Drought. Definitions. Causes. Types.

Drought occurs when there is less water available than what is normally expected in a particular location and season. It can last for days, months or years, and has severe impacts on ecosystems, agriculture and the economy. Droughts are becoming more severe and unpredictable due to climate change. There are three kinds of drought effects: environmental, economic and social. Environmental effects include the drying of wetlands, more and larger wildfires, and loss of biodiversity. Economic consequences include disruption of water supplies, lower agricultural outputs and higher food-production costs. Social and health costs include negative impacts on health, stress from failed harvests and water scarcity. Prolonged droughts have caused mass migrations and humanitarian crises. Some plant species have adapted to tolerate drought, but most arid ecosystems have inherently low productivity. The most prolonged drought in recorded history continues in the Atacama Desert in Chile. Humans have historically viewed droughts as disasters and have attributed them to natural or supernatural forces.

Definition

IPCC defines drought as "drier than normal conditions"

National Integrated Drought Information System defines drought as "a deficiency of precipitation over an extended period of time (usually a season or more), resulting in a water shortage"

National Weather Service office of the NOAA defines drought as "a deficiency of moisture that results in adverse impacts on people, animals, or vegetation over a sizeable area"

Drought is a complex phenomenon related to the absence of water, which is difficult to monitor and define.

Over 150 definitions of "drought" were published by the early 1980s, reflecting differences in regions, needs, and disciplinary approaches.

Types

There are three categories of drought: meteorological, hydrological, and agricultural or ecological drought.

Meteorological drought occurs due to lack of precipitation.

Hydrological drought is related to low runoff, streamflow, and reservoir storage.

Agricultural or ecological drought causes plant stress from a combination of evaporation and low soil moisture.

Socioeconomic drought occurs when the demand for an economic good exceeds supply due to a weather-related shortfall in water supply.

Meteorological drought usually precedes the other kinds of drought.

Hydrological drought tends to show up more slowly because it involves stored water that is used but not replenished.

Agricultural or ecological droughts affect crop production or ecosystems in general.

Agricultural drought can be caused by increased irrigation or poorly planned agricultural endeavors leading to soil conditions and erosion.

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Causes

Precipitation mechanisms include convective, stratiform, and orographic rainfall, and precipitation can be categorized into three types.

Droughts mainly occur in areas with already low rainfall levels and can be triggered by high levels of reflected sunlight, continental winds, and high pressure systems.

The dry season in the tropics increases the occurrence of droughts, and bushfires are common due to the lack of water in the plants.

El Niño and La Niña events can exacerbate drought conditions in various regions around the world.

Climate change is expected to cause droughts with a significant impact on agriculture, increase the frequency of extreme events, and worsen compound warm-season droughts in Europe.