Index name: Deciles.
Ease of use: Green.
Origins: A simple mathematical approach described by Gibbs and Maher in 1967 through their work with the Australian Bureau of Meteorology.
Characteristics: Using the entire period of record of precipitation data for a location, the frequency and distribution of precipitation are ranked. The first decile is composed of the rainfall amounts in which the lowest 10% of the values are not exceeded, and the fifth decile is the median. A wet scale is also available. Daily, weekly, monthly, seasonal and annual values can all be considered in the methodology, as it is flexible when current data are compared to the historical record for any given period.
Input parameters: Precipitation only, and the timescale considered is flexible.
Applications: With the ability to look at different timescales and time steps, deciles can be used in meteorological, agricultural and hydrological drought situations.
Strengths: With a single variable being considered, the methodology is simple and flexible for many situations. Using clearly defined thresholds, the current data are put into a historical context and drought status can be recognized. Useful in both wet and dry situations.
Weaknesses: As with other indicators that use only precipitation, the impacts of temperatures and other variables are not considered during the development of drought. A long record period provides the best results because many wet and dry periods will be included in the distribution.
Resources: There is no specific software code for deciles and several online tools can provide output. Thus, it is important to clarify the underlying methodology, as there are a number of statistical approaches to calculate deciles from meteorological data.
Example: DrinC software.
References: Gibbs, W.J. and J.V. Maher, 1967: Rainfall Deciles as Drought Indicators. Bureau of Meteorology Bulletin No. 48, Melbourne, Australia.
Currently used by: Austria, Brazil, Iran, Macedonia, New Zealand.