5.4 Deficits within the hydrological system

Question 1

What is a drought?

Answer 1

a long period of abnormally low rainfall, especially one that adversely affects growing or living conditions

Question 2

What is a meteorological drought?

Answer 2

when long term precipitation is much lower than normal

Question 3

What is a hydrological drought?

Answer 3

Occurs when there are deficiencies in surface and subsurface water supplies as measured in rivers, reservoirs, lakes and groundwater

Question 4

What is agricultural drought?

Answer 4

when there is insufficient soil moisture to meet the demands of a particular crop

Question 5

What is socio-economic drought?

Answer 5

occurs when the water demand for social and economic purposes exceeds water availability

Question 6

How can we measure drought?

Answer 6

Palmer Drought severity index
Crop moisture index
Palmer hydrological drought Index

Question 7

What are the physical factors which cause drought?

Answer 7

global atmospheric circulation
ENSO cycles
The intertropical convergence zone
Mid latitude blocking anticyclones

Question 8

In global atmospheric circulation, what happens at the equator because of the sun?

Answer 8

Intense solar radiation at the equator warms the air, which rises and starts convection. The air cools as it rises and water vapor condenses to form clouds and rain

Question 9

What is the sub tropical high pressure zone and where is it?

Answer 9

This is 30 degrees N+S of the equator. Here, the air that has risen from the equator has cooled and so sinks to form a high pressure belt, with dry hot conditions

Question 10

What then happens to the air in the sub-tropical high pressure zone?

Answer 10

the air returns to ground level at the equator, creating trade winds

Question 11

Where do trade winds meet?

Answer 11

The trade winds meet at the ITCZ where the warmed air rises

Question 12

How and why does the position of the ITCZ move in global atmospheric circulation?

Answer 12

It moves with the seasons. For example, from December to February, the southern hemisphere is tilted towards the sun and therefore the ITCZ is south of the equator. This movement causes alternate wet and dry seasons at the tropics

Question 13

What happens to the warm air moving from the subtropics to the mid latitudes?

Answer 13

They meet cold polar air at the polar front, where the warm, less dense air rises, causing condensation and rainfall.

Question 14

What happens in the polar cell?

Answer 14

The warmer air rises into the polar front jet stream and is transferred at high altitude levels towards the poles, where it cools and sinks. This creates a movement of air back towards the equator

Question 15

What is evaporation and precipitation like at the ITCZ?

Answer 15

high levels of evaporation, especially from oceans
There is therefore alternate seasons of wet and dry as the ITCZ moves seasonally, according to the angle of the sun

Question 16

Why can severe drought happen at 30 degrees N+S?

Answer 16

the high pressure here associated with global atmospheric circulation, blocks the arrival of a wet season from the ITCZ meaning sever droughts can happen in places like the Sahel

Question 17

What is the ENSO cycle?

Answer 17

A naturally occurring large mass of very warm sea water in the equatorial Pacific ocean

Question 18

Where is the warm water of the ENSO cycle normally placed?

Answer 18

It’s usually located in the western Pacific, where it’s pushed by ocean currents, trade winds and the Walker circulation cell. This means the warm, moist air rises, cools and condenses and rains in Australia, leaving dry conditions in the west coast of South America

Question 19

What happens in an El Nino event?

Answer 19

Around every 7 years, the pushing forces weaken and this allows the mass of warm water to move eastwards towards Southern America. This means there is increased rainfall in South America, but dry conditions and drought in Australia

Question 19

What happens where the warm water is placed?

Answer 19

evaporation rates are higher and precipitation rates are greater

Question 20

What happens in a la Nina event?

Answer 20

The warm mass of air is pushed even further west than normal, which causes drought in Peru and California and sometimes flooding in Australia

Question 21

What happens in the mid latitudes (mid latitude blocking anticyclones)

Answer 21

frontal precipitation is created in low pressure systems that form along the polar front, where warm tropical air rises over cold polar air

Question 22

Where do depressions move from and to in the mid-latitudes and why? (mid latitude blocking anticyclones)

Answer 22

they move from west to east as a result of the corolis force and their track is directed by the polar front jet stream, which is a very fast moving, meandering belt of air in the upper troposphere

Question 23

What happens occasionally to loops in the jet stream that allow drought to happen in the mid latitude countries?

