The contrast of the monthly EDI (Effective Drought Index) as a simple method for monitoring meteorological droughts

Drought indices are used to detect and monitor the drought conditions of an area or region. Various drought indices have been proposed since the mid-sixties, some of them were developed for a specific region or climate and therefore, comparison studies of various drought indices under different climates are always useful and build confidence in such indices, when their results are similar. This study describes in detail the operating procedure of the EDI (Effective Drought Index), applied to monthly precipitation data. The EDI was originally exposed by Byun and Wilhite (1999) to process daily rains, but due to the difficulty of having such records, its monthly version was developed. The three EDI contrasts described in this study are carried out for a drought duration of 12 months: 1) with the SPI (Standardized Precipitation Index) of the climatological station Rio Verde, San Luis Potosi, Mexico; 2) with the SPEI (Standardized Precipitation-Evapotranspiration Index) of the climatological station Zacatecas, Zacatecas, Mexico, and 3) with the SPDI (Standardized Palmer Drought Index) of the climatological station Xilitla, San Luis Potosi, Mexico. The evolution graphs of each contrasted index showed great similarity with the unique EDI graphs, but the latter define in more detail the changes of monthly precipitation. Based on the analysis of the results, the systematic application of the monthly EDI, in the monitoring of meteorological droughts and the comparison studies of indexes, is recommended because it has a simple operating procedure, it does not require the definition of the duration of drought, and it is derived from a one-step calculation.