Drought Monitoring

Radiative Index

Droughts of varying intensity or severity occur in different regions of Thailand. 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

Figure 25.1 Regions of Thailand.

by slight drought. If RI exceeds 1.2 for 30 consecutive days, the region is considered to be affected by moderate drought, and if RI exceeds 1.0 for more than 30 consecutive days, severe drought is said to have occurred in the region. Using these criteria, the percentage of area affected by different drought categories has been determined in Thailand (table 25.1). It can be observed from table 25.1 that the northeastern region is the most drought-prone in Thailand.

Table 25.1 Spatial distribution of drought occurrence in Thailand

Total

Drought severity class (% area)

Region

area (km2)

Severe Moderate Slight None

Source

Northeast

168,854

68 17 9 6

OEPP/KU (1996)

North

169,756

1 7 82 10

OEPP/IWB Siam-Tech. (1998)

Central

104,319

4 57 33 6

OEPP/TRTI (1999)

South

71,040

Incidence of drought reported

SU (1997)

but not quantified

OEPP = Office of Environmental Policy and Planning, TRTI = Thailand Research and Technology Institute, and SU = Songkhla University.

OEPP = Office of Environmental Policy and Planning, TRTI = Thailand Research and Technology Institute, and SU = Songkhla University.

Drought Index

A drought index, D, is also used to monitor drought conditions in Thailand. The index is based on various variables that are given different weights and scores (table 25.2) to compute the index (Eiumnoh, 2000):

HSjW,

i where Wj is the weight for the jth input variable, and Sjj is the score for the 7th class of the jth variable. D ranges from 1 to 42 for slight drought, from 43 to 84 for moderate drought, and exceeds 84 for severe drought.

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