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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B7. Istanbul 2004
where JW is the seasonal water level for the ith well and jth
observation, W,, its seasonal mean, and o is its standard
deviation.
SWI value has been classified and used as a measure of
hydrological drought intensity. Since ground water level is
measured down from the surface, positive anomalies correspond
to drought and negative anomalies correspond to ‘no-drought’
or normal condition. Point values of SWI corresponding to the
wells have been interpolated to generate SWI maps of the
region using the same technique as for SPI.
2.3 Vegetation Indices
Vegetative and agricultural droughts reflect vegetation stress.
NDVI reflects the vegetation condition through the ratio of
responses in near infrared (Ch2) and visible (Chl) bands of
Advanced Very High Resolution Radiometer (AVHRR) of
NOAA. It is expressed as
NDVI = (Ch2 — Chl) / (Ch2 + Chl) (3)
VCI, TCI, and VHI have been developed further using the
following equations as
VCI = 100*(NDVI — NDVIpin) / NDVImax — NDVInin) (4)
TCI & 100*(BT max FT BT) / (Blox m Blow) (5)
VHL=0.(VCD £0:5(TCDH (6)
where NDVI, NDVImin and NDVInax are the seasonal average
of smoothed weekly NDVI, its multiyear absolute minimum and
its maximum respectively; BT, BT, and BTmax are similar
values for brightness temperature (Kogan, 2001).
Brightness temperature values are obtained from the thermal
band (Ch4) of NOAA-AVHRR. NDVI generally provides a
broad overview of the vegetation condition and spatial
vegetation distribution in a region. Vegetative drought is closely
related with weather impacts. However, in NDVI, the weather
component gets subdued by strong ecological component. VCI
separates the short-term weather-related NDVI fluctuations
from the long-term ecosystem changes (Kogan, 1990, 1995).
Therefore, while NDVI shows seasonal vegetation dynamics,
VCI rescales vegetation dynamics in between 0 and 100 to
reflect relative changes in the vegetation condition from
extremely bad to optimal (Kogan, 1995, 2003). VCI and TCI
characterises respectively the moisture condition and thermal
condition of vegetation while VHI represents overall vegetation
health (Kogan, 2001). Since favourable weather provides
optimal moisture condition, high values of VCI correspond to
healthy and unstressed vegetation. On the other hand, low TCI
values correspond to vegetation stress due to dryness by high
temperature. TCI provides opportunity to identify subtle
changes in vegetation health due to thermal effect as drought
proliferates if moisture shortage is accompanied by high
temperature (Kogan, 2002). During calculation of VHI, an equal
weight has been assumed and assigned to both VCI and TCI
since moisture and temperature contribution during a vegetation
cycle is currently not known (Kogan, 2001). Regional VHI
maps have been generated through interpolation of the point
values and classified using the scheme developed by Kogan
(2002) and further modified*. VCI and TCI have been classified
following the same scheme as VHI. Kogan (1995, 2001, 2003)
formulated, calculated, and applied VCI, TCI, and VHI based
on smoothed weekly NDVI and BT values. However, in the
present study all these parameters have been calculated and
analysed season wise by averaging weekly values. For
comparison and correlation among different kinds of drought,
SPI, SWI, and VHI maps of the same years have been displayed
one beneath the other (Figure 5).
3. DROUGHT IN THE ARAVALLI TERRAIN
3.1 Meteorological Drought
Visual observation of the time series SPI maps of the monsoon
and non-monsoon periods indicate that drought is frequent and
even persistent in many parts of the Aravalli terrain. They
further show that meteorological drought scenario in the terrain
changed. continuously with season. Beside years with
exceptional good non-monsoon rainfall, Aravalli terrain
regularly experienced mild drought for most of its parts in the
non-monsoon period. The condition returns to normalcy if the
monsoon wind appears timely and showers heavily. Spatio-
temporal drought scenario in the Aravalli terrain in both the
monsoon and the non-monsoon periods fallows an erratic
pattern due to inconsistent and unstable rainfall pattern
(Bhuiyan er al., 2004).
During the years 1984 - 1985, insufficient rainfall resulted
seasonal drought in some pockets of northern and southwestern
Aravalli. In the year 1986 drought resulted in those province
during both the monsoon and non-monsoon seasons. Poor
monsoon visited the Aravalli terrain in succession in the year
1987 resulting severe drought in major sectors in the north and
south. Some small pockets in the north, south, and central parts
suffered even extreme drought. The drought condition improved
and worsened alternatively in the monsoon and non-monsoon
periods respectively during the years 1988 - 90. Although good
monsoon rainfall helped most parts of the terrain to avoid
drought in the year 1992, some pockets in the north, south, east,
and west of the terrain experienced drought during 1992 — 1993
non-monsoon and 1993 monsoon seasons. In the year 1995,
moderate drought affected some pockets in the eastern and
southern sectors, while during the non-monsoon period drought
appeared in the northern and eastern Aravalli. However, it was
the western Aravalli experiencing drought during 1996
monsoon! The drought reappeared in the terrain during the
monsoon of 1999 too but the situation became worst during the
monsoon season of 2000, when northern, eastern, southern, and
southwestern sectors of the province formed a continuous
drought-belt (Figure 5).
3.2 Hydrological Drought
Time series analysis of SWI maps reveals that hydrological
drought in the Aravalli terrain during both the pre-monsoon and
post-monsoon periods follow an alternate pattern with minor
local variations. During the pre-monsoon period, many discrete
pockets all over the terrain experienced moderate to extreme
hydrological droughts in last two decades. Although during the
years 1984 — 1985, major parts of the terrain were free from
drought, the drought situation degraded gradually in the
following years.
* Through personal communication
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