A new two-dimensional (2D) distributed parameter model is developed to simulate overland flow in two small semiarid shrubland watersheds in the Jornada Basin, southern New Mexico. The model is event-based and operates at the scale of the individual shrub, each watershed being represented as a set of 1-m² cells. In the model flow directions and volumes are computed by a second-order predictor-corrector finite difference scheme (Davis, 1988), which is employed to solve the two-dimensional kinematic wave equation. Thus, flow routing is computed implicitly and may vary in response to flow conditions. The model uses only six parameters for which values are obtained from field surveys and rainfall simulation experiments. The model underpredicts runoff from the watersheds due to the measured values of saturated hydraulic conductivity K_s for intershrub areas being too high. This is because the runoff plot experiments used to estimate K_s were conducted at the beginning of summer on surfaces with degraded seals, whereas most summer storms occur on surfaces that have experienced recent rainfall and have better developed seals. Model performance is much improved when K_s is treated as a calibration parameter. The importance of runon infiltration in supplying water to shrubs is investigated for a range of rainfall and antecedent soil moisture conditions using the 2D model. On average, runon infiltration accounts for between 3 and 20% of the total infiltration in the soil under a shrub. The most favorable conditions for runon infiltration are an initially wet soil and a low mean rainfall rate.