6
Droughts and Floods
Coordinating Lead Authors
Milind Mujumdar, Indian Institute of Tropical Meteorology (IITM-MoES), Pune, India,
e-mail: mujum@tropmet.res.in (corresponding author)
Preethi Bhaskar, Indian Institute of Tropical Meteorology (IITM-MoES), Pune, India
M. V. S. Ramarao, Indian Institute of Tropical Meteorology (IITM-MoES), Pune, India
Lead Authors
Umakanth Uppara, Indian Institute of Tropical Meteorology (IITM-MoES), Pune, India
Mangesh Goswami, Indian Institute of Tropical Meteorology (IITM-MoES), Pune, India
Hemant Borgaonkar, Indian Institute of Tropical Meteorology (IITM-MoES), Pune, India
Supriyo Chakraborty, Indian Institute of Tropical Meteorology (IITM-MoES), Pune, India
Somaru Ram, Indian Institute of Tropical Meteorology, Pune (IITM-MoES), India
Review Editors
Vimal Mishra, Indian Institute of Technology Gandhinagar, Palaj, Gandhinagar, India
M. Rajeevan, Ministry of Earth Sciences (MoES), Government of India, New Delhi, India
Dev Niyogi, Purdue University, West Lafayette, IN and University of Texas at Austin, Austin, TX, USA
Corresponding Author
Milind Mujumdar, Indian Institute of Tropical Meteorology (IITM-MoES), Pune, India,
e-mail: mujum@tropmet.res.in
© The Author(s) 2020
R. Krishnan et al. (eds.), Assessment of Climate Change over the Indian Region,
https://doi.org/10.1007/978-981-15-4327-2_6
117
118
M. Mujumdar et al.
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
6
Droughts and Floods
119
(a) JJAS
(d) JJAS
50
Drought area (%)
2
SPEI
1
0
-1
40
30
20
10
-2
0
(b) OND
(e) OND
80
Drought area (%)
2
SPEI
1
0
-1
-2
60
40
20
0
(c) Annual
(f) Annual
50
Drought area (%)
2
SPEI
1
0
-1
40
30
20
10
-2
0
1905 1920 1935 1950 1965 1980 1995 2010
1905 1920 1935 1950 1965 1980 1995 2010
Fig. 6.1 Time series of a–c SPEI and d–f percentage area affected by
drought (SPEI < −1) for a, d SW monsoon, b, e NE monsoon and c,
f annual scale, during 1901–2016. SPEI in a–c is computed for a all
India during JJAS, b NE monsoon region for OND, and c all India for
the entire year, with respect to the base period 1951–2000. Black lines
indicate 11-year smoothed time series. Blue (red) bars in a–c denote
wet (dry) years. Red lines in (d–f) indicate a linear trend for the period
1951–2016
and Agriculture Organization (FAO; http://www.
fao.org/docrep/x0490e/x0490e06.htm). The PenmanMonteith formulation is based on physical principles of
energy balance over a wet surface and is considered
superior to empirically based formulations, which
usually consider the effects of temperature and/or
radiation only (see Ramarao et al. 2019 and references therein). The SPEI is a normalized index and can
be used to infer both wet (positive SPEI) and dry
(negative SPEI) conditions over the region of interest.
Although the theoretical limits are (-∞, +∞), SPEI
value normally ranges from -2.5 to +2.5. An index of +2
and above indicates extremely wet; (1.5 to 1.99) very
wet; (1.0 to 1.49) moderately wet; (0.99 to -0.99) near
normal; (-1.0 to -1.49) moderately dry; (-1.5 to -1.99)
severely dry; (-2.0 or less) extremely dry. For the
analysis presented in this chapter, SPEI is computed for
4 month, 3 month and 12 month timescales spanning
the JJAS (SPEI-SW), OND (SPEI-NE) and Annual
from January to December (SPEI-ANN) to represent
SW, NE monsoons, and annual scale respectively. The
SPEI-SW and SPEI-ANN are computed for the Indian
subcontinent, whereas SPEI-NE is computed for the
southern peninsular India, the region under the
120
Table 6.1 Various
meteorological drought indices
M. Mujumdar et al.
Index
Computation
Strength and weakness
Percent of normal
precipitation (PNP)
Actual precipitation divided by normal
precipitation—typically a 30 year mean
and multiplied by 100 (%)
Strength: Simple measurement, very
effective in a single region or a single
season, can be calculated for a variety
of timescales
Weakness: Biased by the aridity of
the region, cannot compare with
different locations, cannot identify the
specific impact of drought
Palmer drought
severity index (PDSI)
Computed from precipitation and
temperature (Palmer 1965; Dai et al.
2004)
Strength: Widely used for drought
characterization
Weakness: Lags the detection of
drought over several months due to its
dependency on soil moisture, which is
simplified to one value in each climate
zone
Standardized
precipitation index
(SPI)
SPI is defined based on the cumulative
probability of a given rainfall event. It is
derived from the transformation of fitted
gamma distribution of historical rainfall
to a standard normal distribution (Mckee
et al. 1993)
Strength: Not biased by aridity, better
than PNP and PDSI. It can be
computed for different timescales.
Considers multi-scalar nature of
droughts. Allows comparison of
drought severity at two or more
locations, regardless of climatic
conditions
Weakness: Only precipitation is used
and does not consider other crucial
variables, e.g. temperature
Standardized
precipitation
evapotranspiration
index (SPEI)
SPEI uses accumulations of precipitation
minus potential evapotranspiration
(PET) and thereby accounts for changes
in both supply and demand in moisture
variability over the region of interest
(Vicente-Serrano et al. 2010)
Strength: Similar to SPI. Includes the
effect of temperature via evaporative
demand. More suited to explore
impacts of warming temperatures on
the occurrence of droughts. A more
extensive range of applications than
SPI
Weakness: Sensitive to PET
computation
Several other drought indices have been developed based on different indicator variables such as soil
moisture, run-off and evapotranspiration (Karl and Karl 1983; Mo 2008; Shukla and Wood 2008; Hao and
AghaKouchak 2013)
Table 6.2 List of SW monsoon
droughts from 1901 to 2015.
Years in bold letters represent
severe droughts
Period
Drought years
Total number of droughts (per decade)
1901–1930
1901, 1904, 1905, 1911, 1918, 1920
6 (2)
1931–1960
1941, 1951
2 (0.7)
1961–1990
1965, 1966, 1968, 1972, 1979, 1982, 1986, 1987
8 (2.7)
1991–2015
2002, 2004, 2009, 2014, 2015
5 (1.9)
influence of NE monsoon. The NE monsoon region
comprises of 5 meteorological sub-divisions over the
southern peninsular India, namely, coastal Andhra
Pradesh, Rayalaseema, South interior Karnataka,
Kerala and Tamil Nadu.
For both SW and NE monsoons, the time series of SPEI
shows considerable interannual and multidecadal variations
with a slight negative trend (Fig. 6.1a, b), corresponding to
the respective monsoon rainfall variations. The declining
trend in SPEI time series is indicative of an increase in the
intensity of droughts. The annual scale SPEI time series is
shown in Fig. 6.1c. The variability in the frequency of SW
monsoon droughts during different epochs can be noted in
Table 6.2. The drought frequency for the period 1901–2016
revealed 21, 19 and 18 cases of moderate to extreme
droughts (SPEI −1) for the SW, NE monsoons and
annual timescale, respectively, with almost 2 droughts per
decade on an average. The number of wet monsoon years
(SPEI 1) is found to be 16, 14 and 19 for the SW, NE
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121
Fig. 6.2 Spatial pattern of trends (decade−1) in a SPEI-SW for JJAS,
b SPEI-NE for OND and c SPEI-ANN for annual, during 1951–2016.
