WATER CYCLE
Supporting Life
Key to understand evolution of life
Provides medium that allows organic molecules to mix and form more complex structures
Ubiquity of liquid water on E is due to distance from the sun
Helps create benign thermal conditions in Earth
Oceans moderate temperatures by absorbing, storing and slowly releasing heat
Water helps moderate environment via:
Clouds
Made up of water particles and ice crystals
Reflect 1/5 of incoming solar radiation and lowers surface temperatures
Water vapour
Potent greenhouse has
Absorbs long wl radiation - helps maintain global temps. almost 15 degrees higher than they would be otherwise
Uses
Makes up 65-95% of all living organisms
Crucial to their growth, reproduction and other metabolic functions
Is the medium for all chemical reactions in the body
Plants
Photosynthesis
Occurs in leavs
Combines CO2, sunlight, and water
Makes glucose and starches
Respiration
Converts glucose into energy thru its reaction with 02
Releases CO2 + H20
Maintains rigidity in plants
Transports mineral nutrients form the soil
Transpiration
Transp. from leaf surfaces cools plant via evapotransp.
Economic Activity
Essential resource for economic activity
Generates electricity
Irrigates crops
Provides recreational facilities
Satisfies public demands
Drinking water
Sewage disposal
Industry demands
Food manufacturing
Brewing
Paper making
Steel making
Global Water Cycle
Reservoirs and Stores
No. of reservoirs where water is stored for varying time periods
Size list
Oceans 97%
Store 97% of all global water
Polar ice + glaciers 2%
Freshwater comprises a small proportion
75% of freshwater is stored in icecaps of Greenland and Antarctica
Groundwater 0.7%
Water stored below ground in permeable rocks amounts to 1/5 of all freshwater
Lakes 0.01%
Soils 0.05%
Atmosphere 0.001%
Is a small store but plays an important role
Rapid flux of water into and out of the atmosphere
Average residence time of a water molecule in the atmosphere is 9 days
Rivers 0.0001%
Biosphere 0.00004%
Inputs + Outputs
Global water cycle budget circulates around 505,000 km3 of water/year as inputs and outputs between the principal stores
Inputs to atmosphere
Water vapour evaporated from oceans, soils, lakes and rivers
Vapour transpired from plant leaves
Together these processes = evapotranspiration
Moisture leaves the atmosphere as prceip. and condensation (fog)
Ice sheets, glaciers and snowfields release water by ablation (melting and sublimation)
Precip. and meltwater drain from the land surface as runoff into rivers
Most rivers flow to the oceans; some in the continental drylands, drain in inland basins
A large amount of precip. on land reaches rivers thru infiltration and through flow
After infiltrating and flowing through the soil, water under gravity may percolate into permeable rocks/aquifers
This groundwater eventually reaches the surface as springs or contributes to runoff
Processes
Water Balance
Water balance equation summarises the flows of water in a drainage basin over time
P = E + Q +/- Storage
P = precipitation
E = evapotransp.
Q = streamflow
P is equal to E and Q, plus or minus water entering or leaving storage
Flows
Principal flows that link the various stores are
Precipitation
Evaporation
Transpiration
Runoff
Infiltration
Percolation
Throughflow
Precipitation
Precip. varies in character and this impacts the water cycle at the drainage basin scale
Most rain fows quickly into streams and rivers
In high lattitudes and mountanious catchments, precip. often falls as snow and may remain on the ground for several months = considerable time lag between snowfall and runoff
Intensity is the amount of precip. falling in a given time
High intensity (10-15 mm/hr) moves rapidly overland into streams and rivers
Prolonged precip. events, linked to depressions and frontal systems = large amounts of precip. + river flooding
During the rainy season, river discharge is high and flooding is common where precip. is concentrated in that season
Formation
Clouds form when water vapour cools to its dew point
Dew point is the critical temperature when air becomes saturated (100% relative humidity) and can hold no more vaopur
Dew point = condensation bc excess vapour changes state to form liquid droplets
Aggregated, these droplets form clodus
Rain develops thru the process of droplets coalescing or ice crystals growing within clouds
Rain, snow, hail, sleet, drizzle
Transpiration
Is the diffusion of water vapour to the atmosphere from leaf pores (stomata)
Is responsible for 10% of moisture in the atmosphere
Influenced by temperature, wind speed, water availability to plants
Deciduous trees shed leaves in climates with either dry or cold seasons to reduce moisture thru transp.
