Assessment of Rainfall Variability and Extreme Wet and Dry Events Using the Standardized Precipitation Index in the Bharathapuzha River Basin, Kerala, India

Rainfall variability and extreme wet and dry events were characterized for the Bharathapuzha River Basin, Kerala, India, using 35 years (1991–2025) of India Meteorological Department (IMD) 0.25° gridded daily rainfall data. The Standardized Precipitation Index (SPI) was calculated at 1-, 3-, 6- and 12-month timescales. Trends and change points were evaluated using the Mann–Kendall test, Sen’s slope estimator, and Pettitt’s test. The mean annual rainfall was 2,073 mm with a coefficient of variation of 19.7 percent, and nearly two-thirds of the rainfall was received in the southwest monsoon season. Annual rainfall varied from 1,251 mm in 2003 to 2,923 mm in 2018. SPI-6 registered eight extreme wet and eleven extreme dry events; the most intense wet event occurred between June and November 2018 (peak SPI = +2.81), and the longest dry event lasted eight months from July 2003 to February 2004. Trends in the frequency of extreme wet and dry months were not statistically significant at the SPI-1 timescale (p = 0.220 and p = 0.922, respectively). Annual rainfall was significantly different among the three study subperiods (p = 0.043), but the long-term trend was not significant (p = 0.105). Trends of increasing September rainfall and increasing frequency of wet months at the SPI-3 and SPI-6 timescales were not significant after correction for multiple tests. The results show significant variability in rainfall and the presence of different wet and dry periods, demonstrating the importance of multi-timescale SPI analysis for characterizing meteorological rainfall conditions relevant to flood and drought assessment in monsoon-dominated river basins.