The role of soil in the generation of urban runoff: development and evaluation of a 2D model

A two-dimensional numerical model is developed to determine the role of soil in the formation of urban catchment runoff. The model is based on a modeling unit, called the Urban Hydrological Element (UHE), which corresponds to the cross-section of an urban cadastral parcel. Water flow in the soil of a UHE is explicitly simulated with a finite element code for solving the Richards' equation. Two runoff components, dependent on soil behavior, are represented: runoff from natural surfaces and drainage of groundwater into the rainwater network. In an initial case study, the model is applied to a 4.7-ha suburban catchment. Simulated and observed runoff and soil water pressure heads show reasonable agreement. Soil appears to play a significant role in the formation of runoff at the scale of the small catchment under examination: its contribution represents an average of 14% of the total per-event runoff volume. Soil contribution is particularly important during rainfall events characterized by a shallow water table level, which explains a determinant part of the seasonal trend of catchment response.