Rainfall trends in the transboundary Euphrates-Tigris River Basin

Climate change and anthropogenic factors have a significant impact on rainfall variability. In this study, the trends of monthly total precipitation in the transboundary Euphrates-Tigris River Basin (ETRB), where these effects are intensely observed, were analyzed by trend analysis methods with various perspectives. After decomposing the monthly total rainfall series (1964–2021) obtained from 18 stations representing 12 months of the year, statistical (Mann-Kendall, Wilcoxon, Onyutha), graphical (Innovative Polygon Trend Analysis, IPTA), Trend Polygon Star Concept, TPSC), sub-trend (Onyutha) and spatial trend analyses were performed. Based on the results, it was observed that rainfall in the basin has significant spatial and temporal variability. A monotonic strong decreasing trend was observed in cold months with high rainfall, while an increasing trend was observed in warm months with low rainfall. Extreme rainfall during the winter months has shifted towards the end of the cold months. Graphical and statistical analysis methods have shown strong consistency in identifying monotonic trends. However, the implementation of various methods provides specific insight (transition, sub-trend, etc.) into the temporal and spatial variability of rainfall. Sub-trend analysis shows that rainfall characteristics in the cold months show a significant change before and after the 1990–2000 period. In warmer months, this variability is significantly affected by local and global driving forces. The central and marginal parts of the ETRB have different rainfall characteristics. During cold months, the monotonic trend (decreasing) is significant in the central part. In warm months, the monotonic trend (increasing) increases its intensity in the eastern parts.