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
-2
-1
0
1
2
SPEI
0
10
20
30
40
50
Drought area (%)
-2
-1
0
1
2
SPEI
0
20
40
60
80
Drought area (%)
1905 1920 1935 1950 1965 1980 1995 2010
-2
-1
0
1
2
SPEI
1905 1920 1935 1950 1965 1980 1995 2010
0
10
20
30
40
50
Drought area (%)
(c) Annual
(f) Annual
(b) OND
(e) OND
(a) JJAS
(d) JJAS
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
6 Droughts and Floods
119
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
-2
-1
0
1
2
SPEI
0
10
20
30
40
50
Drought area (%)
-2
-1
0
1
2
SPEI
0
20
40
60
80
Drought area (%)
1905 1920 1935 1950 1965 1980 1995 2010
-2
-1
0
1
2
SPEI
1905 1920 1935 1950 1965 1980 1995 2010
0
10
20
30
40
50
Drought area (%)
(c) Annual
(f) Annual
(b) OND
(e) OND
(a) JJAS
(d) JJAS
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
6 Droughts and Floods
119