- 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.