Runoff Generation Mechanism of Small Forest Watersheds in Humid Regions of China Under Single Rainstorm Conditions

Runoff generation is the production of various runoff components and is a fundamental aspect of the hydrological cycle. Investigating the evolution mechanism and laws governing the formation process of watershed runoff from the perspective of runoff generation mechanisms has consistently been a focal point and challenge within the field of hydrology. However, the variation in the mechanisms of runoff generation under single rainstorm conditions has not been fully elucidated, particularly in the humid regions of China. In the present study, we focus on the Pengchongjian small watershed in Southeastern China, where the average annual precipitation is 1589 mm. Based on long‐term hydrometeorological data, precipitation and runoff characteristics during single rainstorm conditions in the watershed were analysed. Alterations in the runoff generation mechanism were investigated in conjunction with the underlying surface characteristics. The findings revealed that in comparison to the baseline period (1983–2003), there was a 4.6% reduction in average rainfall amount and a 23.8% decrease in average runoff depth during the changing period (2004–2017). Additionally, it was observed that runoff depth exhibited more pronounced fluctuations, with a much higher variation coefficient than rainfall amount. Rainfall amount remained the primary factor influencing changes in runoff depth across different periods. However, its relative contribution decreased during the changing period, while the impact of non‐precipitation factors increased during this period. There was an extremely significant upward trend in the normalised difference vegetation index of the watershed, and the forest coverage increased from 80% to 98%. The water conservation capacity of the main stands (Chinese fir forests) were higher than other forest types. Under the conditions of a single rainstorm, the predominant runoff generation pattern observed during various periods in the watershed was saturation‐excess runoff, indicating a high soil infiltration rate. The proportion of the runoff generation patterns showed a significant change. During the baseline period, the proportion of saturation‐excess runoff was 45.3%, which increased to 52.9% in the changing period. In contrast, the infiltration‐excess runoff decreased from 36.8% to 32.9%. Additionally, the proportion of mixed runoff decreased from 17.9% to 14.2%. Meanwhile, the probability of interflow and groundwater flow generation increased, indicating an enhancement in the regulation and storage effects on watershed runoff. The findings of this research provide a scientific basis for mitigating and controlling flood disasters, optimising the allocation of water resources and evaluating the hydrological effects of the watersheds.