AQA Geography AS Rivers, Floods and Management

It us a closed stystem, there are no outputs  or inputs.
Global hydrological cycle: from the ocean, to the atmosphere, returning to the ocean as rain
Drainage basin hydrological cycles: local hydrological cycles:

• A rivers Drainage basin is the area surrounding in which the rain that falls will collect in the river , it can also be known as the catchment area
• the boundary of the drainage basin is known as the Watershed
• The drainage basin is an open system with inpust and outputs
• water enters the system as precipitattion and leaves via evpouration , transpiration and river discharge

Inputs

• precipitation:  snow , rain , hail , dew , frost

Storage

• Interception: water landing on vegetation or building - this is only temporary storage as it soon after evapourates - can produce significant store of water in wooded area
• vegetation storage water taken up by plkants
• surface storage ponds , puddles and lakes
• Groundwater storage  water stored underground, either in soil or rocks. the water table is the top surface of the zone of saturation which is the zone where all pores are full of water .
• aquifers  pourous rocks that hold lots of water
• channel storage the water held in the river or stream

Flows and Processes

• surface run-off/ overland flow  water flowing over the top of the land.common in arid areas or areas where the ground is saturated
• throughfall  the driopping of what from one leaf to another
• stemflow running of water down a plant stem or tree trunk
• throughflow water moving downhill through the soil
• infiltration  the soaking of water into the soil
• percolation  seeping over water through the soil towards the water table
• grouwwater flow  the slow flowing water below the water table through the permeable rock. this occurs faster with high permeable rocks with lots of joints and gaps that the water can get through
• baseflow  the groundwater flow that feeds into the river through river banks and river beds
• interflow water flwong downhill through permesble rocj aboove the water table
• channel flow water flowing in the river or channel itelf (river discharge)

Outputs

• evaporation  water turning from water into water vapour
• transpiration the evaporation of water from plant leaves
• evapotranspiration the process of evaporation and transpiration together
  • the potential evapotranspirationis the amount of water that could be lost by evapotranspiration. actual evapotranspiration  is what actually happens. for example in the dessert you would expect the evapotranspirstion rates to be high but in reality they are low as not much water is taken up into the plant in the first place.
• river discharge 

The water balance shows the balance betweeen inputs and outputs

• wet seasons the precipitation exceeds the evaporation - water surplus. ground stores fill so there is more surface run off and higher discharge so greater water levels
• drier seasons inputs lower then output, ground stores are depleted as it is used by plants and so flows into the channel
• end of a dry season- water decificit - this is replaced by water during the wet season

river discharge the volume of water flowing in the river in m3/s (cumecs). it is effected by:

• precipitation: the greater the precipitation, the greater the discharge
• hot weather the higher the temperature , the greater the rate of evaporation
• removal of water from the river (abstraction) reducing discharge

Hydrographs  they show river discharge over a period of time. they show how the volume of water flowing in the river changes over time.

• peak dischaarge where the discharge is the greatest
• lag time the delay between peak rainfall and peak discharge. A shorter lag time can increase peak discharge because more water reaches the river during a shorter amount of time.
• rising limb the point of the graph up to peak discharge 
• falling limb th3 point after the peak discharge. discharge is decreasing as less water is flowing into the river. a shallow falling limb shows water flowing in from stores after it has stopped raining.

the storm hydrograph is affected by physical factors:

1. drainage basin characteristics- the physical features of the drainagae basin           affect lag time and peak discharge:

• large drainage basins  catch more prec. so they have greater peak discharge comqared to small drainage basins.
• smaller drainage basin  generally have shorter lag time- less distance to travel.
• steep sided drainage basins have shorter lag times- water flows more quickly downhill increasing the peak dischrage
• circular drainage basins are more likely to have a flashy floodhydrograph rather than long narrow basains. as the water is coming from pionts on the watershed which is roughly the same distance away. this means alot of the water will reach the measuring point at the same time , increasing the peak discharge.
• basins with lots of streams drain quickly, so have shorter lag times (high drainage density)

2.  the amount of water already present in the drainage basin (antecedent conditions) affects lag time

• if the grounds already water logged-less infilitration and more surface run off. run off faster than throughflow or base flow so more water reaches the river faster.

3. rock type - affects lag time and peak discharge

• impermeable rocks do not store water or allow water to flow through- increases surface runnoff and decreases infiltration, increasing peak dischrage and reducing lag time

4. soil type - affects lag time and paek discharge

• sand soils allow infiltration, clay soils less so, low infiltration, greater surface run off- reduces lag time and increases peak discharge

1. In urban areas, much of the soil is covered by impermeable surfaces such as concrete. this means the the water cannot infiltrate the soil , which increases the surface run off, so the water flows more quickly into the river. this decreasing lag time and increases peak discharges. the impermeable surfaces can also lead to flooding as the water hads nowhere to go.

2. Man made drainage systems removes water at a faster rate causing an increased peak discharge and a decreased lag time. this is mainly due tio the fact that it is removed before the water can evaporate or infiltrate the soil which would usually increase the lag time and decrease the peak discharge. Although drainage systems remove the water to from immediate area reduces flooding there, it can cause flooding by the river as the lag time is decreased and the peak flow is increaed. the normal lag time allows the river enough time to respond to the extra water by taking it away downstream, by decreasing this lag time, the amount of water the river recieves at once increase, decreasing the time that it takes for the river to reach bankfull capacity which leads to flooding. 

5. vegetation- affects lag time and peak discharge

• vegetation intercepts and slows it movcement- increases lag time. interception is highest where there is lots of vegetation and deciduous trees have their leaves
• more vegetation, the more water lost through transpiration and evapouration so less reaches the channel

6. precipiation - affects peak discharge

• intense storms- more precipitationcrteating greater peak discharges than light rain showers
• type of precipitation- e.g nwo fallen in winter can melt in spring time and then flwo into the river producing a very long lag time

7. temperture - afffects lag time and peak discharge

• hot dry conditions and cold, freezing both result in hard ground. thus reducing infiltratuib abd increasing surface run off- reducing lag time and increasing peak discharge
• high temperatures can increase evapotransipiration, so less water reaches the river channel, reducing the peak discharge

a flashy hydrograph has is steep and symetrical.

Bankfull dischargeis the point when the water water level reaches the top of the river channel