Subsidence rates in the Po Plain, northern Italy: the relative impact of natural and anthropogenic causation

The central-eastern Po Plain in northern Italy is a rapidly subsiding sedimentary basin that hosts about 30% of the Italian population and 40% of Italyʹs total productive activities. Subsidence rates range from 0 to −70 mm/year, the maximum occurring in synclinal areas at the Po Delta and near Bologna, the minimum located at the top of buried, probable tectonically active anticlines (Mirandola–Ferrara). We show that modern subsidence is at least an order of magnitude higher than due solely to long-term natural processes. This implies that most subsidence in the Po Plain has been induced by human activities. As previously noted, we suggest that the main factor controlling modern subsidence is water withdrawal, which was particularly intense during the second half of the 20th century, coinciding with accelerating economic growth. We also evaluate the impact of rapid subsidence on floods in low areas by comparing subsidence velocity maps with flood maps. This shows that there is a clear-cut correlation between flood frequency and rapid subsidence. In contrast, few floods occurred in low subsidence areas, which generally correspond with the top of buried anticlines. We finally note that the anthropic-caused increase in subsidence has now greatly increased the potential for additional floodings.