Rainfall and Extreme Rainfall Trend Analysis in the Malwathu Oya River Basin, Sri Lanka: A 40-Year Assessment (1985–2024)

Abstract

The Malwathu Oya river basin, Sri Lanka’s second-largest watershed, covering 10,500 km2, lacks a comprehensive rainfall and extreme rainfall trend analysis, despite its critical importance for water resource management and climate adaptation planning. We address this knowledge gap by systematically assessing long-term precipitation patterns and extreme rainfall trends in this climatically sensitive dry zone basin. This research analyzed 40 years (1985–2024) of daily precipitation data from the CHIRPS v2.0 satellite dataset at 0.1◦ resolution across 25 strategically distributed locations throughout the basin. Statistical analyses included Mann–Kendall trend tests for non-parametric trend detection, Sen’s slope estimator for quantifying trend magnitude, extreme value analysis using the Generalized Extreme Value (GEV) distribution, monsoon onset/withdrawal analysis using the pentad method, and assessment of the Precipitation Concentration Index (PCI). The basin was divided into upper, middle, and lower zones for spatial pattern analysis.Significantly increasing trends in annual rainfall were observed at all 25 locations (p < 0.05), with Sen’s slope esti mates ranging from 8.47 to 13.69 mm yr−1. Spatially, the lower basin exhibited the strongest trends (11.0 12.3 mm yr−1), followed by the upper basin (9.6–11.7 mm yr−1) and middle basin (8.5–10.8 mm yr−1). The Southwest monsoon contributed most significantly to annual increases (2.1–3.8 mm yr−1), while the Northeast monsoon showed moderate increases (1.8–3.2 mm yr−1). Extreme rainfall analysis revealed an intensification of daily maximum events, with 23 out of 25 locations showing increasing trends in annual maximum rainfall. Return period analysis indicated that 10-year extreme events now occur with 6–8 year frequencies. Monsoon timing remained relatively stable, with the Southwest monsoon onset occurring around early May and the Northeast monsoon onset in mid-October. PCI values (13.95–16.37) indicated moderate to irregular precipitation concentration patterns, with higher irregularity in upper and lower basin areas.Using satellite-based observational data, this study provides the first comprehensive evidence of significant rainfall intensification across the Malwathu Oya basin over the past four decades. The basin-wide increasing trends, coupled with the intensification of extreme events and spatial variability, indicate clear impacts of climate change on regional precipitation regimes. Annual rainfall has increased by approximately 14% across the basin, with particularly pronounced changes in the characteristics of extreme events. These findings have critical implications for water resource management, flood risk assessment, and agricultural planning, neces sitating adaptive strategies for reservoir operations, early warning systems, and climate-resilient development planning. The results fill a critical knowledge gap in Sri Lankan hydro-climatology and provide essential baseline information for evidence-based climate adaptation and sustainable water resource management in one of the country’s most important river basins. The documented trends support regional climate change projections and highlight the urgent need for adaptive management strategies in water-stressed dry zone environments.