Evaluation of Groundwater Balance in Quaternary Hydrogeological Zones Using Historical Records of Groundwater Levels and Withdrawals in Czechia

In Quaternary hydrogeological zones along large rivers, performing a simple groundwater balance is highly challenging. Quantifying groundwater resources is particularly difficult due to their variability over time, influenced by replenishment possibility and usage intensity. Groundwater flows from slopes in adjacent hydrogeological zones of the base layer, drains from underlying hydrogeological zones, recharges through precipitation, or inflows from rivers. The Quaternary hydrogeological zones contain numerous abstraction areas where withdrawals, along with groundwater inflows from surrounding areas and bedrock, drought, recharge rates, and surface water levels, influence observed groundwater levels. The resulting groundwater level in monitored wells reflects the combined impact of these factors, regardless of their fluctuating contributions.

The only consistently measurable variable is groundwater withdrawal, which has been recorded since 1980. Withdrawals can significantly affect groundwater levels, especially during dry periods such as 1990 – 1994 and 2015–2020. The highest recorded groundwater abstraction in the Quaternary hydrogeological regions occurred in 1989, reaching approximately 5.2 m³/s. Withdrawals began to decline substantially after 1994. During 1990 – 1994, a hydrological drought coincided with high withdrawals. This dry period was comparable in scope and duration to the drought during 2015–2020. Time series of base flow data from the Czech Hydrometeorological Institute indicate that inflows or base flow from underlying hydrogeological zones reached historical minimums during 2015–2020. The second-lowest base flow was recorded for 1990–1994. Low base flows are typically caused by reduced recharge during dry periods, which also lead to a significant drop in groundwater levels. Unsurprisingly, both periods 1990 – 1994 and 2015 – 2020 are characterized by the lowest groundwater levels observed in monitoring network wells over the past 40 years.

The primary distinguishing factor between these two periods is groundwater withdrawals. During 1990 – 1994, withdrawals averaged around 5 m³/s, whereas by 2015 – 2020, abstractions had decreased to half that amount. This reduction often led to groundwater levels in 1990 – 1994 being significantly lower than those in 2015 – 2020. Based on the observed impacts of groundwater withdrawals on levels during dry periods, this study provides an assessment of groundwater balance in individual Quaternary hydrogeological zones.