Many low-lying river deltas, home to over 500 million people, host vast areas of intensely used land surface that are subsiding due to natural causes and human-induced activities and releasing greenhouse gasses (GHG) in the process. For the 21st century, predictions of subsidence rates in populated deltas outweigh those of sea-level rise due to global warming, up to an order of magnitude (e.g. Erkens et al. 2015; Galloway et al. 2016). Reducing subsidence rates and adapt to and mitigate its effects, is possible by implementing feasible, legitimate and sustainable solutions based on an integrative approach of fundamental knowledge of

• physical subsidence processes,
• socio-economic impacts of subsidence, and
• governance and legal frameworks.

Such an approach will increase the availability and performance of measures and shorten the implementation time, leading to reduction of adverse consequences (avoiding economic damage) and enabling societies to mitigate and adapt to subsidence in the short and in the long term.

The physical consequences of subsidence are manifold: increased flood risk, flood water depth and flood duration, relative shallowing of groundwater tables, salinization of ground and surface water and emission of GHG. For delta societies this leads to serious economic loss through damage to buildings and infrastructure, reduced crop yields, land loss and floods generating potential health issues to live stock and citizens. For the Netherlands, in terms of the countries polder water management, economic damage of progressive subsidence could add up to 22 billion Euros by 2050 (Van den Born et al. 2016).

Combined with the attention for water safety and GHG emission reduction, in the Netherlands governmental awareness of the issue of land subsidence has recently been on the rise. It is now considered a national scale problem, requiring upgraded regional management strategies, supported by national knowledge infrastructure, besides national scientific attention (including in this project). Progressing subsidence has spawned a range of mitigation and adaptation measures that are mostly local technical measures, applied ad-hoc in response to incidents.

In urban and agricultural areas alike, such measures patch subsidence problems locally for a few years, but are not mid-term or long-term solutions, because measures and choices of implementation are not informed enough to sustainably tackle the subsidence problem. Hence, problems re-appear or are only moved to neighbors. One reason for this is that the relative contributions of the various drivers of subsidence are not yet understood: physical understanding of the temporal variability and interactive effects of coeval processes causing subsidence is lacking.

This makes that mitigation measures and their effectiveness are hard to assess for stakeholders and responsible authorities, affecting advice and choice of measures and stalling the implementation of proper measures. One may opt once again for a short-term patch and is likely to postpone decisions for the long term in such situation. Implementation of mid-term and long-term measures to mitigate or adapt to subsidence over the large areas on which the problem is occurring, poses serious governance, economic, financial and legal challenges. They may require political choices that deviate drastically from existing and historical policies.

Adaptive and mitigating measures would enable to continuate and reconciliate certain land-use functions (agriculture, nature conservation), but it is challenging to design and finance these within technical, economical and socially-acceptable limits as areal measures tend to impact numerous interests and stakeholders. Hence, beyond physical knowledge and method design, an urgent need exists to develop (innovative) subsidence management strategies and to show politicians and stakeholders what the impact of their management decisions will be on the mid and long term.