Key Messages
• The frequency and spatial extent of droughts over India
have increased significantly during 1951–2015. An
increase in drought severity is observed mainly over the
central parts of India, including parts of Indo-Gangetic
Plains (high confidence). These changes are consistent
with the observed decline in the mean summer monsoon
rainfall.
• Increased frequency of localized heavy rainfall on
sub-daily and daily timescales has enhanced flood risk
over India (high confidence). Increased frequency and
impacts of floods are also on the rise in urban areas.
• Climate model projections indicate an increase in frequency, spatial extent and severity of droughts over India
during the twenty-first century (medium confidence),
while flood propensity is projected to increase over the
major Himalayan river basins (e.g. Indus, Ganga and
Brahmaputra) (high confidence).
6.1 Introduction
Hydroclimatic extremes such as droughts and floods are
inherent aspects of the monsoonal landscape. Droughts over
India are typically associated with prolonged periods of
abnormally low monsoon rainfall that can last over a season
or longer and extend over large spatial scales across the
country (Sikka 1999). The slow evolutionary nature of
monsoon droughts and enhanced surface dryness exert significant impacts on water availability, agriculture and
socio-economic activities over India (Bhalme and Mooley
1980; Swaminathan 1987; Sikka 1999; Gadgil and Gadgil
2006; Asoka et al. 2017; Pai et al. 2017). Compared to
droughts, floods typically occur over smaller locales in
association with heavy precipitation and stream flows on
shorter timescales (Dhar and Nandargi 2003; Kale 2003,
2012; Mishra et al. 2012a; Sharma et al. 2018). Every year,
nearly 8 million hectares of the land area is affected by floods
over India (Ray et al. 2019). Droughts and floods across India
are known to have complex linkages with the space-time
distribution of monsoon rainfall and socio-economic demand
(Sikka 1999; see Chap. 3 for details).
Observations for the recent decades, from post-1950,
clearly show a significant rising trend in frequency and
intensity of both heavy rain events as well as consecutive dry
days (CDD). These trends are particularly notable over
central parts of the Indian subcontinent during the
south-west (SW) monsoon and southern peninsular India
during the north-east (NE) monsoon (see Chap. 3 for
details). The observed rainfall data indicates that there have
been 22 monsoon droughts since 1901 (Fig. 6.1a). Interestingly, studies have shown that drought, as well as flood
frequency, have increased since the 1950s. India experienced
an increase in intensity and percentage of area affected by
moderate droughts along with frequent occurrence of
multi-year droughts during recent decades (Niranjan Kumar
et al. 2013; Mallya et al. 2016). In this chapter, an assessment based on observational evidences from instrumental,
palaeoclimatic records and likely future changes from climate model projections on droughts and floods across India
is presented.
6.2 Observed Variability of Droughts
Droughts are broadly categorized into four major classes:
(1) meteorological drought, as a deficit in precipitation;
(2) hydrological drought, as a deficit in streamflow,
groundwater level or water storage; (3) agricultural drought,
as a deficit in soil moisture; and (4) socio-economic drought,
incorporating water supply and demand (Wilhite and Glantz
1985; Anderson et al. 2011). All these four categories of
droughts usually initiate with a deficiency in precipitation.
Some of the prominent drought indices for the categorization
of meteorological droughts in India are summarized in
Table 6.1. Out of these indices, standardized precipitation
evapotranspiration index (SPEI) has been used for analysing
drought trends and variability over India (Mallya et al. 2016).
The SPEI has also been used for evaluating reanalysis
products during drought monsoon years (Shah and Mishra
2014); for drought monitoring (Aadhar and Mishra 2017),
and adopted by the India Meteorology Department (IMD) for
the operational purpose (http://imdpune.gov.in/hydrology/
hydrg_index.html). As SPEI index is considered better suited
to explore the effects of warming temperatures on droughts
(Table 6.1; also Box 6.1), the present chapter uses SPEI for
assessing the variability of droughts over India.
Box 6.1: Details of SPEI drought indicator
SPEI was computed at horizontal grid spacing of 0.5°
longitude x 0.5° latitude, using monthly rainfall (0.25°
x 0.25°) from IMD and potential evapotranspiration
(PET; 0.5° x 0.5°) from the Climate Research Unit
(CRU) for the period 1901-2016, with respect to the
base period 1951–2000. PET was calculated from a
variant of the Penman–Monteith formula (Sheffield
et al. 2012) recommended by the United Nations Food
118
M. Mujumdar et al.
• The frequency and spatial extent of droughts over India
have increased significantly during 1951–2015. An
increase in drought severity is observed mainly over the
central parts of India, including parts of Indo-Gangetic
Plains (high confidence). These changes are consistent
with the observed decline in the mean summer monsoon
rainfall.
