Use of Minimum Groundwater Levels and Groundwater Abstractions for Assessing the Quantitative Status of Quaternary Groundwater Bodies in the Czech Republic

According to European and Czech legislation, the quantitative status of groundwater must be regularly assessed, and a water management balance of groundwater quantities must be carried out. Good quantitative status of groundwater, assessed at the level of groundwater bodies, is defined based on the groundwater level (GWL), where the GWL in a groundwater body ensures that the available capacity of the groundwater resource is not exceeded by the long-term average annual abstraction.

However, the development of GWL in Quaternary hydrogeological zones (HZs), which in the Czech Republic correspond to groundwater bodies, can vary significantly over time depending on their recharge potential and the extent of their use. Groundwater may flow in from slopes, be drained from underlying aquifers, recharge directly from precipitation, or—due to groundwater abstractions (GA)—flow in from rivers. Furthermore, groundwater level development in individual monitoring wells varies depending on local hydrogeological conditions, distance from abstraction sites, proximity to rivers, and other factors. Consequently, neighboring wells within the same HZ may exhibit very different GWL trends. Establishing simple criteria for assessing the quantitative status of long-term and intensively managed Quaternary HZs is therefore challenging.

To address this, all Quaternary HZs were divided into smaller, more manageable units called subzones. The division was based on the presumed extent of influence of large abstraction sites and the arrangement of boundary conditions. For each subzone, a specific general GWL (sGWL) was determined by averaging standardized monthly GWL values from all wells within the subzone. For each subzone, the minimum sGWL (sGWLmin) and the development of GA were assessed for the most significant drought periods. The most severe drought occurred in 2015–2020, closely followed by droughts in 1990–1994 and 2003–2004. In contrast, GA during 1990–1994 was approximately double that of 2015–2020 and likely caused sGWLmin in many subzones to fall below the level observed in 2015–2020. The drought period of 2003–2004 had GA levels between those of 1990–1994 and 2015–2020.

The comparison was carried out under the assumption that during the driest period, 2015–2020, GA were not restricted due to drought. If higher GA in earlier drought periods caused sGWLmin to fall below its level in 2015–2020, GA influenced the quantitative status of the subzone. If, however, higher GA in earlier drought periods did not cause sGWLmin to fall below the level observed in 2015–2020, GA did not affect the quantitative status of the subzone. By comparing minimum sGWLmin during drought periods with varying historical GA magnitudes, volumetric and level-based criteria were established under which the groundwater status of a subzone can be considered good. This methodology was applied to 65 of 75 subzones. For the remaining 10 subzones with insufficient GA and GWL monitoring data, a specific approach was adopted. Based on the assessment of the quantitative status of subzones, the quantitative status of the HZs was subsequently evaluated.