The importance of system interactions in hydrodynamic models of parts of complex interconnected deltas

Climate change affects river deltas worldwide. Hydrodynamic models are used to study these effects. However, choosing the spatial scale and boundary conditions for these models is complex due to interconnectivity within river deltas. We study how boundary conditions of a model covering only part of such systems are impacted by changing conditions outside of the domain. We couple different components of the Dutch river delta into a model covering the complete delta, and force it with a range of river discharges and sea levels. Results show that the impact depends on the distance to the boundaries, as well as the relative (upstream) discharge in the considered rivers. As these differences are found to propagate far upstream, these findings underline the importance of choosing appropriate downstream boundaries when modelling water levels in parts of interconnected systems influenced by changing conditions outside the modelled domain (e.g. sea level rise or changing hydrographs). • Modelling part of a river delta does not capture relevant interactions within deltas. • Extending the modelling domain reduces the impact of uncertain boundary conditions. • Impact of uncertain boundary conditions increases for decreasing upstream discharge. • Stage-discharge relations are strongly impacted by climate change scenarios.