A multi-scalar drought index sensitive to global warming: the standardized precipitation evapotranspiration index – SPEI

We propose a new climatic drought index: the Standardized Precipitation Evapotranspiration Index (SPEI). The SPEI is based on precipitation and temperature data, and has the advantage of combining a multi-scalar character with the capacity to include the effects of temperature variability on drought assessment. The procedure to calculate the index is detailed, and involves a climatic water balance, the accumulation of deficit/surplus at different time scales, and adjustment to a Log-logistic probability distribution. Mathematically, the SPEI is similar to the Standardized Precipitation Index (SPI), but includes the role of temperature. As the SPEI is based on a water balance, it can be compared to the self-calibrated Palmer Drought Severity Index (sc-PDSI). We compared time series of the three indices for a set of observatories with different climate characteristics, located in different parts of the world. Under global warming conditions only the sc-PDSI and SPEI identified an increase in drought severity associated with higher water demand due to evapotranspiration. Relative to the sc-PDSI, the SPEI has the advantage of being multi-scalar, which is crucial for drought analysis and monitoring.This work has been supported by the research projects CGL2006-11619/HID, CGL2008- 01189/BTE, and CGL2008-1083/CLI financed by the Spanish Commission of Science and Technology and FEDER, EUROGEOSS (FP7-ENV-2008-1-226487) and ACQWA (FP7- ENV-2007-1-212250) financed by the VII Framework Programme of the European Commission, “Las sequías climáticas en la cuenca del Ebro y su respuesta hidrológica” and “La nieve en el Pirineo Aragonés: distribución espacial y su respuesta a las condiciones climáticas” Financed by “Obra Social La Caixa” and the Aragón Government and the “Programa de grupos de investigación consolidados” financed by the Aragón Government.ÿPeer reviewe