The Vipava Valley is a unique region in south-western Slovenia. In addition to surface water, groundwater is also important, although it is hidden from the eye. The paper presents an analysis of groundwater levels in piezometers in the Vipava Valley. The analysis was performed on 10 piezometers, which are still operating today, and includes a display of levels and basic statistics, correlations, the impact of distance from the Vipava riverbed, trends, and seasonality. According to the groundwater level, piezometers in the Vipava Valley can be divided into three groups. The first group with the highest levels includes piezometers Gradišče, Vipavski Križ, and Ajdovščina, the second group piezometers Prvačina, Šempeter, Volčja Draga, Renče, and Vrtojba, and the third group with the lowest groundwater levels includes the piezometers Miren and Orehovlje. The results of the analyses showed good or bad connections between groundwater levels in piezometers, as well as between groundwater levels and the Vipava River water level at various gauging stations. The fluctuation of the groundwater level is conditioned by the distance from the Vipava riverbed and the area’s geological or tectonic structure. An unambiguous trend of groundwater levels cannot be determined. The seasonality of groundwater level fluctuations is not pronounced, but the highest values of groundwater levels occur in autumn and winter, and the lowest in summer.
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