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• Under the action of concentrated flow, the change of surface runoff of karst gently sloping farmland is mainly controlled by the magnitude of concentrated flow. • Under the action of concentrated flow, the underground runoff of karst gentle slope farmland is significantly negatively correlated with the slope. • Concentrated flow can lead to severe soil erosion in karst gently sloping farmland. Exposed bedrock in karst areas considerably alters runoff paths causing partial runoff to converge into a concentrated flow, which is a potential source of soil erosion risk. However, research is lacking on how concentrated flow affects regional soil erosion. In this study, the surface and underground dual structure in karst areas was simulated in a steel tank. Combined with runoff plot monitoring, the runoff conversion process and erosion characteristics of gently sloping farmland in karst areas under different water discharge rates were examined through indoor concentrated flow discharge scouring tests, and the influence of concentrated flow changes on the runoff spatial distribution and sediment yield in gently sloping farmland was clarified. The results demonstrated the following: (i) there existed a significant positive correlation between surface runoff and the water discharge rate (p < 0.01); the partial correlation coefficient value reached 0.87. The higher the water discharge rate, the more surface runoff formation was facilitated. (ii) When the slope remained the same, the underground runoff was relatively high at 3 L/min. Given the same discharge, a significant negative correlation between underground runoff and slope was found (p < 0.05); the partial correlation coefficient value reached −0.71. (iii) In response to a water discharge rate of 5 L/min, surface soil erosion was serious, and the maximum sediment transport modulus was 174.088 g/min∙m2. A significant cubic function relationship existed between the scouring time and surface sediment yield. Overall, a critical flow rate occurred between 3 and 5 L/min, which controlled changes in surface runoff, subsurface runoff and surface sediment yield. Under highly concentrated flow, drastic soil erosion could occur. These results can provide a scientific basis for the control and prevention of concentrated flow-induced erosion in karst areas. [ABSTRACT FROM AUTHOR]