Century-scale spatiotemporal variability and trends of rainfall erosivity and erosivity density in an ecologically sensitive tropical monsoon region of India (1901–2020)

Understanding long-term rainfall erosivity variability is essential for assessing soil erosion risk and supporting sustainable land and watershed management in tropical monsoon environments. This study presents a century-scale (1901–2020) assessment of rainfall erosivity and erosivity density across Kerala, India, using India Meteorological Department (IMD) gridded rainfall data (0.25° × 0.25° resolution) from 50 grid cells. Rainfall erosivity and erosivity density were estimated from monthly rainfall records using the Arnoldus empirical approach to examine their spatial distribution, temporal variability, elevation relationships, long-term trends, and homogeneous erosivity domains. Trend characteristics were evaluated using the modified Mann–Kendall test and Sen’s slope estimator, while agglomerative hierarchical clustering with Ward’s linkage was employed to identify regions exhibiting similar erosivity characteristics. Mean annual rainfall ranged from 824 to 4,123 mm, with mean annual rainfall erosivity and erosivity density ranging from 1,291 to 29,987 MJ mm ha⁻¹ h⁻¹ y⁻¹ and 1.34–7.38 MJ ha⁻¹ h⁻¹, respectively. The results reveal substantial spatial heterogeneity and pronounced interannual and inter-decadal variability in both rainfall erosivity and erosivity density. Despite episodic periods of elevated erosivity, trend analyses indicate that rainfall, rainfall erosivity, and erosivity density generally declined across most grid cells during the study period, with localized increasing trends restricted to a few eastern highland areas. Elevation exhibited weak inverse relationships with rainfall erosivity and erosivity density, suggesting that erosivity variability is influenced by rainfall characteristics, monsoon dynamics, and topographic setting rather than elevation alone. Hierarchical clustering identified four statistically distinct erosivity domains representing intermediate, high, very high, and extreme erosivity conditions, providing a framework for regional erosion-risk assessment and spatially differentiated soil conservation planning. The observed variability in rainfall erosivity is consistent with differences in rainfall intensity distribution, monsoon variability, and regional topographic influences, highlighting the importance of rainfall characteristics beyond annual precipitation totals. Although the monthly rainfall-based approach may not fully capture short-duration erosive storms, it provides a consistent basis for evaluating century-scale regional erosivity variability. The resulting erosivity database and domain classification offer valuable inputs for RUSLE-based soil erosion modelling, watershed prioritization, and climate-informed land management in tropical monsoon environments.