Kerala rainfall erosivity declined across most grid cells from 1901–2020

A century-scale analysis of Kerala, India, finds that rainfall erosivity and erosivity density generally declined across most of the state between 1901 and 2020, despite pronounced year-to-year and decade-to-decade fluctuations and occasional periods of exceptionally high erosivity. The study assessed 50 India Meteorological Department grid cells at a resolution of 0.25° × 0.25°, estimating rainfall erosivity from monthly rainfall totals and examining how the intensity and efficiency of erosive rainfall varied across the state. Mean annual rainfall ranged from 824 to 4,123 mm, mean annual rainfall erosivity from 1,291 to 29,987 MJ mm ha⁻¹ h⁻¹ y⁻¹, and mean erosivity density from 1.34 to 7.38 MJ ha⁻¹ h⁻¹.

The highest long-term values were concentrated in northern Kerala, while the smallest erosion-potential domain covered much of southern and parts of central Kerala. Rainfall erosivity describes the capacity of rainfall to detach and transport soil, while erosivity density expresses that erosive potential relative to the amount of rainfall. The two measures help distinguish the effect of how much rain falls from the effect of how intensely it falls.

For the analysis, the researchers converted daily gridded rainfall observations into monthly totals for each year and applied the Arnoldus empirical approach, a monthly-rainfall method widely used for long-term regional assessments. They calculated annual erosivity density as annual rainfall erosivity divided by annual rainfall. Trend significance was evaluated with the modified Mann–Kendall test, which adjusts for serial correlation, and trend magnitude was estimated with Sen’s slope method.

Agglomerative hierarchical clustering with Ward’s linkage then grouped grid cells with similar 120-year mean rainfall erosivity and erosivity density. The analysis identified a pronounced north–south gradient. Northern Kerala contained the highest minimum, maximum and mean rainfall erosivity, as well as the highest erosivity-density values.

The largest values occurred in and around Kasaragod, Kannur, Wayanad, Kozhikode and parts of eastern Palakkad, with additional high-erosivity areas in parts of Thrissur, Ernakulam and Kottayam. Most of central and southern Kerala had low to moderate values. The mapped patterns broadly followed rainfall totals, but rainfall erosivity had a higher coefficient of variation than rainfall, indicating that the erosive potential of Kerala’s rainfall was more variable across space and time than precipitation amount alone.

Four statistically distinct erosivity domains emerged from the clustering. The largest, C1, contained 23 cells and represented comparatively low conditions, mainly across southern and parts of central Kerala. Its mean rainfall erosivity ranged from 1,291 to 7,044 MJ mm ha⁻¹ h⁻¹ y⁻¹, and its erosivity density ranged from 1.34 to 3.38 MJ ha⁻¹ h⁻¹.

The intermediate C2 domain comprised 17 cells across much of central Kerala, with rainfall erosivity from 8,375 to 12,685 MJ mm ha⁻¹ h⁻¹ y⁻¹ and erosivity density from 2.45 to 3.99 MJ ha⁻¹ h⁻¹. Eight northern cells formed the high-erosivity C3 domain, reaching 21,643 MJ mm ha⁻¹ h⁻¹ y⁻¹ for rainfall erosivity and 5.76 MJ ha⁻¹ h⁻¹ for erosivity density. Two northern cells formed C4, the extreme domain, with rainfall erosivity of 26,974 to 29,987 MJ mm ha⁻¹ h⁻¹ y⁻¹ and erosivity density of 6.54 to 7.38 MJ ha⁻¹ h⁻¹.

Long-term averages nevertheless concealed substantial temporal variability. The 1991–2000 decade showed the widest spread in rainfall erosivity, from 1,269 to 38,824 MJ mm ha⁻¹ h⁻¹ y⁻¹, and the widest spread in erosivity density, from 1.26 to 10.53 MJ ha⁻¹ h⁻¹. Those ranges did not coincide with the highest average values: the decade with the greatest mean rainfall erosivity was 1921–1930, at 12,224 MJ mm ha⁻¹ h⁻¹ y⁻¹, and the highest mean erosivity density was 3.74 MJ ha⁻¹ h⁻¹.

