Water Cycle

• Global water stores


  • Oceans
  • Glaciers, ice caps and ice sheets
  • Groundwater
  • Surface and other freshwater (permafrost, lakes, swamps, marshes, rivers and living organisms)
• Oceans contain 97% of global water
• Only 2.5% of global water stores are freshwater
• 69% of freshwater is in glaciers, ice caps and ice sheets
• 30% of freshwater is groundwater
• Surface and other freshwater only accounts for around 1% of global water stores
• Hydrology in Polar Regions


  • 85% of solar radiation is reflected
  • Permafrost creates impermeable surfaces
  • Lakes and rivers freeze
  • Rapid runoff in spring
  • Seasonal release of biogenic gases into atmosphere
  • Orographic and frontal precipitation
• Hydrology in Tropical Rainforests

  • Dense vegetation consuming 75% of precipitation
  • Limited infiltration
  • Deforestation leads to less evapotranspiration and precipitation
  • Very high temperatures
  • Very humid
  • Convectional rainfall
• Components of the global water cycle


  • Oceans
  • Icecaps
  • Groundwater
  • Rivers and Lakes
  • Soil moisture
  • Atmospheric Moisture

  
• Oceans have a residence time of 3600 years
• Icecaps have a residence time of 15,000 years
• Groundwater has a residence time of 10,000 years
• Rivers and Lakes have a residence time of 2 weeks to 10 years
• Soil moisture has a residence time of 2-50 weeks
• Atmospheric Moisture has a residence time of 10 days
• ITCZ (Inter-tropical Continental Zone)


  • The Earth consist of six cells of circulating air, which form the globe's climate control
  • Hadley Cell - Air rises at The Doldrums, travels upwards, then sinks as it meets the cooler air of the Ferrel Cell. At this meeting point, precipitation tends to occur. The air then travels southwards, heating up as it does. It will then have heated sufficiently to rise up at the Doldrums, commencing the cycle again.
  • Polar Cell - Cold air sinks near the Arctic Circle, cooling and condensing to form precipitation over northern latitudes. The air then travels southwards, heating until it meets warm air from the Ferrel Cell. The air then rises, causing dry conditions for the land beneath, and then travels northwards, cooling as it does.
  • Ferrel Cell - The middle cell of the ITCZ (tends to be at a mid-latitude location). The air circulation is determined by the Hadley and Polar cells either side, similar to a cog system.
• Drainage basin


  An open subsystem operating within the closed global hydrological cycle. It's defined as an area of land drained by a river and its tributaries with a boundary (known as the watershed), usually composing of hills and mountains.
• On a local scale, the water cycle is an open system (a system of processes of water inputs, outputs and throughputs); on a global scale, the water cycle a closed systems (a system that has no inputs or outputs, only throughputs).

  
• The water cycle contains flows/transfers, inputs, outputs and stores/components.

  
• Inputs to the Drainage Basin - Precipitation


  • Precipitation is caused by the cooling and condensation of water moisture in the atmosphere, forming clouds that release moisture in the form of rain, snow, hail, sleet, etc.
  • Seasonality - In some climates (such as monsoon and Mediterranean) there are strong seasonal patterns of rainfall. Therefore the time of year determines the precipitation input within the drainage basin
  • Variability - sudden or long term changes to the climate can happen, which would affect precipitation levels and so the drainage basin as a whole. Secular Variability - long term (for example as a result of climate change trends), Periodic Variability - annual, seasonal or monthly context, Stochastic Variability - random factors like localisation of thunderstorm
  • Latitude - The location of the drainage basin has a major impact on climate, and so the volume and type of precipitation falling. In most cases, the higher the latitude from the Equator, the colder the climate, and so snowfall occurs more often than rainfall. Also, at latitudes where air cells converge (ITCZ), the climate will be categorised by the rise or fall of air

  
• Types of Rainfall


  • Convectional - Often a daily occurance. The morning heat warms the ground, which in turn heats low-level moisture (from plant dew or surface stores). This moisture evaporates and rises. As the air rises, it cools and the moisture within will condense, to form rain and in turn (as more moisture accumulates) rainfall. In tropical climates, convectional rainfall is most common; within tropical rainforests, rainfall occurs mid-morning before the temperature rises too high for condensation to occur.
  • Frontal/Cyclonic - Where two air masses meet, a wedge can occur of hot air within cold air - this is called a depression. At the front (were the two air masses meet), warm moist air is forced to rise above the cold air mass, causing the water moisture within to cool and condense, to form cyclonic precipitation. Depressions are very common to the UK - approx 100 depressions hit the UK each year.
  • Relief/Orographic - When warm, moist air (often travelling onto land from sea) meets land of high relief (e.g. hills), the air mass is forced to rise above the hill to continue travelling. As it rises, the air mass cools and the moisture within condenses, to form clouds and rainfall. Orographic rainfall depends on the relief and location of the land immediate after the sea - many coastal landscapes are too cold, low lying or hills are set too far inland for relief rainfall to occur.
• Fluxes and Flows within the Drainage Basin