Regions with statistically significant (at 95% confidence level) trends
are hatched. d Frequency of annual droughts (SPEI-ANN −1.0) per
decade, from 1951 to 2016
monsoon seasons and annual timescale, respectively, with
about 1–2 wet monsoon years per decade. The second half of
data period (1951–2016) has witnessed frequent droughts
with 14, 11 and 12 cases, compared to the previous period
1901–1950 (7, 8 and 6), for the SW, NE monsoon seasons
and annual timescale, respectively.
The number of severe to extreme drought cases (SPEI
−1.5) for 1951–2016 period is 6, 4 and 5 for the SW, NE
monsoon seasons, and annual timescales respectively,
compared to 2, 5 and 1 during 1901–1950 (Fig. 6.1a–c).
Additionally, an increasing trend in drought area is observed
for the entire period of analysis (1901–2016), with a
122
statistically significant trend (at 95% confidence level) for
JJAS season and annual timescale for 1951–2016 (Fig. 6.1
d–f). The period 1951–2016 also witnessed 1.2%, 1.2% and
1.3% increase in dry area per decade for SW, NE monsoon
seasons and annual timescale, respectively. It is interesting
to note that the drying trends are slightly higher for annual
scale droughts. The analysis thus shows that the period
1951–2016 witnessed an increase in frequency and areal
extent of droughts. Consistent with this, previous studies
also reported an increase in frequency, duration as well as
the intensity of the monsoon droughts for the post-1960
period compared to the pre-1960 period (Mallya et al. 2016;
Mishra et al. 2016). Further, a relative enhancement of
moderate to severe drought frequency has occurred during
the recent epoch of 1977–2010 compared to 1945–1977
(Niranjan Kumar et al. 2013). Interestingly, an increase in
the episodes of two consecutive years with deficient monsoon has also occurred during the post-1960 period
(Fig. 6.1a; Niranjan Kumar et al. 2013). Studies highlight an
increasing trend in dry areas (Niranjan Kumar et al. 2013;
Mallya et al. 2016). The similar conclusions reached by
different studies using different datasets and approaches
provides a “high confidence” finding that frequency, as well
as percentage area under drought, have increased over the
Indian subcontinent during the second half of the twentieth
century when compared to the first half of the century.
Significant drying trend (negative values in SPEI), during
the SW monsoon season, was observed over the humid
regions of Central India, and over some regions of north-east
as well as west coast of India during 1951–2016 (Fig. 6.2a).
A wetting trend is noticed over north-west and few parts of
southern peninsular India (Fig. 6.2a). This indicates that the
humid regions exhibit a tendency towards drying and more
intense droughts during 1951–2016. This drying tendency is
seen prominently during recent decades (Yang et al. 2019).
Long-term (1901–2002) multiple data sources and methods
also revealed that droughts are becoming much more
regional in recent decades and depict a general migration
from west to east and over the Indo-Gangetic plain (Mallya
et al. 2016). This study also identified an increase in the
duration, severity and spatial extent of droughts during the
recent decades, highlighting the Indo-Gangetic plain, parts
of coastal south India and central Maharashtra as regions that
are becoming increasingly vulnerable to droughts. Strong
drying over the central and the north Indian regions
(Fig. 6.2a) has also been revealed from other observational
studies using rainfall observations (Krishnan et al. 2013;
Preethi et al. 2017a) and various drought indices (Pai et al.
2011; Niranjan Kumar et al. 2013; Damberg and AghaKouchak 2014; Yang et al. 2019). It is to be noted that these
regions are also accompanied by an increase in aridity
(Ramarao et al. 2019; Yang et al. 2019). As a result, the
conclusion regarding, the drying and potential for increasing
M. Mujumdar et al.
drought propensity over central and northern India, is a high
confidence finding.
During the NE monsoon season, the spatial trends in
SPEI depict an increase in drought intensity over the
majority of region (Fig. 6.2b). A similar pattern as that of
SPEI-SW is seen for the entire year (Fig. 6.2c) probably due
to the dominance of rainfall contribution from SW monsoon
compared to that of NE monsoon. It is worth noting that the
regions which witnessed significant drying trend, e.g. Central India, Kerala, some regions of the south peninsula, and
north-eastern parts of India, also experience higher annual
frequency of droughts, with more than two droughts per
decade on average for the 1951–2016 period (Fig. 6.2d),
thus confirming that these regions are becoming more vulnerable to droughts during recent decades (high confidence).
The frequent and intense droughts will likely pose significant challenges for food and water security in India by
depleting soil moisture and groundwater storages (Asoka
et al. 2017). Soil moisture droughts hamper crop production
in India, where the majority of the population depends on
agriculture and leads to famines over the region (Mishra
et al. 2019). Past studies have reported that the frequency
and areal extent of soil moisture-based droughts have
increased substantially during 1980–2008 (Mishra et al.
2014), and hence, efforts are being made to provide forecasts
of standardized soil moisture index over India (Mishra et al.
2018;
https://sites.google.com/iitgn.ac.in/expforecastland
surfaceproducts/erf-forecasted-sri-and-ssi).
Apart from the aforementioned observational studies, a
limited number of investigations using climate models are
available that provide additional insight into the drought
occurrence and variability. Among the various climate models participated in the Coupled Model Intercomparison Project 5 (CMIP5), very few could capture the observed
monsoon rainfall variability, particularly the frequent occurrence of droughts and spatial variability of rainfall during
drought years in the recent historical period (Preethi et al.
2019). Further, a marked increase in the propensity of monsoon droughts similar to the observations during the
post-1950s is reasonably well simulated by the high resolution (horizontal grid size *35 km) Laboratoire de
Météorologie Dynamique (LMDZ4) global model with telescopic zooming over the South Asia region (Krishnan et al.
2016). It is reported that the SPEI index at 12-month and
24-month timescales in historical simulation (with both natural and anthropogenic forcings) exhibits an increase in the
frequency and intensity of droughts during 1951–2005, which
is possibly attributed to the influence of anthropogenic forcing on the weakening monsoon circulation and rainfall over
the India subcontinent (Krishnan et al. 2016). It is important
to note that the climate models have a large bias in simulating
monsoon rainfall and its variability on different timescales
(Turner and Annamalai 2012; Chaturvedi et al. 2012;
6
Droughts and Floods
123
Rajeevan et al. 2012; Jayasankar et al. 2015; Preethi et al.
2010, 2017b). Large uncertainties are also found in reconstructing agricultural drought events for the period 1951–
2015 based on simulated soil moisture from three different
land surface models (Mishra et al. 2018). These uncertainties
are mainly due to differences in model parameterizations and
hence the study highlighted the importance of considering the
multi-model ensemble for real-time monitoring and prediction of soil moisture drought over India.
6.2.1 Drought Mechanism
General features associated with SW monsoon droughts are
weaker meridional pressure gradient, a larger northward
seasonal shift of the monsoon trough, more break days,
reduction in the frequency of depressions and shorter westward extent of depression tracks (Mooley 1976; Parthasarathy et al. 1987; Raman and Rao 1981; Sikka 1999).
Droughts during the SW monsoon are, in general, significantly related to external forcings such as sea surface temperature (SST) variations in the tropical oceans, particularly
with the warm phase of El Niño–Southern Oscillation
(ENSO; Sikka 1980, 1999; Pant and Parthasarathy 1981; Pai
Fig. 6.3 Schematic diagram
representing the interactive
mechanisms leading to droughts
(This Schematic is an adaptation
of Fig. 4.4 in Joint COLA/CARE
Technical Report No.2, July 1999
Monsoon Drought in India by D.
