Hydrologic benchmarking of meteorological drought indices at interannual to climate change timescales: a case study over the Amazon and Mississippi river basins

Abstract. Widely used metrics of drought are still derived solely from analyses of meteorological variables such as precipitation and temperature. While drought is generally a consequence of atmospheric anomalies, the impacts to society are more directly related to hydrologic conditions. The present study uses a standardized runoff index (SRI) as a proxy for river discharge and as a benchmark for various meteorological drought indices (scPDSI, SPI, SPEI_th, and SPEI_hg respectively). Only 12-month duration droughts are considered in order to allow a direct (no river routing) comparison between meteorological anomalies and their hydrological counterpart. The analysis is conducted over the Mississippi and Amazon river basins, which provide two contrasted test beds for evaluating drought indices at both interannual (using detrended data) and climate change (using raw data) timescales. Looking first at observations over the second half of the 20th century, the simple SPI based solely on precipitation is no less suitable than more sophisticated meteorological drought indices at detecting interannual SRI variations. Using the detrended runoff and meteorological outputs of a five-member single model ensemble of historical and 21th century climate simulations leads to the same conclusion. Looking at the 21st century projections, the response of the areal fraction in drought to global warming is shown to be strongly metric dependent and potentially overestimated by the drought indices which account for temperature variations. These results suggest that empirical meteorological drought indices should be considered with great caution in a warming climate and that more physical water balance models are needed to account for the impact of the anthropogenic radiative forcings on hydrological droughts.