The impacts of drought are associated with the costs and losses in areas of economic, social, or environmental areas directly or indirectly. Drought monitoring is essential to develop a good prediction system, mitigate losses caused by drought and to set up preparedness strategies. The APCC Global Drought Monitoring is based on the Standardized Precipitation Index (SPI; McKee et al. 1993) maps for the last 1-month, 3-month, 6-month and 12-month periods using monthly precipitation at 2.5°x2.5° resolution. SPI between -1.0 to -1.49 indicates moderate drought, -1.5 to -2.0 severe drought, and less than -2.0 extreme drought conditions. The SPI is estimated by transforming the observed rainfall distribution for the recent 30 yrs, usually fitted to a Gamma distribution, into a standardized normal distribution on an equal probability basis (see Sohn et al. 2012a, 2012b for more details).
Sohn, S.-J., C.-Y. Tam, K. Ashok, and J.-B. Ahn, 2012a: Quantifying the reliability of precipitation datasets for monitoring large-scale East Asian precipitation variations. Int. J. Clim., DOI: 10.1002/joc.2380.
Sohn, S.-J., C.-Y. Tam, and J.-B. Ahn, 2012b: Development of a multimodel-based seasonal prediction system for extreme droughts and floods: a case study for South Korea. Int. J. Clim., DOI: 10.1002/joc.3464.