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What are the values or ranges for EDI (Effective Drought Index)?

Effective Drought Index (EDI) could be recommended for operational drought monitoring in the region; however, the EDI requires daily precipitation, which constitutes a serious limitation for its operational use. Droughts are quantified using indices such as the Effective Drought Index (EDI). EDI is able to quantify droughts in absolute terms. EDI quantifies droughts in terms of droughts classes composed of positive and negative real values; for example, -2.50 indicates extreme drought, +3.28 indicates extreme floods and 0.98 indicates close to normal wetness.

The EDI is calculated in daily time step. The value of EDI generally ranges from -2.5 to 2.5. Near normal conditions are indicated when EDI ranges from -1.0 to 1.0, while extreme drought conditions are indicated when EDI is less than or equal to -2.0.

Compared to other drought indices, the EDI has several advantages. First, it is the only index that was specifically designed to calculate daily drought severity. This enables rapid detection of drought and precise measurement of short-term drought. Second, by utilizing a calculation method that places greater emphasis on recent precipitation, it more accurately calculates the current level of available water resources.

Finally, because it calculates the total precipitation period, considering the continuity of the drought period during the entire calculation process, it is distinguishable from existing drought indices that only provide a calculation for a limited period (e.g., 12 months). Therefore, it becomes possible to diagnose prolonged droughts that continue for several years.

although the EDI is solely based on precipitation, it is calculated in a simply way and considered to be more appropriate for monitoring drought condition in a shorter time interval than other drought indices since it provides drought severity on a daily basis (Kim et al., 2011). EDI expresses the standardized deficit or surplus of stored water quantity (Table 2). It enables one location's drought severity to be compared to that of another location, regardless of climatic differences.

EDI 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 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(Masinde, 2013).


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