The phreatic surface is an important factor of influence on the stability and therefore the safety of regional flood defences. For stability calculations usually a schematisation of the phreatic surface is used, based on the geometry of the flood defence and the water levels. The influence of precipitation and evapotranspiration is not explicitly included in these schematisations. This thesis aims to determine whether it is possible to simulate the phreatic surface in a regional flood defence using a numerical model, which would make it possible to include these factors in the determination of the phreatic surface. Three MODFLOW models were constructed for research cases for which measurements of the phreatic surface were available. Considering the necessary simplifications needed to construct the models, the models simulate the head measurements with sufficient accuracy. However, this sufficient accuracy was only reached after extensive calibration of the model to the measurement set. Using the calibrated models several further analyses were carried out to determine what type of precipitation event causes the largest rise of the phreatic surface. Furthermore, some potential improvements of the schematisation of the phreatic surface were identified.