Runoff trends in a changing climate in the Eastern Carpathians (Bieszczady Mountains, Poland)

The aim of the study is to identify trends in the river runoff and their potential factors in the seminatural catchments of the Bieszczady Mountains (Eastern Carpathians, SE Poland). Five forested and mountainous catchments with full series of runoff, temperature and precipitation data for the period 1986-2015 were selected. Trends in hydrological and meteorological parameters were evaluated with nonparametric Mann-Kendall test and Theil-Sen estimator. Results show that runoff has changed in the seasonal basis while annual runoff remains unaffected. The significant identified trends are an increase in winter runoff and a decrease in autumn (September) runoff. There is a clear increasing trend in air temperature (annual, summer and autumn), whereas sum of precipitation has not changed. The decreasing trend in autumn runoff is the result of a delayed response to the increase in air temperature, evapotranspiration, sunlight hours and duration of dry periods in summer. On the other hand, the winter runoff increase might be caused by several factors: the higher rainfall sum, more frequent precipitation events, shorter river ice cover duration and the increase in air temperature (Nov, Dec). Demonstrated changes in recent streamflow are similar to the results of trend analyses in mountainous catchments in the temperate climate zone in Europe. Hydrological conditions in the Eastern Carpathians seem to be relatively resistant to a changing climate so far.