Currently the South African Weather Service makes use of two drought indices
Percentage of Normal Rainfall
The percentage of normal rainfall is a simple calculation that is aimed at a general audience and is used fairly effectively when comparing conditions within specific regions or for particular seasons or time periods. One of the biggest disadvantages of the Percent of Normal index is that many people misunderstand it. A normal is a rainfall average that is calculated over at least 30 years, but this average is not necessarily the same as the median (middle) value of the rainfall for the 30-year period. One very wet year or one very dry year could result in the average value being either more than or less than the median value respectively. The rainfall value that may be calculated as being 75% of the normal will indicate a meteorological drought, but this value may in actual fact be quite close to the median value and so may not be a true reflection of the water deficit being experienced by the place in question.
The rainfall deciles are used in the monthly Climate Summary publication issued by the South African Weather Service. This index requires rainfall data for long periods of time. The monthly rainfall distribution over a long period of time (usually more than 30 years) is divided into tenths of the distribution. Each of these 10 categories is called a decile. By definition, the fifth decile is the median (middle) rainfall amount and is not exceeded by 50% of the rainfall occurrences over the entire record of the station. The deciles index is a more useful index in assisting decision-makers to determine where financial assistance has to be provided in times of drought. The disadvantage of the index is that it compares the rainfall deficit in the current month with rainfall for the same month in the history of the station and does not consider the cumulative effect of rainfall deficit.
What is the Standardised Precipitation Index (SPI)
The Standardised Precipitation Index (SPI) is an index based on the probability of rainfall for any time scale and can assist in assessing the severity of drought. The SPI can be calculated at various time scales which reflect the impact of the drought on the availability of water resources. The SPI calculation is based on the distribution of rainfall over long time periods (preferably more than 50 years). The long term rainfall record is fit to a probability distribution, which is then normalised so that the mean (average) SPI for any place and time period is zero. SPI values above zero indicate wetter periods and values less than 0 indicate drier periods.
The SPI values that will be adopted at the South African Weather Service are the same as those developed by McKee, Doesken and Kleist in 1993 (for more information, refer to the preprints of the 8th Conference on Applied Climatology, pp 179 - 184, held in January 1993 in Anaheim, California). These values are as follows:
Greater than 0 - Wet (50% occurrence)
0 to (0.99) - Somewhat Dry (34.1% occurrence)
(1) to (1.49) - Moderate Drought (9.2% occurrence)
(1.5) to (1.99) - Severe Drought (4.4% occurrence)
Less than (2) - Extreme Drought (2.3% occurrence)
Why the move to the Standardised Precipitation Index (SPI)?
Neither the Percent of Normal nor the Decile drought indices are able to assist decision-makers with the assessment of the cumulative effect of reduced rainfall over various time periods. Neither of these indices can describe the magnitude of the drought compared with other drought events. The Standardised Precipitation Index can alleviate both of these principal shortcomings of the other indices, while at the same time being less complex to calculate than some of the other drought indices not in use at the South African Weather Service.