Spatiotemporal assessment of groundwater drought in a river valley aquifer using standardized indices in central Poland

The study was conducted within the Kampinos National Park (KPN) and its surrounding buffer zone, located in central Poland. This area is characterized by a mosaic of dune and marshland hydrogeological environments, which provide diverse groundwater conditions. The park represents a complex hydrodynamic system influenced by both natural and anthropogenic factors, making it a representative site for analyzing groundwater drought in shallow valley aquifers. This study aimed to assess spatiotemporal patterns and characteristics of groundwater droughts in the KPN area from 1999 to 2020. Standardized indices—Standardized Precipitation Index (SPI) and Standardized Groundwater Index (SGI)—were used in conjunction with hierarchical cluster analysis to identify zones with distinct drought behaviors. The methodology integrated statistical classification with hydrogeological interpretation, allowing for detailed quantification of drought intensity, duration, magnitude, and response lag to meteorological deficits. The findings revealed significant differences in drought response between dune and marsh environments. Dune zones exhibited prolonged drought durations with moderate intensity due to deeper water tables and thick vadose zones, while marsh areas responded more rapidly but with higher drought intensity. A notable time lag of 3–6 months was observed between meteorological and groundwater droughts. These insights contribute to improved understanding of local hydrological dynamics and support targeted groundwater management strategies under climate change pressures. • SPI is reliable for hydrogeological drought assessment in an unconfined aquifer. • Delayed response of the groundwater to the lack of recharge varies from 3 m to 6 months. • The impact of drought varies between dunes and wetlands under the same precipitation. • Hydrogeological drought episodes in wetlands are short but severe (average SGI<-3).