Land subsidence observed in the Merchtem area (Flanders) – 30 years of SAR data associated to groundwater withdrawal?

A land subsidence affecting several towns at the joining limits of the Belgian Provinces of East Flanders, Antwerp and Flemish Brabant is followed during the last three decades. ERS 1-2, ENVISAT, TerraSAR-X and Sentinel 1A satellites SAR scenes were processed from 1992 till October 2020 to map the land subsidence evolution. The subsidence corresponds to a surface area of 220 km² during the ERS1/2 time interval distributed over three distinct subsidence bowls. During the ENVISAT and TerraSAR-X time interval, only one residual subsidence bowl was mapped affecting a surface area of about 70 km². Several towns (Londerzeel and Steenhuffel) remained in the center of the subsidence bowl. The annual average negative velocity values range between -5.99 and -0.5 mm/year. More recently, during the Sentinel 1A period, the subsidence bowl has lost half of its surface reaching 36 km². The velocity values have also decreased during the period 2016-2020.