R. Sikka, and is used with
permission of the Center for
Ocean-Land-Atmosphere
Studies.)
et al. 2011, 2017; Mishra et al. 2012b; Preethi et al. 2017a
and the references therein) events in the eastern equatorial
Pacific, central Pacific El Niño (Kumar et al. 2006) or El
Niño Modoki events (Ashok et al. 2007) and also the negative Indian Ocean Dipole (IOD) events (Saji et al. 1999;
Ashok et al. 2001). Apart from the tropical teleconnections,
impacts on SW monsoon droughts from extra-tropics are
evident from negative phase of the North Atlantic Oscillation (NAO; Goswami et al. 2006) on interannual timescale,
negative phase of Atlantic Multidecadal Oscillation (AMO;
Goswami et al. 2006) and positive phase of Pacific Decadal
Oscillation (PDO; Krishnan and Sugi 2003) on multidecadal
timescales. On the other hand, NE monsoon droughts are
associated with a negative phase of ENSO (La Niña) and
negative phase of IOD (Kripalani and Kumar 2004). In
addition to the tropical influence, extratropical influence is
evident as a relationship between the positive phase of the
NAO and NE monsoon drought (Balachandran et al. 2006).
Further details can be obtained from Box 3.2 in Chap. 3. It is
to be noted that these teleconnections exhibit a secular
variation, with epochs of strong and weak relationship with
SW as well as NE monsoon rainfall (Kripalani and Kulkarni
1997; Kumar et al. 1999; Pankaj Kumar et al. 2007; Yadav
2012; Rajeevan et al. 2012).
MONSOONAL DROUGHTS
Surface Boundary Conditions
Land Surface
Processes
SST
Eurasian Snow
Cover
ENSO / IOD
Cycle
Other Possible Causes
Interacive Dynamics
Low frequency
Intraseaonal
30-60 day scale
Solar
Volcanic
Northward
Moving Episodes
Eastward
Moving Episodes
Symoptic Scale
< One Week
S. H. mid
Latitudes
Indian and West
Pacific Oceans
N. H. mid
Latitudes
Anthropogenic
Stratospheric ?
124
M. Mujumdar et al.
Table 6.3 List of recent major droughts over India
Year
Region affected
Cause
1987
Central and North India
Warmer SSTs over equatorial Indian and Pacific oceans related to ENSO and IOD phases
were unfavourable and lead to suppressed rainfall over India (Krishnamurti et al. 1989)
2000
North-west and Central India
Enhanced convective activity associated with the warmer equatorial and southern tropical
Indian Ocean SSTs induced anomalous subsidence over the Indian subcontinent and
thereby weakened the monsoon Hadley cell which ultimately decreased the rainfall. The
warmer SSTs also led to a higher probability of occurrence of dry spells and prolonged
break monsoon conditions over the subcontinent (Krishnan et al. 2003)
2002
North-west parts of India
The anomalous atmospheric convective activity over north-west and north-central Pacific
associated with moderate El Niño conditions induced subsidence and rainfall deficiency
over the Indian landmass (Mujumdar et al. 2007). A slower 30–60 days mode dominated
the season and led to deficit monsoon rainfall (Kripalani et al. 2004). Prevailing circulation
features over mid-latitudes of Eurasia and the south Indian ocean, the negative phase of
SOI, warmer SST over South china sea and El Niño conditions have favoured the monsoon
drought (Sikka 2003)
2008
Central India
The abnormal SST warming in southern tropical Indian Ocean due to the combined
influence of a warming trend in the tropical Indian Ocean and warming associated with the
IOD, resulted in enhancement of convection in the south-west tropical Indian Ocean and
forced anticyclonic circulation anomalies over the Bay of Bengal and Central India, leading
to suppressed rainfall over this region (Rao et al. 2010)
2009
Most of the country except north and
south interior Karnataka
The unfavourable phases of the two important modes, viz., El Niño and the equatorial
Indian Ocean Oscillation (EQUINOO) along with the reversal of the SST gradient between
the Bay of Bengal and eastern equatorial Indian Ocean, played a critical role in the rainfall
deficit over the Bay of Bengal and the Indian region (Francis and Gadgil 2010). Also,
monsoon break conditions extended by the incursion of western Asian desert dry air
towards Central India (Krishnamurti et al. 2010) and by the westward propagating
convectively coupled planetary‐scale equatorial Rossby waves (Neena et al. 2011) leading
to a seasonal deficit in rainfall. Thus, weak cross-equatorial flow, monsoon systems not
moving in land, penetration of mid-latitude upper tropospheric westerlies and the
circulation associated with the Walker and Hadley circulation, with descending motion
over the Indian landmass, collectively resulted in less moisture supply, leading to a drought
(Preethi et al. 2011)
2015
Indo-Gangetic plains and western
peninsular India
Enhanced convective activity associated with the pronounced meridional sea surface
temperature (SST) gradient across the central-eastern Pacific ocean induce large-scale
subsidence over the monsoon region (Mujumdar et al. 2017a)
Also, internal variability induced by the intraseasonal
oscillations of monsoon could lead to seasonal droughts that
are not connected to the known external forcing (Goswami
1998; Kripalani et al. 2004). Monsoon droughts are generally associated with at least one very long break with a
duration of more than ten days (Joseph et al. 2010). The
ocean-atmosphere dynamical coupling, between the monsoon flow and thermocline depth on intraseasonal timescales, in the equatorial Indian Ocean, plays an important
role in forcing extended monsoon breaks and causes
droughts over the Indian subcontinent (Krishnan et al. 2006).
Complex thermodynamical interactions among equatorial
Indo-Pacific and off-equatorial northern Indian Ocean (between 10° N—25° N) convective systems on intraseasonal
timescale as well as the ocean-atmosphere coupling on
interannual timescale can also trigger the occurrence of very
long breaks (Joseph et al. 2010). Moreover, the initiation of
extended breaks resulting in drought conditions could be
influenced by the extratropical systems as well (Krishnan
et al. 2009). A schematic diagram representing the interactive mechanisms leading to large-scale droughts is provided
in Fig. 6.3. Major SW monsoon droughts along with their
possible causes are also listed, in Table 6.3.
In recent decades, warming of the Indian Ocean, at a
faster rate than the global oceans (Roxy et al. 2014) could
have implications on the variability of rainfall over India,
contributing to the declining trend of SW monsoon rainfall
(Roxy et al. 2014; Preethi et al. 2017a) and aiding occurrence of frequent droughts (Niranjan Kumar et al. 2013).
Niyogi et al. (2010) have shown using observational analysis
that landscape changes due to agricultural intensification and
irrigation could also contribute to declining monsoon rains
and aid drought occurrences particularly in northern India.
Also, the increase in anthropogenic aerosol emissions might
have contributed to the observed decline in monsoon rainfall
(see Chap. 3 and Box 5.3 of Chap. 5). In this context, it is
6
Droughts and Floods
important to have an estimation of likely future changes in
rainfall, particularly droughts, under warming scenario.
Additionally, quantitative information on rainfall and related
atmospheric and oceanic parameters prior to the period of
recorded meteorological data is also essential for understanding and possibly mitigating the effects of projected
climate change. Hence, a look into the palaeoclimatic
records has also been made, in the following section, for
understanding the variability of monsoon droughts in the
past.
125
Niño events, probably related to deficient Indian monsoon
rainfall (Fig. 6.4a; Borgaonkar et al. 2010). It is noteworthy,
however, that the mid-eighteenth century is a time where
drought is indicated in northern Thailand (Buckley et al.
2007) and northern Vietnam (Sano et al. 2009), suggestive
of a weakened monsoon in the late eighteenth century. Most
of these periods, including those prior to the mid-eighteenth
century, have also been reported to have widespread
droughts in India (Pant et al. 1993). Aforementioned studies
thus indicate a strong influence of Indo-Pacific SSTs on past
monsoon droughts at decadal to millennial timescales.
6.2.2 Palaeoclimatic Evidences
6.3
Evidences from proxy records indicate that past monsoonal
variations were dominated by decadal- to millennial-scale
variability and long-term trends (Kelkar 2006; Sinha et al.
