The fragmentary development of saturated zones in the early stage during the storm event was controlled by soil depth; subsequently, the lateral transfer of saturation was controlled by the topographic wetness index at the soil-bedrock interface, showing an "upper-lower" and "fragment-integration" process. We suggest that it is preferable to define the variable source area by soil depth and the topographic wetness index at the soil-bedrock interface than by the distance to the valley bottom, as in the traditional variable source area concept