The Wageningen Lowland Runoff Simulator (WALRUS): a lumped rainfall–runoff model for catchments with shallow groundwater

Abstract

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) wetnessdependent 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.

The Wageningen Lowland Runoff Simulator (WALRUS): application to the Hupsel Brook catchment and the Cabauw polder

“…The initial groundwater depth was calibrated together with the parameters. The other initial states followed from the observed discharge at the start of the period, the stage-discharge relation and the model equations and parameters (see Brauer et al, 2014). Several choices of objective functions are compared in Sect.…”

“…The Wageningen Lowland Runoff Simulator (WALRUS) is a new parametric (conceptual) rainfall-runoff model which accounts explicitly for processes that are important in lowland areas, such as groundwater-unsaturated zone coupling, wetness-dependent flowroutes, groundwatersurface water feedbacks, and seepage and surface water supply (see companion paper by Brauer et al, 2014). Lowland catchments can be divided into slightly sloping, freely draining catchments and flat polders with controlled water levels.…”

The evolution of process-based hydrologic models: historical challenges and the collective quest for physical realism

“…Another class of methods consists of scale-dependent parameterizations, where new flux parameterizations are defined directly on the scale of interest. Examples of this class of methods include the empirically derived storage-discharge relationships described earlier, where the large-scale transmission of water is often defined as a linear (or near-linear) function of water storage (Ambroise et al, 1996;Clark et al, 2008;Fenicia et al, 2011;Brauer et al, 2014). Similarly, large-scale stability corrections, used in computations of land-atmosphere energy fluxes, implicitly represent the impact of local pockets of instability on large-scale fluxes (Mahrt, 1987).…”

Effect of Unsaturated Flow Model Conceptualization on the Dynamic Response of an Integrated Distributed Hydrological Model

“…Physically based distributed models operate on a numerical mesh and are traditionally based on Richards' equation for unsaturated flow. In recent developments this equation forms the basis for solving subsurface flow in three dimensions (Brauer et al, 2014) while earlier developments assume one-dimensional flow in the unsaturated zone and independent flow solutions are thus used for transmitting precipitation falling at the ground surface to the groundwater table (Barron et al, 2013).…”