Two ways of connecting numerical hydrological models that have significantly differently sized calculation time steps are compared. A model for rainfall-runoff and surface water flow (SOBEK) is connected to a model for groundwater flow (TRIWACO). Data exchange between the two models takes place at the end of each larger time step. In the "explicit" connection, both models calculate every time step only once, after which the algorithm moves to the next time step. In the "implicit" connection, the same time step is recalculated with the exchanged data until the resulting exchanged values converge. Due to the iteration, implicit connections require more calculation time, which can only be justified if it leads to significant improvements of model predictions. By simulating the hydrological situation of the Huewelerbach basin in Luxembourg, this work shows that implicit and explicit connections can indeed lead to significantly different calculation results.