Key Considerations in Choosing the Optimal Reference Period for Hydrological Applications in Slovakia

The selection of an appropriate reference period for determining the hydrological characteristics in Slovakia is of the utmost priority, particularly in the light of the observed changes in the hydrological regime of the watercourses in the last decades, since the last valid reference period 1961-2000 was established. We have analyzed the long-term discharge data from 113 water-gauging stations, to assess the deviations in moving averages of mean annual flows over 10-, 20-, 30-, 40-, and 50-year long periods.

Based on the results of comparison against the long-term mean annual flow values for the selected reference periods 1961-2000 and 1961-2020, we recommend considering the length of 30- to 40-years for the future reference period for hydrological applications. It is also important that the selected representative period will include the period after the year 2000.

We emphasize the need for further analysis of other hydrological characteristics, particularly in the area of the low flows, before finalizing the future reference period, as the dry periods after the year 2000 may lead to lowering the limits connected with minimum discharges. This may significantly affect sectors like water planning, water use, waste-water treatment, irrigation, nuclear reactor cooling, reservoir management, etc. The transition to a new reference period for hydrological characteristics should involve a thorough professional discussion, along with an analysis of potential economic impacts.