Sustainable Intensification of Rice Agriculture in Vulnerable Mega-Deltas: A Global Challenge The world's major river deltas - hotspots of agricultural production that support rural livelihoods and feed much of the global population - are facing a major sustainability crisis. This is because they are under threat from being 'drowned' by rising sea levels, with potentially severe consequences for the 500 million people who live and work there. In particular, the process of 'drowning' means that deltas are rapidly losing land (up to 20% of land is projected to be lost to sea-level rise by 2100 in the major deltas of south and southeast Asia alone), while simultaneously exacerbating problems of flooding and soil salinization. These problems are creating a 'perfect storm' that makes agriculture increasingly challenging, at precisely the time when the pressure is increasing for these rich, fertile, landscapes to produce more food to support rapidly growing global populations.   The world's third largest delta, the Mekong, is SE Asia's rice basket and home to almost 20 million people, but it is being exposed to severe environmental risks as a result of climate change and rapid economic development, most notably from the development of hydropower dams in the Mekong's catchment upstream which are cutting off the supply of sediment to the delta. The Mekong is therefore not only representative of many of the issues facing the world's deltas, but reliable data are urgently needed to help inform the sustainable management plans required to provide a safe operating space for the delta's inhabitants. In our prior work we have demonstrated that flows of water, sediment and associated nutrients within and through deltas are critical to the resilience of rice cultivation strategies. The sediments that are deposited in the delta help to offset sea level rise and they are very fertile because of the abundant nutrients (nitrogen and phosphorous) they contain. The key issue that is the focus of this proposal is that many of the Mekong delta's poor farmers (over 3.5 million farmers live below the poverty line) rely on the free fertilisation provided by river sediment deposition to reduce their 'input' costs (the portion of their income that is spent on purchasing and applying artificial fertiliser to maintain rice production). There is therefore a trade-off between the positive effects (delta building and free fertilisation) of natural sediment deposition versus the negative effects of the flooding process that causes it. As sediment and nutrient fluxes decline in the future (as a result of sediment trapping by dams upstream), new approaches are needed to inform adaptation strategies (such as managed flooding) to ensure that vulnerable communities can continue to farm sustainably in the future.   In this proposal we will collaborate with Vietnamese partners to bring UK expertise in (i) the modelling of floods, sediment transport and nutrient fluxes; (ii) agricultural livelihoods; (iii) participatory stakeholder engagement processes and (iv) social-ecological systems dynamics to bear on this challenge. By bringing together this blend of expertise and working closely with our Vietnamese colleagues and stakeholders we will be able to define policy relevant scenarios of future change, quantify the links between flooding, sediment and nutrient deposition and agricultural livelihoods, and develop new modelling tools that will be able to evaluate the trade-offs between flooding and nutrient supply as the future environment changes. We will be able to concept-proof a new approach that can deliver an integrated assessment of the factors driving changes to livelihoods and explore the effects of adaptations that could enable more sustainable intensification of rice agriculture. This will be done within a globally significant, iconic, delta, providing a template for similar analyses in other vulnerable deltas in the Global South. The Global Challenges Research Fund (GCRF) supports cutting-edge research to address challenges faced by developing countries. The fund addresses the UN sustainable development goals. It aims to maximise the impact of research and innovation to improve lives and opportunity in the developing world. The fund addresses the UN sustainable development goals. It aims to maximise the impact of research and innovation to improve lives and opportunity in the developing world.