2018; Band et al. 2018 and references therein). Reconstruction of SW monsoon variability based on stalagmite
oxygen isotope ratios from Central India indicates a gradual
decrease in monsoon during the beginning of the
mid-Holocene from 8.5 to 7.3 ka BP (Before Present: 1950
AD), followed by a steady increase in monsoon intensity
between 6.3 and 5.6 ka BP. This overall trend of monsoon
during the mid-Holocene is punctuated by abrupt megadrought events spanning 70–100 years. During the past
1500 years, centennial-scale climate oscillations include the
Medieval Warm Period (MWP) during 900–1300 AD—with
relatively stronger monsoon, and the Little Ice Age
(LIA) during 1400–1850 AD—with the relatively weaker
monsoon (e.g. Sinha et al. 2007, 2011a, b; Goswami et al.
2015; Kathayat et al. 2017 and references therein). Severe
drought, in India, lasting decades occurred during fourteenth
and mid-fifteenth centuries in LIA. Nearly every major
famine, including the devastating Durga Devi famine during
1396–1409 AD, coincides with a period of reduced monsoon
rainfall, reconstructed from d18O of speleothems collected
from Central India (Sinha et al. 2007). A possible influence
of ENSO is suggested for the Indian monsoon variability
during the mid-Holocene, MWP and LIA (Mann et al. 2009;
Band et al. 2018; Tejavath et al. 2019).
Indian monsoon drought history for past 500 years and its
association with El Nino was derived by Borgaonkar et al.
(2010) from 523-year (1481–2003 AD) tree-ring chronology
from Kerala, south India (Fig. 6.4a). This chronology exhibits a significant positive relationship with the observed SW
monsoon rainfall for the instrumental period (1871–2003
AD; Fig. 6.4b, c). LIA with weaker monsoon is also evident
(Fig. 6.4a: 1600–1700 AD). Higher frequency of low tree
growth occurrences (Fig. 6.4b) was observed in years of
monsoon droughts (Fig. 6.4c), these events are associated
with El Niño since the late eighteenth century. Prior to that,
many low tree growth years were detected during known El
Observed Variability of Floods
Floods, as compared to droughts, have regional characteristics and are typically confined to shorter timescales ranging
from several hours to days. Floods are classified into different types such as riverine (extreme rainfall for longer
periods), flash (heavy rainfall in cities or steep slopes), urban
(lack of drainage), coastal (storm surge) and pluvial (rainfall
over a flat surface) flooding. Regions prone to frequent
floods mainly include river basins, hilly, coastal areas and in
some instances, cities. In India, different types of floods
frequently occur primarily during the SW monsoon season,
the major rainy season. In addition, south peninsular India
experiences floods during the NE monsoon season (Dhar and
Nandargi 2000, 2003). The majority of floods in India are
closely associated with heavy rainfall events, and not all of
these heavy rain events translate into floods. Apart from the
rainfall extremes, flood occurrences are linked to other factors such as antecedent soil moisture, storm duration,
snowmelt, drainage basin conditions, urbanization, dams and
reservoirs, and also proximity to the coast (Rosenzweig et al.
2010; Mishra et al. 2012a; Sharma et al. 2018). In addition,
several other factors, such as infrastructure, siltation of rivers, deforestation, and backwater effect, can accelerate the
impacts of floods.
In India, the spatial variation of floods mostly follows the
monsoon intraseasonal oscillations. For example, Central
India experiences the majority of flood events during the
active monsoon phase, primarily due to heavy rainfall
received from monsoon disturbances (Dhar and Nandargi
2003; Kale 2003, 2012; Ranade et al. 2007; Sontakke et al.
2008). On the contrary, regions near the foothills of the
Himalayas typically experience floods during the break
monsoon condition, due to heavy rainfall associated with the
movement of monsoon trough towards the foothills, orographic uplift of moist monsoon flows and also due to
tropical and mid-latitude interactions (Dhar and Nandargi
2000; Krishnan et al. 2000, 2009; Vellore et al. 2014).
Occasionally, low pressure systems and western disturbances interact to give rise to heavy rains and floods (Sikka
126
Fig. 6.4 Correspondence between tree-ring width index anomaly of
Kerala tree-ring chronology (KTRC) and SW monsoon rainfall.
a KTRC anomaly for the period 1481–2003. Anomalies in
(b) KTRC and (c) SW monsoon rainfall for the instrumental period
of 1871–2003. Smooth line denotes 10-year cubic spline fit. Dashed
lines in the figures indicate “mean ± std.dev.” limits. In Figure (a),
M. Mujumdar et al.
circles indicate low growth events during the years of deficient rainfall
(droughts) associated with El Niño and Squares are low growth
associated with El Niño years. Circles in Figure (b) are low growth
years and have one to one correspondence with deficient ISM
rainfall (drought) years associated with El Niño, shown as circle in
Figure (c)
6
Droughts and Floods
127
Fig. 6.5 a Schematic diagram
representing various types of
floods and causative interactive
mechanisms. b Time series of the
frequency of severe flood events
over India, during 1985–2019
based on flood database of the
Dartmouth Flood Observatory
(http://www.dartmouth.edu/
*floods/Archives/index.html).
The red line in (b) indicates linear
trend
et al. 2015). Low pressure systems during the monsoon
season or active monsoon conditions or monsoon breaks are
the root cause of extreme floods in the South Asian rivers
(Ramaswamy 1962; Dhar and Nandargi 2003). Further
details on heavy rainfall occurrences and monsoon disturbances are given in Chaps. 3 and 7, respectively. In addition
to the intraseasonal variability, floods exhibit variations on
interannual to multidecadal timescales, in association with
the flood producing extreme rainfall events. The variations
in floods are reported to be also linked to the large-scale
climatic drivers such as ENSO, NAO, AMO, PDO
(Chowdhury 2003; Mirza 2003; Ward et al. 2016; Najibi and
Devineni 2018). In particular, flood duration appears to be
more sensitive to ENSO conditions in current climate. Long
duration floods mainly occur during El Niño and La Niña
years, compared to neutral years (Ward et al. 2016), while
the influence of ENSO on flood frequency is not so strong as
that on the flood duration. A schematic diagram representing
the various types of floods and causative interactive mechanisms is provided in Fig. 6.5a.
Under changing climate, an intensification of the global
water cycle could accelerate the risk of floods and exposure to
flooding on a global scale (Milly et al. 2002; Dentener et al.
2006; Trenberth 2011; Schiermeier 2011; Hirabayashi et al.
2013). Observations for the period 1985–2015 reported an
increase in frequency as well as long duration floods over the
globe with a fourfold increase in frequency of floods in
tropics after 2000 (Najibi and Devineni 2018). The increasing
trend in extreme rainfall over the Indian subcontinent, in spite
of the weakening of monsoon circulation observed during the
post-1950s, also hints towards an increase in flood risk in a
warming environment (Rajeevan et al. 2008; Guhathakurta
et al. 2011; Roxy et al. 2017). A noticeable increase in the
flood events has also occurred over the Indian subcontinent.
In particular, urban and river floods (discussed in detail in the
following subsections) have increased considerably along
with the increasing trend in heavy rainfall events. A brief
description of the major flood events that occurred over the
Indian subcontinent since 2000 can be found in Table 6.4.
