Characterizing Drought and Its Severity
In technical terms, droughts differ from one another in three essential characteristics: intensity, duration, and spatial coverage. Intensity refers to the degree of the precipitation shortfall and/or the severity of impacts associated with the shortfall. It is generally measured by the departure of some climatic parameter (e.g., precipitation), indicator (e.g., reservoir levels), or index
(e.g., Standardized Precipitation Index or SPI) from normal and is closely linked to duration in the determination of impact. These tools for monitoring drought are discussed in detail in Chapters 7 and 8. Another distinguishing feature of drought is its duration. Droughts usually require a minimum of 2 to 3 months to become established but then can continue for months or years. The magnitude of drought impacts is associated with the timing of the onset of the precipitation shortage, its intensity, and the duration of the event. It should also be noted that quick onset or "flash" droughts occur, especially when precipitation deficiencies are associated with high temperature stress and internal atmospheric variability, such as that occurred during the 2012 drought in the United States.
Droughts also differ in terms of their spatial characteristics. The areas affected by severe drought evolve gradually, and regions of maximum intensity (i.e., epicenter) shift from season to season. In larger countries, such as Brazil, China, India, the United States, and Australia, drought rarely, if ever, affects the entire country. During the severe drought of the 1930s in the United States, for example, the area affected by severe and extreme drought reached 65 percent of the country in 1934. The 2012 drought in the United States was of a comparable spatial extent. These two droughts represent the maximum spatial extent of drought in the period from 1895 to 2016. The climatic diversity and size of countries such as the United States suggest that drought is likely to occur somewhere in the country each year.
From a planning perspective, the spatial characteristics of drought pose serious problems. For example, nations should determine the probability that drought may simultaneously affect all or several major crop-producing regions or river basins within their borders and develop contingencies for such an event. Likewise, it is important for governments to calculate the chances of a regional drought simultaneously affecting agricultural productivity and water supplies in their country and adjacent or nearby nations on which they may depend for food supplies or water transfers. A drought mitigation strategy that relies on the importation of food from neighboring countries or even distant markets may not be viable when regional-scale drought occurs. For example, the South African region experienced food insecurity from the simultaneous reduction of local corn production and the drop in rice imports from Southeast Asia, as a result of droughts associated with the 2015-2016 El Nino Southern Oscillation (ENSO) event.