The Wageningen Lowland Runoff Simulator (WALRUS) was tested on the Reusel catchment to investigate if the model is applicable to medium sized catchments. Hydrological routing and coupling of subcatchments were applied to account for variation in local catchment characteristics.
Making WALRUS applicable for large catchments: a case study in the Reusel catchment
The Wageningen Lowland Runoff Simulator (WALRUS) is a lumped rainfall-runoff model, which only needs five calibrated parameters. In previous studies, it has already been tested on relatively small catchments (<10 km2). The objective is to investigate if WALRUS is able to simulate discharges correctly in a larger catchment, since the five parameters are representing local catchment characteristics. The Reusel catchment (145 km2) in the Netherlands is used as research area for this study. First the WALRUS parameters were calibrated for the entire catchment. After validation, an average Nash-Sutcliffe efficiency of 0.79 was found. WALRUS was also applied to three subcatchments, each with its own parameter set. An average Nash-Sutcliffe efficiency of 0.53 was the result of validation. A nonlinear reservoir model was used to investigate if routing of discharge time series from the subcatchments (observed or simulated) could improve simulation results for the entire catchment. Results of two (only available) hydrological years were contradictory, probably caused by observational errors. Another focus was on the coupling of subcatchments in WALRUS by using (observed or simulated) discharge of one subcatchment as input for the next subcatchment downstream. Testing this coupling on two hydrological years leads to acceptable results. Finally, WALRUS results were compared to already existing simulation data of the Wageningen Model for the entire catchment and one subcatchment. It was found that WALRUS is performing better, especially when focusing on peak discharges. In terms of Nash-Sutcliffe efficiency, average values of 0.70 (WALRUS) and 0.53 (Wageningen Model) were found for the entire catchment and 0.44 (WALRUS) and -0.67 (Wageningen Model) for the subcatchment. It can be concluded that WALRUS is able to simulate discharges in larger catchments, but more research can be done on hydrological routing and coupling of subcatchments.