Abstract
Abstract. 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, groundwater–surface 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. Here, we apply WALRUS to two contrasting Dutch catchments: the Hupsel Brook catchment and the Cabauw polder. In both catchments, WALRUS performs well: Nash–Sutcliffe efficiencies obtained after calibration on 1 year of discharge observations are 0.87 for the Hupsel Brook catchment and 0.83 for the Cabauw polder, with values of 0.74 and 0.76 for validation. The model also performs well during floods and droughts and can forecast the effect of control operations. Through the dynamic division between quick and slow flowroutes controlled by a wetness index, temporal and spatial variability in groundwater depths can be accounted for, which results in adequate simulation of discharge peaks as well as low flows. The performance of WALRUS is most sensitive to the parameter controlling the wetness index and the groundwater reservoir constant, and to a lesser extent to the quickflow reservoir constant. The effects of these three parameters can be identified in the discharge time series, which indicates that the model is not overparameterised (parsimonious). Forcing uncertainty was found to have a larger effect on modelled discharge than parameter uncertainty and uncertainty in initial conditions.
Highlights
The parameter values obtained during the calibration runs described in the previous section were used in validation studies for whole years, a short period to focus on groundwater dynamics, major flood and drought events, and a case with management operations
This does not mean that cV is superfluous – cV controls the delaying influence of the unsaturated zone, which is not visible unless one zooms in on individual discharge peaks
A first step in this direction is given in Fig. 4, where the Nash–Sutcliffe efficiency is plotted as a function of each individual parameter
Summary
Lowlands exist all over the world (often in river deltas; Fan et al, 2013) They are generally densely populated and centres of agricultural production, economic activity and transportation. There is growing awareness that for simulation and prediction of water and energy fluxes in lowland areas, models need to account explicitly for the dynamic groundwater table (Alley et al, 2002; Maxwell and Miller, 2005; Kollet and Maxwell, 2006; Bierkens and van den Hurk, 2007; Maxwell and Kollet, 2008). Surface water networks are generally dense, and surface water levels influence drainage fluxes and groundwater levels (Sophocleous and Perkins, 2000). These groundwater– surface water interactions are important in both freely draining catchments and polders.
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