Analysis of long-term rainfall trend and extreme in upper northern Thailand

Understanding long-term precipitation trends is critical for climate adaptation and water resource management, particularly in regions prone to extreme weather events. This study investigates seasonal, annual, and extreme rainfall trends in upper northern Thailand using data from nine meteorological stations spanning 1981 to 2021. Trend analyses are conducted using simple linear regression (SLR), the Mann-Kendall and modified Mann-Kendall (MK/MMK) tests with Sen's slope estimator (SSE), and innovative trend analysis (ITA). Extreme precipitation is assessed based on the 1-day maximum rainfall index (RX1Day). The results reveal distinct seasonal disparities: Lampang exhibits a significant increasing trend in summer, while Uttaradit shows a declining trend. During the rainy season, upward trends are observed in Chiang Mai, Lamphun, and Phrae, with no significant changes detected in winter. Annual rainfall trends show increases in Chiang Rai, Lamphun, and Phrae. Regarding extreme precipitation, significant upward trends in Lamphun and Phayao suggest an elevated risk of flash flooding. Importantly, these statistical trends suggest a potential increase in the risk of flood and flash flood events, consistent with observations from 2020 to 2024 in Chiang Mai, Chiang Rai, Lamphun, and Phrae, particularly during active monsoon periods and tropical storm events. The widespread flooding across all nine provinces in 2024 further corroborates the observed increasing trends in rainfall and extreme events. The consistency across methods confirms the robustness of the trends and provides essential insights for climate adaptation, water planning, and disaster preparedness in upper northern Thailand.