WALRUS: Wageningen Lowland Runoff Simulator

The Wageningen Lowland Runoff Simulator (WALRUS) is a novel rainfall-runoff model for catchments with shallow groundwater. It fills the gap between complex, spatially distributed models used in lowland catchments and simple parametric models developed for sloping catchments. WALRUS explicitly accounts for processes important in lowland areas: groundwater–unsaturated zone coupling, wetness-dependent flow routes, groundwater–surface water feedbacks, and seepage and surface water supply. WALRUS consists of a coupled groundwater-vadose zone reservoir, a quickflow reservoir, and a surface water reservoir. It is suitable for operational use because it is computationally efficient and numerically stable. Only four parameters require calibration, and defaults can be changed for research purposes. The WALRUS code and user manual are available as an R-package on GitHub. WALRUS and its first applications have been published in peer-reviewed scientific journals, a PhD thesis, and a Dutch journal.