Disentangling hydrological thresholds and runoff mechanisms across contrasting two climatic zones: Evidence from mountain watersheds

Understanding hydrological thresholds and runoff generation mechanisms across contrasting climatic zones remains a major challenge for improving flood prediction under climate change. However, existing studies are largely constrained to single climatic regions and rely on empirical rainfall thresholds, limiting their transferability across diverse environments. This study addresses this gap by comparatively analysing event-based hydrometeorological observations from 2006 to 2022 in two representative mountainous watersheds under contrasting climatic conditions. The results reveal two distinct types of hydrological threshold control. In arid environments (the Rujigou watershed in the Helan Mountains), runoff is governed by intensity-controlled thresholds, where high rainfall intensity exceeding infiltration capacity triggers rapid hydrological responses with short lag times. By contrast, humid environments (the Longxi River watershed in the Hengduan Mountains) exhibit storage-controlled thresholds, where runoff generation depends on the combined effects of antecedent soil moisture and cumulative rainfall, reflecting saturation-excess and subsurface flow processes. A coupled threshold between antecedent storage and event precipitation is identified: when the sum of rainfall and antecedent soil moisture reaches 111.23 mm, it marks the threshold for runoff generation; when this sum reaches 260.72 mm, it indicates the threshold at which runoff begins to increase sharply. This significantly improves the explanation of runoff variability beyond conventional rainfall-only metrics. By integrating multiple indicators, including rainfall characteristics, runoff coefficients, curve numbers and lag-time correlations, this study develops a transferable diagnostic framework for identifying dominant runoff generation mechanisms. The findings demonstrate that hydrological thresholds are climate-dependent and shaped by interactions among precipitation regimes, soil properties and vegetation. Under intensifying extreme rainfall associated with climate change, these contrasting controls are likely to amplify, highlighting the need for region-specific, process-based strategies for flood early warning. This framework provides a basis for improving flood risk assessment and adaptive management in mountainous regions worldwide, particularly in data-scarce environments.