Study region: Liulin Experimental Watershed, a semi-arid, monsoonal, forested headwater watershed in North China. Study focus: Flood processes in semi-arid watersheds often exhibit strong rainfall-runoff nonlinearity that cannot be adequately captured by traditional time-invariant unit hydrograph (UH) approaches. To address this limitation, a variable unit hydrograph (VUH) framework is developed. The framework introduces dynamic adjustment of UH shape through two physically interpretable factors: an antecedent recession factor (alpha), representing the influence of prior runoff conditions, and a rainfall spatial distribution factor (beta), reflecting storm-center location. The VUH is coupled with a lumped runoff generation module and evaluated using 80 flood events. New hydrological insight for the region: Results indicate that runoff concentration plays a more dominant role than runoff generation in producing nonlinear flood responses in the Liulin Experimental Watershed. Antecedent flow conditions tend to increase peak discharge and advance peak timing, whereas upstream shifts of the storm center reduce peak discharge and delay peak timing. Compared with the conventional Nash UH, VUH improves validation performance (mean NSE from 0.51 to 0.60) and reduces both peak discharge error and peak timing error. The improvement primarily reflects enhanced representation of routing dynamics rather than correction of structural deficiencies in runoff generation. These findings underscore the importance of incorporating antecedent recession state and rainfall spatial variability into UH formulations for reliable flood simulation in semi-arid monsoonal watersheds.