Variability of Meteorological Droughts Over India
D.S. Pai, Pulak Guhathakurta, Ashwini Kulkarni and M.N. Rajeevan
Drought is a complex, natural and recurrent feature of climate with significant impacts on various sectors like agriculture and water resources and the Indian economy. Droughts are generally observed in all the climatic zones. Drought is a relative, rather than absolute condition that should be defined for each region separately. Each drought differs in intensity, duration and the spatial extent. In general, drought over a geographic area represents a temporary condition of scarcity of water for an extended period of time caused by significantly low precipitation, high evapotranspiration and over-exploitation of water resources or a combination of all these (Bhuiyan et al. 2006; WMO 2006). A drought differs from aridity, which is a permanent feature of the climate over regions where climatological normal is low. In terms of the number of fatalities, drought ranks first among all natural hazards (Obasi 1994; Hewitt 1997).
Drought can be categorized into four major categories (Wilhite and Glantz 1985): meteorological, agricultural, hydrological and socioeconomic droughts. Meteorological drought is defined by the deficiency of precipitation from expected or “normal” amount over an extended period of time. The agricultural drought is characterized by a deficiency in water availability for crop or plant growth. It is usually associated with deficiencies in soil moisture, which is the most critical factor in defining crop production potential. Hot temperatures, low relative
D.S. Pai (H) • P. Guhathakurta
Indian Institute of Tropical Meteorology, Earth System Science Organization, Pune, India M.N. Rajeevan
Earth System Science Organization, Ministry of Earth Sciences, New Delhi, India © Springer Science+Business Media Singapore 2017
M.N. Rajeevan and S. Nayak (eds.), Observed Climate Variability and Change Over the Indian Region, Springer Geology, DOI 10.1007/978-981-10-2531-0_5
humidity and strong winds often accompany the lack of rainfall which adds to further reduction in crop yield. It has been observed that there is substantial loss in agricultural output during the severe droughts (Parthasarathy et al. 1994). Gadgil and Gadgil (2006) have also shown that there is a significant reduction in the gross domestic product (GDP) of India during the drought years.
The hydrological drought is defined as the deficiency in surface and subsurface water supplies that lead to a lack of water availability to meet normal and specific water demands. Although climate is a primary contributor to hydrological drought, other factors such as changes in land use (deforestation), land degradation and dam construction also contribute. The socioeconomic droughts occur when physical water shortages start to affect the health, well-being and quality of life of people. Droughts put enormous demand on rural and urban water resources and immense burden on agricultural and energy production.
A drought index indicates the severity of a drought, useful in understanding the drought conditions over an area. There are several drought indices as seen in the literature (Palmer 1965,1968; Bhalme and Mooley 1980; Rao et al. 1981; Sastri 1993; Heddinghaus 1991; Tate et al. 2000; Lloyd-Hughes and Saunders 2002; Kogan 1995, 1997). For more details, Heim (2002), Mishra and Singh (2010) and Sivakumar et al. (2011) can be referred. Rainfall is the primary factor that leads to the generation and maintenance of drought conditions. However, evapotranspiration also impacts the drought severity. The indices based on only rainfall data are simple to compute and perform better compared to more complex hydrological indices (Oladipio 1985).
One of the widely used drought indices is Palmer Drought Severity Index (PDSI). This index is the most prominent meteorological drought index used in the USA (Dai 2011). This index considers precipitation, evapotranspiration and soil water-holding capacity. However, this index cannot capture the multi-scalar nature of droughts. Also it may lag behind the emerging droughts by several months. Standardized Precipitation Index (SPI) (Mckee et al. 1993) considers the multi-scalar nature of droughts. However, SPI does not include temperatures which have substantial effects on severity of droughts. Standardized Precipitation-Evapotranspiration Index (SPEI) is a drought index that can account for the moisture demands of the atmosphere due to global warming (Begueria et al. 2010; Vicente-Serrano et al. 2010a, b).
In India, several studies have been carried out which are related to drought using drought indices based on rainfall data. These studies (Ramdas 1950; Banerji and Chabra 1964; Appa Rao 1991; Chowdhury et al. 1989; Sen and Sinha Ray 1997; Gore and Sinha Ray 2002; Sinha Ray and Shewale 2001; Guhathakurta 2003; Gore et al. 2010) mainly dealt with the droughts during the southwest monsoon season (June-September). Guhathakurta (2003) found the highest probability (>60 %) of droughts in some districts from northern India. Pai et al. (2011) examined the climatology droughts of different intensities over India using two most simple drought indices, PNP and SPI both based on rainfall only. Recently, Niranjan Kumar et al. (2013) examined observed variability of the droughts over India using the SPEI and showed that there is a general increase in percent area and intensity of droughts over India in the recent decades.
In this chapter, long-term climatology and interannual variability of the meteorological droughts over India for the southwest monsoon season (June to September) in the district-wide and all-India (country-wide) scales are discussed. For this purpose, district and all-India rainfall data for the period 1901-2010 have been used. Impact of sea surface temperatures (SSTs) over the equatorial Pacific and Indian Oceans on the variability of droughts has also been discussed.