COMPARISON OF CHARACTERISTIC DISCHARGES OF SLOVENIAN RIVERS OF THE NEW REFERENCE PERIOD 1991-2020 WITH PREVIOUS REFERENCE PERIODS

Abstract

The choice of the long-term 30-year reference period has a strong influence on the values of characteristic periodic discharges. Following the recommendation of the World Meteorological Organization that the 30-year standard reference periods should be updated every decade, the Slovenian Environmental Agency has begun introducing a new reference period, 1991-2020, to compare current observational data with long-term averages. This paper presents the impact of the 30-year reference periods recommended in historical practice by the World Meteorological Organization on the values of mean, low and high discharges for the six water gauging stations with the longest continuous data sets. The results show that the highest values of the mean periodic discharges are mostly for the period 1931-1960 and the lowest for the period 1981-2010. The new reference period, 1991-2020, gives a slight increase in mean discharges. Mean low periodic discharges are highest for the period 1961-1990 and lowest for the periods 1981-2010 and 1991-2020. Mean high periodic discharges are highest for the periods 1961-1990 and 1991-2020 and lowest for the period 1931-1960. A comparison of the characteristic discharges of the new reference period, 1991- 2020, with the previous period, 1981-2010, shows that there are no significant differences between these periods on an annual scale. However, there are major differences on the monthly scale. In the period 1991-2020, compared to 1981-2010, February discharges increased the most. From April to August, the values of all the characteristic discharges of the period 1991-2020 fell below the values of the period 1981-2010. Mean discharges also increased in September and November. In other months, there were no significant differences.