Air and Water Temperature Trend Analysis at the Confluence of the Sava and Danube Rivers in Belgrade, Serbia

This study highlights significant recent increases in water and air temperatures in the rapidly developing city of Belgrade, Serbia to raise awareness of the anticipated negative impacts and the urgent need to develop appropriate mitigation strategies. We investigate the mean annual water temperature trends at the confluence of the Sava and Danube Rivers, along with air temperature trends at the Belgrade meteorological station, spanning from 1956 to 2020. Results reveal a consistent increase in temperature across all three measuring stations, with the Danube experiencing a rise of 0.34 °C/decade, the Sava at 0.44 °C/decade, and Belgrade's air temperature increasing by 0.39 °C/decade. Employing the Rescaled Adjusted Partial Sums method, sharp rises in water temperature were pinpointed in 1989 for the Sava and 1990 for the Danube, while Belgrade's air temperature surge began in 1998. The highest intensity of air temperature increase within the recent period (1998–2020) was observed at the Belgrade observatory, reaching 0.76 °C/decade. Notably, the Sava exhibited a faster increase in water temperature over the last thirty years compared to the Danube. August marked the peak average water temperature for both rivers, while July recorded the highest average air temperature in Belgrade. Despite differing flow rates, both rivers exhibit similar hydrological regimes, with maximum flows occurring in April and minimum flows in August for the Sava, and October for the Danube. Seasonal temperature increases were most pronounced in summer, notably in August, with the smallest rises occurring during cold periods. Additionally, an inverse proportional relationship between mean annual water temperatures and discharges was observed at both river stations. The overall findings suggest that the increase in both air and water temperatures is more pronounced during the warmer part of the year.