The analysis of severe flood events using the flood database
of Dartmouth Flood Observatory indicates a statistically
significant increasing trend (1 flood event per decade) in the
frequency of severe flood events over India during the period
1985–2019 (Fig. 6.5b). The severity of the flood events is
calculated following the formulation used by the Dartmouth
Flood Observatory. Studies have shown that extreme floods
over South Asia cluster during excess monsoons and these
extremes are rising post-1950s in the river basins across India
128
M. Mujumdar et al.
Table 6.4 List of recent major flood events over India
Year
Region
Cause
2005 (July)
Mumbai flood
Heavy downpour resulted in a huge rainfall as much as 994 mm of rain fell in just 24 h and
684 mm in only 12 h. Resulted in massive flooding of the Mithi river. The impact was
further amplified by the inadequate drainage and sewage resulting in massive flooding
(Gupta and Nikam 2013)
2007 (August)
Bihar flood
Extremely heavy and long-term rainfall flooded various rivers in Bihar and Uttar Pradesh
2008 (August)
Bihar flood
The flooding of the Kosi river valley in the northern Bihar due to breaking of Kosi
embankment
2012 (June)
Brahmaputra floods
Extremely heavy monsoon rainfall resulted in over-flowing of Brahmaputra river and its
tributaries
2013 (June)
North India floods
(Uttarakhand)
Notable natural disaster in Uttarakhand. Continuous heavy monsoon rainfall followed by
landslides in the hills led to flash flooding (Vellore et al. 2016). This region has recently
experienced frequent flooding and landslides (e.g. flash floods in 2010; floods and landslides
in 2011; and Himalayan flash floods in 2012)
2013 (July)
Brahmaputra floods
Similar to the 2012 event, extremely heavy monsoon rainfall resulted in over-flowing of
Brahmaputra river and its tributaries
2014 (September)
Kashmir floods
Continuous rainfall for more than three days resulted in floods and landslides in Jammu and
Kashmir after the Jhelum river reached above the dangerous level
2015 (June and
August)
Assam floods
Extremely heavy monsoon rainfall resulted in the bursting of Brahmaputra river and its
tributaries causing landslides in the region
2015 (July)
Gujarat flood
Monsoon deep depression over the Arabian Sea caused intense rainfall and flooding across
the coast of Gujarat
2015 (November)
Chennai floods
The transition of low pressure into a deep depression after crossing the coast resulted in very
rainfall. It is likely due to blocking of clouds by the Eastern Ghats which led continuous
rainfall and produced massive urban flooding. (Assessment AR 2016; Van Oldenborgh et al.
2016). Also, rampant urban development could have played a vital role
2016 (July)
Assam floods
Extremely heavy monsoon rainfall mostly during monsoon breaks resulted in the bursting of
Brahmaputra river and its tributaries
2017 (June and July)
North-east India
floods
Extremely heavy monsoon rainfall mostly during monsoon breaks resulted in the bursting of
Brahmaputra river and its tributaries
2017 (July)
Bihar flood
The torrential rain in the Nepal region resulted in a sudden increase in the discharge in all the
eight rivers in Bihar, which led to massive flooding
2017 (July)
West Bengal floods
Week-long continuous rainfall due to cyclone Komen during monsoon resulted in dangerous
floods in West Bengal and Jharkhand
2017 (July)
Gujarat flood
Simultaneous occurrence of rainfall due to low pressure systems from Arabian sea as well as
Bay of Bengal. It is also likely that the heavy inflow into dams Dharoj and Dantiwada
resulted in the massive flooding
2017 (August)
Mumbai flood
Massive Mumbai flood after 2005. The high tide and the extreme rainfall (468 mm in 12 h)
along with inadequate drainage and sewage resulted in massive flooding
2018 (August)
Kerala floods
The unusual rainfall during the monsoon season has resulted in massive flooding. Other
reasons are a sudden discharge of water from the reservoir, land-use changes and landslides
(Mishra and Shah 2018)
2019 (July, August
and September)
Widespread over
Indian regions
A series of devastating floods over areas of several states (such as Maharashtra, Karnataka,
Kerala, Gujarat, Rajasthan, Andhra Pradesh, Orissa, Uttarakhand, Madhya-Pradesh, Bihar,
Uttar Pradesh, West Bengal, Assam and Punjab) due to persisting monsoonal deluges with
excessive rain rates, stream flow and run-off during peak monsoon months extending into
September (Global Disaster Alert and Coordination System, GDACS www.gdacs.org,
https://erccportal.jrc.ec.europa.eu/ and http://floodlist.com/tag/india)
(Kale 2012; Nandargi and Shelar 2018; Mirza 2011; Ali et al.
2019). Increase in extreme rainfall events (Goswami et al.
2006; Rajeevan et al. 2008; Guhathakurta et al. 2011), rate of
intensification of cyclones into severe cyclones (Niyas et al.
2009; Kishtawal et al. 2012; Chap. 8 on Extreme storms) and
prolonged breaks (Ramesh Kumar et al. 2009; Chap. 3 on
6
Droughts and Floods
Precipitation changes in India) are suggested to be the possible reasons for the intensification of river floods during
post-1950 period, in addition to the anthropogenic induced
changes in the catchment and river hydrology (Kale 2012).
The increasing trend in floods is also possibly attributed to
long-term climate variability (Ward et al. 2016; Najibi and
Devineni 2018).
129
the region (Assessment AR 2016, vandenborgh et al. 2016).
In spite of numerous flood occurrences, there is a knowledge
gap in assessing the impact of climate change on flooding
over urban areas. However, floods in Mumbai and Kolkata
are attributed to the impact of climate shifts, urbanization,
sea-level rise and other regional factors.
6.3.2 River Floods
6.3.1 Urban/Coastal Floods
In general, urban areas are prone to river or flash flooding.
Additionally, the major factors for urban floods include the
effect of anthropogenic geographical alterations, inadequate
drainage and storm water management system as well as
high structural inhomogeneity due to intense land-use
changes in proportion to increased urban population, and
also the increasing population (Carvalho et al. 2002; Shepherd 2005; Goswami et al. 2010; Yang et al. 2015; Liu and
Niyogi 2019). Under global warming, the observed
increasing trend in heavy rainfall events has resulted in more
frequent and intense flash floods over urban areas (Kishtawal
et al. 2010; Guhathakurta et al. 2011; Mishra and Lilhare
2016). It is also reported that the regions which are not
traditionally prone to floods experience severe inundation
due to downpour and cloud burst during recent decades.
The major urban flood events of India have occurred in
Mumbai (2005, 2014, 2017), Bangalore (2005, 2007, 2015),
Chennai (2002, 2004, 2005, 2006, 2007, 2015), Ahmadabad
(2017) and Kolkata (2007, 2017). It is to be noticed that
three major metropolitan Indian cities experienced severe
flooding in the same year 2005, i.e. Mumbai in July 2005,
Bangalore and Chennai in October and December 2005,
respectively (Guhathakurta et al. 2011). In the case of
Mumbai flood of 2005, apart from about 944 mm rainfall
recorded in 24-h, the intrusion of sea water into the city
resulted in mass inundation due to the complex drainage
system (Gupta and Nikam 2013). Studies also suggest that
Mumbai region is highly vulnerable to climate change due to
sea-level rise, storm surge and extreme precipitation (Hallegatte et al. 2010). The coastal city of Kolkata is also prone
to flooding due to extreme rainfall activities associated with
tropical cyclones. The subsidence of land in this region
combined with high-tide results in heavy flooding and is a
major problem for the river-side dwellers of the city which
could be exacerbated in this era of climate change (Dasgupta
et al. 2013). The Chennai city is more prone to tropical
disturbances, and cyclones, which often leads to flooding of
major rivers and clogging of drainage systems (Boyaj et al.
