Groundwater levels at Cabauw have been measured since August 2003 in two fields south of the tower. Nine piezometers were placed in a row. Initially, 6 of those piezometers contained pressure transducers (of the brand Keller) to measure levels automatically every 4 hours. On 9 July 2010, the temporal resolution was increased to hourly. Several sensors have stopped working since 2003. Piezometers that were not measured automatically were measured during each filed visit, which was initially every 2 weeks, but has become longer towards the end of the measurement period.