The research team, led by Ninu Krishnan Modon Valappil of Universiti Sains Malaysia, together with Yusri Yusup and Vijith Hamza, drew on the India Meteorological Department’s high-resolution daily gridded rainfall dataset, which covers the subcontinent at a quarter-degree grid spacing and extends back to the beginning of the twentieth century. From these daily records, aggregated into monthly totals, the team computed two closely related quantities. The first is rainfall erosivity, often called the R-factor, a term in the Universal Soil Loss Equation family of models that quantifies the kinetic punch delivered by raindrops and the runoff they generate. The second is erosivity density, which normalizes that punch by the amount of rain, effectively asking how destructive each millimeter of rainfall is on average.

To estimate these quantities across the full century, the researchers employed the monthly rainfall-based empirical model introduced by Arnoldus in 1980, a widely used approach when sub-hourly rainfall intensity data are unavailable, as they are for most of the historical record. The resulting values reveal an extraordinary range. Annual rainfall across Kerala varied from as little as 134 millimeters to as much as 5,424 millimeters in individual grid cells and years. Rainfall erosivity ranged from 61 to 58,063 megajoule millimeters per hectare per hour per year, a spread of nearly three orders of magnitude, while erosivity density spanned 0.38 to 17.61 megajoules per hectare per hour. That enormous variability is precisely why the authors argue that averages and totals conceal more than they reveal about erosion risk.

Geographically, the study found a persistent north-south divide. Higher rainfall, higher erosivity, and higher erosivity density were consistently concentrated in northern Kerala, where the Western Ghats force moisture-laden monsoon winds upward and squeeze out intense orographic precipitation. Lower values predominated across much of the southern region. This spatial pattern matters because the Western Ghats are recognized as one of the world’s biodiversity hotspots, and previous work has documented substantial soil loss across the region, including dramatic erosion episodes following the severe Kerala floods of 2018. Knowing where the erosive energy of the climate is concentrated provides a scientific basis for targeting watershed management and soil conservation measures where they will do the most good.

The temporal analysis was where the study broke new ground. Using linear trend analysis alongside seasonal, decadal, and inter-decadal comparisons, the team found that rainfall, erosivity, and erosivity density did not move in lockstep. Instead, the record alternated between phases of increasing and decreasing values on decadal timescales, and seasonal hotspot analysis, performed with the Getis-Ord Gi* statistic, a method for identifying statistically significant spatial clustering, revealed pronounced shifts between monsoon and non-monsoon periods. In other words, the places and times where erosive power concentrates are not fixed features of the landscape but migrate through the decades and across the calendar, responding to the shifting rhythms of the monsoon system.

The conceptual centerpiece of the paper is the decoupling index, a measure borrowed from economics, where decoupling analysis was developed to examine whether economic growth could be separated from environmental damage. Applied here, the index asks a simple question: when rainfall amount changes, does erosivity change proportionally? The answer, across most of Kerala’s twentieth century, was no. Weak coupling predominated throughout the study period, meaning that changes in how much rain fell were only loosely reflected in changes in how erosive that rain was. More strikingly, the results suggest that climatic rainfall erosivity became progressively less dependent on rainfall amount alone as the century wore on, hinting that the character of the rain itself, its intensity, concentration, and timing, has been changing in ways that totals cannot capture.

This decoupling has a physical explanation rooted in how raindrops transfer energy to the ground. Erosivity scales with the kinetic energy of falling drops and with rainfall intensity, not merely with volume. A season that delivers the same total rainfall as another, but in fewer, fiercer bursts, will strip far more soil. Climate change is widely expected to intensify precisely this pattern across the tropics, with warming seas and atmospheres loading more moisture into individual storm events even where total rainfall stagnates or declines. Related studies cited by the authors have documented intensifying erosivity in West Africa, and research along the Western Ghats and the southwest coast of India has documented changes in extreme rainfall, mesoscale convective systems, and moisture transport in recent decades, all consistent with a monsoon regime whose extremes are sharpening.

For Kerala, the practical stakes are considerable. The state’s steep slopes, lateritic soils, dense river networks, and reservoir-dependent agriculture make it acutely sensitive to sediment loss, which chokes reservoirs, degrades farmland, and compounds landslide and flood hazards. The study’s authors frame their results as a foundation for regional soil erosion assessment, watershed management, and climate adaptation in tropical monsoon environments. If planners continue to infer erosion risk from rainfall totals, they may systematically misjudge which decades and districts face the greatest threat. A decade of modest total rainfall punctuated by violent downpours could be more erosive than a wetter, gentler decade, and the decoupling index offers a way to detect exactly that divergence in the historical record.