2018). A major flood event occurred in December 2015 was
reported as one of the most disastrous floods in the history of
The major river basins of South Asia such as the Brahmaputra, Ganga, Meghna, Narmada, Godavari and Mahanadi are
mainly driven by the SW and NE monsoons apart from snow
and glacier melt for Himalayan rivers (Mirza 2011). River
basin scale flooding is generally due to the occurrence of
extreme rainfall as well as variations in the factors associated
with the basin catchment characteristics (e.g. Mishra and
Lilhare 2016). Several intense floods were recorded in all the
large river basins in South Asia during the second half of the
twentieth century, such as the 1968 flood in the Tapi river, the
1970 flooding of the Narmada, the 1978 and 1987 floods on
the Ganga, the 1956 and 1986 floods in the Indus river, the
1979 flood of the Luni river, the 1982 flooding of the
Mahanadi river, the 1986 flooding of the Godavari river, the
1988 and 1998 floods of the Brahmaputra and the catastrophic
flood of 2010 along the Indus basin (Kale 2012). River basins
located in Central India, i.e. Ganga, Narmada-Tapi and
Godavari, exhibit a significant increasing trend in the area
covered by heavy rainfall episodes, during the monsoon
season for the period 1951–2014 (Deshpande et al. 2016),
which has lead to increased flooding over these basins. The
increase in flood events over the Ganga-Brahmaputra basin is
compounded by subsidence of land (Higgins et al. 2014) as
well as glacier and water from snowmelt feeding into these
rivers (Lutz et al. 2014). Hence, in a global warming scenario,
melting of glaciers and snow could get accelerated and could
lead to larger flood risks in the Himalayan rivers.
River floods are found to have a close association with
ENSO events. A strong connection between rainfall over the
Ganga-Brahmaputra-Meghna basin and the Southern Oscillation Index (SOI) was identified (Chowdhury 2003), with
less than normal rainfall during the negative phase of the
SOI (El Niño) whereas severe flooding due to significant
increase in rainfall during positive SOI (La Niña). Moreover,
major floods over the basin have occurred during La Niña
years and also La Niña years co-occurring with negative
IOD events (Pervez and Henebry 2015). The extreme floods
across the Brahmaputra river in 1988 (Bhattacharyya and
Bora 1997), 1998 (Dhar and Nandargi 2000), 2012, 2016,
2017; over the Narmada river in 1970, 2012, 2016 and over
the Indus river in 1956, 1973, 1976, 2010 (Houze et al. 2011;
130
M. Mujumdar et al.
Webster et al. 2011; Mujumdar et al. 2012; Priya et al. 2015)
have also occurred during La Niña years. Thus indicating
that, in addition to the regional factors, remote forcing also
has a strong influence on the flood occurrences in the Indian
river basins.
In general, the increasing trend in the heavy rainfall
events is found to be the major factor for the rising trend in
flood occurrences in India. However, with the limited
observational flood records, it is difficult to ascertain whether
the increasing trend in floods is attributed to natural climate
variability or to anthropogenically driven climate change. In
this context, an assessment of palaeoclimatic records from
the Indian subcontinent can provide crucial information on
the natural variations in floods during the pre-instrumental
era and the same is provided in the next section.
6.3.3 Palaeoclimatic Evidences
Palaeoclimate records from Indian peninsular rivers have
indicated the occurrence of floods in the ancient period as
well (Kale and Baker 2006; Kale 2012). Moreover, considerable variations in the frequency and magnitude of large
floods during the last two millennia are observed in some of
the western, central and south Indian rivers such as Luni,
Narmada, Tapi, Godavari, Krishna, Pennar and Kaveri. The
Late Holocene period witnessed clustering of large floods
whereas extreme floods were absent during the late MWP
and LIA (Kale and Baker, 2006; Kale 2012). This suggests a
close association of century-scale variations in river floods
with the variations in monsoon rainfall across the Indian
subcontinent. However, a comparison of the Late Holocene
floods with the post-1950 floods over palaeoflood sites in the
Indian peninsular rivers indicates that the recent flood events
are more intense than those during the past (Kale and Baker
2006; Kale 2012).
6.4
Future Projections
India has witnessed an increase in the frequency of droughts
and floods during the past few decades. Notably, the humid
regions of the central parts of India have become
drought-prone regions. Also, the flood risk has increased
over the east coast, West Bengal, eastern Uttar Pradesh,
Gujarat and Konkan region, as well as a majority of urban
areas such as Mumbai, Kolkata and Chennai (Guhathakurta
et al. 2011). Given the adverse impacts of droughts and
floods on food and water security in India, it is imperative to
understand the future changes in drought and flood characteristics projected to develop suitable adaptation and mitigation policies.
6.4.1 Droughts
Climate model projections indicate an increase in monsoon
rainfall, however, the models also show a large inter-model
spread leading to uncertainty (Turner and Annamalai 2012;
Chaturvedi et al. 2012; Jayasankar et al. 2015). Apart from
this, a probable increase in the severity and frequency of both
strong and weak monsoon as indicated by strong interannual
variability in future climate is suggested by a reliable set of
CMIP5 models, identified based on their ability to simulate
monsoon variability in the current climate (Menon et al. 2013;
Sharmila et al. 2015; Jayasankar et al. 2015). Along with this,
an increase in consecutive dry days has also been projected
for the future (see Chap. 3 for details). However, drought
severity and frequency in the future warming climate remain
largely unexplored over India and are considered in a limited
number of studies. Hence, to bring out characteristics of future
droughts, additional analysis is undertaken using six
dynamically downscaled simulations using the regional climate model RegCM4 for historical, RCP 4.5 and RCP 8.5
scenarios till the end of the twenty-first century. These simulations are available from CORDEX South Asia experiments, and details are provided in Box 2.3, Table 2.6 (see list
of IITM-RegCM4).
Similar to the observations, discussed in Sect. 6.2 (see
Box 6.1), the SPEI drought index is computed for 4-month,
3-month and 12-month timescales spanning the JJAS
(SPEI-SW), OND (SPEI-NE) seasons and annual from
January to December (SPEI-ANN), for 1951–2099 with
respect to the base period 1976–2005. Monthly rainfall and
PET computed using the Penman-Monteith formula, from
the six downscaled historical, RCP 4.5 and RCP 8.5
experiments, are used to derive SPEI. Consistent with the
observation (Fig. 6.1a–c), the ensemble mean of CORDEX
simulations (Fig. 6.6a–c) depicts a weak negative trend in
SPEI for both the monsoon seasons and annual timescale.
The large spread among the different members indicates low
skill in simulating the rainfall variability over the Indian
subcontinent, as mentioned earlier. The future projections,
however, depict a spread larger than the historical period for
both the scenarios RCP 4.5 and RCP 8.5, for all the seasons
(Fig. 6.6a–c). The spread is seen more notably, especially
for the SW monsoon season (Fig. 6.6a) and for RCP 8.5
scenario (Fig. 6.6a–c). In spite of the large spread, the
ensemble mean projected a weak declining trend till 2070 for
all the time series in both the scenarios. A stronger
decreasing trend is projected for the post-2070 period by the
high emission RCP 8.5 scenario compared to the medium
emission scenario of RCP 4.5. Also, a weak increasing trend
in drought area is simulated for the historical period, compared to that of observations (Fig. 6.1d–f). Similar to
drought intensity (Fig. 6.6a–c), large spread among the
6
Droughts and Floods
131
Fig. 6.6 Time series of a–c SPEI and d–f difference in percent area
affected by drought during 1951–2099 (relative to 1976–2005), from
six CORDEX South Asia downscaled regional climate simulations for
a, d SW monsoon b, e NE monsoon and c, f annual scale. The
historical simulations (grey) and the downscaled projections are shown
for RCP4.5 (blue) and RCP8.5 (red) scenarios for the multi-RCM
ensemble mean (solid lines) and the minimum to the maximum range of
the individual RCMs (shading)
ensemble members is noted in all the timescales (Fig. 6.6d–
f). Compared to the historical period, the spread is larger in
the future projections, indicating the difficulty in estimating
the drought characteristics for the future. For 2006–2099, the
dry area is projected to increase by 0.84%, 0.35% and 1.64%
per decade under RCP 4.5 scenario for SW, NE monsoons
and annual timescale, respectively. A significant (95%
confidence level) increase of 1.87, 3.22 and 3.81% per
decade are projected for the respective seasons under RCP
8.5 scenario. It is interesting to note that the projected trends
in dry areas are slightly higher for annual scale droughts.
