Drought

Types of drought

There are three types of drought:

• Meteorological drought: when the degree of dryness compared to ‘normal’ precipitation occurs due to dry weather patterns dominating an area.

• Agricultural drought: when there is insufficient water for crops, leading to crop failure.

• Hydrological drought: when the drainage basins suffer shortfalls due to rainfall deficit.

Sequence of drought and impact Source: National Drought Mitigation Center

Case study: drought in Brazil

In 2014–15, Brazil suffered its worst drought in 80 years. The drought caused power supplies to stop as HEP schemes were too low, agriculture fell into crisis, and urban water supplies froze.

Usually, rainfall in Brazil comes from the moist air that moves in a westerly direction from the South Atlantic across the Amazon Basin. This air encounters the high Andes mountain range to the West of the continent, which then is forced to turn southwards and maintain the moisture around the Basin.

Map of South America: note Brazil, the Amazon Basin, and the Andes. Source: Library of Congress

Cause of the drought

In 2014-2015 a series of high-pressure systems diverted the rain-bearing winds further north, away from the Amazon, and also prevented them from moving southwards towards the Andes. The heavy rains occurred in Bolivia and Paraguay and dry air remained in Brazil instead.

Impact of the drought

The drought was devastating. More than four million people were affected by rationing and rolling power cuts. The production of Arabica coffee, a commodity that Brazil supplies in bulk, fell by 15% which raised the global price by half. Seventeen of the largest reservoirs were at their lowest point and increased groundwater abstraction led to aquifers becoming dangerously low.¹

Deforestation and drought

Climate scientists who studied this case reported that the deforestation in the Amazon may have passed the tipping point, changing the hydrological and climatic cycles permanently.

Usually, a rainforest can recycle half of its rainfall. However, the positive feedback loop caused by deforestation and less rainfall is reducing the ability of the rainforest to regenerate. The fragile rainforest ecosystems are less resilient and have reduced soil water storage and evapotranspiration. This leads to lower precipitation and has a domino effect of feedback loops which reinforce the drying of the tropics.

The combination of global climate change and deforestation will cause extreme weather more frequently. This could mean:

• The decline in the Amazon’s capacity to absorb carbon.

• Changes in regional water cycles and increases in soil temperatures.

• The transformation of the Amazon rainforest into savannah-like grasslands.

• An increase in wildfires, which would lead to an increase in carbon in the atmosphere.

• Reduced rainfall that affects HEP, which generates 70% of Brazil’s electricity.

• The loss of a major carbon sink and source of moisture.

Human activity and drought

Human activities, such as the over-abstraction of surface water resources and groundwater aquifers, can add to the risk of drought. In the case of Brazil’s 2014–15 drought, the water companies tried to maintain the water supply for factories and services while residents tried to avoid cuts in supply. Groundwater became the only water source for the urban poor and the residents of remote rural areas started to drill illegal wells to get water.

The impacts of drought on rainforest ecosystems

Going back to our case study, the Amazon rainforest is seen as a giant water pump. It lifts up vast amounts of moisture into the air which then circulate through to the South and West of Brazil falling as rain in the central and southern parts of the country.² The rainforest releases 20 billion tons of water daily and these are transported by these ‘flying rivers.’³

Drought can cause a series of chain reactions in a forest. Due to it, the younger trees can die. It also reduces the canopy cover, which then leads to humidity, water vapour, and eventually rainfall. Without the canopy cover, surface tree litter and dying vegetation create a potential tinderbox that can easily catch fire with lightning storms and high winds. Long term drought means thinner canopies and shorter trees.

The impacts of drought on wetland ecosystems

A wetland is a distinct ecosystem that is flooded by water, either permanently or seasonally. In Brazil, there is the Pantanal, a large wetland area that lies in the Upper Paraguay River Basin. The Pantanal is home to aquatic and bird wildlife which makes it one of the world’s most significant freshwater ecosystems.

In the graph below, you can see that the seasonal rainfall floods in Pantanal are between November and April. At this time, the Pantanal goes from terrestrial to aquatic habitats. The flooding covers 80% of the Pantanal but the river is permanent with the wetlands near the river retaining up to 60% of floodwater throughout the year.

Annual rainfall in the Panatal. Data provided by WorldWeatherOnline

The land near the river is filled with forest and gradually changes into savanna grassland as it becomes further from the river. The 2014-2015 drought had a major effect on the Pantanal and tested its resilience.

• Tree mortality increased which reduced habitats for wild animals.

• During the drought, wildfire became a major threat as cattle farmers would set old grass on fire during the dry season to clear vegetation left ungrazed by their cattle and would spread out of control into the wetlands and forests.

1. The Guardian. https://www.theguardian.com/world/2015/jan/23/brazil-worst-drought-history

2. https://www.wri.org/insights/3-maps-help-explain-sao-paulo-brazils-water-crisis

3. https://www.fao.org/in-action/forest-and-water-programme/news/news-detail/en/c/1190278/