Abstract. We present the Wageningen Lowland Runoff Simulator (WALRUS), a novel rainfall–runoff model to fill the gap between complex, spatially distributed models which are often used in lowland catchments and simple, parametric (conceptual) models which have mostly been developed for sloping catchments. WALRUS explicitly accounts for processes that are important in lowland areas, notably (1) groundwater–unsaturated zone coupling, (2) wetness-dependent flow routes, (3) groundwater–surface water feedbacks and (4) seepage and surface water supply. WALRUS consists of a coupled groundwater–vadose zone reservoir, a quickflow reservoir and a surface water reservoir. WALRUS is suitable for operational use because it is computationally efficient and numerically stable (achieved with a flexible time step approach). In the open source model code default relations have been implemented, leaving only four parameters which require calibration. For research purposes, these defaults can easily be changed. Numerical experiments show that the implemented feedbacks have the desired effect on the system variables.