Under the RCP 4.5 scenario, the drought area is projected to
reduce slightly after the 2070s until the end of the century.
Under RCP 4.5 scenario, the ensemble mean frequency of
SW monsoon droughts is projected to increase by 1–2 events
per decade over central and northern parts of India with more
than two droughts over eastern parts of India in both near
(2040–2069) and far (2070–2099) future, against the
reference period of 1976–2005. On the other hand, a
decrease of 1–2 drought events per decade is projected over
southern peninsular India under RCP 4.5 scenario (Fig. 6.7a,
d). Under RCP 8.5, more frequent occurrence of moderate
and severe SW monsoon droughts (>2 events per decade) is
projected over Gangetic plains, north-west and central parts
of India while a reduction in drought frequency is projected
over south peninsular India (Fig. 6.7g, j). For NE monsoon,
the drought frequency is projected to increase by 1–2 events
under RCP 4.5 scenario (Fig. 6.7b, e), whereas an increase
by more than two events per decade in the far future is
indicated under RCP 8.5 scenario (Fig. 6.7h, k). An increase
of more than three annual scale drought events per decade is
suggested for north-west India during the twenty-first century under RCP 4.5 scenario (Fig. 6.7c, f). The increase in
drought frequency (>3 events per decade) for the near future
(Fig. 6.7i) is larger under RCP 8.5 scenario in comparison
with RCP 4.5. Moreover, the area with more than three
132
M. Mujumdar et al.
Fig. 6.7 Changes in frequency (per decade) of moderate and severe
droughts (SPEI −1) for a, d, g, j JJAS, b, e, h, k OND and c, f, i,
l annual timescale, projected by multi-model ensemble of six CORDEX
simulations for a–c, g–i near future (2040–2069) and d–f, j–l far future
(2070–2099) from a–f RCP4.5 and g–l RCP8.5 scenarios, with respect
to the historical period (1976–2005)
drought events per decade is projected to expand across most
of the regions in India by the end of the twenty-first century
under RCP 8.5 scenario (Fig. 6.7l). Thus, more frequent
droughts are projected under RCP 8.5 in comparison with
RCP 4.5 for all the timescales.
Similar results are obtained from various climate model
studies. A probable increase in drought intensity over the
Indian region towards the end of the twenty-first century is
suggested under the RCP 4.5 scenario (Krishnan et al. 2016).
SPEI index derived from 5 CMIP5 climate models indicates
a possibility of more frequent occurrences of severe droughts
by the end of the twenty-first century under both RCP 4.5
and RCP 8.5 scenarios. The similar results are reproduced
using CORDEX South Asia experiments (Spinoni et al.
6
Droughts and Floods
2020). The area affected by severe drought is also projected
to increase by 150% with warming under RCP 8.5 scenario
(Aadhar and Mishra 2018). Another study using a subset of
9 CMIP5 model simulations also suggests a high likelihood
of above moderate drought conditions along with a significant rising trend in a drought area and an increase in the
average drought length in a warming climate under RCP 4.5
and RCP 8.5 scenarios (Bisht et al. 2019). Using multiple
drought indices like PNP, SPI and percentage area of
droughts, two CMIP5 models, which adequately simulate
frequent droughts during recent decades, have projected
frequent occurrence of droughts during near and mid future,
with a pronounced intensification over Central India,
dynamically consistent with the modulation of the monsoon
trough under RCP 4.5 scenario (Preethi et al. 2019).
On the other hand, few studies have revealed contradicting drought projections. For example, a study using
drought projections based on SPI shows a decrease in the
drought frequency in the twenty-first century (Aadhar and
Mishra 2018). A global analysis of the CMIP5 projections of
a drought hazard index based on precipitation in a warming
climate (Carrão et al. 2018) evaluated that although drought
has been reported in the agriculture dominated parts of India
at least once in every 3 years during the past five decades,
the CMIP5 ensemble mean for the present time period is
found to be less consistent with the observed drought hazard
index over subtropical western India. Further, this study
concluded that although the clear signals of wetting are
found in the CMIP5 simulations for the core monsoon zone
in South Asia-east India, the projected future changes in
drought hazard are neither robust nor significant for this
region. Also the SPEI analysis, based on CORDEX simulations, suggests that the projected future change in drought
frequency is not robust over India (Spinoni et al. 2020).
It is interesting to note that recent climate modelling
studies suggest a possible frequent occurrence of El Niño
events in the future (Cai et al. 2014; Azad and Rajeevan
2016) with a stable inverse relationship between El Niño and
monsoon rainfall (Azad and Rajeevan 2016). This is in turn
indicative of the persistent influence of El Niño events on the
Indian monsoon droughts in the future. Moreover, in a
warming climate, the rise in atmospheric water demand (or
PET) can lead to depletion of soil moisture and prolonged
drought conditions (Scheff and Frierson 2014; Ramarao
et al. 2015; Krishnan et al. 2016). In summary, the above
studies using regional as well as global models indicate that
there is a high likelihood of an increase in the frequency,
intensity and area under drought conditions even in a wetter
and warmer future climate scenario. However, the large
spread in the model simulations and implementation of
different drought indices introduces uncertainties in analysis
which eventually brings down confidence in future projections of droughts. In spite of that, an increase in droughts in
133
the future can pose a severe threat to the availability of
regional water resources in India and highlight the need for
better adaptation and water management strategies.
6.4.2 Floods
Many studies have projected a possible increase in extreme
precipitation events in a warming environment (see Chaps. 3
, 7, and 8 for further details), which could likely increase the
flood risk over the Indian subcontinent. Analysis of precipitation extremes under 1.5 and 2.0 °C global warming levels
(GWL), committed under the “Paris Agreement”, suggested
a rise in the short duration rainfall extremes and associated
flood risk over urban areas of India (Ali and Mishra 2018).
The increase in temperature over Indus-Ganga-Brahmaputra
river basins, which are highly sensitive to climate change, is
projected to be in the range of 1.4–2.6 °C (2.0–3.4 °C)
under 1.5 °C (2 °C) GWL. A further amplified warming is
projected under RCP 4.5 and RCP 8.5 scenarios, possibly
leading to severe impacts on streamflow and water availability over these river basins (Lutz et al. 2019). Due to the
proximity of Indus-Ganga-Brahmaputra river basins to the
foothills of the Himalayas, the run-off is projected to
increase primarily by an increase in precipitation and
accelerated meltwater in a warming environment, at least
until 2050 (Lutz et al. 2014). Other major river basins of
India also suggest an increase in run-off in the future, with
the most significant change over the Meghna basin, indicating a high probability for flood occurrences (Mirza et al.
2003; Mirza 2011; Masood et al. 2015).
