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
136
M. Mujumdar et al.
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
136
M. Mujumdar et al.