The frequency and intensity of drought periods have increased since the 1950s over most land areas . In fact, between 1998 and 2017, drought was the sixth natural hazard associated with disasters (4.8% of the total number of disasters) but the second in terms of the total number of affected people (33% of the total number of affected people), causing more than 21,000 deaths . For example, in 1992, an intense and prolonged drought devastated a region of more than 800 million hectares in Africa and directly affected the lives of 20 million people . Also, widespread drought in south-east Australia (1997–2009) has affected New Zealand direct and off-farm output of about NZ$3.6 billion . In Europe, the joint impacts of drought and forest fires of 2003 exceed 13 billion € .
Over the 21st century, scientists have forecasted with high confidence that the total land area subject to increasing drought frequency, and severity will expand . Future aridification in the Mediterranean, southwestern South America, and western North America will far exceed the magnitude of change seen in the last millennium . Figure 1 shows the different impacts of different extreme weather events in Europe, which is becoming more vulnerable to the rise of sea level and the occurrence of more extreme weather events due to climate change . Such impacts will be worse in some regions than in others. In particular, droughts are increasing in frequency and intensity in the Mediterranean area . This increase might be due to weather conditions being strongly influenced by:
(i) position and magnitude of the Azores anticyclone,
(ii) the moderating effect of the Atlantic Ocean and,
(iii) the influence of the Mediterranean Sea and North Africa.When developing drought risk prevention, mitigation, and preparation measures, it is thus important to address one or more components of risk, which is commonly defined as the product of hazard, exposure, and vulnerability . In this regard, it is important to refer to an accurate definition of “drought”. Contextually, it is essential to define the different types of drought impacts [8, 9]. With this in mind, I will guide you through an overview of the several definitions of drought types. We will look at the most common indices adopted to assess drought hazard, how to assess them, and a detailed list of the most important drought impacts.
Drought types definitions
As a natural phenomenon, drought is defined as a negative precipitation anomaly over a certain period and specific area [9, 10]. In other words, it can arise from much drier conditions than usual or from a dramatic reduction of moisture availability . Drought is a hydrometeorological hazard, which may have a cascading impact on the occurrence of other hazards such as landslides and wildfires .
Drought type definition often depends on the precipitation characteristics or on the environmental or socioeconomic consequences . The most common drought types are meteorological, hydrological, agricultural and socio-economic drought. Meteorological drought, which is only influenced by climate, can be defined as a precipitation deficit in relation to historical records . The other three types of droughts can have considerable anthropogenic influences and are directly linked to potential impacts . Hydrological drought can be associated with a deficiency in water supply volume, which includes stream flow, reservoir storage, and/or groundwater heights [3, 9]. Agricultural drought usually starts after a meteorological drought . It can be characterised as a period with abnormal soil moisture deficit resulting from the combination of excess evapotranspiration and scarcity of precipitation, impacting crop production and/or ecosystem function in general . Socio-economic drought is associated with the failure of water resource systems to meet human demands .
Drought risk assessment
Drought models have the main aim to predict/assess drought impacts. Usually, this is done by computing (i) drought hazard, which comprises the probability of a drought occurring in an area in a certain period  and causing loss of life or other health impacts, as well as damage and loss to property, infrastructure, ecosystems, and resources . (ii) Drought vulnerability that highlights the expected lowest or highest drought sensitivity areas . And (iii) drought risk, which is the potential drought consequences where something of value is at stake and the outcome is uncertain . In other words, most of the existing models employ qualitative or semi-quantitative methods that allow us to estimate the number of people or livestock exposed to droughts and predict when and where drought will occur . These methods use drought indicators and indices found in the Handbook of Drought Indicators and Indices  and also in an online catalogue of the existing drought hazard and risk tools: https://www.droughtcatalogue.com/.
Drought impact and adaptation
As a climate hazard, drought can impact multiple sectors of essential services to society, the environment, and the economy. In fact, drought impact can be seen as a consequence or a symptom of the vulnerability of drought occurrence on the human and economic activities and on the environment . For example, when a drought occurs, water quantity and quality are likely to be affected and compromised, which might mean less water volume for human consumption and more costly and difficult water treatment . Several other drought impacts are described in Table 1. With this in mind, you can understand how extremely crucial it is to model and know such impacts to support the decision of the adaptation strategies.
In recent decades, the impacts from recent climate-related extremes (Figure 2), such as droughts and wildfires, have revealed significant vulnerability and exposure of some ecosystems and many human systems to current climate variability . Such impacts are consistent with a significant lack of preparedness for present climate variability in all countries .
History shows that people and societies have adapted to climate, climate variability, and extremes, with varying degrees of success. Adaptation answers are becoming part of engineered and technological options, often integrated within existing planning processes, such as disaster risk management and water management . For example, adjustments in technologies and infrastructure and ecosystem-based approaches are part of African national governments’ adaptation plans and policies . The agricultural sector is changing in Central and South America, and resilient crop varieties and climate forecasts have been adopted . Policy instruments and binding technical guidance publications have been made at the European Union level. One example is the Water Framework Directive and the Drought Management Plan Report Including Agricultural, Drought Indicators and Climate Change Aspects . Also, the European Climate Adaptation Platform Climate-ADAPT was created. Users can access information from the platform on the current and future vulnerability of regions and sectors or on tools that support adaptation planning. Finally, individual countries have been taking their measures. For example, in Portugal, several counties have been united with research institutions and other organizations, to develop Municipal Climate Change Adaptation Strategies. This useful information can be accessed and adapted from the individual person to a government institution. If you made till here, you might now understand better how drought works and the challenges they pose to our society. Research and policies are working in the right direction, but we still need to improve water management in water-scarce areas and develop drought early warning and prediction systems, which may help to reduce its impacts and harms on food production.
Post edited by Giulia Roder, Valeria Cigala and Gabriele Amato.
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