What is the time scale in EDI (Effective Drought Index)?

Various drought indices have been developed to quantify drought status. The estimation of drought features is based on the method proposed by Byun and Wilhite (1999). In that method, a new concept of drought index was proposed to solve the weaknesses of other current drought indices and to improve drought monitoring.

The new features in this method are the use of daily precipitation height values and the introduction of a new concept, effective precipitation (EP), which is the summed value of daily precipitation with a time-dependant reduction function. Thus, the only data required for the calculation of the drought index were daily precipitation height values.

In Effective Drought Index, RDIT (Rain based Drought Indices Tool) calculates index based on daily data. After that RDIT converts EDI values to daily, monthly, seasonally, or yearly. The output time scale is based on request of user. If you enter rainfall data in monthly or yearly, you can't use Effective Drought Index in RDIT (Rain based Drought Indices Tool).

The "drought range" of the EDI indicates extreme drought at EDI equal less than -2, severe drought at greater than -2 and equal less than -1.5, and moderate drought at EDI greater than -1.5 and equal less than -1. Near normal conditions are indicated by EDI greater than -1 and equal less than 1. EDI expresses the standardized deficit or surplus of stored water quantity. It enables one location's drought severity to be compared to that of another location, regardless of climatic differences.

The EDI makes it possible to simulate daily drought severity and to provide the explicit duration of drought event. The EDI represents daily drought severity by comparing accumulated precipitation for the specific duration, i.e., i=365 days.