K. G. A. Dilhara, V.P.A. Weerasinghe
The Rainfall Erosivity Factor (R) is crucial in assessing raindrop impact on soil erosion and improving disaster management in tropical regions. This study investigated changes in mean annual rainfall and raindrop impact on soil erosion in Sri Lanka’s Gampaha District over 40 years, identifying vulnerable areas for soil erosion. Maps showing rainfall and the R factor for four decadal periods (1982-1992, 1992-2002, 2002-2012, 2012-2022) were prepared using ArcGIS 10.8 and CHIRPS v.2 precipitation data (5 km resolution). Analysis across four decades (1982-2022) revealed significant increases in both rainfall and erosivity. The mean R factor grew by 59.2 MJ mm ha⁻¹ h⁻¹ yr⁻¹ (26.92%), while average annual rainfall increased by 572.01 mm (26.93%) in Gampaha District. High-R factor zones (> 285 MJ mm ha⁻¹ h⁻¹ yr⁻¹) and high-rainfall areas (> 2422 mm) expanded notably from the 1982–1992 to 2012–2022 period. Mean annual rainfall ranged between 2124.05–2696.06 mm, and mean rainfall erosivity ranged from 219.91-279.11 MJ mm ha⁻¹ h⁻¹ yr⁻¹ in Gampaha District. These trends suggest climatic shifts potentially linked to climate change, as evidenced by significant increases in mean annual rainfall and R factor between 1992-2002 and 2002-2012. The Dompe, Attanagalla, and Mahara Divisional Secretariat Divisions are particularly vulnerable to fluctuations in rainfall erosivity, primarily driven by the southwest monsoon, necessitating focused erosion mitigation efforts. Overall, the study demonstrates that climatic fluctuations have amplified rainfall-induced soil erosion over the past four decades, highlighting the urgent need for adaptive management strategies.