Methodologically, the study also demonstrates the value of squeezing more from the data that exist. True erosivity calculations ideally require high-temporal-resolution rainfall intensity measurements, which global efforts such as the Global Rainfall Erosivity Database have assembled for recent decades. But century-scale assessment demands the long observational records that only monthly or daily data can provide, and the Arnoldus monthly model, though an approximation, allows researchers to extend erosion-relevant analysis back through periods when no rain gauge recorded intensity. The trade-off is acknowledged in the literature, and the authors’ use of trend analysis, hotspot statistics, and the decoupling index together provides a more robust picture than any single metric could, triangulating on the underlying behavior of the monsoon system.

The broader message extends well beyond Kerala. Rainfall-driven soil erosion is a major cause of land degradation in tropical monsoon regions, where hundreds of millions of people depend on rain-fed agriculture. As global assessments of rainfall erosivity grow more sophisticated, the Kerala study adds a century-scale caution: the relationship between the amount of water falling from the sky and the damage that water does is neither fixed nor guaranteed. In a warming world, that relationship appears to be loosening, and the erosion threat may be growing fastest precisely where rainfall statistics look unremarkable. For the steep, green slopes of the Western Ghats and for monsoon landscapes across Asia, Africa, and South America, the rain that matters most may be the rain that falls hardest, not the rain that falls most.

Subject of Research: Century-scale changes in rainfall erosivity and its decoupling from rainfall amount in Kerala, India

Article Title: Decoupling rainfall amount and rainfall erosivity: century-scale changes in climatic rainfall erosivity across Kerala, India

Article References: Valappil, N. K. M., Yusup, Y., & Hamza, V. (2026). Decoupling rainfall amount and rainfall erosivity: century-scale changes in climatic rainfall erosivity across Kerala, India. Theoretical and Applied Climatology, 157(10), Article 675. https://doi.org/10.1007/s00704-026-06587-z

Image Credits: AI Generated

DOI: 10.1007/s00704-026-06587-z

Keywords: rainfall erosivity, soil erosion, Kerala, monsoon, Western Ghats, erosivity density, decoupling index, climate change, India Meteorological Department, watershed management, RUSLE, land degradation

The study, conducted by Prodip Mandal and Mayank Joshi of the Govind Ballabh Pant National Institute of Himalayan Environment and Siddharth Prizomwala of the Institute of Seismological Research, applied the Revised Universal Soil Loss Equation, widely known as RUSLE, to a 75.52 square kilometre area around the rapidly growing district of Namchi. Their results, published in the journal Discover Geoscience, estimate an average annual soil loss of 39.10 tonnes per hectare per year, with localized hotspots reaching up to 138.62 tonnes per hectare per year. While those figures far exceed the global average erosion rate of roughly 2.4 tonnes per hectare per year, they are modest compared with rates exceeding 800 tonnes per hectare per year recorded in parts of India’s Northeastern Himalaya, where shifting cultivation and deforested wastelands leave slopes fully exposed.

RUSLE is an empirical model originally developed by the United States Department of Agriculture that computes average annual soil loss as the product of five factors: rainfall erosivity, soil erodibility, slope length and steepness, land-cover management, and conservation practices. Its enduring appeal lies in its adaptability. Because each factor can be derived from remotely sensed data and processed within a geographic information system, the model works well in data-scarce mountain regions where rain gauges with sub-hourly intensity measurements and detailed field soil surveys simply do not exist. That is precisely the situation in much of the Sikkim Himalaya, a tectonically young fold mountain belt built of weakly consolidated rocks from the Daling Group, threaded with active faults and hammered by prolonged monsoonal rainfall that makes the region a natural hotspot for erosion and landslides alike.

To build each factor layer, the researchers assembled a mix of datasets. Rainfall erosivity, which captures the capacity of falling rain to detach and transport soil particles, was estimated from daily rainfall records at six stations in and around the study area using an empirical relationship suited to Indian conditions, yielding values between 717.47 and 826.03 megajoule millimetres per hectare per hour per year, highest in the wetter southern portion of the map. Soil erodibility was computed with the EPIC equation from sand, silt, clay and organic carbon contents drawn from the ISRIC Soil Grids database, producing values between roughly 0.0137 and 0.0156, with the more erodible soils at higher elevations where organic carbon is abundant. The topographic factor, combining slope length and steepness, was derived from a 12.5-metre ALOS PALSAR digital elevation model and reached values as high as 19.05 in dissected terrain near drainage channels.

The remaining two factors encode human influence on the land surface. The cover management factor, ranging from 0.004 to 1.0, was assigned to five land-use classes mapped from European Space Agency satellite imagery acquired in October 2024 and refined with high-resolution imagery from Google Earth: dense forest, moderately dense forest, sparse forest, agricultural land and built-up areas. Forested slopes, where canopy intercepts raindrops and roots bind the soil, received the lowest values, while bare and open land received the highest. The conservation support practice factor, spanning 0.55 to 1.0, reflects measures such as terracing, contour farming and retaining walls that locally slow runoff. When the five layers were multiplied together in raster format and the resulting erosion values classified with the Jenks natural breaks method, the picture that emerged was strikingly uneven: 89.52 percent of the study area falls in the low-risk class, 6.52 percent in the moderate class and only 3.96 percent in the high-risk class.

