Integration of models using shared state variables: Implementation in the regional hydrologic modelling system SIMGRO

SIMGRO is an integration hub that connects to diverse hydrologic models. Its coupling method aims to describe the hydrologic feedbacks adequately at an acceptable cost of the increased computational load. The achieved balance between accuracy and efficiency is investigated in the paper. The coupling method centres around a shared state variable of two connecting subsystems. A prime example of such a variable is the elevation of the phreatic surface that forms the natural demarcation between the saturated groundwater flow and the unsaturated flow in the soil. In our scheme the shared variable is alternately updated by the connecting submodels, with both models using the same combined storage relationship. This method has also been implemented for the linkage between the ponding water of column models and surface water models. Using a basin-scale model, the scheme was subjected to a time-step sensitivity analysis. Results were compared to a benchmark, for which we took the same scheme, but with a short time step of 0.125 d. Increasing the time step to 1 d caused a numerical error of less than 5% in the simulated regional design discharge. Errors in the simulated 95th percentile of highest groundwater levels are less than 0.05 m. The coupling method presented here is a key element of the Netherlands Hydrological Modelling Instrument. The current version involves half a million spatial units of 250 × 250 m.