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What is EDI (Effective Drought Index)?

EDI (Byun and Wilhite, 1999) in its original form used daily rainfall data to analyse drought severity and duration. It is a function of the precipitation needed for a return to normal (PRN) condition, signifying the precipitation necessary for recovery from accumulated deficit since the beginning of the drought. The Effective Drought Index (EDI) was proposed by Byun and Wilhite (1999) to monitor the duration and severity of drought.

EDI was developed to monitor drought condition on daily time step (Akhtari et al., 2009; Kim and Byun, 2009). The EDI(Effective Drought Index) and SPI use similar classification of drought severity. The EDI(Effective Drought Index) is based on the concept of effective precipitation (EP) that is calculated by a time dependent reduction function on daily/monthly precipitation and requires specifying at least 30-years data for calculation of mean effective precipitation.Droughts are quantified using indices such as the Effective Drought Index (EDI). EDI(Effective Drought Index) is able to quantify drought in absolute terms and also provide answers to: (1) the when, (2) the how long (onset to cessation) as well as (3) the severity of droughts/floods (Byun and Wilhite 1999;Mishra and Singn 2010).

They defined effective precipitation as a function of current month's rainfall and weighted rainfall over a defined preceding period computed using a time dependent reduction function. They computed .EDI(Effective Drought Index) as a function of the amount of precipitation required to return to normal (PRN). Where, PRN is calculated from monthly effective precipitation and its deviation from the mean for each month.

The EDI(Effective Drought Index) is calculated in daily time step and its values are standardized in a similar way with that for calculating SPI values. The EDI(Effective Drought Index) was originally developed by Byun and Wilhite (1999) to overcome some limitations of other indices. The value of EDI(Effective Drought Index) generally ranges from -2.5 to 2.5. Near normal conditions are indicated when EDI(Effective Drought Index) ranges from -1.0 to 1.0, while extreme drought conditions are indicated when EDI(Effective Drought Index) is less than or equal to -2.0. Effective precipitation should be calculated firstly before obtaining the EDI(Effective Drought Index). AgMerra Drought

The EDI(Effective Drought Index) is different from the rest of the indices in a number of ways; it was developed to address weaknesses identified in the existing (at the time) drought indices. Some desirable features of EDI(Effective Drought Index) are: (1) More accurately calculates current level of available water resources; (2) It considers drought continuity, not just for a limited period; it can therefore diagnose prolonged droughts that continue for several years; (3) It is computed using precipitation alone; and (4) It considers daily water accumulation with a weighting function for time passage.



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