The effects of non-stationarity on SPI for an operational drought monitoring in Europe

It is a good practice to follow common guidelines in the computation of Standardized Precipitation Index (SPI) datasets as part of operational drought monitoring systems. In the European Drought Observatory (EDO, https://edo.jrc.ec.europa.eu/) reference statistics are computed following the WMO Guidelines on the Calculation of Climate Normals, where it is introduced a definition of reference period for monitoring applications as the most-recent 30-year period finishing in a year ending with 0. In this study, the temporal consistency of the SPI time series computed using the ERA5 precipitation dataset is tested over Europe to quantify the effect of the transition from one baseline period (1981-2010) to another (1991-2020), as well as to evaluate the capability of these static baselines to reproduce the behavior of non-stationary SPIs. The results of the comparison suggest that the threshold commonly used to identify droughts (SPI = -1) is only marginally affected by the change in reference period (MAD = 0.15±0.1) for short-term SPI, whereas larger differences (MAD up to 0.6) can be observed over certain areas (i.e. Southern Italy and Eastern Europe) for larger accumulation periods (i.e. SPI-9 or SPI-12). Examples show that changes in drought classification from extremely dry (SPI < -2) to moderately dry (SPI < -1) are not uncommon, which may lead to misinterpretation from users. Finally, analyses against non-stationary SPI highlight an overall good correspondence between stationary and non-stationary SPIs, even if both static baselines displayed difficulties in reliably capturing the magnitude of non-stationary SPI for the entire 10-year period for which they should be used. In this regard, it has been demonstrated that more spatially-uniform results can be achieved with 5-year updates, with a good matching (MAD < 0.25) for SPI-1 and acceptable matching (MAD < 0.50) for SPI-12 over more than 80% of Europe.

CAMMALLERI Carmelo;  SPINONI Jonathan;  MARINHO FERREIRA BARBOSA Paulo;  TORETI Andrea;  VOGT Juergen; 

2022-05-05

WILEY

JRC125888

0899-8418 (online),   

https://rmets.onlinelibrary.wiley.com/doi/10.1002/joc.7424,    https://publications.jrc.ec.europa.eu/repository/handle/JRC125888,   

10.1002/joc.7424 (online),