EQ2- What factors influence the hydrological system over short and long-term timescales?

Drought

An extended period of deficient rainfall relative to the statistical multi-year average for a region.

-short term precipitation deficit due to high air pressure; cool sinking air can’t form clouds + produce precipitation.

2 types:

Meteorological: Long term precipitation is lower than normal.

Hydrological: Lack of precipitation impacts water supply: reduced stream flow, lowered groundwater levels + reduced water stores.

ENSO cycles: El Niño + La Niña

Change in weather patterns in the Pacific ocean.

NORMAL YEAR

• Trade winds + water travels from east→west, creating warm water pooling at A.

• EAST (South America): High atmospheric pressure causes cool dry air to sink, causing low rainfall.

• WEST (Australia): Low atmospheric pressure causes warm moist air to rise, creating high rainfall.

• warm water travels to allow cold water upwelling at SA.

EL NIÑO

• Trade winds reverse, causing water to reverse which now travels from west→east. Warm water pooling at SA.

• S. America: Low pressure causes flooding.

• Australia: High pressure causes droughts.

LA NIÑA

• Trade winds strengthen + more water travels east→west, creating excess warm water pooling at A.

• S. America: High pressure causes droughts.

• Australia: Low pressure causes flooding.

Causes + impacts of human activity on droughts: Australia- The Big dry 1997-2009

CAUSES

Increased population on east coast Sydney, Brisbane, put pressure on water-60% of pop live there.

Water extraction from MDR: Adelaide get 40% drinking water from here- high water consumption per capita.

Excessive irrigation of the Murray Darling Basin.

IMPACTS

50% farmland affected, decreasing food supplies. 23% increase in food prices.

Reservoirs fell to 40% full

High lack of water from the river Murray- 30% affected

Impact of drought on ecosystems (wetlands + forests)

WETLANDS

An area which is either permanently or seasonally saturated with water.

-support carbon cycle, prevent erosion, support a food web for animals, retain+recycle nutrients which maintains water quality.

Impacts of droughts

Areas of water dry up/ shrink, causing habitats to be damaged or lost.

Soil moisture drops, causing soil erosion to occur as more land is exposed.

Food webs can change as certain species are more/ less resilient to droughts e.g. ducks don’t survive well

FORESTS

Functions include timber, regulation of hydrological processes, providing habitats, food chains.

Impact of droughts

Trees can take 2-4 years to recover from droughts, some species are less resilient

Trees don’t grow properly and can be more susceptible to pests+diseases.

Meteorological causes of flooding + examples

Intense storms: Leads to flash floods (occur very quickly without warning). Not much chance for infiltration, therefore rapid runoff + short lag time. e.g. 2013 storms in Sardina, Italy caused $1.14bn in damage.

Unusually heavy or prolonged rainfall: depressions around mid-latitude e.g. UK form from low pressure. Bring showers on warm front + rain on cold front. Rain is prolonged→ground becomes saturated→more runoff into river.

Monsoons: A seasonal change in the direction of prevailing winds in a region. Brings heavy rainfall. e.g. 2015 Myanmar Monsoon led to flooding + 103 deaths

Rapid snowmelt: In high altitudes, snowmelt adds to river flow. Occurs due to lower regions remaining frozen, reducing infiltration rate. e.g. 2013 Norfolk sudden change to weather caused localised floods

Human activities that exacerbate (worsen) flooding

Urbanisation: Impermeable material (roads, pavements) prevents infiltration + increases surface runoff which decreases lag time. Causes river to overflow + flood surrounding land.

Agriculture+deforestation

River mismanagement

• channelisation is when the river is widened/ deepened to increase discharge of river and straightening it by cutting off meanders. However, this causes more rapid flow of water + overwhelms unchanneled river downstream.

• river embankments are built beside rivers to reduce flood risk. However, these may fail if floods exceed their capacity.

Impacts of flooding (reference to Cumbria floods, 2015)

Environmental

Polluted floodwater: With heavy metals, chemicals, etc. poison wildlife living in soil, farm animals and can degrade aquatic habitats.

Increase connectivity: Floods move sediment + nutrients around landscape + into marine environment, triggering breeding, migration + dispersal and connecting aquatic habitats.

Socio-economic

Economic activity: business owners lose productivity + income when workers can’t travel to work. Damaged shops may lose their stock. Local economy lost £200M in Cumbria.

Infrastructure: Roads, railways, bridges damaged + unsafe to use. Affects businesses, tourism, etc. 100 bridges damaged in Cumbria, impacting community. Road + rail travel were suspended during floods due to network damage.

Settlements: Homes flooded + without power as power stations become flooded. 5000 homes flooded within 3 days of rainfall in Cumbria.

Effect of climate change on hydrological cycle

Trends in precipitation

Likelihood of tropical storms increase: Due to sea level rise + temp rise. Higher rainfall in tropical regions near coastlines at flood risk.

Can also reduce precipitation in regions close to the poles due to high pressure.

Trends in evaporation

Rising temp in some regions lead to more evaporation (+vice versa).

If temps decrease to below freezing, water remains frozen + stored as ice.

Impact of climate change on stores + flows

Size of snow+glacier masses: Increase in temp has led to ice forming seasons reducing. Less accumulation as precipitation is falling as rain.

Lakes+reservoirs: Not being recharged (replenished) as they previously were, due to ↓ in rainfall + higher temp in some areas. e.g. Lake Chad is 1/20 size of what it was 40yrs ago.

Amount of permafrost: If soil temp increases above freezing, ice within can melt (thaw) leaving water + soil. This leads to percolation + through flow until water leaves ecosystem and permafrost decreases.

Soil moisture levels: During droughts, soil moisture levels decrease as less water infiltrates soil.

Rate of runoff + streamflow: More extreme flows (high flows cause floods, low flows cause droughts). Runoff increases when precipitation increases due to soil becoming saturated.

Why is there uncertainty in the hydrological cycle?

• ENSO cycles produce uncertainty in weather patterns as conditions change in different years. e.g. La Niña causes extreme drought in S. America.

• Global warming leads to more tropical storms + floods associated with them.

Impact of climate change in Sahel region (borders Sahara desert)

• Low rainfall at 100-600mm per year. Declined by 40% during 1970-1980.

• However, there have been several wetter years since, providing opportunity for ‘re-greening’ which involved regeneration of water, retaining vegetation + low cost afforestation.

Drought risk projections (predictions)

• USA, Central America + Mediterranean may see reduction in rainfall (around similar latitude)

  • Mediterranean (Spain+Greece) are at higher risk due to high population + reliance on tourism.

Flood risk projections (predictions)

• SE Asia (Bangladesh, India) seeing increase in likelihood of flooding.

  • Himalayan glacial meltwater due to rising temp may increase flooding.

  • Increased likelihood of tropical storms increases risk of flooding.