Exploring the role and connections between rainfall and soil moisture over cascading hazards in the Himalayas

In recent years, the frequency and severity of extreme rainfall events have increased in the Himalayas, triggering landslides and debris flows often as a cascading hazard. Understanding the interactions between rainfall, initial soil moisture, and the triggering of landslides or debris flows is essential for mitigating risks to communities and infrastructure. However, the limited availability of observed hydrometeorological data poses serious challenges for accurate hazard assessment and early warning. To address these, numerical modelling-based reanalysis of rainfall-induced cascading hazards are chosen as a good choice. This study examines the influence of key hydrometeorological parameters, particularly rainfall and initial soil moisture, on the initiation and progression of shallow landslides and runoff-generated debris flows within a small mountainous catchment in the Himalayas, utilizing a basin-scale numerical modeling approach. We utilize multiple precipitation data sources, including reanalysis products, satellite-based retrievals, and outputs from numerical weather prediction models, to conduct a retrospective analysis of a historical cascading hazard event. This approach enables us to assess how these parameters impact the timing and severity of individual hazards, such as landslides and debris flows, within the cascading hazard chain. Our findings reveal distinct temporal patterns and triggering mechanisms for shallow landslides and runoff-generated debris flows, shedding light on their cascading behaviour in data-scarce, topographically complex regions. We also observe that initial soil moisture has a strong influence on hazard severity, and understanding its connection with rainfall is crucial for reliable hazard assessment.