Re-evaluating the WALRUS rainfall-runoff model design after twelve years of application

In 2014 we developed the Wageningen Lowland Runoff Simulator (WALRUS), a conceptual rainfall-runoff model for catchments with shallow groundwater. Water managers and consultants were involved in model development. In addition, they sponsored the steps necessary for application: making an R package, user manual and tutorial, publishing these on GitHub and organising user days. WALRUS is now used operationally by several Dutch water authorities and for scientific studies in the Netherlands and abroad. When developing the model, we made certain design choices. Now, after twelve years of application in water management, science and education, we re-evaluate the consequences of those choices.

The lessons can be divided into things we learned about the model’s functioning and things we learned from how people use the model. Concerning the model’s functioning, we found that keeping the model representation close to reality has advantages and disadvantages. It makes it easy to understand what happens and why, but it also causes unrealistic expectations. Certain physically based relations hampered model performance because they contained thresholds, and deriving parameter values from field observations resulted in uncertainty and discussions about spatial representativeness.

Concerning the practical use, we found that the easy-to-use, open source R package with manual was indispensable for new users. Nearly all users preferred default options over the implemented user-defined functions to allow tailor-made solutions. Parameter calibration was more difficult than expected because the feedbacks necessary to simulate the hydrological processes in lowlands increase the risk of equifinality. In addition, lack of suitable discharge data for calibration prompted the request for default parameter values. Finally, the model was subject to unintended model use, sometimes violating basic assumptions and sometimes showing unique opportunities we had not thought of ourselves.

C.C. Brauer, A.J. Teuling, P.J.J.F. Torfs, R. Uijlenhoet (2014): The Wageningen Lowland Runoff Simulator (WALRUS): a lumped rainfall-runoff model for catchments with shallow groundwater, Geosci. Model Dev., 7, 2313-2332, doi:10.5194/gmd-7-2313-2014