Contemporary runoff trends in some highland stream basins in Bulgaria

Abstract

Hydrological changes in four stream basins situated on the northern slopes of the Stara Planina range (North Bulgaria) and in the Rodopi massif (South Bulgaria) have been investigated during the 1951–1989 time period. Parallel analysis of runoff and precipitation mean annual values have been made, calculating the respective annual ratios compared to the mean of the entire range mean value. Curves of runoff and precipitation frequencies have been developed and analyzed. This investigation establishes that during the 1980s a consistent trend towards significant humidity reduction has occurred. Mean annual deviation values of the last decade compared to the entire period mean annual figure amount to 5–20% and 10–25% for the precipitation and runoff, respectively, with humidity reduction of greater amplitude in the North Bulgarian stream basins. Annual Mean deviations are due to significant moisture reduction during the warm half of the year in North Bulgaria and in the cold half in South Bulgaria. Both regions reflect zonal differences caused by climatic patterns of European if not planetary scale.