LANDMARK (LAND MAnagement for flood RisK reduction in lowland catchments) will evaluate the effectiveness of realistic and scalable land-based NFM measures to reduce the risk from flooding from surface runoff, rivers and groundwater in groundwater-fed lowland catchments. We will study measures like crop choice, tillage practices and tree planting, that have been identified by people who own and manage land, to have the greatest realisable potential. NFM measures will be evaluated for their ability to increase infiltration, evaporative losses and/or below-ground water storage, thereby helping to store precipitation to reduce surface runoff and slow down the movement of water to reduce peak levels in groundwater and rivers. However, we need to carefully examine the balance between increased infiltration, soil water storage and evaporative losses under different types of NFM measures, because long-term increases in infiltration could actually increase groundwater and river flood risk if there is less capacity within the ground and in rivers to store excess precipitation from storm events. Also, following a review of the available research to date, other researchers (Dadson et al, 2017) came to the conclusion that land-based NFM measures would only provide effective protection against small flood events in small catchments. As the catchment size and flood events increase, the effectiveness of land-based NFM measures in reducing flood risk would decrease significantly. However, this idea needs to be tested further. Currently, there are many unanswered gaps in knowledge that make it hard to include land-based NFM measures in flood risk mitigation schemes. The Environment Agency tell us that there are no case studies on land-based NFM measures to support decision making, with most focusing on leaky barriers made from trees. Yet, land-based NFM measures have potential to do more than just reduce flood risk, including improving water quality, biodiversity and sustainable food and fibre production. So in LANDMARK, we will carry out research to help to fill this evidence gap, and test the ideas Dadson et al. proposed about land-based NFM using the West Thames River Basin as a case-study area. We will work at three spatial scales (field, catchment and large river basin) and explore modelling scenarios, developed with people who own and manage land and live at risk of flooding, to look at how land-based NFM could affect flooding. Scenarios will include experiences in the recent past in July 2007 and over the winter of 2013-14, and how future land use and management could affect flood risk in 2050 as the climate changes. We will consider how government policy could change after we leave the EU to support land-based NFM. Work will be carried out in five stages: (1) we will bring together available maps, data and local knowledge on current land use and management, and use this to create scenarios for modelling experiments to explore land use and management measures impact on events from the past and in the future; (2) we will make measurements to see how below-ground water storage and infiltration vary between different land-based NFM in fields where innovative land management is being practiced; (3) we will collect data from sensors sitting above the ground, flying on drones and on satellites to see how vegetation and soil moisture vary across large catchment areas; (4) we will use all the data collected from 1-3 to run modelling experiments across a range of scales, linking together models that capture soil and vegetation processes, overland and groundwater flows and catchment hydrology, exploring variation in model outputs; and (5) we will create web applications to display and explore the outputs from the modelling experiments. All this work will be supported by workshops, field visits, reports and resources to support people and their learning about how land-based NFM measures work and could be used to reduce flood risk.