251
(India Meteorological department, 1981–2010), it is evident that the climatic condition of the study area is hot and dry in summers, pleasant and cool in winters, and
experiences rainy monsoons. The maximum temperature ranges from 28 °C to
40 °C, and minimum temperature ranges between 12 °C and 9 °C. The average
annual rainfall varies between 1100 and 1500 mm. There are two cropping seasons
in the study area, Kharif and Rabi (Panigrahy et al. 2005). Kharif crops are dependent on monsoonal rainfall for water and are sown in June–July and harvested in the
months of September and October. Rabi crops are cultivated in winter. They are
sown in October–November and harvested in the months of March–April. Drought
is a common phenomenon here (Haldar and Saha 2015).
14.3 Materials and Methods
For this study, three major indices were used, namely, VCI, RAI, and YAI, from
which the intensity and extent of the drought was characterized.
NOAA-STAR (National Oceanic and Atmospheric Administration Center for
Satellite Application and Research, www.star.nesdis.noaa.gov) derived weekly VCI
data was used which was freely downloadable for study area. It is a vegetation
health (VH) product. The satellite-based global VH system was designed to monitor, diagnose, and predict long- and short-term land environmental condition and
climate-dependent socioeconomic activities. The data are globally available with
4 km resolution and are a 7-day composite data. The system contains vegetation
health indices and products like VCI, TCI, etc. Kogan (1995) developed this VCI
using the range of NDVI which is a good indicator for assessing the severity of
agricultural drought. It is defined as shown in Eq. (14.1):
VCI
NDVI NDVI
NDVI
NDVI
=
−
(
)
−
∗
min
m ax
min
/
)100
(14.1)
Where NDVI is the actual value of NDVI and NDVI Max and NDVI Min are smoothed
weekly NDVI absolute maximum and minimum values, respectively.
Primarily maximum growth of Kharif crops in this region starts in the fourth
week of July and stays till the entire August. So, this week was chosen for study.
The weekly data were downloaded and analyzed for the period of 20 years
(1998–2017) for July to August. When study area is very large, then this type of data
provides much more accuracy (Dutta et al. 2015). The VCI of the fourth week of
July and fourth week of August for 20 years was layer stacked separately in ENVI
4.7 software. VCI value ranged from 1 to 100 in the merged data. The values ranging within 50–30% indicated moderate drought condition, and less than 30% indicated severe drought condition (Kogan 1995). In the file of VCI images, the value of
certain pixels showed −9999; these were on the oceans. So, in order to overcome
this situation at first, a mask with min value 0 and max value of 100 was applied,
and then these images were opened in Arc. GIS 10.1. With the help of the
14 Spatiotemporal Extent of Agricultural Drought Over Western Part of West Bengal
(India Meteorological department, 1981–2010), it is evident that the climatic condition of the study area is hot and dry in summers, pleasant and cool in winters, and
experiences rainy monsoons. The maximum temperature ranges from 28 °C to
40 °C, and minimum temperature ranges between 12 °C and 9 °C. The average
annual rainfall varies between 1100 and 1500 mm. There are two cropping seasons
in the study area, Kharif and Rabi (Panigrahy et al. 2005). Kharif crops are dependent on monsoonal rainfall for water and are sown in June–July and harvested in the
months of September and October. Rabi crops are cultivated in winter. They are
sown in October–November and harvested in the months of March–April. Drought
is a common phenomenon here (Haldar and Saha 2015).
14.3 Materials and Methods
For this study, three major indices were used, namely, VCI, RAI, and YAI, from
which the intensity and extent of the drought was characterized.
NOAA-STAR (National Oceanic and Atmospheric Administration Center for
Satellite Application and Research, www.star.nesdis.noaa.gov) derived weekly VCI
data was used which was freely downloadable for study area. It is a vegetation
health (VH) product. The satellite-based global VH system was designed to monitor, diagnose, and predict long- and short-term land environmental condition and
climate-dependent socioeconomic activities. The data are globally available with
4 km resolution and are a 7-day composite data. The system contains vegetation
health indices and products like VCI, TCI, etc. Kogan (1995) developed this VCI
using the range of NDVI which is a good indicator for assessing the severity of
agricultural drought. It is defined as shown in Eq. (14.1):
VCI
NDVI NDVI
NDVI
NDVI
=
−
(
)
−
∗
min
m ax
min
/
)100
(14.1)
Where NDVI is the actual value of NDVI and NDVI Max and NDVI Min are smoothed
weekly NDVI absolute maximum and minimum values, respectively.
Primarily maximum growth of Kharif crops in this region starts in the fourth
week of July and stays till the entire August. So, this week was chosen for study.
The weekly data were downloaded and analyzed for the period of 20 years
(1998–2017) for July to August. When study area is very large, then this type of data
provides much more accuracy (Dutta et al. 2015). The VCI of the fourth week of
July and fourth week of August for 20 years was layer stacked separately in ENVI
4.7 software. VCI value ranged from 1 to 100 in the merged data. The values ranging within 50–30% indicated moderate drought condition, and less than 30% indicated severe drought condition (Kogan 1995). In the file of VCI images, the value of
certain pixels showed −9999; these were on the oceans. So, in order to overcome
this situation at first, a mask with min value 0 and max value of 100 was applied,
and then these images were opened in Arc. GIS 10.1. With the help of the
14 Spatiotemporal Extent of Agricultural Drought Over Western Part of West Bengal