Motivated by the growing hazard of rainfall-induced shallow landslides both in humid regions transitioning from dry to wet seasons and in semi-arid or arid regions experiencing intensified storms under climate change, this study develops a novel, simple, yet physically based approach for potential instability prediction in long unsaturated slopes subjected to anticipated rainfall. The method is built upon a closed-form solution of Richards’ equation coupled with infinite-slope stability analysis, enabling a real-time evaluation of transient hydromechanical responses. It explicitly accounts for two governing triggering mechanisms, namely suction loss and the groundwater-table rise, and quantitative criteria for identifying the dominant trigger at failure onset are provided. For each prescribed combination of slope geometry, soil properties, and antecedent moisture conditions, this approach yields a single controlling rainfall intensity-duration threshold curve together with the corresponding failure depths along the curve, thereby simplifying prediction procedures while enriching hazard interpretation. A parametric analysis further quantifies the sensitivity of the derived threshold curves to key hydro- and geo- mechanical factors, including slope geometry, shear strength, antecedent moisture, and soil type. Validation against two well-documented cases demonstrates good agreement with the observed failure timing, depth, and mechanism. With its computational efficiency and modest data requirements, the proposed approach shows promise for shallow landslide forecasting and rapid, large-area hazard assessment.

Shallow landslide; Slope stability; Rainfall threshold; Triggering mechanism; Climate change.