GROUND WATER MANAGEMENT AND ITS CONSEQUENCES IN DELFT, THE NETHERLANDS AS OBSERVED BY PERSISTENT SCATTERER INTERFEROMETRY Freek J. van Leijen and Ramon F. Hanssen Delft University of Technology, Delft Institute of Earth Observation and Space Systems (DEOS), Kluyverweg 1, 2629 HS Delft, The Netherlands, Email: F.J.vanLeijen@tudelft.nl ABSTRACT The city of Delft, The Netherlands, is very vulnerable for ground water. A better understanding of the rela- tion between ground water level (fluctuations) and sur- face displacement would assist future water management. The displacement field in Delft is analyzed by Persistent Scatterer Interferometry (PSI) using a periodic displace- ment model. Both ERS-1/2 and Envisat SAR data are ob- served, covering a period between 1992 and 2007. Com- parison with ground water level measurements show high correlation with the PSI time series. The displacement signals have a delay of about 1 month with respect to the ground water fluctuations. Key words: Persistent Scatterer interferometry; Ground water. 1. INTRODUCTION Infrastructure in Delft, the Netherlands, is threatened by ground water. As large parts of the country, Delft is sit- uated below mean sea level and maintenance of ground water levels is therefore of high importance. Subsidence in combination with ground water level rise has resulted in severe problems, especially in the last 20 years. Cur- rently, the difference between ground level and mean ground water level is at certain locations only 50-60 cm, with even lower margins during periods of heavy rain, see Fig. 1. Especially buildings in the historical city center are vulnerable for these relative high ground water levels. Apart from natural causes, artificially induced ground water level fluctuations and subsidence deteriorate the sit- uation. An industrial complex in Delft has been with- drawing ground water since 1916. The amount of ex- traction has changed over time and has a seasonal cycle. The withdrawal effects the relative ground water level and causes subsidence. However, due to changes in the pro- duction process in 2004 the use of ground water has be- come unnecessary. As instant termination of the extrac- tion process would have a severe impact on the already sensitive ground water situation, a gradual decrease of the extraction volume is proposed. However, because of the unique soil decomposition in the Dutch coastal area (sand, clay, peat), even the effects of a gradual decrease cannot be predicted very well. A more profound under- standing of the relation between ground water extraction, subsidence (or uplift) and ground water level is therefore necessary. In this contribution Persistent Scatterer Interferometry (PSI) results of the city of Delft are used to deduct the re- lation between ground water fluctuations and subsidence. Since 2000 ground water levels at 155 locations in Delft are measured automatically on an hourly basis, providing a unique opportunity to relate subsidence with the actual ground water level. Using ERS-1/2 as well as Envisat SAR data, the full period between 1992 to 2007 is ana- lyzed. Figure 1. Examples of high level of (ground) water in Delft, The Netherlands. The lower right image shows the historical city center of Delft, which contains a lot of canals. 2. PSI USING PERIODIC FUNCTIONS The ERS-1/2 and Envisat data sets of Delft are analyzed accounting for a periodic signal in the data. This addi- ____________________________________________________________ Proc. of FRINGE 2007 Workshop, Frascati, Italy, 26 – 30 November 2007 (ESA SP-649, February 2008)