This paper proposes an Economic Drought Management Index (EDMI) that could assist water managers to inter-temporally manage water reservoirs. The index's main appeal is that it can be easily interpreted and that encompasses in a single number hydrological processes, structural constraints, water institutions' rules and the economic benefits of the customers served from the supply system. An empirical application of EDMIs is performed for two irrigation districts in Andalusia (Southern Spain), that are managed under different institutional arrangements. Results for one district show that the region's vulnerability to drought could be reduced following the interpretation of the EDMIs. For the other district, the index shows that water stocks are managed under nearly optimal criteria. This last result shows that there is an efficient level of drought vulnerability, and that increasing supply security levels would result in welfare losses.