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Terrestrial watersheds are considered to be the main mercury source of the receiving water bodies.Surface runoff is the primary pattern of mercury output for most terrestrial watersheds.Early studies demonstrated that the soil erosion by runoff is an important source of mercury to the receiving water bodies.However,there still exist huge margins in the study on migration and output of mercury in rainfall runoff in agricultural watersheds.We conducted high-frequency sampling and monitoring of gulley of overland runoff from a typical small agricultural watershed in Three Gorges Reservoir Area,aiming to understand the effect of the first rainfall after the summer drought on total mercury(THg)and methylmercury(TMeHg)outputs in runoff and analyze the first flush effect of mercury.The concentrations of THg and TMeHg in rainfalls were 13.12±0.91 ng L-1 and 0.177±0.034 ng L-1,and the wet deposition flows of THg and TMeHg were 3.67 mg ha-1 and 0.050 mg ha-1,respectively.The concentration ranges of THg and TMeHg in rainfall runoff were 3.89-52.55 ng L-1 and 0.063-2.974 ng L-1,respectively.The change in runoff quickly led into a change in THg concentration from the runoff.The maximum value of THg concentration appeared at the early stage of runoff(within 6 hrs),and then decreased quickly to a relatively low level.The concentrations of THg and TMeHg at the early stage of runoff were remarkably higher than those at the later stage of runoff.During the observation period,the output flows of THg and TMeHg were 7.33 mg ha-1 and 0.354 mg ha-1,respectively,primarily in particulate.Although the runoff rate in the study area was only 14.56%,the THg and TMeHg both showed the characteristic of net output,with retention rates being-22.27%and-82.05%,respectively.However,we could only make a rough estimate on the wet deposition of mercury in this rainfall because this rainfall only represented 8%of the total average annual rainfall.In addition,the concentrations of THg and TMeHg in rainfall were both lower than those in relevant studies,therefore,this estimate may be relatively low,and requires further study.Furthermore,THg and TMeHg both showed relatively obvious first flushing effects.