Spatiotemporal Dynamics of Rain Spell Persistence across the Indian Ganga Basin (IGB)

Understanding the evolving dynamics of rainfall extremes is critical for assessing hydroclimatic risks in the Indian Ganga Basin (IGB), one of the world’s most densely populated and monsoon-dependent river systems. This study presents a comprehensive, century-long (1901–2023) assessment of rain spell dynamics across the IGB using a multivariate and probabilistic framework. Rain spells are characterized through the joint consideration of duration, intensity, and rainfall volume, enabling a clear distinction between short-duration (1–3 days) and long-duration (>3 days) events. For each category, a joint probability-based severity index is developed to quantify rainfall extremeness in an integrated manner. The analysis reveals a pronounced basin-scale reorganization of rainfall regimes over the last century. Historically, the IGB was dominated by spatially coherent and persistent long-duration rainfall events. However, recent decades show a marked shift toward increasingly frequent, intense, and spatially fragmented short-duration spells. Since the 1990s, short-duration rainfall events have exhibited rising persistence, increased recurrence rates, and enhanced severity across most parts of the basin. In contrast, long-duration wet spells display declining spatial continuity, reduced stability, and weakening basin-wide coherence. Notably, the entire basin now experiences an elevated occurrence of short, high-intensity events, indicating a fundamental transformation in monsoon rainfall behaviour. These evolving patterns significantly amplify hydrological hazards, including flash floods, rapid surface runoff, soil erosion, and landslides. Concurrently, the decline in sustained rainfall limits groundwater recharge, reduces soil moisture replenishment, and poses challenges for agricultural productivity and water security. The novelty of this study lies in its integration of multivariate rain spell characteristics within a joint probability framework to assess the long-term evolution of rainfall regimes. The findings provide robust evidence of hydroclimatic reorganization across the IGB and establish a probabilistic foundation to inform water resource management, disaster risk reduction, and climate adaptation strategies under a changing monsoon system.