Water cycle

Water cycling system on a global scale

• closed system meaning the total amount of water from earth stays the same and there’s transfer of energy but not mass

• input - elements that can enter a system to be processed

• transfers - movements of energy or matter through the system that enable inputs to become outputs

• stores - matter stored and not transferred until the appropriate processes are in place to move them

• outputs - outcome of processing within the system e.g. energy used up, water leaving the sea via evaporation

Water cycle system on a local scale

• the drainage basin water cycle is open meaning there is a transfer of energy and mass

• the suns energy comes into and leaves the drainage basin but mass also enters and leaves

Major water stores

Percentage of water in 3 main stores:

• Atmosphere (0.001%)

• Ocean (97%)

• Land (2.8%)

Residence time:

• the short time spent by water in transit through the atmosphere results in regional weather patterns

• long residence times of 3000-10000 years in deep-ocean circulations, groundwater aquifers, glacial ice act to moderate temperatures and climate

Global stores of water

• Hydrosphere

• Cryosphere

• Atmosphere

• Lithosphere

• Biosphere

Water’s role in moderating temperatures and climates

• Oceans - take a long time to warm up and cool down. Therefore areas near coasts tend to be warmer in winter and colder in summer

• Clouds - are highly reflective and reflect a lot of the suns energy preventing the earth from getting too hot

• Water vapour - is a greenhouse gas and therefore helps to keep the temperatures on earth warm because the greenhouse layer intercepts, absorbs and re-emits heat, keeping the Earth habitable

Distribution of water around the planet

• Less than 10 countries possess 60% of the worlds available freshwater supply

• 30% of all freshwater is stored as groundwater in rocks deep below the surface forming vast underground reservoirs called aquifers

• these aquifers most commonly form in chalk and sandstone which are permeable and hence they are unevenly distributed

Precipitation

Forms when vapour in the atmosphere cools to its dew point and condenses into tiny water droplets or ice particles to form clouds. Eventually these droplets or ice particles aggregate, reach a critical size and leave the cloud as precipitation

• ablation

• most rain reaching the ground flows quickly into streams and rivers

• but in high latitudes and high altitude drainage basins, precipitation often falls as snow and may remain on the ground for several months until spring

• therefore, there can be considerable time-lag between snowfall and runoff into streams and rivers

How does season snowmelt affect the water cycle in the Arctic Tundra:

• Spring → Summer

• water sits on top of soil because soil is frozen (permafrost) and becomes impermeable and can’t be infiltrated

• some evapotranspiration occurs due to higher temps however rainfall is minimal because temps are still too low

Intensity and duration of precipitation

Intensity:

• the amount of precipitation falling, measured in mm/hr

• high-intensity precipitation is c. 10-15mm/hr

• low intensity is c.2mm of rain a day

• high intensity rainfall on steep slopes, may lead to flash floods

• on flat areas it may lead to surface storage when soil is saturated

• or flash floods when the drainage in the urban area is insufficient for the intensity of the rain

Duration:

• duration is the length of time that a precipitation event lasts

• prolonged events, linked to frontal rain in the UK, may deposit high amounts of precipitation and cause flooding

• in some parts of the world, e.g. East Africa, Mediterranean, precipitation is concentrated in a rainy season

• during this season, river flow is high and flooding is common

• in the dry season, rivers may cease to flow altogether

Ablation

• the loss of water from snow, ice sheets and mountain glaciers due to a melting, evaporation and sublimation

• ablation is a key input into drainage basin systems in glacial regions or high latitude regions in spring and summer

Evaporation (system output)

• The process of turning from a liquid into a gas

• it is the phase change of liquid water to vapour

• the main transfer from land/ocean to the atmosphere

It happens due to:

• the suns energy which break the molecular bonds of water

• air movement i.e. kinetic energy (wind)

• availability of water - rivers, wetlands, oceans, soils

Transpiration and Evapotranspiration

• transpiration is a biological process by which water is lost from a plant through stomata

• it is the diffusion of water vapour to the atmosphere from plants

• Evapotranspiration is the combination of evaporation and transpiration. Evaporation is water movement from wet soil and leaf surfaces. Transpiration is water movement from the stomata of the leaves

• it is responsible for c.10% of water in the atmosphere therefore it is a fundamental process in the water cycle

