5.2 Flashcards
When does orographic rainfall occur
Orographic rainfall occurs when air is forced to rise over a barrier e.g mountain , causing it to cool and condense forming clouds and rain. When the air is rising lots of rainfall is distributed onto the immediate area however as the rising air descends not much rain is distributed. This is known as the rain shadow affect
When does cyclonic rainfall occur
Cyclonic rainfall occurs when warmer,less dense , lighter air is forced to rise above colder, denser air. As it rises the warmer air cools and its ability to hold water vapour decreases. It then condenses and clouds and rain forms
When is interception is worse
Interception is worse when there is lots of rainfall for a long duration of time as he branches are highly saturated so more excess water falls away
When is interception greatest
Interception is greatest when there is rainfall for a short duration as dry branches have the greatest water storage ability
When does convectional rainfall occur
Convectional rainfall occurs when land rises in temperature causing the air above it to become hot, rise and expand. As it rises the airs ability to hold water vapour decreases and condensation occurs. If it then continues to rise rain will fall
Which type of trees allow for greater interception
Dense coniferous needles allow for greater accumulation of water compared with deciduous trees which allow for less interception. This is shown by evidence as coniferous forests e.g the Qillian mountain conifer forest intercept 25-35 % of annual rainfall whereas deciduos forests only intercept 15-25 % of annual rainfall. This contrast is partly to do with the fact that deciduous trees shed their leaves but coniferous trees do not shed their needles
Which crop type allows for greater interception
Denser coniferous crops allow for greater interceotion than sparser deciduous crops
What wind conditions allow for greatest interception
Low wind conditions minimise the amount of interception dislodged from the vegetation and reduce evaporation rates. Whereas high winds maximise interception dislodgement and increase evapotranspiration rates
What conditions will allow for high infiltration
Precipitation-For infiltration to be at its highest there needs to be moderate antecedent rainfall to ensure soil permeability so the soil is not too dry or too saturated
Gradient-for infiltration to be at its highest the ground needs to have a shallow gradient as too steep gradient makes water run off the surface
Soil depth and type- for high infiltration there needs to be Sandy soils with uncompacted surfaces and a high depth as the precipitation can flow through and around the small particles
Vegetation-To allow for maximum infiltration there needs to be high forest density or moorland with low seasonal variation to maximise interception
What conditions will allow for low infiltration
Precipitation-Lots of precipitation will cause the ground to become saturated and less infiltration to occur. Also if there has been lots of rainfall previously then the ground is more likely to be saturated and less infiltration can occur.
Gradient-very steep slopes encourage overland run off ( surface run off)
Vegetation-permenant pasture on land decreases infiltration rates significantly as it compacts the soil and allows for less infiltration
What is evapotranspiration
Evapotranspiration is the amount of water in moisture removed from a drainage basin by process of evaporation and transpiration
It is an output from the drainage basin open system
What is evaporation
Evaporation is the process by which liquid water is transformed into gaseous water vapour
What is transpiration
Transpiration is the biological process by which water is drawn upwards from the soil by plants and evaporated through stomata in the leaves
What is potential evapotranspiration
Potential evapotranspiration is the amount of evapotranspiration that could take place given unlimited supplies of water in the environment
What is channel flow (discharge)
Channel flow is water that has collected to flow in a water flow system (e.g a rivulet, stream or river)
It is an output from the drainage basin open system
The discharge of a river is the volume of water passing a specific point per unit of time
Discharge is dependent on precipitation amount and contributions of water from drainage basin stores (surface run off, through flow or groundwater flow )
What are factors affecting the rate of evapotranspiration
Temperature - rate of evaporation increases with temperature
Wind - wind increases rate of evaporation by reducing relative humidity and preventing air saturation
Vegetation cover - transpiration rate increases with high vegetation density and vegetation. Vegetation with a low albedo affect will absorb solar radiation, increasing evaporation
What is saturated over land flow
Saturated over land flow is when water accumulates in the soil until the water table reaches the surface , forcing further rainwater to run off the surface
-This is common where there are thin soils of moderate permeability especially in high moisture concavities near streams or river banks
What is infiltration excess overland flow
Infiltration excess overland flow is when rainfall intensity exceeds the infiltration capacity. When this happens excess water flows over the ground surface and is quickly delivered to river channels. This however increases the risk of downstream flash flooding
What is soil through flow
Soil through flow is the internal transfer of water downslope through soil via natural pipes and percolines ( lines of concentrated water flow between soil horizons to the river channel )
What is groundwater flow
This is the slow transfer of water ( percolated) through previous ( joints and bedding planes) and porous ( pore spaces) rocks
-It is a vital regulatory component in maintaining a steady level of channel flow in varying conditions
What is percolation
Percolation is the continuation of infiltration deeper below ground
- It is the deep transfer of water into permeable rocks ( pervious e.g Carboniferous limestone or porous e.g. chalk and stand stone)
- Percolated water reaches a saturated zone ( water table ) above impermeable rock and creates groundwater storage and aquifers
What is cloud seeding
Cloud seeding is an attempt to try and change the amount of type of precipitation by dispersing substances ( silver iodide pellets or ammonium nitrate) into the air to serve as cloud condensation nuclei ( hygroscopic nuclei). It is improving with new technology and is used in drought stricken countries to increase rainfall.
-It is an input disruption to the drainage basin
What is a case study for somewhere where cloud seeding has been used
- China used cloud seeding in Beijing just before the olympics in 2008 in order to create rain to clear the air of pollution
- The USA used it during the Vietnam war in order to draw out the monsoon season and create difficult, muddy conditions for opposition fighters
- It was also used in Texas in 2015 to reduce the impact of drought
How does urbanisation disrupt the drainage basin
Urbanisation creates impermeable surfaces that reduce infiltration and increase surface runoff and through flow through artificial drains ; streams and river discharges often increase rapidly as a result
Case study: across the UK, urbanisation has increased flood risk in many towns and cities such as Winchester and Maidenhead ( 2014 floods) and Manchester ( 2015 floods)
