TREND AND VARIABILITY IN DISCHARGE AND CLIMATE VARIABLES IN THE CROATIAN LOWER DRAVA RIVER BASIN

Changes in hydrological values as a result of climate change directly influence people and ecosystems. This paper investigates changes in the annual and half-year values of mean air temperature and precipitation parameters and mean discharges in the Drava River basin in Croatia according to the data of the meteorological stations Varazdin (1949-2008) and Osijek (1926-2004) as well as the hydrological station Donji Miholjac, for which there are existing discharge data since 1926. The objective is to evaluate the trends and variations in the 20th and at the beginning of the 21st century in climate, which at the global and regional scales show warming up, while precipitation trends are spatially less coherent and do not show the same or similarly intense changes (IPCC, 2007). The linear trends in the mean annual air temperatures show increase in the period 1949-2008, which is statistically significant according to the non-parametric Mann-Kendall test with a 5% significance level in the area of the Upper Drava annually and semi-annually (Varazdin) and the Lower Drava annually (Osijek). According to the Sneyers progressive trend test, the warming up starts at the beginning of the second half of the 1990's, and is significant from the beginning of the 21st century. Simultaneously, with regards to precipitation quantities, there is no similar trend or the same intensity present in the Upper and the Lower Drava, with the determined trends not statistically significant. The number of precipitation days significantly decreases in the cold half-year in the 1980's (since 1981 in Osijek and since 1988 in Varazdin), and, with a mild increase in the warm half-year, contributes to the negative annual trend. A decrease in the frequency of precipitation is accompanied by a decrease in their variability. The mean annual discharge at Donji Miholjac shows a statistically significant decreasing trend in both monitored periods and is the result of a decreasing trend in both periods, whereas the decreasing trend in the mean discharges in the warm half-year is statistically significant and appears since 1997. The trend analyses of the successive 30-years sub-periods, with the beginnings shifted by ten years, indicate parallel exchanges of positive and negative annual precipitation trends (Varazdin) and discharges (Donji Miholjac). In all monitored series, there is a present temporal variability. The trend analysis of the variation coefficient series for 10-year intervals indicates a significantly increased variability of precipitation quantities in the warm half-year in Varazdin. At the same time, there is a present significant decrease in the variability of the mean annual discharges and a significant increase in the variability of the mean discharges in the cold half-year for the Drava at Donji Miholjac, which is a consequence of the natural changes and anthropogenic influences.