Monitoring drought in Greece

In recent years, Greece has experienced significant natural disasters. Examples include the devastating wildfires in August 2021, the destructive flash floods in Mandra, Attica in 2017, and, more recently, the Rhodes Island wildfires in 2023. Scientists attribute these events to climate change and caution that their frequency may escalate in the future climate change and caution that their frequency may escalate in the future. Following last year’s droughts in Southern and Central Europe, inquiries arose regarding the seemingly absent occurrences in Greece. The question now revolves around whether comparable risks exist and, if so, how frequently they manifest.

Collecting the data

The data for the annual measurement of the Standardized Precipitation and Evapotranspiration Index (SPEI) are sourced from the official website of the scientists who developed the drought index. This index relies on data related to precipitation and potential evapotranspiration, which represents how plants release water from the soil into the atmosphere. These data are collected both globally and by region, spanning from 1950 to the present. To calculate the index, the data are modeled and provided in map form through the SPEI Global Drought Monitor, which allows for the selection and downloading of region-specific data based on coordinates, with a spatial resolution of 0.5o. This resolution corresponds to approximately 55 km per unit square.

In order to highlight drought trends in different regions, the so-called hydrological year has been taken into account. Dimitris Tigkas, a PhD student at the National Technical University of Athens, specializing in water resources and drought management, notes that, for Greece, the hydrological year starts in October and ends in September. On this basis, the SPEI-12 values for October have been used.

On the website, the data are provided for retrieval in .csv files, which include SPEI values for a time scale of 1 to 48 months. We have compiled data files for the regions of Attica, eastern Crete, eastern Peloponnese, Epirus, Cyclades and Thessaly up to October 2022. The selection of the areas was made after discussion with experts Niki Evelpidou, Professor at the Faculty of Geology and Geoenvironment at the University of Athens, and Dimitris Tigas, as well as observation of the areas with the most severe problems.

Positive values indicate higher humidity levels, while negative values suggest drier conditions compared to the region's normal weather patterns. "Normal conditions" are defined as the long-term average over at least 30 years, following the standards set by the World Meteorological Organization (WMO). To be classified as a drought period, the values must tend towards or be less than -1.

Calling attention

Some important points to consider for data management are as follows:

• The year 2023 is excluded from our study as the hydrological year is not yet completed.
• The data used are modeled, meaning that the values are approximate. As Dimitris Tigas highlights, the accuracy of the index values for each region can be influenced by the adequacy of measurements and the spatial distribution of weather stations, which serve as the basis for modeling the primary data.
• The SPEI does not provide information about the type of precipitation or its relevance to the soils of the region.