  • Interception - The direct intervention of plants' leaves in changing the direction or temporarily stopping precipitation as it falls to the surface. Any moisture retained by the surface of the leaf (interception store) is generally greatest at the start of storms. A plant's interception capacity varies depending on the type of vegetation.
  • Infiltration - The movement of water from the surface into the soil. The infiltration capacity is the maximum rate at which water can be absorbed by the soil, and can be affected by soil composition, previous precipitation, type and amount of vegetation, compaction of soils, and relief of land.
  • Surface Runoff - Water flows overland, rather than permeating deeper levels of the ground. Overland flow occurs faster where the gradient of land is greater. Surface runoff if the primary transfer of water to river channels, hence heavily influencing their discharge - Moderate/Fast
  • Throughflow - Water moves through the soil and into streams or rivers. Speed of flow is dependent on the type of soil. Clay soils with a high field capacity and smaller pore spaces have a slower flow rate. Sandy soils drain quickly because they have a lower field capacity, larger pore spaces and natural channels from animals such as worms. - Moderate/Fast
  • Percolation - Water moves from the ground or soil into porous rock or rock fractures (deeper bedrock and aquifers). The percolation rate is dependent on the fractures that may be present in the rock and the permeability of the rock - Slow
  • Groundwater Flow - The gradual transfer of water through porous rock, under the influence of gravity. Water can sometimes become trapped within these deeper layers of bedrock, creating aquifers and long water stores for the drainage basin - Slow
• Outputs of the Drainage Basin


  • Evaporation - The direct loss of water moisture from the surface of a body of water, the soil and interception storage (on top of leaves) to the atmosphere. Evaporation rates increase when the weather is warmer, windier and dryer. Other factors that influence evaporation rates include volume and surface area of the water body, vegetation cover or built environment surrounding the water, and the colour of the surface beneath the water.
  • Transpiration - A biological process where water is lost to the atmosphere through the pores of plants (stomata). Transpiration rates are affected by seasonality, the type of vegetation, moisture content of the air and the time of day (morning dew is the release of moisture through transpiration in temperate climates).
• Stores in the drainage basin


  • Soil Water - Water stored in the soil which is utilised by plants - Mid-term
  • Groundwater - Water that is stored in the pore spaces of rock - Long-term
  • River Channel - Water that is stored in a river - Short-term
  • Interception - Water intercepted by plants on their branches and leaves before reaching the ground - Short-term
  • Surface Storage - Water stored in puddles, ponds, lakes etc. - Variable
• Water table


  The upper level at which the pore spaces and fractures in the ground become saturated. It is used by researchers to assess drought conditions, health of wetland systems, success of forest restoration programmes etc.
• Physical factors influencing the drainage basin

  • Climate - influences amount of rainfall and vegetation growth
  • Soil Composition - influences rate of infiltration and throughflow
  • Geology - affects percolation and groundwater flow
  • Relief - steeper gradients of land will encourage faster rates of surface runoff
  • Vegetation - affects interception, overland flow
  • Size - larger basins collect more precipitation generally
• Anthropogenic factors influencing the drainage basin


  • Cloud seeding - substances dispersed into the air to provide something for condensation to occur on
  • Deforestation - Less vegetation means less interception, less infiltration, more overland flow leading to more flooding, cycle speeds up
  • Afforestation - More vegetation means interception, less overland flow, more evapotranspiration
  • Dam construction - Dams reduce downstream river flow and discharge, increase surface stores so more evaporation
  • Change in land use - Infiltration is 5 times faster under forests compared to grasslands. Converting land to farmland means less interception, increased soil compaction and more surface runoff
  • Ground water abstraction - When water is taken out faster that the water is recharged, groundwater flow decreases and the water table drops
• Example


  • Was used in China right before 2008 Beijing games to try and reduce pollution levels
• Deforestation


  • Less vegetation means less interception, less infiltration, more overland flow leading to more flooding, cycle speeds up
• Afforestation


  • More vegetation means interception, less overland flow, more evapotranspiration
• Dam construction


  • Dams reduce downstream river flow and discharge, increase surface stores so more evaporation
• Example


  • Lake Nasser behind Aswan dam in Egypt – 10-16 billion m3 water loss from Nile
• Change in land use


  • Infiltration is 5 times faster under forests compared to grasslands. Converting land to farmland means less interception, increased soil compaction and more surface runoff
• Ground water abstraction


  • When water is taken out faster that the water is recharged, groundwater flow decreases and the water table drops
• Example


  • In China, groundwater irrigates 40% of farmland whilst 70% of drinking water comes from groundwater
• Irrigation


  • Drop in water tables due to high water usage
• Example


  • Aral Sea in Kazakhstan shrank in 1960s due to farmers using the water to grow cotton
• Urbanisation


  • Impermeable surfaces reduce infiltration, increase surface runoff, river discharge increase. Cycle speeds up
• Water Budget


  Precipitation = Discharge + Evaporation ± changes in stores
• Precipitation is greater than evaporation


  Soil moisture increases creating a soil moisture surplus