How many types of drought are there?

Drought is perhaps the most complex natural hazard. It is often generally defined as a temporary meteorological event that stems from the lack of precipitation over an extended period of time compared with some long-term average condition (e.g. precipitation). Drought is described to be a phenomenon which is complex, multidimensional and least understood yet has a lot of paramount effects on socio-economic and agricultural activities in most areas which are agri-based (Modarres, 2010). The American Meteorological Society (1997) groups drought definitions and types of drought into four categories: meteorological or climatological, agricultural drought, hydrological drought, and socio-economic drought. Different studies (Wilhite & Glantz, 1985; Hayes & Svoboda, 2000 ; Bhuyan-erhardt, 2018 ;Mckee et.al., 1993) agree that droughts can either be meteorological, agricultural, hydrological or socio-economic. Though termed differently, they are interrelated and consequential. All droughts share one thing in common and that is, they are as a result of deviations from normal conditions be it precipitation, soil moisture, stream flow or groundwater or aquifer recharge (Loukas & Vasiliades, 2004). However, according to the classification of WMO and drought mitigation center, we accept that there are four types of drought as follows:

1. The first type of drought is 'Meteorological Drought': Meteorological drought is characterized by lack of precipitation over weeks, months, or years (Heim, 2002; Potop et al., 2014; Escalante-Sandoval and Nunez-Garcia, 2017). Meteorological drought is usually defined as a shortage of precipitation (or moisture supply) over some period of time (e.g. weekly, monthly, seasonal or annual timescales). Definitions of meteorological drought, therefore, are location specific since normal precipitation is a function of the climate. Some definitions of meteorological drought focus on the length of time since the last precipitation event (e.g. number of consecutive dry days), while others focus on the magnitude of the precipitation departure from normal. Just like there are numerous definitions of meteorological drought, there are also numerous meteorological drought indices. Monitoring meteorological drought is a vital and important part of drought risk mitigation (Li et al., 2017) on a global scale (WMO, 2013; Li et al., 2014; Salehnia et al., 2017).
2. The second type of drought is 'Agricultural Drought': Short-term (i.e., a few weeks duration) dryness in the surface layers (root zone), which occurs at a critical time during the growing season, can result in an agricultural drought that severely reduces crop yields, even though deeper soil levels may be saturated. Hot temperatures, low relative humidity, and desiccating winds often add to the impact of the lack of rainfall (Condra 1944). The onset of an agricultural drought may lag that of a meteorological drought, depending on the prior moisture status of the surface soil layers. So, shortage of precipitation during the growing season impinges on crop production or ecosystem function in general (due to soil moisture drought, also termed agricultural drought). IPCC uses the term 'soil moisture drought' instead of 'agricultural drought,' despite the widespread use of the latter term (e.g., Heim Jr., 2002; Wang, 2005), because soil moisture deficits have several additional effects beside those on agro-ecosystems, most importantly on other natural or managed ecosystems (including both forests and pastures).
3. The third type of drought is 'Hydrological Drought': Precipitation deficits over a prolonged period that affect surface or subsurface water supply, thus reducing streamflow, groundwater, reservoir, and lake levels, will result in a hydrological drought, which will persist long after a meteorological drought has ended. So, shortage of precipitation during the runoff and percolation season primarily affects water supplies (hydrological drought). The water is collected in lakes, reservoirs, rivers in a given territory (catchment area). If the territory is suffered by meteorological drought, then the water level of reservoirs, rivers, the streamflows, etc. will be reduced. This situation is called hydrological drought. This drought event can distribute to other territories easily.
4. The forth type of drought is 'Socioeconomic Drought': Socioeconomic drought associates the supply and demand of some economic good with elements of meteorological, agricultural, and hydrological drought. This category of drought occurs when physical water shortages start to affect the health, well-being and quality of life of people. This drought starts to affect the supply and demand of economic products such as water, fish production, hydroelectric power generation, etc. Drought places enormous demand on rural and urban water resources, and immense burden on agricultural and energy production. Therefore, timely determination of the level of drought will assist the decision making process in reducing the impacts of drought.

Relationship between types of drought conditions

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