Analysis of historical drought in the Lisbon region, in the west of Portugal, using Reconnaissance Drought Index

Abstract

Drought is a natural phenomenon whose likelihood is increasing due to climate change, which is gradually altering temperature and precipitation patterns. While various drought indices exist for monitoring extreme dry conditions, this study employs the Reconnaissance Drought Index (RDI) due to its accuracy and dependency on both precipitation and temperature. The research aims to assess historical droughts in the Lisbon region (Portugal) by applying RDI to a 157-year time series (1864-2021) using monthly precipitation and temperature data from the Lisboa-Geofísico climatological station. The influence of potential evapotranspiration (PET) on drought identification was analysed, alongside temporal drought assessments at short-term (3-month RDI, RDI-3), mid-term (6-month RDI, RDI-6), and long-term (12-month RDI, RDI-12) scales. RDI was computed monthly using the Drought Indices Calculator (DrinC), with three PET methods-Hargreaves, Thornthwaite, and Blaney-Criddle-compared for their performance. The standardized RDI, calculated preferably using the Hargreaves method for the Lisbon region, served as the index for spatial and temporal drought assessment. Results revealed frequent extreme drought events (when RDI values were less than minus two), with the most intense drought occurring in 2005 across all time scales. For meteorological drought (RDI-3 for short-term atmospheric conditions), 39 extreme events occurred, with a total of 51 months under drought conditions, with the longest event (5 months) in 2005. Agricultural drought (RDI-6 for soil moisture deficits) showed 18 extreme events lasting 28 months, with the longest (7 months) in 2005. Hydrological drought (RDI-12 for water resource depletion) exhibited 9 extreme events spanning 25 months, with the longest (9 months) also in 2005. The average return time for extreme drought in Lisbon was estimated at 4, 7, and 8 years for meteorological, agricultural, and hydrological droughts, respectively. This comprehensive regional drought risk assessment based on the standardized RDI index provides valuable insights for effective drought management in the Lisbon region.

 

Keywords: Drought risk assessment, empirical methods, PET, RDI, Lisbon, Portugal

 

Acknowledgement

This work was supported by the Slovak Research and Development Agency under the Contract no. APVV-20-0281 a project funded by the Ministry of Education of the Slovak Republic. This work was also supported by the Foundation for Science and Technology (FCT) through funding UIDB/04625/2020 from the research unit CERIS and by the European Union’s Horizon 2020 research and innovation programme SCORE under grant agreement No 101003534.