the water cycle

lithosphere

water stored in the earth's crust e.g. rocks

hydrosphere

liquid water e.g. rivers, lakes and oceans

cryosphere

frozen water e.g. glaciers and icebergs

atmosphere

water vapour stored in the air e.g. clouds

where is the earth's water stored?

97% is stored in oceans, the next biggest store is ice caps (the cryosphere)

processes driving change in the stores of the water cycle

- evaporation
- condenstation
- cloud formation
- precipitation
- cryospheric processes

evaporation

the process of liquid water turning into water vapour, occurs when energy from the sun hits the surface of water causing the liquid to change to a gas
- depends on: amount of solar energy, availability of water, humidity of air and temperature of air

condensation

the conversion of water vapour into a liquid
- occurs because either the air has cooled or there has been a fall in pressure

the dew point

the temperature at which water vapour in the air turns into liquid water

cloud formation

when the air reaches the dew point, water droplets form around condensation nuclei (dust or pollen etc.), these water droplets collide with others to form larger droplets and clouds
- when the water droplets get too big to support themselves gravity causes them to fall as rain

types of rainfall

frontal, relief, convectional

frontal rainfall

1) cold polar air mass meets a warm tropical rain mass
2) the cold air mass is forced under the warm air mass because it is heavier
3) where the hot and cold air masses meet a front is formed
4) as the warm air is forced upwards it cools and condenses to form clouds which rain

relief rainfall

1) prevailing winds bring warm air masses over the land
2) the air if forced to rise over high areas
3) as the air rises it cools and condenses into clouds causing it to rain

convectional rainfall

1) the sun heats the land which then heats the air above it
2) the air expands and rises as it warms up
3) as the air rises it cools and condenses into clouds causing it to rain

cryospheric processes

ice is formed as layer upon layer of snow falls and adds pressure onto the pile of snow to compress it and force the air out
- cryospheric processes are affected by temperature and are responsible for sea level changes
- when it is warmer there is more ablation and ice melts, when it is colder there is more accumulation of ice and sea levels fall

glacial period

a period when glacial advancement occurs

interglacial period

a warmer period between ice ages when glacier retreat and sea levels rise

flows in the water system

- precipitation
- evaporation
- interception
- stem flow
- evapotranspiration
- overland flow
- through flow
- groundwater flow
- infiltration
- percolation

why is a drainage basin an open system?

it starts with an input from the water cycle as precipitation after the process of cloud formation

hill slope water cycle

1) precipitation falls from clouds
2) the water can be intercepted by trees and plants where it can be stored in vegetation storage or move down the stems via stem flow
3) water can also be returned to the atmosphere from vegetation via evapotranspiration
4) water can fall straight to the ground where it can infiltrate into the soil and then percolate into the rocks
5) if the rocks are not permeable or the infiltration capacity of the soil is full then water will move via overland or through flow
6) runoff will reach streams and rivers where it is carried out of the drainage basin via channel flow

the water balance

the balance between water intake and output within a drainage basin

flood hydrograph

a way of displaying the discharge of a river following a particular storm event

discharge

the volume of water flowing past a certain point in a given unit of time

features of a flood hydrographic

- peak rainfall
- peak discharge
- lag time
- rising limb
- falling limb
- storm flow
- base flow
- bankfull discharge

factors affecting flood hydrographs

- basin size (smaller means high peak + short lag time)
- drainage density (high density means high peak + short lag time)
- rock type (impermeable means high peak + short lag time)
- land use (urbanisation means high peak + short lag time)
- relief (steep means high peak + short lag time)
- rainfall intensity (heavy rain means high peak + short lag time)

rivers regime

the annual pattern of the rivers rise and fall

changes in the water cycle due to natural variation

storm events, droughts, seasonal changes

changes in the water cycle due to human impact

farming practices, land use changes, water abstraction

storm events

* lots of precipitation means soils become saturated fast, infiltration capacity is reached and overland flow is encouraged
* low temperatures reduce evapotranspiration
* channel flow is rapid and flooding is more likely

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e.g. September 2015, STORM DESMOND

* broke the UK record for the most rainfall in 24 hours at 341.4mm of rain
* left houses destroyed, roads damaged and many people without power

droughts

- soils dry out and become less permeable, groundwater and through flow are reduced
- vegetation dies out so evapotranspiration and interception are reduced
- if a storm comes infiltration is very slow so overland flow is rapid which can cause flash flooding

seasonal changes in summer

- total rainfall decreases but storms are more likely
- vegetation grows rapidly so interception and evapotranspiration are increased
- high temperatures also encourage evapotranspiration
- dry soils encourage infiltration but hard baked soils encourage overland flow
- low flow conditions are more likely

seasonal changes in winter

- greater quantities of rainfall and maybe snow
- vegetation dies back reducing interception and evapotranspiration
- low temperatures reduce evapotranspiration
- soils become saturated fast so overland flow is encouraged
- high flow conditions are more likely

land use changes

- urbanisation replaces vegetation with impermeable concrete that water cannot infiltrate, overland flow and flooding is more likely (soil and groundwater stores are reduced)
- deforestation removes trees so there is no interception, soil erosion and overland flow are more common

farming practices

* farmers control the local water cycle using irrigation of land drainage
* soil covered by plants has higher infiltration rates and runoff is reduced

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e.g. EAST ANGLIAN FENS

* the area was drained in order to create more productive land for farming
* this changes the local water system as there is no more interception and previous stores of water are depleted making flooding more likely

water abstraction

water is taken from rivers or aquifers for irrigation, domestic use or industry
- aquifers can become depleted
- results in low flow conditions which can be harmful for ecosystems

CASE STUDY: THE RIVER EXE

* flows for 82.7km from the hills of Exmoor to the sea at Exmouth 
* it has an extensive network of tributaries and therefore a high drainage density 
* around 84.4% of the catchment is underlain by impermeable rock (mainly Devonian sandstone)
* 67% of land use if agricultural grassland, 15% is woodland
* rainfall is very high, runoff accounts for 65% of the water balance due to impermeable bedrock

Recently:

* the construction of the Wimbleball Reservoir attempts to regulate water flow and prevents peaks and troughs of discharge
* restoration of peatland increases water storage and quality downstream, helps retain carbon
* the Exmoor Mires Project has been successful as more moisture is being retained and water tables are starting to rise