Agriculture Reference
In-Depth Information
CHAPTER TWENTY-FIVE
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A
PISIT EIUMNOH, RAJENDRA P. SHRESTHA,
A
ND VIJENDRA K. BOKEN
[Firs
[323
Line
——
-0.5
——
Norm
PgEn
Thailand is located between 5°30' and 20°30'N latitudes and between
97°30' and 105°30'E longitudes. Geographically, the country can be di-
vided into northern, northeastern, central, and southern regions (figure
25.1). Most of the country experiences distinct wet and dry climates, except
some parts of the southern region, which experience a wet and humid cli-
mate. Of the country's total area (514,000 km
2
), 41% is under agricultural
use (Office of Agricultural Economics, 1999) with 92% of it being rain-
fed. Drought normally occurs during the hot season (March-April) and
sometimes during dry season (November-April) due to inadequate rains.
In recent times, the occurrence of drought has increased in Thailand, threat-
ening sustainability of agricultural production. According to Department
of Local Administration (1998), droughts of varying intensity occur in 67
out of 76 provinces of Thailand almost every year. During the period from
1987 to 1997, drought impacted a total of 5.44 million ha of agricultural
land, causing $1.4 billion in losses.
[323
Drought Monitoring
R
adiative Index
Droughts of varying intensity or severity occur in different regions of Thai-
land. A drought is categorized as severe, moderate, slight, or none drought
using a radiative index (RI) determined during the rainy season (May-
October)
.
The RI for a region is determined using the number of rainy days,
percentage of irrigated area, groundwater availability, topography, land
use, soil, drainage density, and watershed size. If RI ranges from 1.0 to 1.2
for 15 consecutive days for a region or area, the region is said to be affected
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