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What is DI(Deciles Index)?

The DI was defined a ranking of the precipitation in a particular time interval over the entire historic period (Gibbs and Maher, 1967). For drought monitor and quantify it, we need to apply drought indices. One of the simplest indices is Deciles drought index. Deciles is a meteorological drought measurement tool which uses only rainfall. The deciles index provides an accurate statistical measurement of precipitation provided that long climatic data is available. Specifically, monthly historical precipitation data are sorted from lowest to highest and divided into ten equal categories or deciles. So, precipitation in a given month can be placed into the historical context by decile. AgMerra Drought

In calculating Deciles, the long-term monthly rainfall records were first ranked from highest to lowest to construct a cumulative frequency distribution. This distribution was then split into ten parts (or deciles) based on equal probabilities (Gibbs and Maher, 1967). 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 threshold ranges of Deciles used to define drought conditions are presented as follows:

Decile 1-2: Much below Normal

Decile 3-4: Below Normal

Decile 5-6: Near Normal

Decile 7-8: Above Normal

Decile 8-9: Much above Normal

In another means, the distribution is then split into 10 parts (tenths of distribution or deciles). The first decile is the precipitation value not exceeded by the lowest 10% of all precipitation values in a record. The second decile is between the lowest 10 and 20% etc. Comparing the amount of precipitation in a month (or during a period of several months) with the long-term cumulative distribution of precipitation amounts in that period, the severity of drought can be assessed. The deciles are grouped into five classes, two deciles per class. If precipitation falls into the lowest 20% (deciles 1 and 2), it is classified as much below normal. Deciles 3 to 4 (20 to 40%) indicate below normal precipitation, deciles 5 to 6 (40 to 60%) indicate near normal precipitation, 7 and 8 (60 to 80%) indicate above normal precipitation and 9 and 10 (80 to 100%) indicate much above normal precipitation. Morid et al. (2004)

The deciles is located in meteorological drought, but it can be used in meteorological, agricultural and hydrological drought situations. The strengths of DI is: With a single variable (precipitation) being considered it can easy to use, and 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. DI is Useful and applicable in both wet and dry situations.

Also, it has some weaknesses: DI uses 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.

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