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
130
M. Mujumdar et al.
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
130
M. Mujumdar et al.