Hillslope Hydrological Extremes under Changing Monsoon Patterns and Land Use Dynamics

Complex terrains account for approximately 25% of India’s total geographical area, which plays a crucial role in contributing to the country’s water resources. Such high-altitude zones like the Himalayas and Western Ghats have been frequented with hydrological extremes (viz., GLOFs, flash floods, landslides) in recent years due to the alteration of land use/land cover (LULC) change. The changing frequency and magnitude of monsoon rainfall further exacerbate the hydrological response of these high-altitude zones. This study analyses long-term (1951–2020) gridded rainfall data from the India Meteorological Department (IMD) at a 0.25° × 0.25° resolution to introduce a novel Monsoon Index (MI) aimed at quantifying and identifying monsoon dominance areas across India. Frequency analysis indicates that 80% of the high-altitude regions have become more monsoon-dominant during the period 1991–2020 compared to 1951–1990. A case study in the monsoon-dominated Ranikhola watershed (Gangtok, Sikkim, India) reveals a marked rise in surface runoff during the monsoon season (May to October), largely attributed to an 11% reduction in forest cover between 1991 and 2000. This vegetation loss in the small hilly catchment resulted in a 32% increase in peak runoff, while annual monsoon runoff rose by 23% when comparing the observed period (2015–2020) with the short-term projection for 2021–2026. The vulnerability of the watershed to potential hydrological extremes—such as landslides and flash floods—was found to increase due to the rising magnitude and uneven distribution of surface runoff during the peak of the monsoon season. Through this study, the need for an integrated GIS-based flood (or flash flood) route mapping is emphasized for sustainable watershed management and reconstruction of the LULC change policies.