Comparison of the reference periods of the hydrological years 1981 - 2010 and 1991 - 2020 in groundwater in Slovakia

In the Slovakia for assessment of climate change is used as references period hydrological years 1981 - 2010. Although the first observations in groundwater began in 1956, the monitoring network was not dense enough and the objects were not evenly distributed in Slovakia. Even the expansion of monitoring network in the 1970s was not enough to obtain longer, 30-year time series. Based on the new reference period 1991 - 2020 recommended by the World Meteorological Organization WMO, this period was also evaluated from the point of view of groundwater. The aim of the contribution was to compare reference periods 1981 2010 and 1991 - 2020 and their changes in the groundwater in Slovakia. These two reference periods were compared with each other based on the ratio values of long-term time series of minimum and average values of the groundwater level and springs yield. The new reference period 1991 - 2020 and period 1981 - 2010 were also evaluated by trend analysis using the non-parametric Mann-Kendall test. The Mann-Kendall statistical test was use to assess whether a set of data values is increasing or decreasing over time and whether the trend in either direction is statistically significant. The Mann-Kendall test does not assess the magnitude of change. The advantage of this test is, that it is not affected by the current distribution of the data and at the same time is less sensitive to extreme values in the time series. The long-term average values of the groundwater level and the springs yield for the period 1991 - 2020 compared to 1981 - 2010 were lower in the outer West and Northwest, in the Northern parts of Slovakia, in the region of central Slovakia, and in the Southeast and outer East. Based on the comparison of the long-term minimum values of the compared periods, the values were steady in compared to the previous period at most of the evaluated objects. When evaluating the trends of long-term averages for the period 1991 - 2022, significant decreases in the average groundwater level and springs yield were detected in the outer West and Northwest, in the strip from Northern to central Slovakia, in the North of Eastern Slovakia, and in the Southeast and outer east of the territory. When evaluating the trends of long-term minima for the period of hydrological years 1991 - 2022, the situation was similar to the evaluation of long-term averages. By comparing the period of 30 annual series 1991 - 2020 to the period 1981 - 2010 based on the evaluated ratio values, we did not notice significant deviations. Keywords: groundwater, spring yield, references periods