International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXX V, Part B7. Istanbul 2004
2. THE STUDY AREA
The Aravalli Range, one of the oldest mountain ranges of the
world stretches itself from northern Gujarat to central Rajasthan
states of India, running through more than 700 km. distance and
covering nearly 40,000 sq. km. area to separate *"Thar', the
Great Indian Desert from the eastern plain land. The study area
(latitude N23°30’- N26°18” and longitude E72°24"-E74°36")
comprises of about 25,000 km? of the main block of the Aravalli
Range (Figure 1). The terrain exhibits a semi-arid climate with
high heat flow during summer.
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Figure 1
Rainfall occurs mainly during June — September through the
monsoon wind: non-monsoon rainfall is limited and irregular.
Therefore, water resources, vegetation and agriculture are under
control of the monsoon. The ground water condition in this
terrain varies from place to place due to variations in soil,
lithology, land-use, geomorphology, topography and climatic
conditions. Gneiss and schist covers most parts in the north,
central and southeast of the terrain, whereas phyllites and
phyllitic-schist is dominant in the south, southcentral and some
pockets of the eastern Aravalli. Western part of the terrain is
composed mainly of granite and quartzite along with calcite-
schist as the subordinate rock type (DST, 1994). The normal
water table depth also varies in different lithologic domains. In
general, ground water fluctuates seasonally within the range of
5m to 20 m (GWD, 2000). In the Aravalli region, agricultural
activities are influenced and controlled by rainfall and
availability of water resources. Monsoon crops or Kharif crops
are cultivated during June to September whereas winter crops or
Rabi crops are sewed in the middle of October and harvested in
March. In many parts of the region, summer crops and
vegetables are cultivated during March — May depending upon
availability of water. Double cropping is practiced in some parts
whereas many pockets have uncultivated fallow land.
3. DROUGHT INDICES
In the present paper, spatiotemporal patterns of seasonal
drought indicated through meteorological, hydrological, and
vegetative parameters have been discussed. Analysis has been
carried out by dividing the year into two main seasons — the
monsoon and the non-monsoon or pre-monsoon since drought
in the study area is a function of the monsoon rainfall. The
monsoon period consists of four months from June to
September while non-monsoon or pre-monsoon period is
constituted of the months, intermediate of two successive
monsoon periods. Thus, the non-monsoon 1984 - 85 or the pre-
monsoon 1985 consists of the months October — December of
1984 and January — May of 1985 and so on. The term "non-
monsoon’ has been used for meteorological and vegetative
drought analysis since they incorporate all the non-monsoon
months. On the other hand, the term ‘pre-monsoon’ has been
used for hydrological drought analysis since ground water level
is measured twice a year, once before the commencement of the
monsoon and again after the termination of the monsoon.
2.1 Standardised Precipitation Index (SPI)
In order to analyse the impact of rainfall deficiency on drought
development in this terrain, SPI has been used to quantify the
precipitation deficit in the monsoon and the non-monsoon
periods since 1984 up to 2000. The SPI is calculated using the
following equation, written as
where, X; is the seasonal precipitation at the ith rain-gauge
station and jth observation, X;, is its long-term seasonal mean
and oc is its standard deviation.
Although McKee et al., (1995) in the original classification
scheme proposed ‘mild drought’ for SPI values less than 0.00,
in the modified SPI classification scheme of Agnew (1999),
there is a straight jump from ‘no drought’ to ‘moderate
drought’. In the present study, SPI maps have been classified
using the modified scheme of Agnew (1999) to represent
various hydro-meteorological drought intensities, however,
‘mild drought’ has been recognised corresponding to the SPI
values less than -0.50, which has a probability of occurrence
0.309 (Agnew, 1999). Seasonal normals of 35 years (1966 —
2000) have been used for calculation of SPI. Instead of
averaging anomalies for the entire terrain, SPI has been
computed separately for each of the 35 rain-gauge stations
falling within and around the study area. Since drought is a
regional phenomenon, SPI values of the rain-gauge stations
have been interpolated using Spline interpolation technique in
Arc view 3.2a GIS to demarcate its spatial extent.
2.2 Standardised Water level Index (SWI)
For hydrological drought analysis, examination of stream-flow
statistics and run series analysis is the widely used technique.
However, since the streams and channels of the Aravalli terrain
are dry for most of the time and since domestic and agricultural
activities are solely dependent on ground water resources,
monitoring and analysis of water table fluctuation has been
considered for hydrological drought analysis. In order to
monitor hydrological drought in the Aravalli terrain, the pre-
monsoon and post-monsoon ground water levels of 541 wells of
the region have been analysed. SWI has been developed to scale
the ground water recharge deficit. The SWI expression stands as
SWI = (Wi Wim)! 6 (2)
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