Where does the sediment actually come from? The answer, overwhelmingly, is the steepest farmland. Among land-use classes, agricultural land and open land showed the highest erosion rates, peaking at 138.62 tonnes per hectare per year, whereas dense forest lost only 21.53 tonnes and moderately dense forest 20.23 tonnes per hectare per year. Together, dense and moderately dense forest blanket more than 55 percent of the study area, and the analysis makes clear that this vegetation is the single most important brake on soil loss, absorbing raindrop kinetic energy and reducing runoff velocity before it can carve rills into hillsides. Interestingly, the urban core of Namchi itself recorded relatively low erosion of 85.76 tonnes per hectare per year, an unexpected finding the researchers attribute to impervious concrete surfaces and the retaining walls residents build to protect hillside properties, which shield the soil from direct raindrop impact even as construction churns the surrounding periphery.

Slope proved equally decisive. The highest erosion rates occurred on slopes between 35 and 50 degrees, where gravity accelerates runoff and shallow, weakly stable soils sit atop fragile Himalayan bedrock. Somewhat counterintuitively, the steepest slopes of all, above 50 degrees, showed the lowest measured rates at just 6.33 tonnes per hectare per year, because exposed hard rock and difficult access limit both soil availability and human interference there. The geographic pattern also follows the rain: the southern part of the study area, closer to the moisture-laden winds arriving from the Bay of Bengal, receives more rainfall and more sunshine on its south-facing slopes, and consequently loses more soil than the northern segment.

Seeking to understand what drives these patterns, the team extracted six topographic and hydrological factors from the digital elevation model, including elevation, slope, aspect, relative relief, topographic wetness index and drainage density, and examined their pairwise relationships with Pearson correlation coefficients after checking for multicollinearity. Elevation showed weak but statistically significant positive correlations with slope, relative relief and drainage density, and a weak negative correlation with the topographic wetness index, which is higher in low-lying zones where moisture accumulates and deposition rather than erosion dominates. Aspect correlated positively with slope and relief, and the strongest relationship in the matrix linked rainfall erosivity with drainage density, underscoring how climatic forcing and landscape dissection interact to concentrate erosion along densely channelled terrain. Overall, relative relief and slope emerged as the most influential contributors to soil loss in the region.

The implications reach well beyond geomorphology. The authors frame their findings against the United Nations Sustainable Development Goals, noting that erosion degrades spring catchments that mountain communities depend on for drinking water, undermining SDG 6 on clean water; strips fertile topsoil from farms, deepening rural inequality under SDG 10; and threatens the sustainability of fast-growing Himalayan towns under SDG 11. Because erosion here is most severe during the monsoon, targeted interventions such as contour farming, terracing and slope protection on the moderate-to-steep agricultural slopes of the southern study area could deliver outsized benefits. Prior studies cited in the work suggest that well-implemented conservation practices can cut erosion by a factor of 2.5 to 7, and the Namchi results hint that even informal measures by local residents already mute erosion around settlements.

The study is candid about its limits. Empirical erosivity equations built on daily rainfall cannot fully capture the short, violent cloudbursts that characterize early-monsoon events in the Eastern Himalaya and frequently trigger landslides, and globally generalized soil databases may smooth over local heterogeneity in the rugged terrain. Climate change, intensifying extreme precipitation and accelerating infrastructure development are all expected to shift erosion rates in ways the current model does not project. Even so, the team argues that RUSLE, paired with freely available satellite data, offers a practical blueprint for other data-scarce Himalayan districts: by identifying exactly which slopes, land uses and aspect zones bleed soil fastest, it gives planners a prioritized map for conservation investment in one of the most erosion-prone mountain belts on the planet.

Subject of Research: Quantitative assessment of monsoon-driven soil erosion using the RUSLE model in the Sikkim Himalaya

Article Title: RUSLE-based quantitative soil erosion assessment in the monsoon-dominated region of Sikkim Himalaya

Article References: Mandal, P., Joshi, M., & Prizomwala, S. (2026). RUSLE-based quantitative soil erosion assessment in the monsoon-dominated region of Sikkim Himalaya. Discover Geoscience, 4(1), Article 348. https://doi.org/10.1007/s44288-026-00706-z

Image Credits: AI Generated

DOI: 10.1007/s44288-026-00706-z

Keywords: soil erosion, RUSLE, Sikkim Himalaya, monsoon rainfall, land use, slope classes, soil conservation, GIS and remote sensing, Namchi, rainfall erosivity, forest cover, sustainable development goals