Alterra and TNO have recently developed an OpenMI (Open Modeling Interface) style coupling between MODFLOW and Alterra's top-system model SIMGRO. SIMGRO includes the simulation of plantatmosphere interactions, the unsaturated-saturated zone (phreatic storage coefficients that depend dynamically on the soil water content), surface water flows and levels, drainage and surface runoff, thus providing an alternative for the MODFLOW packages RCH/EVP, DRN, RIV, and GHB. The resulting coupled model is especially appropriate for regions with shallow water tables where groundwater and surface water interact strongly and actually influence each other’s state variables, like in river deltas. SIMGRO calculates surface water levels, drainage and recharge using its own small time step; the fluxes are then summated over the time step of MODFLOW. The use of modeling functions is data driven and thus can be varied per cell and layer combination. If one is not interested in the surface water interactions of SIMGRO they can be left out. If on the other hand one is more interested in state-of-art surface water modeling, the MODFLOW-SIMGRO model can be used as a refined rainfall-runoff model, which can for instance be integrated with the OpenMI compliant version of the SOBEK channel flow model of Delft Hydraulics.