Observed Changes in the Precipitation Regime in the Bulgarian–Serbian Cross-Border Region for the period 1961-2020

This study examines long-term changes in precipitation patterns in the Bulgarian–Serbian cross-border region over 1961–2020, using monthly and annual data from 23 meteorological stations. The analysis focuses on differences between two standard climatological periods (1961–1990 and 1991–2020) to identify recent shifts in precipitation regimes, including the calculation of the Precipitation Concentration Index (PCI) and the evaluation of seasonal and annual precipitation variability. The Precipitation Concentration Index (PCI) is a statistical indicator that quantifies the temporal distribution of precipitation over a year by assessing the extent to which annual precipitation is concentrated in a limited number of months (Oliver, 1980). Unlike precipitation totals, PCI provides information on rainfall seasonality and the uniformity of monthly precipitation distribution.

The results confirm that the study area is characterized by a pluvial-continental regime, with two precipitation maxima—more pronounced in summer—and two minima, with a dominant winter minimum. In the more recent period, a noticeable shift in the timing of secondary maxima and minima is observed across most of the region, accompanied by a general increase in annual precipitation totals. The Precipitation Concentration Index (PCI) reveals predominantly uniform precipitation distribution (PCI < 10) at all stations in both periods. However, higher PCI values are characteristic of the earlier period (1961–1990), while in 1991–2020 a slight eastward spatial shift in PCI is evident. Seasonal analysis further indicates that precipitation is more evenly distributed during the cold half of the year, with relatively stable PCI values between the two periods. In contrast, the warm season exhibits higher concentration levels, although a decrease in precipitation concentration is observed in the later period.

The findings point to a clear reorganization of precipitation regimes in the region during 1991–2020, expressed through shifts in seasonal distribution, changes in precipitation concentration, and increasing annual totals, indicating ongoing regional climate variability.