• like evaporation, transpiration is influenced by thermal energy and wind - kinetic energy

• it is also influenced by water availability to plants. For example, deciduous trees shed their leaves in climates with either dry or cold seasons to reduce moisture loss through transpiration

Types of rainfall

Frontal rainfall:

• forms when cold air and hot air meet

• less dense hot air is lifted over the cold denser air

• this causes the water vapour to cool and condense into water to form rain

Relief/orographic rainfall:

• forms when moisture laden air is forced upwards over high ground which produces clouds

• precipitation over high ground removes moisture form the air and causes drier conditions on the leeward side

Convectional rainfall:

• forms when there is an unstable atmosphere when buoyant air rises upwards into the atmosphere

• this happens when air is heated from below by warm land or sea

• the bubble of air cools and condenses into clouds

• convective rain falls as showers with rapidly changing intensity and can last much longer

• convective rain also falls over a much smaller area

Interception

• occurs when precipitation lands on vegetation instead of reaching the ground surface

• the water can be stored on leaves of trees, usually in the form of small droplets

• some of this intercepted precipitation might eventually reach the ground as it flows along leaves and branches (stem flow) or falls through the leaves (throughfall)

• some of the precipitation will evaporate directly from the vegetation and be lost from the drainage basin which is called interception loss

• interception lowers the intensity of precipitation that reaches the ground and therefore help to limit overland flow

Factors affecting interception loss:

• Interception storage capacity - when vegetation becomes saturated with precipitation, output through stemflow and throughfall increases

• wind speed - rates of evaporation increase with windspeed

• vegetation type - vegetation with larger surface area and aerodynamic roughness have higher interception losses

• tree species - trees that have leaves all year around will have higher interception loss e.g. conifers

Runoff

Order in decreasing speed due to increasing friction:

Overland flow - the lateral movement of water on the surface after precipitation (very fast)

Throughflow - the lateral movement of water through the soil and its layers (fast)

Groundwater flow - the lateral movement of water through the cracks in rocks (slow)

Types of overland flow

• Overland flow is the principle way that rainwater is transferred to the river channel. this is water that does not pass into the soil via infiltration or into the rock and groundwater store via percolation

Overland flow will occur when:

• prolonged rainfall causes the soil to be saturated

• high intensity rainfall exceeds the infiltration capacity of the soil

• urban areas with impermeable surfaces

• when the soil has baked dry during very hot periods

• glacial snow melt when the ground is frozen and therefore impermeable

• steep slopes - GPE enhances overland flow

Saturated overland flow:

• prolonged rainfall fills the soil spaces with water so the soil is saturated

• there are no more pore spaces to fill with water

• soil acts as though it is impermeable so water cannot infiltrate

• saturated overland flow then takes place

Infiltration excess overland flow:

• heavy rainfall can exceed the infiltration capacity of the soil

• previous weather conditions affect infiltration capacity e.g. antecedent moisture

• infiltration excess overland flow then takes place

Condensation and clouds

• condensation occurs when air is cooled to its dew points

• the dew point is the temperature at which water vapour in the air turns into liquid water

• at this critical temperature, air becomes saturated with vapour resulting in condensation

• condensation nuclei, tiny particles of dust are required for condensation to occur, attract water vapour

• clouds form when moist air cools which can occur in a number of ways

• the first is by convection, forming cumulus clouds

• the second is by frontal, forming stratus and cirrus clouds

• the third is by orographic

water extraction

How it disturbs the water stores

• excessive pumping - lowers the water table so much that the wells no longer supply water so they can go dry and impacts the river flow

• abstraction - alters the natural flow of rivers by lowering groundwater levels which affects flows into springs, wetlands and rivers

• pollution - agricultural pesticides and fertilisers can percolate into groundwater

• salt water intrusion - common in coastal locations, salt-water seeps into pore spaces in aquifers and salty water cannot be used for crop growth or human use

• 1500 million people worldwide rely on aquifers for water supply

• groundwater extraction in the Kennet exceeded recharge, falling water table has reduced flows in river kennet by 10-14%

  • 2003 drought flows fell by 20%

  • lower groundwater levels have caused springs and seepages to dry up and reduced incidence of saturated overland flow on the chalk