The Origins and Sources of Droughts

Drought is a natural, recurring and temporary phenomenon of climate, which can occur in all types of climate. It definitely differs from aridity, which is a permanent feature and hence it should not be confused with the water scarcity occurring from an excess of water demand over available supply. Rather it is more reasonably linked with the distribution and frequency of rainfall over a region. Although, there are no generally accepted definitions for drought (Wilhite and Glantz, 1985), the American Meteorological Society has categorized it into four types namely: meteorological or climatological, agricultural, hydrological and socioeconomic (Heim, 2002). A prolonged drought lasts several months or even years while the absence or reduction of precipitation creates meteorological droughts. On the other hand, short-term (few weeks) dryness in surface layer could results an agricultural drought (Heim, 2002). However, when prolonged meteorological droughts reduce the ground water level severely then hydrological droughts occur. Finally, all first three droughts with a deficit in water availability are named as socioeconomic drought. Among these four, the agricultural drought might be a serious issue when the farming or crop producing in humid or sub humid zones are concerned. With the world population about 7.7 billion, farmers across all continents need to produce 40% more yield by 2020 (Sivakumar, 2011). India is a country, where agriculture and its allied activities act as major source of livelihood and hence it is expected to be deeply affected by drought occurrences. India receives ~80% of the annual rainfall during the southwest monsoon season which produces major impact on crop production.

Generally drought events originate from the deficiency in precipitation, and water shortage over a particular region and time (Wilhite and Glantz 1985). As rainfall observation data is available from past two centuries, mostly all the calculations of drought indices includes this variable either single headedly or in combination with other meteorological parameters (WMO, 1975; Tannehill, 1947). Some early drought index were simply represented the drought duration or intensity upon satisfying the drought defining criteria, e.g. Munger (1916) defined the drought index as the length of period without 24 hours precipitation with a minimum of 1.27 mm. Similarly, Kincer (1919) used 30 or more consecutive days with less than 6.35 mm daily rainfall for the process of drought identification. Marcovitch (1930) used temperature data along with the precipitation while Blumenstock (1942) used the length of drought in days, where the count was terminated upon occurrence of 2.54 mm of rainfall over a span of 48 hours. Likewise, many other drought index can be found in the past literature where precipitation has been used as a primary factor (Bates, 1935; Palmer, 1965, 1968; Gibbs and Maher, 1967; Frere and Popov, 1979; Bhalme and Mooley, 1980; Petrasovits, 1990; Rao et al., 1981; Heddinghaus, 1991; Tate et al., 2000; Lloyd-Hughes and Saunders,2002).

Recently, the multi-scaler drought index like Standardized Precipitation Index (McKee et al., 1993) is widely used by several researchers in analysing the drought characteristics. However, no single index has the ability to precisely represent the drought duration and intensity and its possible impacts (Wilhite and Glantz, 1985). Again, apart from the rainfall, there are also some other parameters that affects the drought severity, e.g. potential evapotranspiration (PET) and soil water holding capacity (Dai et al., 2004). The Palmer Drought Severity Index (Palmer, 1965) is an effective parameter which uses all these three parameters; however, it has some limitations when applying over climatic zones like India (Niranjan et al., 2013). In addition, gathering all these parameters in gridded form and then quantifying the drought index will be very difficult over the Indian region. On the other hand, the standardized precipitation–evapotranspiration index (SPEI) uses only precipitation and temperature, and is considered to be better for analysing drought occurrence (Begueria et al., 2010; Vicente-Serrano et al., 2010a, 2010b; Das et al., 2016).
India happens to be one of the most vulnerable drought-prone countries, as severe droughts occur at least once in a three year time span since the past few decades (Mishra and Singh, 2010). In addition, there are numerous instances of severe drought conditions during Monsoon as reported in recent past (Pai and Sreejith, 2010). Consequently, several studies have been carried out in the recent years in order to understand the drought occurrences during the Indian summer monsoon period (Ramdas, 1950; Banerji and Chabra, 1964; Chowdhury et al., 1989; Appa Rao, 1991; Gore and Sinha Ray, 2002).

