International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B7. Istanbul 2004
In the year 1986, post-monsoon water level was normal or near
normal in major parts of the terrain except some discrete
pockets in the west, and south. Before commencement of the
1987 monsoon, many regions all over the terrain were hit by
mild drought and one major pocket in the south-west suffered
from extreme hydrological drought. The drought situation
further aggravated during the monsoon period of 1987, when
except the western sector rest of the terrain was affected by
moderate to severe hydrological drought. Except the western
sector, the terrain underwent a temporary recovery from the
water stress during the monsoon of 1989. The year 1989 is also
marked by a shift in drought from the east towards the west.
This shift in drought pattern caused some western pockets to
suffer from moderate to extreme droughts consistently since
1989 until 1996. Since the year 1997, the drought shifted again
from the west towards the eastern parts of the terrain (Figure 2).
Js 7 ^
* g f €
5 ? A M - 7 5
WM Jt & e /^ Lt
d > ^ < > m. E
X - { Y > S { # NM , Ü
M A arto y - ^" y "
p b Eh, A km X1
d A Yo v ~~ AP XH EN a w
) À A rt JO
Figure 2
The drought condition worsened gradually from the pre-
monsoon of 1998 onwards, and in the post-monsoon period of
2000, almost the entire terrain was in the grief of acute drought.
The SWI maps show that some pockets particularly in the
western sector of the terrain frequently suffered from water
stress either in the monsoon or non-monsoon period (Figure 3 &
5) or in both seasons (Figure 5).
LP
? 45
f aam
€ {
"At
n A
£ = >
FAS [3 f
£
CC -
1 *
4 * "
2 %
Ps
CN
"
}
ALL
a À À 1E d
1985 1986 1987
Figure 3
3.3 Vegetative Drought
Vegetative drought as observed and understood through NDVI,
VCI, TCI, and VHI implies that like meteorological drought, it
also appears in the Aravalli province in irregular pattern. Major
parts of the Aravalli terrain show favourable vegetation .
condition for most of the years under consideration. There is no
direct relationship among VCI, TCI, and precipitation (Singh
et al., 2003). Extremely unhealthy vegetation (very low VHI) is
generally associated with severe moisture (low VCI) and
thermal stress (low TCI) and vice versa. However, vegetation
health could be represented by many other combinations of VCI
and TCI. For example, in the year 1993 inspite of thermal stress
(magenta, light, and dark orange) the entire Aravalli terrain
could maintain healthy vegetation due to extremely favourable
moisture condition. On the contrary, good vegetation health was
maintained in the next year too inspite of extreme moisture
stress (yellow, orange and red), this time due to optimal thermal
condition (Figure 4).
VCI TCI VHI
AP
1994
Figure 4
The Aravalli terrain experienced vegetative drought for the
three consecutive years of 1985, 1986 and 1987 before
regaining back vegetation health during the monsoon of 1988.
The vegetation stress reached culmination in the year 1987
when almost the entire terrain suffered from severe to extreme
vegetative degradation. Vegetation suffered mild stress in the
years 1989, 1991, 1992, and 1995 but only in some pockets. In
both the seasons of the intermediate years, vegetation in the
Aravalli terrain was free of stress. Vegetation in some discrete
pockets all over the terrain was under stress again during the
monsoon period of 2000.
3.4 Comparison and Correlation
Precipitation and evapotranspiration are the chief causative
parameters while water level is the main responsive parameter
of drought. Vegetative drought is a manifestation of
meteorological and hydrological droughts. The time-series
drought maps generated through various drought indices
indicate that meteorological, hydrological and vegetative
droughts in the Aravalli terrain are not linearly correlated with
one another. Moreover, speed of drought development and
drought duration are also different in different spheres.
Therefore, it is quite common that when one drought index
identifies drought at a particular place, another drought index
indicates a normal condition at the same place and time.
During the years 1986 - 1987, insufficient rainfall in both the
monsoon and non-monsoon periods gradually resulted severe
meteorological and hydrological droughts in almost the entire
terrain except some pockets. Gradual development of
meteorological and hydrological drought in turn imparted
extreme stress on vegetation health resulting acute vegetative
drought during the monsoon of 1987 (Figure 5). In the year
1992, the entire terrain received good monsoon rainfall.
Deficiency in rainfall during non-monsoon period has resulted
water stress in the western sectors. Hydrological drought
covered more area during the monsoon of 1993 inspite of good
rainfall. Vegetation enjoyed an excellent health owing to the
support of good monsoon. In the year 1995, monsoon rainfall
was better in the western sector than the east.
1286
