also increases
-wind and humidity levels vary, influencing the type and amount of precipitation and evaporation
how do the soils impact the drainage basin?
high moisture level soils often hold more water
moisture levels determine the rates of transpiration, infiltration and throughflow which are all influenced
how does vegetation impact the drainage basin?
highly vegetated areas often have high interception rates, increasing infiltration and transpiration rates, as well as the occurrence of overland flow
how does the geology of an area impact the drainage basin?
permeable or impermeable surfaces affect percolation and groundwater flow
how does the relief of an area impact the drainage basin?
as the slope angle increases and relief increases, the surface run-off speed increases, while infiltration levels decrease
-altitude impacts precipitation totals
-mountainous areas often experience more orographic rainfall
how does precipitation impact the drainage basin?
high precipitation intensity levels can cause flooding as levels of the drainage basin increase
-in summer, drainage basins can dry out as rainfall levels are lower
human factors impacting the drainage basin
deforestation, urbanisation and changing land use, abstraction
how has deforestation impacted the drainage basin system?
as dense tree canopy is removed in tropical rainforests in particular, interception levels are reduced, as the top soil layer is washed away. evapotranspiration rate are reduced, surface run-off increases, increasing flooding risk
how has urbanisation and land use impacted the drainage basin system?
natural ground cover leads to high evapotranspiration levels, low surface run off levels and higher shallow infiltration
therefore impermeable surfaces cause lower evapo-trans levels, higher surface run off and lower shallow infiltration levels
-drains and sewers cause discharge to increase as runoff and throughflow levels rise
