Abstract: To investigate the inpacts of runoff on tidal asymmetry in small‑ and medium‑sized estuaries, this study takes the Dafengjiang Estuary as the research object. Three sets of numerical experiments (no‑runoff, annual runoff, and climatological runoff) were designed to simulate the tidal variations throughout the year of 2023. The contributions of different tidal constituent combinations to tidal asymmetry were quantitatively calculated, and the regulatory roles of amplitude and phase‑lag variations of the O1, K1, and M2 constituents on skewness were analyzed. The influence mechanism of a flood event in October 2023 was also examined. The results indicate that: (1) The tidal asymmetry in the Dafengjiang Estuary is dominated by the O1/K1/M2 constituent combination, with its skewness contribution rate increasing gradually from the estuary mouth to the offshore area (38%–79%), exhibiting an overall ebb‑dominant characteristic. (2) Runoff input raises the residual water level near the estuary mouth by 0.1–0.3 m and increases the peak ebb current velocity by up to 0.9 m/s. Consequently, the amplitudes of O1 and K1 decrease, the K1 phase lag changes significantly, while the M2 amplitude slightly increases and its phase lag decreases by approximately 4°. (3) Statistical correlation analysis shows that both the amplitude and phase‑lag of the M₂ tidal constituent exhibit consistent significant negative correlations with skewness, serving as a steady source driving ebb‑dominated tidal asymmetry. The phase‑lags of O₁ and K₁ are positively correlated with skewness in the middle‑upper reaches, providing statistical evidence that runoff modulates tidal asymmetry via phase adjustment. Variations in correlation patterns at offshore stations reflect the transition of dynamic mechanisms from “runoff‑modulated phase” to “domination by diurnal tides themselves”. (4) During the flood event, the skewness of the O1/K1/M2 combination first decreases by 0.07 and then increases by 0.17, with a response lag of about 5–7 days, the larger the runoff discharge, the more pronounced the effect. This study reveals the physical mechanisms by which runoff modulates tidal asymmetry in small‑ and medium‑sized estuaries and provides a new theoretical perspective for tidal dynamic studies of mixed‑tide estuaries along the coast of the Beibu Gulf.

Key words: Dafengjiang Estuary, tidal asymmetry, runoff, numerical simulation, skewness.