Condensation
Is the phase change of vapour to liquid water
Occurs when air is cooled to its dew point
At the dew point where it reaches its critical temperature, air becomes saturated with vapour = condensation
Clouds form through condensation in the atmosphere
Cumuliform clouds
Flat bases and considerable vertical development
- Most often form when air is heated locally through contact with the E's surface
- = heated air particles rise freely thru the atmosphere (convection), expand (due to fall in pressure with altitude) and cool
- When cooling reaches the dew point, condensation begins and clouds form
Stratiform clouds
Air mass moves horizontally across a cooler surface (often ocean)
This process, mixing and turbulence is ADVECTION
Layered
Cirrus clouds
Form at higyh altitude
Consist of tin ice crystals
Have little influence on water cycle bc they do not produce precipitation
Wispy
Condensationa t/nrea ground = dew/fog
Deposits large amounts of moisture on veg. + other surfaces
Cloud Formation
Clouds are visible aggregates of water and/or ice that float in the free air
Clouds form when vapour is cooled to its dew point. Cooling occurs when:
Air warmed by contact w ground/sea, rises and so pressure falls = it cools by expansion ADIABATIC EXPANSION
The vertical movement of air is CONVECTION
Air masses move horizontally across as relative cooler surface - ADVECTION
A relatively warm air mixes with a cooler one
Lapse rates (LR)
Vertical distribution of temperature in the lower atmosphere
The temperature changes that occur within an air parcel as it rises vertically away form the ground
Three types
Environmental LR
Is the vertical temp. profile of the lower atmosphere at any given time
Temp. falls by 6.5 degrees for every km of height gained
Dry Adiabatic LR
Rate of which a parcel of dry air cools
Cooling by adiabatic expansion is 10 degrees/km
Saturated Adiabatic LR
Rate at which a saturated parcel of air cools as it rises thru the atmospehre
Bc condensation releases latent heat, the SALR is lower than the DALR, at 7 degrees/km
Saturated parcels - condensation is occurring
Dry parcel - under 100% humidity so condensation is not occurring
Their interaction explains the formation of clouds
Clouds formed by convection
Ground heated by Sun warms the air in contact with the surface to 18 degrees
The air is less dense and more bouyant bc it is warmer than its surroundings (13 degrees)
This atmospheric instability = air rising freely in a convection current
When its internal temp. reaches dew point (8 degrees), condensation occurrs adn clouds form
The air continues to rises so long as its internal temperature is higher than the surrounding atmosphere
Catchment Hydrology
Evaporation
Phase change of liquid to vapour
Main pathway for water to enter atmosphere
The energy input does not increase temp.
Heat brings about evaporation and breaks molecular bonds
Energy is absorbed ass latent heat and is released later in condensation
This process allows huge quantities of heat to be transferred globally
Interception
Vegetation intercepts a proportion of precipitation, storing it temporarily on branches, leave and stems
The moisture either evaporates (interception loss) or falls to ground
Rainwater briefly intercepted before reaching ground is throughfall
Prolonged/intense rainfall = intercepted rainwater to flow to ground along stems + branches (stemflow)
Infiltration, Thruflow, Groundwater flow, Runoff
Rain reaching ground and not entering storage follows one of the 2 flowpaths to streams + rivers
Infiltration
By gravity into the soil and lateral movement or throughflow to stream and river channels
Overland flow
Across ground surface, either as a sheet or trickles and rivulets to stream and river channels
When intensity exceeds infil. capacity
Only occurs when soil becomes saturated and the water table rises to the surface - saturated overland flow
Groundwater flow
Water percolates deep underground when soil is underlain by permeable rocks
The water migrates slowly thru the pores and joints as groundwater flow
Emerges at surface as springs + seepages
Groundwater levels on the chalk in S England has a distinct seasonal pattern
Late Oct. - water tables beginning to rise as temps. + evapotransp. fall
This recharge continues until late Jan.
Groundwater levels then decline thru late winter, spring + summer (lowest point in early autumn)
Factors of Interception Loss
Interception storage capacity
Before rainfall, veg. surfaces are dry and unsaturated. As rainfall continues, surfaces become saturated = increased stemflow and throughfall
Wind speed
Evapo. rates increase w. wind speed. Turbulence also increases = additional thrufall
Vegetation Type
Greater interception loss from grasses than agricultural crops. Trees have higher interception losses bc of surface area + aerodynamic roughness
Tree Species
Greater interception losses from evergreens than broad-leaved deciduous trees
Most conifers have leaves year-round + water adheres to spaces between conifer needles = increased evap.
Cyrospheric Processes
Ablation - loss of ice from snow, ice sheets and glaciers due to melting, evaporation and sublimation
Meltwater is imp. to river flow in high latitudes and mountain catchments