Given the complicated spatial distribution of impervious ground and its multiple flow routing paths in urban areas

the effectiveness of the impervious area and its runoff generation mechanism has become one of the hot topics in urban hydrology. In this work

we examine the core urban area of Beijing

construct a method for identifying its directly connected impervious (DCI) areas based on flow routing paths

and fit empirical equations for calculating its total impervious area and the DCI area. Based on the equations

the influence of DCI areas on the city’s urban rainwater runoff is quantitatively analyzed. The results show the accuracy of simulations is improved significantly by considering DCI areas. In different stages of urbanization

if different proportions of DCI areas are used to simulate the scenarios under the rainfalls with various return periods

DCI areas have a greater effect on the runoff process of a shorter recurrence period than that of a longer one. The increase in the DCI area in the early urbanization period leads to large peak discharge growth rates

but its effect decreases in the late stage. This work is useful to urban landform pattern design and urban flooding management.