Stormflow generation in a humid forest watershed controlled by antecedent wetness and rainfall amounts

Understanding the meanings and identifying the controls for the stormflow generation with complex and nonlinear behaviors is essential for the development of threshold-based hydrological theory, as well as accurate assessment and prediction for flash flood risks. However, the study of catchment emergent patterns with threelinear threshold behaviors associated with hydrological connectivity has received little attention. Therefore, utilizing soil water storage, rainfall, and streamflow data spanning 3 years in a humid forest experimental watershed, Dujiangyan city, China, we elucidated how and where stormflow was generated with nonlinear behaviors, which were affected by antecedent wetness and rainfall amounts. Stormflow threshold behavior was taken as a function of combined gross precipitation and antecedent soil water storage, which was isolated using piecewise regression analysis with the identification of two breakpoints (i.e., generation threshold, Tg and rising threshold, Tr). It was found that the initial emergent behavior of rainfall-runoff was generally activated at the Tg, and then an abrupt shift from slow to fast flood response was possibly triggered at the Tr. These processes are important to understand the formation and development of flash floods at the watershed scale. It was noted that, above the Tr, considerably higher stormflow amounts generally occurred due to the lateral-connectivity extension of runoff contributing area from stream to neighboring hillslopes. Meanwhile, gravity-driven water movements in soil and better hydrological connectivity during the above-Tr phase readily triggered the huge flash flood disasters. The above-Tr flash floods with abrupt shifts were predominantly controlled by rainfall amounts, while initial below-Tg stormflow generation was mainly controlled by unsaturated soil water storage. More noteworthy, under heavy rainstorm conditions, the above-Tr stormflow was dominantly generated by subsurface flow, as was demonstrated at hillslope and watershed scales. These findings contribute to increasing our understanding of the controls on three-linear threshold-based hydrological behaviors, as well as of subsurface stormflow generation mechanism associated with hydrological connectivity in humid forest watersheds.