Hydrological Cycle - Water Deficits (Droughts)

  • Droughts are deficits in the hydrological cycle
    • They are a prolonged period of abnormally low rainfall, leading to a storage of water
  • Meteorological droughts happen when dry weather patterns dominate an area
  • Hydrological droughts occur when low water supply becomes evident, especially in streams, reservoirs and groundwater levels usually months after a meteorological drought.
  • Agricultural droughts happen when crops become affected
  • But the precipitation level is not abnormally lower than normal in deserts a drought is not happening.
Pressure:
  • High pressure = sinking air = colder dry air = clouds do not form as there is no condensation = no precipitation
  • Low pressure = warmer air = rising air = clouds forming = precipitation.
Causes Of Droughts:
  • Mainly occur due to less precipitation over a given amount of time, which is a meteorological cause.
    • Metrology is the study of weather
      • Can be a short term deficit e.g hot dry summer for a few weeks in the UK. Caused by the blocking ‘anti-cyclone’ high pressure systems that sits over the UK.
  • Places in Asia like India have dry seasons until the rain comes (Monsoon season) if the rain is delayed or smaller than normal, a drought results.
  • Droughts will be more severe if this happens over a longer time.
ENSO Causes Of Droughts:
  • El Nino Southern Oscillation
    • Occurs from a reversal in the Walker circulation (Trade winds reverse)
    • ENSO is a cycle that occurs naturally every 3 – 7 years
    • Lasts several months to a year
  • Disrupts normal climates e.g. areas that usually have rain become dry for several months
  • Essentially the Pacific Ocean warmest water moves from West to the East
    • This creates low pressure rising moist air over East Pacific, and high-pressure drier sinking air of West Pacific
      • This creates heavy rains in Peru (unusual)
  • Droughts occur in S.E.Asia e.g. Philippines, Indonesia, Australia as higher pressure, drier air and less rain
Human Contributions To Droughts:
  • Around the world river systems are over-exploited.
  • Groundwater aquifers are also unsustainably used in many places.
  • Rivers have been used for too much agriculture (irrigation) so the channel flow is lower or completely exploited so nothing left.
    • This has happened around Lake Chad.
    • Also happened along the Colorado River (USA/Mexico), so river channel dries up before reaching the Pacific Ocean in Mexico, and rivers in Australia.
  • Growing food demand, water needs, higher population and agriculture, has meant groundwater is increasingly pumped out of aquifers in the ground.
    • This lowers the water table (saturated rock zone) and can lead to ground sinking and loss of surface water stores and flows like lakes and rivers.
Case Study 1: The Sahel Region:
  • The land south of the Sahara Desert is largely LDCs, dry land with seasonal rains.
  • Subsistence farmers depend on rains to fill lakes, rivers and aquifers.
  • Droughts are common due to unreliable seasonal rainfall.
    • This causes meteorological droughts.
    • Also, humans have over-exploited river water and groundwater. This is unsustainable and has caused hydrological droughts.
Case Study 2: Lake Chad:
  • Lake Chad has dramatically declined in size
  • Evaporation and drainage to the ground exceeds input from rivers and direct rainfall into the lake
    • Humans largely to blame due to over-extraction of water from the lake and rivers feeding it
  • Also local groundwater pumping as water table lowered
  • Meteorological droughts in recent years increased problem
  • Future is bleak – climate change and overpopulation will increase depletion of remaining water
  • Overgrazing reduces vegetation (too many animals eating plants) and therefore this reduces evapotranspiration and so convectional rain decreases even more
  • ‘Desertification’ can be a result
Impacts Of Droughts:
Forest Stress:
  • Trees cannot reproduce effectively if water levels too low as don’t produce fruit or seeds
    • Trees and plants might die if bad enough
  • Large areas could be destroyed by forest fires causing long term damage to the ecosystems
    • Droughts reduce the resilience of the forest against disease or to recover from fires, causing ecosystems to change to grasslands
Wetland Impacts From Droughts – Functioning & Resilience:
  • Wetlands rely on river input and also precipitation in the area.
    • Droughts therefore can cause wetland lakes and soil moisture to dry up.
  • The functioning of an ecosystem is basically its characteristics: what living organisms are there (ecology), how do they interact and what is the food web like.
    • The resilience of an ecosystem is how well it can withstand pressures, and how fast it can recover.
  • Droughts damage functioning of wetlands as plants and animals need the water to survive.
    • If lakes dry up, fish die, and birds have less to eat.
      • Also animals like crocodiles that require water to hunt for food can also die.
      • The functioning of the ecosystem changes.
    • Droughts therefore mean wetland resilience is lower, as the ecosystem is more at risk from permanent damage if too many animals die or fires destroy the area.
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