Answer 23

The loops in the jet stream occasionally stabilise and allows the high pressure area (anticyclone) to move northwards. These anticyclones bring stable weather with very little precipitation, while rain bearing depressions are forced around them, causing drought in mid-latitude countries

Question 24

How long can an anticyclone last and why?

Answer 24

The stability of the anticyclone with their sinking air and calm conditions means they can persist and block weather systems from the west for up to 2 weeks. If this situation repeats, drought can occur

Question 25

How does land being deforested cause drought?

Answer 25

land is deforested and so vegetation cover is reduced. This causes evapotranspiration and atmospheric moisture to be reduced. This causes less precipitation

Question 26

How does land being overgrazed cause drought?

Answer 26

land is overgrazed and so soil conditions change through compaction and less organic material. This means there is less infiltration and more surface run off and therefore, soil moisture content and water storage is reduced

Question 27

Where is the Pantanal wetland area and why is it important?

Answer 27

It’s in central southern America in the upper Paraguay river basin and its aquatic and bird wildlife make it one of the world’s most significant freshwater ecosystems

Question 28

How has the Pantanal wetland area been affected by drought?

Answer 28

The 2014-15 drought affected the Pantanal. It increased tree mortality, which reduced habitats for wild animals. Wildfires became a major threat, caused by cattle ranchers deliberately setting old grass on fire during the dry season to clear vegetation left ungrazed (their normal management method). However during the drought, those fires easily got out of control

Question 29

What is the Pantanal normally like?

Answer 29

Flooding generally covers 80% and this water is vital for aquatic and bird life survival

Question 30

How much of the earth do wetlands cover?

Answer 30

around 10%

Question 31

Why are wetlands purposefully destroyed?

Answer 31

for agriculture and development, such as in the Florida Everglades

Question 32

What are the key functions of wetlands?

Answer 32

-act like giant water filters by trapping and recycling nutrients
-recharge aquifers
-mitigate river floods
-support a diverse food web, producing nurseries for fish and refuges for migrating birds

Question 33

What are the effects of drought on wetland areas?

Answer 33

-areas shrink up or dry, leading to habitat loss
-leads to soil erosion
-reduced ability to store water in times of flood or release water in times of drought
-Organic soils may oxidize, releasing carbon and concentration of pollutants may increase
-eliminate some species from the food web, leaving other species with food shortages
-water table falls

Question 34

How has the marshland in Southern Iraq been completely destroyed?

Answer 34

by dams on the Tigris and Euphrates, substantially reducing flows as well as by Saddam Hussein’s drainage schemes, largely designed to destroy the lifestyle of 25,000 Marsh Arabs who lived there

Question 35

Why is the Amazon so important?

Answer 35

known as the lungs of the earth as it absorbs CO2 and returns O2 to the atmosphere. It’s 400 billion trees also transport humidity inland from the Atlantic

Question 36

How does prolonged drought cause forest stress?

Answer 36

1. young trees die which reduces canopy cover
2. reduction of humidity, water vapor and therefore rainfall
3. exposed to sunlight, drying vegetation and surface tree litter can catch fire easily
4. lightning storms and high winds frequently turn a small fire into a wildfire
5. long term drought means shorter trees and thinner canopies

Question 37

What does harm to drought stressed trees cause?

Answer 37

-foliage loss
-impairing growth
-increased accumulation of pests/disease
-lasting damage to vascular tissues, which impairs water transport

Question 38

What drought happened in Brazil 2014-15

Answer 38

The 2014-15 drought saw the worst drought in Brazil in 80 years. Water levels in some of the world’s largest HEP schemes were so low that power supplies were suspended, agriculture was in crisis and urban taps ran dry.

Question 39

Why is rainfall in Brazil usually predictable?

