Modelling the interactions between transient saturated and unsaturated groundwater flow. Off-line coupling of LGM and SWAP

The groundwater flow model for the Netherlands (LGM), and a one-dimensional model of soil water flow (SWAP) were coupled. With this combined model, it is possible to calculate fluxes and residence times of nutrients and pesticides in both the unsaturated zone and the phreatic aquifer. The model can also predict the seasonal dynamics of the groundwater table. In order to correctly simulate these seasonal dynamics, both LGM and SWAP are used in transient mode. The performance of the model was tested in a regional-scale model application. There was generally a good agreement between the mean depth of the groundwater table, as simulated with SWAP, and the mean depth of the groundwater depth as simulated by LGM. The seasonal dynamics of the groundwater table were, however, underestimated by LGM. Further investigation showed that correct transfer of the phreatic storage coefficient was the key factor for correctly predicting the seasonal dynamics of the groundwater table. With the correct storage coefficient, the correspondence between the groundwater heads simulated by SWAP and the groundwater heads simulated by LGM were nearly perfect. An additional study should show whether there is also a good agreement with observed groundwater heads. Results from this study can be used to conclude upon the applicability of the adopted methodology for both ecohydrological studies and water quality assessments as required by the Netherlands Environmental Assessment Agency