5.2 drainage basins Flashcards
Is a drainage basin an open or closed system?
Open
What is a drainage basin?
an area of land which is drained by one main river and its tributaries
What physical factors affect drainage basin flow?
Interception
Infiltration
direct runoff
saturated overland flow
throughflow
percolation
groundwater flow
What is interception?
The process by which raindrops are prevented from falling directly onto the ground by the presence of a layer of vegetation.
What happens to intercepted water
The water later reaches the soil via stem flow of throughfall. The undergrowth may intercept again (secondary interception) and some of the water will return to the atmosphere via evapotranspiration
What is the rate of interception dependent on?
precipitation and vegetation: Interception is greatest when precipitation is light and of short duration. As vegetation becomes wetter, more water will drip or flow to the ground and interception effectiveness will decrease. Type of vegetation also matters- denser vegetation like coniferous forests intercept more than sparse deciduous forests
What is infiltration?
the movement of water vertically downward through pores in the soil
What is infiltration capacity?
the maximum rate at which the soil can absorb precipitation
What does the rate of infiltration depend on?
the amount of water already in the soil (the degree of saturation)
slope gradient
soil/rock type (porosity/ permeability)
water table depth
duration of rainfall
raindrop size (the bigger the raindrop, the faster it’s infiltrated)
Vegetation cover
What are the 6 key physical factors affecting the drainage basin?
- amount of precipitation
- type of precipitation
- Seasonality
- intensity of precipitation
- Variability
6 The distribution of rainfall in a drainage basin
What are the three different types of variability?
-secular variability: happens long term, for example as a result of climate change
-periodic variability: happens in a monthly season/ annual/ diurnal
-stochastic variability: random factors
What is convectional rainfall?
When the land becomes hot, the air becomes warmer, expands and rises. As the air rises, it cools and its ability to hold water vapor decreases. Condensation occurs and clouds develop. If the air continues to rise, it will rain. Cool air then descends and replaces the warm air
What is cyclonic rainfall?
This happens when warm air, which is lighter and less dense, is forced to rise over cold, denser air. As it rises, the air cools and its ability to hold water vapor decreases. Condensation occurs and rain falls
What is orographic rainfall?
When air is forced to rise over a barrier, such as a mountain, it cools and condensation takes place forming rain. The leeward slope receives relatively little rain which is known as the rain shadow effect
What are the main physical factor which affects inputs to the drainage basin
ITCZ
Continentality
Relief
Where is the highest level of rainfall found?
in the tropics, because of the ITCZ
What happens in the ITCZ?
intense solar radiation fuels the convection of warm, humid air, resulting in condensation and precipitation
What is continentality and how does it effect the inputs of the drainage basin?
How far land is from the sea
These areas are far from the moisture of maritime air masses
What are examples of regions affected by continentality?
Gobi desert in Asia
Alice Springs region in Australia
How does relief affect drainage basin inputs?
relief like mountains and prevailing winds complicate the pattern, with high levels of precipitation occurring where prevailing winds are forced to rise over higher altitudes, forming orographic rainfall
What and where is the highest average rainfall average
Mawsynram in India with 11,873mm per year, mostly between monsoon season between June and September
Where is some of the lowest levels of average annual rainfall found?
In areas of high atmosphere pressure, such as Quillagua in the Atacama desert, which is the driest place on earth, receiving less that 0.2mm a year
What are the factors which affect drainage basic outputs?
Evaporation and transpiration and channel flow
What is evapotranspiration?
The total amount of moisture removed from the drainage basin by evaporation and transpiration
What is evaporation?
the process by which water is transformed into water vapor
What is transpiration?
A biological process by which liquid water is drawn upwards from the soil by plants and evaporated through pores called stomata
What is channel flow?
Water that has collected to flow is a stream or river. It measured by discharge
What is the volume of channel flow dependent on?
the amount of precipitation falling directly into the channel
Contributions from drainage basin stores via surface run off, throughflow or groundwater flow
What are the factors affecting evapotranspiration?
- Temperature- the rate increases with temperature
- wind- wind will increase the rate of evapotranspiration by reducing the relative humidity and preventing saturation of the air
- Vegetation cover- the rate increases with more vegetation. Vegetation with a low albedo (reflectivity) such as dark forests will absorb more solar radiation, increasing evapotranspiration
- soil moisture content- this will determine the amount of water available for transpiration. It’s dependent on soil and rock permeability
What physical factors affect drainage basin inputs flows and outputs?
climate
soils
geology
relief
vegetation
What human factors affect the hydrology cycle?
groundwater abstraction
urbanisation
cloud seeding
dam construction
How does groundwater abstraction affect the drainage basin?
Groundwater is abstracted from aquifers faster than it is replaced, causing reduced groundwater flow and a lower water table. In other locations, reduced industrial activity or deforestation has increased groundwater storages, increasing the risk of groundwater flooding
What is an example of groundwater abstraction?
groundwater is used to irrigate more than 40% of China’s farmland and provides about 70% of drinking water in the dry north western regions. This means ground water levels in the arid North China Plain dropped by as much as a metre between 1974 and 2000.
Where has groundwater rebound occurred?
In some of the UK’s major cities like London, Birmingham, Nottingham and Liverpool as a result of reduced abstraction for industry
How does urbanisation influence the drainage basin?
it creates an impermeable surface that reduces infiltration and increases surface runoff and throughflow through artificial drains; stream and river discharges often increase rapidly as a result
Where has urbanisation influenced drainage basins?
Winchester and Maidenhead (2014 floods)
Carlisle, York and Manchester (2015 floods)
How does dam construction affect the drainage basin?
dams increase surface water stores and evaporation and and reduce downstream river discharge
Where has dam construction influenced the drainage basin?
Lake Nasser behind the Aswan dam in Egypt is estimated to have evaporation losses of 10 to 16 billion cubic metres every year. This represents a 20-30% loss of Egyptian water volume from the Nile
What is cloud seeding?
This is the attempt to change the amount or type of precipitation by dispersing substances into the air that serve as cloud condensation nuclei
Where has cloud seeding been used?
China used it before the 2008 Beijing Olympics to create rain to clear the air of pollution. It’s used in the Alpine Meadows ski area in California to improve snow cover and was used in Texas 2015 to reduce the levels of drought.
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