Monitoring the evolution of sealed surfaces in Flanders (Belgium) with annual high-resolution soil sealing maps

Soil sealing results in an increased flood risk, reduced water infiltration, higher temperatures in urban areas, decreased carbon storage and biodiversity losses. As the Flanders region in Belgium has one of the highest soil sealing rates in Europe, it is especially prone to these effects. Hence, sustainable spatial management is crucial to ensure a healthy living environment and address climate change. The Flemish government therefore wants to promote de-sealing and limit additional sealing. An example of a concrete policy measure is that by 2050 sealed surfaces in open space zoning have to be reduced by 20% compared to 2015, which corresponds with a decrease of 9 000 ha.  
In order to monitor the evolution of sealed surfaces, and thus the effectiveness of sealing-related policy measures, a method was developed for automatically generating annual and spatially detailed soil sealing maps of Flanders. These maps combine “known” sealing from administrative databases (buildings and transport infrastructure) with modelled sealing based on artificial intelligence. Administrative databases do not (adequately) cover parking lots, private driveways and garden terraces, which are a substantial part of the sealed area in Flanders. Hence, a machine learning model was built for deriving this remaining sealing from 25 cm resolution aerial imagery. For this purpose, an assessor manually labeled the sealed parts on a subset of the images. Based on this training set, a convolutional neural network model was used to produce a sealing probability map, which was converted to a binary modelled sealing map. Finally, a continuity correction was applied to ensure a temporally consistent result across the yearly maps.            
According to this method, 1 m resolution soil sealing maps were obtained for 2013 until 2021. These maps show that an additional area of sealed surfaces of approximately 15 000 ha was constructed in Flanders between 2013 and 2020. They also reveal that de-sealing takes place in Flanders every year, but also is always over-compensated by new sealing. As a result, there is a net increase of soil sealing year after year. The annual soil sealing maps also show an upward trend in open space zoning that moves in the opposite direction to the target of decreasing by 9 000 ha.