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

  1. Most often form when air is heated locally through contact with the E's surface
  1. = heated air particles rise freely thru the atmosphere (convection), expand (due to fall in pressure with altitude) and cool
  1. 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