Effective Drought Index (EDI)
Index name: Effective Drought Index (EDI).
Ease of use: Yellow.
Origins: Developed through work done by Byum and Wilhite, along with staff at NDMC.
Characteristics: Uses daily precipitation data to develop and compute several parameters: effective precipitation (EP), daily mean EP, deviation of EP (DEP) and the standardized value of DEP. These parameters can identify the onset and end of water deficit periods. Using the input parameters, EDI calculations can be performed for any location in the world in which the results are standardized for comparison, giving a clear definition of the onset, ending and duration of drought. At the time of EDI’s development, most drought indices were being calculated using monthly data, so the switch to daily data was unique and important to the utility of the index.
Input parameters: Daily precipitation.
Applications: A good index for operational monitoring of both meteorological and agricultural drought situations because calculations are updated daily.
Strengths: With a single input required for calculations, it is possible to calculate EDI for any location where precipitation is recorded. Supporting documents explaining the processes are available for the program. EDI is standardized so that outputs from all climate regimes can be compared. It is effective in identifying the beginning, ending and duration of drought events.
Weaknesses: With precipitation alone accounted for, the impact of temperature on drought situations is not directly integrated. Using daily data may make it difficult to use EDI in an operational situation, as daily updates to input data may not be possible.
Resources: The authors state that the code is available by contacting them directly. The calculations are available and described in the original paper referenced below. EDI calculations are part of a suite of indices calculated as part of the Spatial and Time Series Informational Modeling (SPATSIM) software package.
Reference: Byun, H.R. and D.A. Wilhite, 1999: Objective quantification of drought severity and duration. Journal of Climate, 12: 2747–2756. DOI: 10.1175/1520-0442(1999)012<2747:OQODSA>2.0.CO;2.
Currently used by: Iran.