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
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
6 Droughts and Floods
133
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
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
6 Droughts and Floods
133