Regionalisation of Summer Hydrological Droughts Variability in Poland in Period 1993–2022

ABSTRACT Droughts still is a growing challenge, particularly in Poland's temperate climate zone, impacting agriculture, water management, and ecosystems. This study investigates the spatiotemporal variability of summer hydrological droughts (May–October) from 1993 to 2022 across Poland's seven physiographic regions. The motivation stems from the need to understand drought patterns amidst climate change and to inform strategies for mitigating water shortages. Despite extensive research on droughts in Europe, limited studies have addressed the spatial and temporal variability of hydrological droughts across Poland's diverse physiographic regions, particularly concerning the interplay of precipitation, potential evapotranspiration ( PET ), standardised runoff index ( SRI ), and temperature. Data from over 52 synoptic stations and 125 gauging stations were analysed, focusing on regional monthly averages of air temperature, precipitation, PET , SRI and the annual number of drought days, defined using the Q 90% threshold flow rate. Results revealed marked regional variability in drought conditions, with the northern and central regions experiencing the highest number of drought days, especially after 2015. Mountainous areas, including the Carpathians and Sudetes, recorded fewer drought days due to higher precipitation and lower PET compared to lowland regions. Trend analysis indicated rising temperatures across all regions, driving increased PET , particularly in northern areas. Precipitation trends were inconsistent, with some regions observing limited changes. Notably, a strong correlation between PET and precipitation was observed in lowland regions, while mountainous areas exhibited a negative correlation, where higher precipitation curtailed evaporation. The SRI highlighted a clear intensification of hydrological drought across Poland, especially in lowlands, driven by rising temperatures, limited precipitation recovery, and human activities that reduce water retention. This research addresses the gap in understanding regional drought dynamics and confirms the critical role of temperature and evapotranspiration in intensifying droughts. Findings provide valuable insights into hydrological drought patterns in Poland, contributing to more informed water resource management amidst ongoing climate changes.