Inter-decadal comparisons showed alternating periods of increase and decrease rather than a smooth progression. Rainfall erosivity rose most strongly between the second and third decades, while pronounced decreases occurred in several later transitions, particularly between 1991–2000 and 2001–2010. Comparing the first and last decades likewise showed that declines predominated statewide.

Statewide linear analysis confirmed the general direction of change. Mean annual rainfall decreased at a rate of 3.53 mm per year, with an R² of 0.1119. Mean rainfall erosivity declined by 34.34 MJ mm ha⁻¹ h⁻¹ y⁻¹ per year, with an R² of 0.1302, while erosivity density fell by 0.0074 MJ ha⁻¹ h⁻¹ per year, with an R² of 0.0947.

Grid-level results were more varied. For rainfall erosivity, 39 of 50 cells showed decreasing trends, including 33 statistically significant declines; 10 showed increases, five of them significant, and one showed no trend. Sen’s slopes ranged from −171.72 to 38.79 MJ mm ha⁻¹ h⁻¹ y⁻¹ per year.

For erosivity density, 40 cells declined, including 34 significant decreases, nine increased, including four significant increases, and one showed no trend. The strongest declines occurred in northern districts, whereas increases were confined to a few eastern highland locations. Elevation did not explain the regional pattern by itself.

In an analysis using 300 spatially distributed points sampled from interpolated surfaces, elevation had a moderate negative relationship with mean rainfall, with r = −0.49 and R² = 0.2418. Its relationships with rainfall erosivity and erosivity density were much weaker: r = −0.24 and R² = 0.0589 for erosivity, and r = −0.18 and R² = 0.0325 for erosivity density. Low-elevation locations generally received more than 3,000 mm of annual rainfall and often had greater erosivity, whereas high-elevation sites commonly received 1,000–1,500 mm, but considerable variation occurred within every elevation band.

The results indicate that monsoon circulation, rainfall intensity, seasonal distribution, topographic position and local atmospheric conditions jointly shaped erosivity rather than altitude acting as a simple control. The coexistence of declining long-term averages and episodic high-erosivity periods is an important feature of the record. A reduction in mean rainfall erosivity does not necessarily imply a proportional reduction in erosion hazard, because average conditions and extreme events represent different parts of the rainfall regime.

The monthly-rainfall method also cannot resolve the kinetic energy, duration or within-storm intensity of individual short-duration storms. Although this approach provides a consistent century-scale comparison, it may under- or overestimate local annual erosivity where extreme storms make a disproportionate contribution. The estimates describe climatic potential for soil detachment, not observed soil loss, which also depends on soil properties, vegetation, land use, slope, infiltration and sediment connectivity.

Likewise, the annual erosivity-density calculation describes long-term rainfall aggressiveness per unit rainfall rather than seasonal concentration or event-scale behavior. The spatial divergence also carries implications for erosion modelling. Because Kerala’s rainfall erosivity varies sharply over a comparatively small and topographically diverse area, applying one statewide R-factor could obscure localized high-erosivity conditions.

The four-domain classification provides an alternative framework for parameterizing Revised Universal Soil Loss Equation models, prioritizing watersheds and supporting spatially differentiated conservation planning. It does not by itself establish current erosion rates, however, and the four-cluster solution was selected through dendrogram interpretation rather than quantitative validation with indices such as silhouette width. High-temporal-resolution rainfall observations, climate projections and further analysis of extreme events will be needed to clarify future erosion susceptibility, but the current evidence shows that Kerala’s erosive rainfall has been highly variable despite its broad long-term decline.

Subject of Research: Century-scale analysis of rainfall erosivity and erosivity density across Kerala, India

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

Article References: Valappil, N. K. M., Yusup, Y., Dewi, J. R., Huda, N., & Hamza, V. (2026). Century-scale spatiotemporal variability and trends of rainfall erosivity and erosivity density in an ecologically sensitive tropical monsoon region of India (1901–2020). Environmental Earth Sciences, 85(17), Article 437. https://doi.org/10.1007/s12665-026-13119-5

Image Credits: AI Generated

DOI: 10.1007/s12665-026-13119-5

Keywords: rainfall erosivity, erosivity density, Kerala, monsoon rainfall, soil erosion, Western Ghats, climate variability, Mann–Kendall test, hydroclimatology, RUSLE, spatial analysis, long-term trends