Challenges in selecting the new reference period for long-term hydrological characteristics in Slovakia

Abstract

Establishment of a new reference period is an ongoing topic in Slovakia as the currently used reference period is 1961-2000. This contribution focuses on the assessment of spatial and temporal changes in the mean long-term yearly discharges and long-term mean monthly discharges at 140 water-gauging stations. The years 2021 and 2022 have been further evaluated at selected 13 water-gauging stations to assess the possible impacts of using different reference periods (1991-2020, 1981-2020, 1971-2020 and 1961-2000) on the water management. The analyses have confirmed that there are significant changes between long-term values, especially for the period 1991-2020 compared with the reference period 1961-2000, with the highest negative changes in monthly discharges being in April and highest positive changes being in January. Evaluation of mean monthly and annual discharges in 2021 and 2022 for 13 selected water-gauging stations, according to the four above mentioned reference periods, has identified changes especially in set categories of monthly discharges. The same average monthly but also annual flows in one station have fallen into different assessment categories, in several cases. For annual flows the changes have manifested in 6 water-gauging stations in the year 2021 and 2 stations in the year 2022. The number of stations with changes in categories between different reference periods used for particular months have varied from 2 to 8 water-gauging stations in 2021 (62 cases for the whole year) and from 2 to 6 watergauging stations in 2022 (40 cases for the whole year). The highest number of changes for both years were detected in June ( 14) and August and December (11). The selection of the reference period can therefore lead into different set of applied water management rules.