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Soil Moisture Trends

Figure 6a shows the patterns of spatial variation of annual mean surface soil moisture over India. Annual mean surface soil moisture is higher (>12 %) over north, central, west coast and north-east India. In contrast, western India and the east coast are having lower (<6 %) annual mean surface soil moisture. Trends in annual surface soil moisture are shown in Fig. 6b. Out of the 27 stations considered for the analysis, 15 stations are showing increasing trends. Stations showing significantly increasing trends are Dantiwada (0.75 % per decade), Kalyani (1.24 % per decade), Pune (3.98 % per decade), Bhubaneswar (0.97 % per decade), Dharwad (1.87 % per decade) and Tirupati (1.99 % per decade) while soil moisture is significantly decreasing at Nagpur (-10.65 % per decade), Bhopal (-2.73 % per decade), Sabour (-2.04 % per decade), Anakpalle (-0.78 % per decade), Solapur (-2.38 % per decade) and Vittal (-2.11 % per decade) as shown in Table 3.

Moisture Content in the Atmosphere

The relative humidity data at three representative levels, namely 850, 500 and 200 hPa, are analysed using the ECMWF Reanalyses (ERA) data. The data averaged over the central Indian region (18-28°N, 73-82°E) (Singh et al. 2014) are shown in Fig. 7a from 1980 to 2012. RH at all the levels (except 200 hPa) shows an increasing trend (significant at a confidence level of 99 % confidence level) during the period. This seems to be consistent with the fact of surface temperatures show increasing trend as reported by Jaswal and Koppar (2011). The increase in relative humidity is also consistent with the fact that the atmosphere shows increasing moist instability over the region for the period as mentioned by Goswami et al. (2006). To test the robustness of the result, another reanalyses data set MERRA relative humidity is analysed for the same period (Fig. 7b). The MERRA data also show an

Table 3 Annual means, standard deviation (Sdev) and trends of surface soil moisture at 27 stations

STATION

Period

Latitude (N)

Longitude (E)

Mean (%)

Std.

dev. (%)

Trend

(%/decade)

Karnal

1998-2008

29° 43'

76° 58'

11.76

3.32

-4.33

Agra

1991-2007

27° 10'

78° 02'

4.16

0.93

0.19

Durgapura

1993-2012

26° 51'

75° 47'

1.78

0.45

-0.19

Basti

1991-2008

26° 48'

82° 46'

15.11

1.34

-0.22

Sabour

1991-2008

25° 14'

87° 04'

18.84

2.21

-2.04a

Dantiwada

1996-2008

24° 10'

72° 29'

1.02

0.52

0.75a

Udaipur(RCA)

1993-2008

24° 35'

73° 42'

5.44

1.49

0.42

Bhopal

1991-2010

23° 16'

77° 25'

11.97

2.69

-2.73a

Sagar

1991-2008

23° 51'

78° 45'

16.31

2.79

1.10

Kanke

1998-2008

23° 17'

85° 19'

11.87

0.53

-0.41

Rajkot

1996-2008

22° 17'

70° 48'

11.83

1.70

-1.13

Anand

1996-2008

22° 35'

72° 55'

3.72

0.74

0.10

Kalyani

1993-2008

22° 05'

82° 20'

15.16

1.30

1.24a

Nagpur

1997-2008

20° 15'

79° 35'

14.51

4.87

-10.65a

Niphad

1991-2008

20° 06'

74° 06'

9.28

1.13

0.25

Bhubaneshwar

1994-2011

20° 15'

85° 52'

4.99

0.77

0.97a

Rahuri

1996-2013

19° 24'

74° 39'

13.10

0.75

0.42

Pune

1996-2012

18° 32'

73° 51'

12.94

3.04

3.98a

Anakapalle

1996-2013

17° 38'

83° 01'

3.51

0.66

-0.78a

Solapur

1996-2013

17° 04'

75° 54'

10.48

1.84

-2.38a

Dharwad (ARS)

1996-2008

15° 26'

75° 07'

8.32

1.05

1.87a

Tirupati

1991-2008

13° 27'

79° 76'

4.46

1.49

1.99a

Vittal

1991-2008

12° 57'

75° 25'

14.48

2.28

-2.11a

Hebbal

1996-2008

13° 00'

77° 37'

5.98

0.51

0.12

Vellanikara

1991-2007

10° 31'

76° 13'

12.68

1.55

0.11

Vedasandur

1992-2008

10° 32'

77° 57'

3.18

0.33

-0.20

Kovilpatti

1996-2011

09° 12'

77° 53'

10.39

0.15

0.05

Statistically significant trends are marked ‘a’

increasing trend (significant at the 99 % confidence level) for the three representative levels, e.g. 850, 500 and 200 hPa, over the central Indian region. Therefore, it indicates that the central Indian region which is considered to be the core monsoon zone shows an increasing trend in relative humidity in the middle and upper troposphere.

Along with the relative humidity, the specific humidity from ERA and MERRA analyses shows an increasing trend for the same period (Fig. 8a, b). The signal of enhanced humidity or moisture content in the atmosphere appears to be consistent with the fact as reported by Goswami et al. (2006) about increasing extreme events (Chap. 3). The precipitable water from ERA and MERRA data also shows an increasing trend (significant at the 99 % confidence level) during the period of

Fig. 7 Relative humidity at 850, 500 and 200 hPa from a ERA and b MERRA analysis averaged over the monsoon core region (18°- 28°N, 73°-82°E)

Specific humidity at 850 and 500 hPa from a ERA and b MERRA analysis averaged over the monsoon core region (18°-28°N, 73°- 82°E)

Fig. 8 Specific humidity at 850 and 500 hPa from a ERA and b MERRA analysis averaged over the monsoon core region (18°-28°N, 73°- 82°E)

Precipitable water (cm) from a ERA and b MERRA analysis averaged over monsoon core region (18°-28°N, 73°-82°E)

Fig. 9 Precipitable water (cm) from a ERA and b MERRA analysis averaged over monsoon core region (18°-28°N, 73°-82°E)

1980-2014 (Fig. 9a, b). The increasing precipitable water appears to be consistent with the analyses by Jaswal (Chap. 7) who showed that there is an increasing cloudiness over the core monsoonal region.

 
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