This chapter presents an expert qualitative assessment of climate impacts on diffuse substances from UK agriculture, focusing on hydrological mobilization processes. The assessment was carried out through five activities: definition of objectives and system with stakeholders, selection of expert working group, facilitated systemic expert group elicitation, systematic review of the peer reviewed literature and synthesis, critical reflection and communication. The assessment is based on a review of projected changes in UK climate by 2020 and three representative model farm systems (arable, lowland dairy, and upland sheep). In general, mobilization of diffuse substances is likely to be most vulnerable to climate change in lowland dairy systems, followed by upland sheep and arable systems, respectively. Mobilization in the form of solubilization was greater than detachment in all systems and has the greatest levels of uncertainty. Research is required to increase our knowledge of how climatic drivers at the event, season, and annual scales will alter solubilization from the soil pore to farm scales. There is a need for long term research platforms that combine process and system experimentation and modeling to compare the responses of different agricultural systems to the impacts of climate change. This methodology is potentially applicable to agricultural systems in other regions of the world, given a peer reviewed knowledge base for the systematic review and appropriate expertise that covers the domain of the assessment.