In a landfill, the biodegradation of the organic fraction of the waste is driven by its moisture. Once the waste dump is covered, the infiltration through the cover layer is a source of additional water. In this study, the changes in water content in a landfill cover layer were investigated with long time-lapse electrical resistivity tomography (ERT) profiles. Measurements carried out during a rainfall event allowed the detection of zones with large infiltration events. We addressed all interpretation limits inherent to such a large scale, low resolution investigation with the use of synthetic modelling. Firstly, the smoothed resistivity changes distribution failed to detect small infiltration areas and induced a misestimation of the infiltration depth and volume in large infiltration zones. Secondly, when the background water content is not homogeneous, relative changes, as commonly used in literature, reflect both the background moisture distribution and the monitored changes. Thirdly, the temperature and pore fluid conductivity distribution are needed to compute the absolute water content changes. The latter better reflects the infiltration pattern. Lastly, rain water infiltration results in pore water dilution. When the dilution effect is not considered, the infiltrated volume is underestimated. This approach enabled us to detect a large infiltration zone that could be responsible for an important part of the annual water infiltration.