LAUSR

Abstract The objective of this study was to seek better insight into the natural factors that may contribute to hydrogeological drought in shallow hydrogeological ecosystems that exhibit significant seasonal and multiannual groundwater-level dynamics. With this aim, we monitored groundwater levels in two groundwater-dependent ecosystems (GDEs) situated close together in the central part of the Vistula Valley regularly at 14-day intervals over a long-term period (1999–2013), using a monitoring network of 17 piezometers of the monitoring network (recording about 6600 observations overall). The piezometers were situated such that they were not under the direct influence of anthropopressure, thus allowing the results to be interpreted as the influence of natural factors on the quantitative state of GDEs. We found that the groundwater level changes over the study period can be classified as exhibiting hydroperiods of the periodic type, with a typical seasonal-level fluctuation pattern, without any distinct long-term trend. Statistical analysis showed that hydrogeological droughts, characteristic of the summer months, occur only during years when the lowest groundwater levels were observed. Shallow hydrogeological droughts occur periodically, from summer to spring, and the average duration of shallow droughts was 8 weeks. Overall, this study illustrates how analysis of groundwater-level fluctuations in GDEs on multiannual and seasonal scales, determining the fluctuation trends and the values of critical levels, provides better understanding of the risk of groundwater drought hazard and occurrence. Assessment of the natural range of groundwater-level fluctuations can then assist in effecting better local water resource management and may aid agricultural organizations and development/planning authorities to improve their understanding of sustainable groundwater resource management.