Water- PMT notes

• Global water stores


  • Oceans (96.9%)
  • Icecaps (1.9%)
  • Groundwater (1.1%)
  • Rivers and Lakes (0.01%)
  • Soil moisture (0.01%)
  • Atmospheric Moisture (0.001%)

  
• 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
• Cells of circulating air in the ITCZ

  • Hadley Cell
  • Polar Cell
  • Ferrel Cell
• Drainage basin


  An open subsystem operating within the closed global hydrological cycle, defined as an area of land drained by a river and its tributaries with a boundary (watershed) usually composing of hills and mountains
• On a local scale, the water cycle is an open system; on a global scale, the water cycle is a closed system
• Inputs to the Drainage Basin

  • Precipitation
• Factors affecting precipitation


  • Seasonality
  • Variability (secular, periodic, stochastic)
  • Latitude
• Types of Rainfall


  • Convectional
  • Frontal/Cyclonic
  • Relief/Orographic
• Fluxes and Flows within the Drainage Basin

  • Interception
  • Infiltration
  • Surface Runoff
  • Throughflow
  • Percolation
  • Groundwater Flow

  
• Interception


  The direct intervention of plants' leaves in changing the direction or temporarily stopping precipitation as it falls to the surface
• Infiltration


  The movement of water from the surface into the soil
• Factors affecting infiltration


  • Soil Composition
  • Previous precipitation
  • Type and amount of vegetation
  • Compaction of soils
  • Relief of land
• Surface Runoff


  Water flows overland, rather than permeating deeper levels of the ground

  
• Throughflow


  Water moves through the soil and into streams or rivers

  
• Percolation


  Water moves from the ground or soil into porous rock or rock fractures (deeper bedrock and aquifers)
• Groundwater Flow


  The gradual transfer of water through porous rock, under the influence of gravity
• Outputs of the Drainage Basin

  • Evaporation
  • Transpiration

  
• Factors affecting evaporation


  • Volume and surface area of the water body
  • Vegetation cover or built environment surrounding the water
  • The colour of the surface beneath the water
• Stores in the Drainage Basin

  • Soil Water
  • Groundwater
  • River Channel
  • Interception
  • Surface Storage
• Water table


  The upper level at which the pore spaces and fractures in the ground become saturated
• Physical Factors influencing the Drainage Basin

  • Climate
  • Soil Composition
  • Geology
  • Relief
  • Vegetation
  • Size
• Anthropogenic Factors influencing the Drainage Basin

  • Cloud seeding
  • Deforestation
  • Afforestation
  • Dam construction
  • Change in land use
  • Ground water abstraction
• to occur on in 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
• 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
• Irrigation


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


  • Impermeable surfaces reduce infiltration, increase surface runoff, river discharge increase. Cycle speeds up
• Precipitation = Discharge + Evaporation ± changes in stores
• The Water Budget


  1. A. Precipitation is greater than evaporation, therefore the soil's moisture increases creating a soil moisture surplus

  2. B. As temperature increases, the rate of evaporation will increase. The soil uses the moisture gained during its surplus, through soil moisture utilisation

  3. C. The point of maximum evaporation. This is the point of highest risk of drought

  4. D. Despite evaporation decreasing, there isn't enough precipitation to provide for vegetation use of water loss, therefore there is a soil moisture deficit

  5. E & F. As precipitation exceeds evaporation rates, the soil will regain moisture and reduce its deficit, through soil moisture recharge
• Factors affecting the characteristic of a river's regime

  • Channel capacity of the river
  • Area and relief of the drainage basin
  • Volume, pattern and intensity of precipitation
  • Climate
  • Geology of the soil (affecting the input of groundwater)
  • Anthropogenic (human) activities, such as building dams or terracing the land
• Factors influencing Flashy vs Subdued Storm Hydrographs

  • Weather/Climate: Intense storm which exceeds infiltration capacity of soil vs Steady rainfall which is less than the infiltration capacity of soil
  • Rock type: Impermeable rocks like granite which encourage rapid surface runoff vs Permeable rocks like limestone which allow for infiltration and reduce runoff
  • Soil: Low infiltration rate vs High infiltration rate
  • Relief: High and steep slopes → More runoff vs Low and gentle slopes → Less runoff
  • Basin size: Usually small basin vs Usually large basin
  • Vegetation: Low density vegetation, less interception and more rapid movement of water vs High density vegetation, more interception, more evapotranspiration
  • Pre-existing conditions (Antecedent conditions): Basin already wet from previous rainfall vs Basin dry, Low water table vs High water table, Soil saturated, less infiltration vs Soil unsaturated, more infiltration
  • Human activity: Urbanisation, Deforestation, Channelisation, Dams vs Afforestation
• Management of Drainage Basins

  1. Growing vegetation on roofs, to increase interception and temporarily stores some water within plants

  2. Create permeable pavements (gaps within paving blocks) to increase infiltration and reduce surface runoff

  3. Rainwater Harvesting – collecting rainwater to use as domestic greywater

  4. Creating wetlands (areas with marsh and wetland vegetation) that will act as natural sponges and increase temporary water storage

  5. Deforestation, tree felling and slash-and-burn - Soil becomes exposed and roots are lost (which bind the soil together), which leads to more soil erosion and so more surface runoff

  6. Impermeable surfaces – As more tarmac and concrete is laid, less infiltration into the topsoil can occur, and so more surface runoff occurs

  7. Bridges can act as dams for rivers, restricting channel flow (especially storm flow) and increasing the pressure the river's water is under, therefore worsening flood impacts

  8. Drainage and sewage systems will reduce lag time, and so a quick flow of water back to rivers, increasing the risk of flash flooding
• Deficit in the Hydrological Cycle

  An imbalance in inputs and outputs of water can have serious implications for the hydrological cycle. A deficit (more commonly known as a drought) refers to when input is less than output. This deficit can be caused by natural and/or human factors.
• Types of droughts and their characteristics

  • Rainfall deficit: Loss of soil moisture, Irrigation supply drops, Reduction in water available for consumption
  • Stream flow deficit: Reduced storage in lakes and reservoirs, Less water for urban supply, Poorer water quality, Threats to wetlands and habitats
  • Soil moisture deficit: Poor yields from rainfed crops, Failing irrigation systems, Livestock productivity falls, Rural industries affected, Government aid may be required
  • Food deficit: Widespread failure of agricultural systems, Food shortages, Rural economy collapses, Rural to urban migration, International aid required, Humanitarian crisis, Loss of vegetation, Increased risk of wildfires, Soil erosion, Desertification
• El Nino Southern Oscillation

  El Nino is the change in water body patterns within the Southern hemisphere, leading to unusual weather conditions. The causes of El Nino aren't fully understood.