Water cycle - geography

• Drainage basin


  Area of land surrounding a river, from which the river receives water and subsequently drains it
• Stores of water


  • Groundwater storage
  • Groundwater
  • Soil water
  • Surface water
• Flows of water


  • Stem flow
  • Interception
  • Surface runoff
  • Throughflow
  • Infiltration
  • Percolation
  • Groundwater flow
• Outputs of water


  • Transpiration
  • Evaporation
• Inputs of water


  • Precipitation
• Evaporation


  1. Solar radiation causes surface water to change state from a liquid to a gas

  2. Evaporation rate depends on solar energy amount, availability of water, air humidity and air temperature

  3. Plants lose water through transpiration

  4. Evaporation uses latent heat so it cools the surroundings
• Condensation


  1. Cooler air holds less water vapour

  2. If cooled sufficiently it becomes saturated

  3. Excess water vapour converts to liquid

  4. Water molecules condense onto tiny particles or surfaces below dew point temperature

  5. It is the direct cause of precipitation, taking place when air temp is reduced below dew point but volume remains constant
• Sublimation


  Conversion between solid and gas phases with no intermediate liquid stage
• Approximately 97% of water in the hydrosphere is oceanic and the remaining 3% is fresh water locked up in land ice, glaciers, permafrost, groundwater, lakes, soil, wetland, rivers, biomass and atmospheric water
• Oceanic water


  • Dominates water stores covering 72% of Earth's surface
  • Contains salts, allowing it to stay a liquid below 0 degrees
  • Is alkaline but there is a constant PH fall, linked to the increase in atmospheric carbon and will influence marine ecosystems
• Types of cryosphere


  • Sea ice
  • Ice caps
  • Ice sheets
  • Alpine glaciers
  • Permafrost
• Types of terrestrial water

  • Surface water
  • Wetlands
  • Groundwater
  • Soil water
  • Biological water
• Water movement to a river

  1. When rain intensity is larger than the infiltration rate, soil reaches saturation point faster

  2. When this builds on the surface it can become stores

  3. Most surface water evaporates into the atmosphere

  4. When stores are full, overland flow begins on slopes

  5. Lateral movement of water (throughflow) is much slower than overland flow but water will still reach a channel

  6. Water moves vertically down through soil and unsaturated rock by percolation then is held in pore spaces of rock as groundwater

  7. It passes slowly to zones of saturation and moves vertically and laterally by groundwater flow

  8. It feeds the river through drought periods, aquifers can store a lot of water due to being porous
• Water budget


  • Difference between inputs and outputs in the drainage basin
  • If precipitation exceeds runoff and evaporation, there will be a positive water balance
• Water balance equation


  Precipitation = runoff + evapotranspiration
• Soil moisture budget


  • The change in amount of water stored in the soil throughout the year
  • Main factors that affect it are precipitation and evaporation
  • Potential evapotranspiration is the amount of evapotranspiration that could occur if there was sufficient water available in the summer
  • Increased temperature means more evapotranspiration
• Soil moisture states


  • Soil moisture recharge
  • Soil moisture utilisation
  • Soil moisture deficit
  • Soil moisture surplus
• Runoff


  • The amount of water in a river
  • Varies in rivers and in one river throughout the year
  • Runoff at a given time uses river discharge
• Discharge


  Cross section area (m2) x by speed (m/s)
• Knowledge of discharge is important in assessing and managing water resources, design of water structures and flood warning schemes. It helps to develop hydroelectric power and protect both ecological health of water courses and wetlands and their amenity/recreational value. Climate change will unevenly impact river discharge patterns
• River regime


  • Variability is a response to precipitation, temperature, evaporation and drainage basin characteristics
  • Precipitation is the only input of a river so its density, duration and total amount can impact the river regime
  • Increased temperature leads to more evapotranspiration therefore water won't make it to the river, low temps lead to soil saturation
  • Gradient, geology, human use and vegetation all affects how fast water reaches a river
• Storm hydrograph


  A graph showing the discharge of a river when normal flow is affected by a storm event
• Components of a storm hydrograph

  • Bankfull
  • Peak Discharge
  • Storm flow
  • Base flow
  • Lag time
  • Discharge
• Factors affecting the storm hydrograph

  • Drainage basin shape
  • Steeper sides
  • High drainage density
  • Saturated basin
  • Impermeable rock
  • Vegetation
  • Amount and intensity of rainfall
• Deforestation


  1. After vegetation is removed, new growth has less leaves and shallower roots, therefore more water runs off the land and stem flow increases

  2. Larger effects are had if 50-100% of the forest is deforested

  3. Rivers of all sizes will be affected if deforestation is extensive enough

  4. Changes won't be restricted to the catchment of deforestation, it's spread unevenly throughout the basin by atmospheric circulation
• Soil drainage


  • Subsurface drainage removes water from the soil profile
  • It helps impermeable rock become more productive
• Advantages of soil drainage

  • Improved aeration means that microorganisms thrive, increasing the rate of organic matter breakdown and supply's air for root cell respiration
  • Improved soil structure makes roots penetrate deeper and faster
  • Better aeration can allow for better germination
  • Rescues the risk of compacted soils from machinery
• Disadvantages of soil drainage

  • Artificial drainage increases throughflow which increases the speed at which water will reach the water course as well as increasing the range of flow in rivers
  • Dry topsoil can be eroded by the wind if not protected properly
  • Nitrate loss leads to eutrophication
• Water abstraction


  • Problems occur when demand for water exceeds the amount of water available
  • Groundwater is important because it is the dominant water source in manyEuropean countries and is used for irrigation systems
  • The main reason for over-abstraction of water is irrigation in agricultural areas
• Over abstraction and dry winters can cause chalk rivers to dry up completely, having economic impacts including no fishing
• GARDIT was developed to control water levels in the London basin, largely achieving its aim by 2000