What are droughts and which tools are used to track them?
According to the Intergovernmental Panel on Climate Change’s (IPCC) fourth assessment report, drought is a “deficiency of precipitation that results in water shortage for some activity or for some group,” [1]. For instance, the lack of precipitation leads to changes in soil moisture and groundwater storage. This is made worse by increased evapotranspiration, the process by which water moves from the land surface to the atmosphere via evaporation and transpiration[1, 2]. During the growing season, these changes negatively affect crop production and ecosystem functioning, eventually leading to agricultural drought. Conversely, hydrological droughts are caused by the lack of precipitation during the percolation and runoff season – when water moves through the soil before reaching ground water reservoirs. In meteorological terms, a drought occurs when precipitation falls under normal levels, in other words a period of precipitation deficit is defined as a meteorological drought [1]. Lastly, a socioeconomic drought takes into account the impact of these multiple drought conditions (meteorological, agricultural, or hydrological) on supply and demand of economic goods. It occurs when the demand for an economic good exceeds supply as a result of a weather-related deficit in water supply [3].
Drought indices have been developed in order to track and quantify different drought parameters such as intensity, duration, severity and the geographic spread of droughts. They combine thousands of data points on rainfall, snowfall, streamflow, and other variables on the availability of water. The Standardized Precipitation Index (SPI) and Palmer Drought Severity Index (PDSI), and other drought index values are often a single number which provides an overall picture on the availability of water that is more useful for decision-making than raw data [4].
How do droughts occur and what are their impacts on people, environment and assets?
Despite the fact that weather extremes are becoming more frequent and intense due to climate change, risks also vary depending on the exposure and vulnerability of a community. And it is the disasters that follow, which have a negative impact on people’s lives, assets and the economy, that force government intervention, thus shown by a recent introductory article on ClimaTalk examining the difference between extreme weather and disaster.
The sun’s radiation heats the ocean and land surface from which water evaporates. It then moves around with winds in the atmosphere, condenses to form clouds, and falls back to the Earth’s surface as rain or snow, only for some of it to flow to oceans via rivers, thereby completing the global water cycle [5]. Yet under high temperatures induced by climate change water evaporates more quickly from the soil and ocean surface. This results in a drier, more compact soil and high temperatures of sea surface levels, as is the case in East Africa where Ethiopia, Somalia and Kenya have experienced four consecutive failing rainy seasons as of late 2020, which has impacted an estimated 16.7 million people with acute food insecurity [11, 12].
This impact is also shown by a study in the Northern Hemisphere, outside of the tropics, where human-induced climate change has affected soil moisture levels. Droughts have become more likely by a factor of at least 20 for soil moisture in the root zone (as deep as 100cm) and at least 5 for surface soil moisture [6]. On the other hand, during the rainfall seasons compact soil hinders the movement of water through the soil. Combined with other factors it induces floods as shown in a study on floods in Nigeria where it has been concluded that climate change has made flooding events about 80 times more likely and approximately 20% more intense [7].
Consequently, droughts are environmental disasters that cost ecosystems, agriculture, human civilizations, and other sectors significantly, making them one of the most expensive natural disasters in the world [8]. As droughts intensify in Somalia, there have been an estimated 43,?000 more deaths compared to mortality levels in 2015–2016, a rate exacerbated by political instability, ethnic tensions and insecurity.
There are also severe health risks brought about by droughts. The reduction in potable water supply favours the concentration of pathogens, notably Escherichia coli and cholera thus leading to increases in cases of diarreha, scabies and conjuctivitis [9, 10].
Multidisciplinary research outlines the impacts of droughts, yet in the absence of a singular, consistent, and universal definition of drought, policymakers and other relevant entities face more difficulties recognizing and planning for drought than they do for other natural catastrophes. There is an urgency to increase adaptation efforts and develop more efficient strategies to deal with the crises brought by drought.
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