Meteorological drought variability in Poland by means of the ERA5-Land dataset

In this work, meteorological drought in Poland has been characterized considering the Standardized Precipitation Index (SPI) evaluated at different timescales (3, 6, 12 and 24 months) from the ERA5-Land monthly dataset provided by ECMWF under the framework of Copernicus Climate Change Service Programme. With this aim, trend detection employed Sen’s slope estimator and Mann-Kendall test, and drought characteristics (e.g., quantity, duration, severity, and intensity) were derived using the run theory applied to the SPI values calculated in 4,084 grid points. As a result of the trend analysis, the short-term SPI (3-month) exhibits pronounced spatial and temporal variability, with trends that are generally weak and less spatially coherent. As the aggregation scale increases to 6 months, trend patterns become more structured, reflecting seasonal to interannual precipitation variability. The long-term SPI scales (12- and 24-month) show more consistent and spatially persistent trends, indicating clearer long-term wetting tendencies across the country.

As regards the drought characteristics, considering the average values, the number of drought events decreases markedly as the SPI time scale increases, with the highest number of events observed for the 3-month SPI and the lowest for the 24-month SPI. In contrast, the average drought duration increases with increasing SPI time scale. Droughts identified using longer accumulation periods persist for longer durations, with the 24-month SPI showing the highest median and variability in duration. A similar increasing trend is observed for the average drought severity, where longer SPI scales are associated with more severe drought events, reflecting the cumulative nature of long-term precipitation deficits. The average drought intensity shows a slightly decreasing trend as the SPI time scale increases. Although intensity remains relatively stable across time scales, droughts identified at shorter SPI periods tend to be marginally more intense than those detected at longer accumulation periods. 

With respect to the drought characteristics, considering the extreme values, drought frequency remains relatively stable across the SPI time scales, with only minor variations in median values. In contrast, maximum drought duration exhibits a clear increasing trend with increasing SPI time scales. The short-term SPI identifies extreme droughts with relatively limited durations, whereas the 24-month SPI substantially captures longer extreme drought events, with both higher median values and greater variability, reflecting the ability of longer SPI time scales to represent prolonged drought persistence. A similar pattern is observed for maximum drought severity, which increases markedly with SPI accumulation period. Extreme droughts identified at longer time scales accumulate larger precipitation deficits, resulting in significantly higher severity values, particularly for the 24-month SPI, also showing the widest range of variability. Conversely, maximum drought intensity shows a decreasing trend as the SPI time scale increases. Higher intensity values are associated with shorter SPI periods, while longer accumulation periods tend to smooth short-term variability, leading to less intense but more persistent extreme drought events. Finally, the spatial distribution of the drought characteristics in Poland allows us to identify the areas that could also face water stress conditions in the future, thus requiring drought monitoring and adequate adaptation strategies.