• Increased frequency of localized heavy rainfall on
sub-daily and daily timescales has enhanced flood risk
over India (high confidence). Increased frequency and
impacts of floods are also on the rise in urban areas.
• Climate model projections indicate an increase in frequency, spatial extent and severity of droughts over India
during the twenty-first century (medium confidence),
while flood propensity is projected to increase over the
major Himalayan river basins (e.g. Indus, Ganga and
Brahmaputra) (high confidence).
6.1 Introduction
Hydroclimatic extremes such as droughts and floods are
inherent aspects of the monsoonal landscape. Droughts over
India are typically associated with prolonged periods of
abnormally low monsoon rainfall that can last over a season
or longer and extend over large spatial scales across the
country (Sikka 1999). The slow evolutionary nature of
monsoon droughts and enhanced surface dryness exert significant impacts on water availability, agriculture and
socio-economic activities over India (Bhalme and Mooley
1980; Swaminathan 1987; Sikka 1999; Gadgil and Gadgil
2006; Asoka et al. 2017; Pai et al. 2017). Compared to
droughts, floods typically occur over smaller locales in
association with heavy precipitation and stream flows on
shorter timescales (Dhar and Nandargi 2003; Kale 2003,
2012; Mishra et al. 2012a; Sharma et al. 2018). Every year,
nearly 8 million hectares of the land area is affected by floods
over India (Ray et al. 2019). Droughts and floods across India
are known to have complex linkages with the space-time
distribution of monsoon rainfall and socio-economic demand
(Sikka 1999; see Chap. 3 for details).
Observations for the recent decades, from post-1950,
clearly show a significant rising trend in frequency and
intensity of both heavy rain events as well as consecutive dry
days (CDD). These trends are particularly notable over
central parts of the Indian subcontinent during the
south-west (SW) monsoon and southern peninsular India
during the north-east (NE) monsoon (see Chap. 3 for
details). The observed rainfall data indicates that there have
been 22 monsoon droughts since 1901 (Fig. 6.1a). Interestingly, studies have shown that drought, as well as flood
frequency, have increased since the 1950s. India experienced
an increase in intensity and percentage of area affected by
moderate droughts along with frequent occurrence of
multi-year droughts during recent decades (Niranjan Kumar
et al. 2013; Mallya et al. 2016). In this chapter, an assessment based on observational evidences from instrumental,
palaeoclimatic records and likely future changes from climate model projections on droughts and floods across India
is presented.
6.2 Observed Variability of Droughts
Droughts are broadly categorized into four major classes:
(1) meteorological drought, as a deficit in precipitation;
(2) hydrological drought, as a deficit in streamflow,
groundwater level or water storage; (3) agricultural drought,
as a deficit in soil moisture; and (4) socio-economic drought,
incorporating water supply and demand (Wilhite and Glantz
1985; Anderson et al. 2011). All these four categories of
droughts usually initiate with a deficiency in precipitation.
Some of the prominent drought indices for the categorization
of meteorological droughts in India are summarized in
Table 6.1. Out of these indices, standardized precipitation
evapotranspiration index (SPEI) has been used for analysing
drought trends and variability over India (Mallya et al. 2016).
The SPEI has also been used for evaluating reanalysis
products during drought monsoon years (Shah and Mishra
2014); for drought monitoring (Aadhar and Mishra 2017),
and adopted by the India Meteorology Department (IMD) for
the operational purpose (http://imdpune.gov.in/hydrology/
hydrg_index.html). As SPEI index is considered better suited
to explore the effects of warming temperatures on droughts
(Table 6.1; also Box 6.1), the present chapter uses SPEI for
assessing the variability of droughts over India.
Box 6.1: Details of SPEI drought indicator
SPEI was computed at horizontal grid spacing of 0.5°
longitude x 0.5° latitude, using monthly rainfall (0.25°
x 0.25°) from IMD and potential evapotranspiration
(PET; 0.5° x 0.5°) from the Climate Research Unit
(CRU) for the period 1901-2016, with respect to the
base period 1951–2000. PET was calculated from a
variant of the Penman–Monteith formula (Sheffield
et al. 2012) recommended by the United Nations Food
118
M. Mujumdar et al.