A Century of Rain in Kerala Reveals That How It Falls Matters More Than How Much

For more than a century, the tropical monsoon state of Kerala, along India’s southwestern coast, has been measured almost entirely by one number: how much rain fell. A new study argues that this single number has been quietly misleading the people who manage the region’s soil. By analyzing 120 years of daily rainfall records spanning 1901 to 2020, researchers have shown that the erosive power of Kerala’s rainfall, its capacity to tear soil particles loose and sweep them away, has drifted progressively out of step with the total amount of rain delivered. The finding, published in Theoretical and Applied Climatology, carries a stark implication for tropical monsoon regions worldwide: annual rainfall totals alone are no longer sufficient to explain long-term changes in erosion potential, and land management built on that assumption may be underestimating the threat.

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