Bhalme and Mooley (1980) defined the Drought Area Index for drought intensity assessment using monthly rainfall distribution. Raman and Rao (1981) suggested a possible relation between summer droughts and prolonged brake phase of southwest monsoon over the Indian sub-continent. Parthasarathy et al. (1987) identified the extreme drought years by analysing the decade long anomalies in the Indian summer monsoon rainfall. Tyalagadi et al. (2015) analysed more than 100 years of rainfall and identified 21 drought years, half of which were associated with El Niño. Gadgil et al. (2003) explained the excess rainfall or drought in terms of Equatorial Indian Ocean Oscillation (EQUINOO) during 1972 – 2002, especially during monsoon season. Francis and Gadgil (2010) also suggested the role of El Niño Southern Oscillation (ENSO) and EQUINOO behind the 48% deficit of June rainfall over India, although there are contradictions behind this theory (Neena et al., 2011). Apart from these oscillations like ENSO or IOD (Indian Ocean Dipole) there are also lots of other parameters which may have prominent influences on drought occurrence, e.g. Himalayan ice cover, Eurasian snow cover, the passage of intra-seasonal waves, effects of accumulated pollution etc., e.g. Krishnamurti et al. (2010) reported the intrusion of desert air mass to be responsible towards the drought occurrences over the central Indian region.

In general, most of the previous studies on monsoon droughts are discussed on the basis of rainfall accumulation, and there are very few, which quantify its relation with the direct or indirect radiative effects of aerosols (Atwater, 1970; Ensor et al., 1971; Twomey, 1977; Albrecht, 1989; Charlson et al., 1992) while considering both rainfall and PET. Absorbing aerosols such as black carbon (BC) or dust have the capabilities of atmospheric heating by absorbing solar radiation, while non-absorbing aerosols (e.g. sulphates) scatter the solar radiation have less effect over the same (Lau and Kim, 2006). Additionally, they have the capability of modulating the cloud characteristics by altering cloud radiative properties (Li et al., 2010; Gu et al., 2012; Dipu et al., 2013; Wencai et al., 2015). Previous studies have shown the presence of the aerosols (mainly dust and BC), and their ability to impact the rainfall (depending upon their sizes) during Indian summer monsoon as described by elevated heat pump hypothesis (Lau and Kim, 2006; Manoj et al., 2011; Vinoj et al., 2014; Das et al., 2015; Solmon et al., 2015). During late pre-monsoon or early monsoon season, the aerosol loading over India is nearly three times higher than the average due to the dust abundance, which is partly dependent upon the winds, precipitation and surface temperature (Dey, 2004; Grini and Zender, 2004;Deyand Girolamo, 2010; Wang et al., 2015; Parajuli et al., 2016). However, the vice versa can also be true (e.g. Moorthy et al., 2007; Lau and Kim, 2006). Very recently some new attempts were also undertaken to study the long and short term implications of both natural and anthropogenic components in producing a hindrance to convective rainfall especially over urbanized coastal locations which may also lead to subsequent drought occurrences (Chakraborty et al., 2016, 2017a, 2017b, Guha et al., 2017 and Talukdar et al., 2018). Keeping all these assertions in mind, the present study has put an effort in establishing a possible relationship between aerosol loading and summer monsoon rainfall, consequently, over drought occurrences during this period in past few decades.

Hence a detailed investigation is presented to study the evolution of dry phase leading to drought conditions during mid-monsoon over three Indian regions based on the balance between precipitation and PET during the monsoon season. Next, a new parameter called dry day frequency is used to understand the trends of drought potential over the mentioned Indian regions. This is followed by a three pronged investigation to identify the most dominant factor behind these trends after which future projections of DDF is observed and explained for these locations during the mid-monsoon period.