Analysis of Long-Term Seasonal Rainfall Trends in Sri Lanka after Eliminating the Serial Correlation Effect

This research analyzed the annual and seasonal rainfall trends in Sri Lanka. It used daily rainfall data from 266 rain gauge stations across the island, covering 60 years from 1960 to 2020. A trend-free pre-whitening method utilizing Mann-Kendall's nonparametric trend test and Sen's slope estimator was employed for the analysis. This technique helped remove the data's correlation effects and estimate clear trends. The results showed that rainfall trends vary by region and season. The Kalu, Kelani, Nilwala, Attanagalu, Deduru Oya, and Upper Mahaweli basins in the central hills experienced a mild decrease in rainfall over the past 60 years. In contrast, the Maha Oya and Deduru Oya basins showed an increase in average annual and seasonal rainfall. Other basins did not show significant trends. Additionally, stations like Nuwara-Eliya, Nakkala, and Padiyathalawa have experienced consistently declining extreme rainfall trends for six hours over the last decades.