the water cycle

the percentage of water stored globally

• oceans - 97%
• polar ice and glaciers - 2%
• groundwater and aquifers - 0.7%
• lakes - 0.1%
• soils - 0.005%
• atmosphere - 0.001%
• rivers - 0.0001%
• biosphere - 0.00004

fresh water comprises only a tiny amount of the world's water; three quarters of which is stored in ice caps and glaciers, and a fifth in permeable rocks

• inputs to atmosphere - evaporated water from oceans, lakes, soils, rivers and vapour transpired from plant leaves. known as evapotranspiration
• outputs from atmosphere - precipitation (rain, snow) and condensation (fog)
• ice sheets, glaciers and snowfields release water by ablation (melting) and sublimation (ice skips liquid and turns into water vapour)
• inputs to rivers - precipitation and meltwater through runoff. most rivers flow to oceans, and some to inland basins. precipitation often reaches rivers from soil infiltration and flowing.
• after infiltration, water may percolate permeable rocks or join aquifers

the water balance 

• water balance equation summarises the flows of water in a drainage basin over time. 
• precipitation is equal to evapotranspiration and streamflow, plus or minus the water entering or leaving storage
  • precipitation (p) = evapotranspiration (e) + streamflow = storage

flows: precipitation 

• water and ice that falls from clouds - rain, snow, hail, sleet, drizzle
  
• forms when water vapour in the atmosphere cools to its dew point and condenses to form clouds
• when they become too heavy, clouds sink and release water
• most rain flows into streams and rivers, whereas snow may fall in places of high latitude and may remain on ground for several months - time lag between snowfall and runoff
• intensity is the amount of precipitation falling in a given time
• duration is the amount of time the precipitation event lasts for
• in some places, precipitation concentrated to a rainy season - river discharge higher, flooding common. in dry season, rivers may cease to flow altogether

transpiration

• the diffusion of water vapour to the atmosphere from the leaf pores
• responsible for 10% of moisture in atmosphere
• influenced by temperature, windspeed, and water availibility to plants

condensation

• change from water vapour to liquid water
• occurs when air is cooled to its dew point
• air becomes saturated with vapour resulting in condensation
• clouds form through condensation in the atmospherenear ground, condensation causes fog and dew.
  • cumuliform - flat based, form when air is heated locally through contact with the earth's surface. heated air rises through atmosphere (convection), expands and cools. as it reaches dew point, clouds from
  • stratisform - develop when an air mass moves horizontally across a cooler surface (often the ocean). this is known as advection
  • cirrus - whispy clouds; form at high altitudes and consist of tiny ice crystals. do not produce precipitation
• deposits large amounts of moisture on vegetation and other surfaces

cloud formation

• cooling of water vapour to form clouds occurs when:
  • air warmed by contact with ground or sea surface, rises freely through atmosphere. as air rises, pressure falls, cools by expansion - adiabatic expansion. vertical movement of air known as convection
  • air masses move horizontally across cooler surfaces - known as advection
  • warm air mass mixes with a cool one
  • air masses forced to rise when they cross physical barriers

lapse rates

• lapse rate - vertical distibution of temperature in the lower atmosphere, and temperature changes of an air parcel as it rises vertically away from ground
• 3 types of lapse ratewhen air is warmer than its surroundings, it is less dense and therefore buoyant. called atmospheric instability - air rises freely as convection current 
  • environmental lapse rate: vertical temperature profile of lower atmosphere at any given time. average - the temperature falls by 6.5 degrees for every km of height gained
  • dry adiabatic lapse rate: rate the parcel of dry air (condensation free) cools. approximately 10 degree drop per km
  • saturated adiabatic lapse rate: rate a saturated parcel of air (condensation full) rises through atmosphere. around 7 degrees per km due to latent heat released
• when it reaches dew point (8 degrees) condensation occurs and clouds form 
• cloud continues to rise so long as internal temp is higher than surrounding atmosphere
• equilibrium attained at 4000 m, temp -15
• air cannot rise in this zone as it is cooler and therefore heavier than its surroundings  

evaporation

• phase change of liquid water to vapour 
• main passage of water into the atmosphere 
• heat needed to break molecular bonds of water 
• energy input does not produce rise in water temperature - energy absorbed as latent heat and released later as condensation 

interception

• vegetation intercepts a proportion of precipitation and stores it temporarily on branches, leaves and stems 
• eventually moisture either evaporates or falls to the ground 
• rainwater briefly intercepted before dripping to floor known as throughfall 
• in intense rainfall, water may run along edges of branches or stems, known as stemflow 
• factors affecting interception loss 
  • interception storage capacity - when veg is dry, ability to retain water is maximum. as vegetation becomes saturated, output of water through stemfall and throughflow increased. interception is dependent on duration and intensity of rainfall event 
  • wind speed - evaporation increases with windspeed, as well as turbulence, increasing throughfall 
  • vegetation type - interception losses greater from grasses than agricultural crops. trees with larger surface area and roughness have higher interception losses than grasses 
  • tree species - interception losses greater in narrow leafed trees like pine compared to broads leafed trees such as oak 

infiltration, throughflow, groundwater flow and runoff

• precipitation not entering a body of water follows one of 2 paths when soils or other ground materials reach max water capacity, overland flows occurs
  • infiltration into the soil, and from there movement or throughflow into streams and rivers
  • overland flow across ground surfaces either as a sheet or a trickle into streams and rivers
• when soils underlay rock, water percolates and flows deep into underground water stores such as aquifers. 
  • this water may emerge in the form of a spring 

cryospheric processes

• ablation - loss of ice from ice sheets, glaciers, snow due to melting, evaporation and sublimation 
• meltwater important component of river flow in high latitudes in spring and summer 
• rapid melting often causes flooding