The projected changes in the frequency of extreme
flooding events of 1-day, 3-day and 5-day duration for the
periods 2020–2059 and 2060–2099 estimated based on the
20-year return period streamflow values with respect to the
historical base period (1966–2005) are provided in Fig. 6.8
(modified from Ali et al. 2019). A higher increase in 1-day
flood events is projected for the far future than that of the
near future under RCP 8.5 scenario (Fig. 6.8a). The highest
increase is located over the Brahmaputra basin as well as the
river basins in the central parts of the Indian subcontinent,
while the least increase is seen over the Indus basin. It can
also be noticed that the projected increase in multi-day (3
and 5 days) flood events is more compared to one-day
events across all the river basins under both RCP 2.6 and
RCP 8.5 scenarios (Fig. 6.8). The increase in the frequency
of all the flood events of different duration is more in the
high emission scenario of RCP 8.5 compared to low emission scenarios of RCP 2.6. In another study, a rise in flood
frequency, with respect to the magnitude of floods of
100-year return periods in the historical simulation, is projected over the majority of the Indian subcontinent in the
twenty-first century under the RCP8.5 scenario by CMIP5
134
Fig. 6.8 Changes in frequency of a–d 1-day, e–h 3-day and i–l 5-day
duration extreme flood events, projected for a, e, i; c, g, k near future
2020–2059 and b, f, j; d, h, l far future 2060–2099, exceeding 20-year
M. Mujumdar et al.
return level based on the historic period 1966–2005, as derived from
the ensemble mean of five GCMs for a, b; e, f; i, j RCP2.6 and c, d; g,
h; k, l) RCP8.5 scenario (Modified from Ali et al. 2019)
6
Droughts and Floods
Fig. 6.9 a COSMOS-INDIA network, b COSMOS-IITM site. Notation A, B, C, D and E are used to indicate different
hydro-meteorological sensor installed at COSMOS-IITM site Pune. A
models. The southern peninsular India, Ganges, and
Brahmaputra basins are projected to experience floods of
similar magnitude at a higher frequency (<15 years) in the
twenty-first century, with high consistency among the
models (Hirabayashi et al. 2013). The majority of studies
projected an increasing flood risk for Indus-GangaBrahmaputra river basins (Higgins et al. 2014; Shrestha
et al. 2015; Kay et al. 2015; Wijngaard et al. 2017; Lutz
et al. 2019) and these basins are considered as hotspots in a
changing climate (De Souza et al. 2015). The increased flood
risk in terms of frequency and duration of flood events can
exert profound impacts on food production, water resources
and management. Additionally, the human-induced influences such as land-use changes, irrigation, mismanagement
of dams and reservoirs can aggravate the magnitude and the
frequency of flood events in the future.
135
—COSMOS Probe; B—Data logger; C1,2,3—Multiweather component
sensor; at 2 heights 10 and 20 m; D—Net radiometer E—Eddy
covariance system
6.5
Knowledge Gaps
This section highlights some of the knowledge gaps that
would be of relevance for future studies on drought and
flood assessments.
1. Lack of dense observational networks for essential climate variables like soil moisture, surface and sub-surface
energy, water fluxes, stream flow, etc. limits our scientific understanding of the complex multiscale (spatial and
temporal) interactions taking place in the climate system.
To better understand the processes involved in the variations of intensity and duration of floods and droughts
over India in a warming climate, novel observational
datasets are highly required. For example, network of the
136
M. Mujumdar et al.
newly developed neutron scattering method, used in
non-invasive Cosmic ray soil moisture monitoring system (COSMOS), could potentially help scale gap
between the conventional point scale, remote sensing
techniques and model simulations of surface soil moisture (see Fig. 6.9 and Mujumdar et al. 2017b).
2. Attribution of anthropogenically induced climate change
to the variability of drought and floods in historical as
well as future projections remains a challenging issue and
an open problem for further scientific research.
3. Model uncertainties in reproducing the observed variability of droughts and floods, as well as the spread
among the models, also hamper our confidence in
assessing future changes. Thus efforts are needed for
reducing the model uncertainties.
4. Assessing the impact of increasing urbanization, as well
as agricultural intensification on the hydroclimatic
extremes of heavy rains/floods and droughts, continues to
be a challenge for the Indian monsoon region, and
additional multiscale assessments are critically needed.
6.6
Summary
A detailed assessment of the long-term variability of
droughts and floods in the current as well as future climate is
presented in this chapter, in view to support a better framing
of climate mitigation and adaptation strategies in India.
Indian subcontinent witnessed a decline in monsoon rainfall
along with frequent occurrences of droughts and flood
events in the past few decades, in association with the
changes in regional and remote forcings. Besides, many
studies projected a probable increase in these hydroclimatic
extreme events in a warming environment.
The analysis of SPEI over India for the period 1901–2016
identified more droughts (*2 per decade) compared to wet
(1–2 per decade) monsoon years. For the post-1950 period, a
high frequency of droughts along with an expansion of dry
area at a rate of 1.2, 1.2 and 1.3% per decade is observed in
SW, NE monsoon seasons and annual timescale, respectively. In the humid regions of the country, particularly the
parts of Central India, Indo-Gangetic plains, south peninsula
and north-east India experienced significant drying trend
with more intense droughts during SW monsoon season. On
the other hand, the east coast and southern tip of India show
slight wetting trend during the NE monsoon season. In
recent decades (post-1950 period), droughts have been more
frequent (>2 droughts per decade on average) over Central
India, Kerala, some regions of the south peninsula, and
north-eastern parts of India, making these regions more
vulnerable. These results are consistent among various
studies (Pai et al. 2011, 2017; Niranjan Kumar et al. 2013;
Damberg and AghaKouchak 2014; Mallya et al. 2016;
Krishnan et al. 2016; Mishra et al. 2016; Preethi et al. 2019;
Yang et al. 2019). Thus, it is assessed with high confidence
that the frequency and spatial extent of droughts over the
country have increased significantly along with an increase
in intensity, mainly confining to the central parts including
the Indo-Gangetic plains of India, during 1951–2016. These
changes are observed in association with the decline in
monsoon rainfall, which is likely due to an increase in
anthropogenic aerosol emissions in the northern hemisphere,
regional land-use changes as well as warming of the Indian
Ocean. During this period, an increasing trend in floods is
also reported over the majority of the Indian river basins
associated with the rise in heavy rainfall episodes. In addition to the enhanced stream flow due to increase in extreme
precipitation events, the floods over the Himalayan rivers are
compounded by subsidence of land as well as glacier and
snowmelt water feeding into these rivers. The observed
increasing trend in heavy rainfall events combined with the
intense land-use changes has resulted in more frequent and
intense flash floods over urban areas, like Mumbai, Chennai,
Bangalore, Kolkata, etc. (Guhathakurta et al. 2011). Though
there is high confidence in the rising trend in extreme rainfall
events and the associated flood risk over India, its attribution
of climate change remains a challenging issue and an open
problem for further scientific research.
Future projections of regional as well as global climate
models indicate a high likelihood of an increase in frequency, intensity and area under drought conditions over
India, with medium confidence due to large spread in model
projections (Aadhar and Mishra 2018; Bisht et al. 2019;
Preethi et al. 2019). Though the climate models project an
enhanced mean monsoon rainfall, the projected increase in
droughts could be due to the larger interannual variability of
rainfall and the increase in atmospheric water vapour
demand (potential evapotranspiration) over the country
(Menon et al. 2013; Scheff and Frierson 2014; Jayasankar
et al. 2015; Sharmila et al. 2015; Krishnan et al. 2016).
Moreover, climate model projections also indicate frequent
El Niño events in the Pacific Ocean with a stable inverse
relation with the monsoon, which could also result in more
number of monsoon droughts in future (Cai et al. 2014;
Azad and Rajeevan 2016). The climate projections for India
also indicate an increase in frequency of urban and river
floods, under different levels of warming, 1.5 and 2.0 °C, as
well as for different emission scenarios in association with
an expected rise in heavy rainfall occurrences (Hirabayashi
et al. 2013; Ali and Mishra 2018; Lutz et al. 2019). However, larger changes in flood frequency are projected in the
high emission scenario of RCP 8.5. Flood frequency and
associated risk are projected to increase over the major river
basins of India, with a higher risk for the
Indus-Ganges-Brahmaputra river basins in a warming
6
Droughts and Floods
climate (Lutz et al. 2014). The enhanced flood risk is likely
due to increasing stream flow and run-off associated with the
projected increase in frequency of extreme rainfall events
over the major Indian river basins and is compounded by
glacier and snowmelt over the Indus-Ganges-Brahmaputra.
The projected enhanced droughts and flood risk over India
highlight the potential need for a better adaptation and
mitigation strategies.
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