Abflusstrends slowakischer Flüsse und mögliche Zusammenhänge mit ENSO/NAO-Erscheinungen / Runoff trends in Slovak rivers and possible relationships with ENSO/NAO phenomena

A study was conducted on runoff trends in the period 1931-2000 for 27 Slovak rivers included in the Slovak National Climate Program. Comparison of the monthly runoff series for the 27 rivers helped to identify three regions with different runoff trends within the territory of Slovakia (constant, slightly decreasing, rapidly decreasing). The study also included the identification of different cyclic components in the runoff series. The length of the available records allowed identification of periods not greater than 22 years. The longer runoff series of the Danube river show that several dry periods occured in Central Europe and Slovakia In the middle of the 19th century. These were also used to detect longer runoff cycles of about 31 and 46 years. In addition, a regression analysis was carried out to study the relationship between runoff variability and the NAO and ENSO phenomena. The main cycles of the NAO and ENSO indices were obained by use of combined periodograms and compared with the length of the runoff cycles. Cycles of similar length occur in both series. The relationship between runoff series for Slovak rivers and the NAO and ENSO series was also studied.