Answer 39

-Most air moves in a westerly direction from the South Atlantic across the Amazon Basin
-When moist air encounters the high Andes mountain range to the west of the continent, it’s forced to turn southwards-maintaining the flow of moisture around the Basin

Question 40

How was the precipitation pattern different in 2014-15 that caused droughts?

Answer 40

A series of high-pressure systems diverted rain bearing winds further north, away from the Amazon, and prevented them from diverting southwards from the Andes
Heavy rains then occurred in Bolivia and Paraguay, whilst dry air remained over Brazil

Question 41

What were some of the impacts of drought in Brazil in 2014-15

Answer 41

-Its impacts led to street protests in Sao Paulo, Brazil’s largest city
-Water rationing for 4 million people; water supplies were cut off for three days a week in some towns
-The halting of HEP production, which led to power cuts
-The depletion of Brazil’s 17 largest reservoirs to dangerously low levels- some down to a capacity of just 1%
-Increased groundwater abstraction, which led aquifers to become dangerously low
-A reduced crop of Arabica coffee beans, which pushed up global coffee prices by 50%

Question 42

How did water companies and people in Brazil cause the drought and make it worse?

Answer 42

Human activity also contributed, due to over-abstraction, as water companies tried to maintain the water supply for factories and services, and residents tried to avoid cuts in supply. In south-east Brazil, groundwater became the only water source for the urban poor and remote rural areas.

Question 43

How did the Brazilian government make the 2014-15 drought worse?

Answer 43

The problem was caused by the high fees (US$3000) charged by Brazil’s government for granting a licence to drill a well. In addition, drilling a single well can cost between US$35,000 and US$100,000
This cost meant that many people avoided payment and instead drilled illegal wells, which were not monitored for water safety

Question 44

How many wells were made in Brazil during the drought?

Answer 44

Between January and October 2014, 25,000 licences to allow drilling were made by Sao Paulo’s state government. However, hydrologists believe that this was only 30% of the real number and they were concerned at shrinking groundwater levels

Question 45

Why were illegal wells a problem during the 2014-15 drought in Brazil

Answer 45

Illegal wells are generally shallower and less filtered by bedrock, so they contain industrial pollutants and higher levels of bacteria.

Question 46

How are humans creating drought in the Sahel?

Answer 46

-The demand for food and fuel wood is accelerating
-the natural dryland ecosystems are progressively being converted into farmland, which is being overcultivated and overgrazed, causing desertification.
-Therefore, there is reduced vegetation cover and soil moisture, which may create self perpetuating droughts, as desertification increases human vulnerability to drought hazards.

Question 47

What has happened to rainfall in the Sahel since the 60s?

Answer 47

Pronounced decline in annual rainfall

Question 48

When was the most recent drought in the Sahel?

Answer 48

2011/12

Question 49

How has air pollution caused drought in the Sahel

Answer 49

Caused by air pollution (Sulphur-based aerosols) generated in Europe and North America. These pollutants are thought to have caused atmospheric cooling, changing the global heat budget and atmospheric heat circulation so that tropical rains associated with the ITCZ didn’t arrive, causing drought

Question 50

How has climate change caused drought in the Sahel?

Answer 50

Could be a result of higher sea surface temperatures caused by climate change. The rain-bearing winds that move over the Sahel appear to fail when the sea-surface temperatures in the Atlantic are warmer than usual, as they favour strong convectional uplift

Question 51

How did ENSO cycles effect the Sahel in 2015/16?

Answer 51

It was forecasted that the 2015/16 El Nino event would increase water stress in the Sahel, when temperatures were 2 degrees higher than the average

Question 52

Why is the Sahel already a drought sensitive zone?

Answer 52

The Sahel is drought sensitive as it occupies a transitional climate zone. The mean annual rainfall is usually all concentrated in summer. On the edge of the Sahara, the average is only 100mm

Question 53

What were the effects of the 1999-2000 Ethiopia-Eritrean drought crisis?

Answer 53

-about 10 million people needed food assistance, made worse by the fact the population there had doubled every 20-30 years
-Additionally, the two countries were at war, blocking food access for many