Which of the drought indices are the best one, for monitoring drought events?

Several drought indices have been derived in recent decades. Commonly, a drought index is a prime variable for assessing the effect of a drought and defining different drought parameters, which include intensity, duration, severity and spatial extent. It should be noted that a drought variable should be able to quantify the drought for different time scales for which a long time series is essential.A large number of drought indices exist, each providing a somewhat different measure of drought (Heim 2002). This can lead to confusion about how to properly utilize and interpret drought indices. Unfortunately, many drought indices have limited utility because they are often difficult to calculate and interpret, and they do not provide location-specific drought information.

Several drought indices have been derived in recent decades. A number of different indices have been developed to quantify a drought, each with its own strengths and weaknesses. According to different researches (Rossi et al., 2003; Morid et al., 2006; Mishra and Singh, 2010; Salehnia et al., 2017 AgMerra Drought), it depends on different factors, such as the case of study's climate, field of research, the period of historical data, the quality of input data, the quantity of climate data available and on the ability of the index to consistently detect spatial and temporal variations. Every of drought indices is calculated from a unique equation. Therefore, we can’t specify which of them are the best, maybe it is better to apply all of them then according to the result and the past drought events, we can judge the proper one.

Overall, no single index can represent all aspects of meteorological drought so it is best to use a multi-index approach for operational drought monitor.